chapter 3 basics of entomology - extension.unh.edu

CHAPTER 3BASICS OF ENTOMOLOGY

Basics of Classification .............................................................................................................................. 1Table 1. Classes of the Phylum Arthropoda .................................................................................................................... 2Table 2. Orders of the Class Insecta ................................................................................................................................ 2

Insect Form and Structure - Morphology .................................................................................................. 2

Head, Thorax and Abdomen ............................................................................................................................... 2Legs .................................................................................................................................................................................. 3Wings ............................................................................................................................................................................... 3Antennae .......................................................................................................................................................................... 4Mouthparts ....................................................................................................................................................................... 4

Insect Development-Metamorphosis ......................................................................................................... 4

Identifying Insects ....................................................................................................................................... 5

Insect Orders Important to the Gardener: ........................................................................................................... 6Coleoptera - Beetles, Weevils ......................................................................................................................................... 6Dermaptera - Earwigs ...................................................................................................................................................... 6Diptera - Flies, Mosquitoes, Gnats, Midges ..................................................................................................................... 6Hemiptera - Stink Bug, Plant Bug, Squash Bug, Boxelder Bug ...................................................................................... 6Homoptera - Scale Insects, Mealybugs, Whiteflies, Aphids, Cicadas, Leafhoppers. ..................................................... 7Hymenoptera - Bees, Ants, Wasps, Sawflies, Horntails .................................................................................................. 7Lepidoptera - Butterflies, Moths ....................................................................................................................................... 7Neuroptera - Lacewings, Antlions, Snakeflies, Mantispids, Dobsonfly, Dustywing, Alderfly .......................................... 7Orthoptera - Grasshopper, Cricket, ....................................................................................................................................Praying Mantid ................................................................................................................................................................. 8Thysanoptera - Thrips ...................................................................................................................................................... 8

Insect Orders of Lesser Importance to the Gardener: ........................................................................................ 8Order Examples Common “Non-Insect” Pests Found in New England: .......................................................................... 8Arachnida - Spiders, Spider Mites, Ticks ......................................................................................................................... 8Diplopoda - Millipedes ...................................................................................................................................................... 9Chilopoda - Centipedes ................................................................................................................................................... 9Crustacea - Sowbugs, Pillbugs ........................................................................................................................................ 9

Types of Insect Injury ................................................................................................................................ 10Injury by Chewing Insects .............................................................................................................................................. 10Injury by Piercing-Sucking Insects ................................................................................................................................. 10Injury by Internal Feeders .............................................................................................................................................. 10Injury by Subterranean Insects ...................................................................................................................................... 10Injury by Laying Eggs ..................................................................................................................................................... 10Use of Plants for Nest Materials .................................................................................................................................... 11Ways in Which Insects Injure Plants ............................................................................................................................. 11

Insects as Disseminators of Plant Diseases .......................................................................................... 12Disease Vector ............................................................................................................................................................... 12

Benefits and Value of Insects ................................................................................................................... 12Insects are Beneficial to the Gardener in Several Ways: .............................................................................................. 12Soil Preparation .............................................................................................................................................................. 13Plant Selection ............................................................................................................................................................... 13Cultural Practices ........................................................................................................................................................... 13Mechanical Controls ....................................................................................................................................................... 14Biological Controls ......................................................................................................................................................... 15

Pesticides ................................................................................................................................................... 16Nonsynthetic Pesticides ................................................................................................................................................. 16Synthetic Pesticides ....................................................................................................................................................... 16

Summary ..................................................................................................................................................... 17

Chapter 3 Basics of Entomology 1

CHAPTER 3Basics of Entomology

Edited and revised by Dr. Alan Eaton, University of New Hampshire Cooperative Extension

Insects and mites are among the oldest, most numerous, and most successfulcreatures on earth. It is estimated that over 100,000 different species live in NorthAmerica. In the typical backyard, there are probably 1,000 insects at any giventime. While insects which cause problems for humans are heard about mostoften, it is important to note that the vast majority are either beneficial or harm-less. Insects pollinate fruits and vegetables, provide food for birds and fish, andproduce useful products such as honey, wax, shellac, and silk. In addition, someinsects are beneficial because they feed on other insects that are considered pestsby humans.

Although the number of pest species compared to the total number of insectspecies is very small (less than 3% of all insects are classified as pests), thetroubles for humans wrought by this group reach astonishing proportions. In-sects annually destroy millions of dollars worth of crops, fruits, shade trees andornamental plants, stored products, household items, and other materials val-ued by man. They transmit diseases of humans and domestic animals. Theyattack humans and pets, causing irritation, blood loss, and in some instances,death.

This chapter is designed to present basic principles that apply to the identifica-tion of insects and mites on horticultural crops, especially those commonlyencountered in New England.

Basics of ClassificationIdentification of the thousands of species of insectswould be impossible if they were not organizedaround a standard classification system. By group-ing organisms based on the degrees of similarityamong them, we can arrive at a system of classifica-tion. At the highest level of this classificationsystem, organisms are divided into five kingdoms.Insects are placed in the Animal Kingdom. TheAnimal Kingdom has major divisions known asphyla. Several of the phyla which contain agricul-tural pests are:

• Arthropoda (insects, spiders, crayfish, milli-pedes)

• Nematoda (roundworms, trichina)

• Platyhelminthes (flatworms, flukes, tapeworms)

• Mollusca (snails, slugs, clams)

Insects belong to the phylum Arthropoda.Arthropods are a very important group of animals,as they represent more than three-fourths of theanimal species known to exist. Characteristics thatplace an animal in the phylum Arthropoda includebody segmentation and skeletons outside(exoskeleton) of their bodies.

The phylum Arthropoda is divided into classes.Table 1 describes a few of the more importantclasses and presents some characteristics that areused to distinguish between the various Arthropodclasses. Insects belong to the class Insecta. For anArthropod to be further classified in the classInsecta, it must have 3 body segments and 3 pairsof legs.


Classes are further divided into orders. The moreimportant orders of the class Insecta are describedin Table 2.

Table 1. Classes of the Phylum Arthropoda

Body Pairs AgriculturalClass Examples Regions of Legs Importance

Crustacea Crayfish 2 5 Sowbugs canSowbugs be minor

pests.

Arachnida Spiders, Mites, 2 4 Some mitesTicks are major

plant pests.

Symphyla Symphylan 2 12 Symphylanscan be seriouspests.

Insecta Bugs, Beetles, 3 3 Large numberButterflies are pests.

Table 2. Some Orders of the Class Insecta

Order Common Meta- Mouth- WingsName morphosis parts

Collembola Springtails none chewing none

Orthoptera Crickets gradual chewing 2 pairGrasshoppers

Isoptera Termites gradual chewing 2 pair

Thysanoptera Thrips gradual rasping- 2 pairsucking

Hemiptera True Bugs gradual piercing- 2 pairsucking

Homoptera Aphids, gradual piercing- 2 pairScales sucking

Coleoptera Beetles, complete chewing 2 pairWeevils

Lepidoptera Butterflies, complete chewing 2 pairMoths or

siphoning

Hymenoptera Bees, complete chewing 2 pairWasps,Ants

Diptera Flies complete various 1 pair

Siphonaptera Fleas complete sucking none

Dermaptera Earwigs gradual chewing 2 pair

Thysanura Silverfish gradual chewing none

Insect orders are further broken down into a classifi-cation known as family. The family is a more finitegrouping of very closely related insects. Familynames end with “idae.” Aphidae (aphids), Mus-cidae (houseflies), and Blattidae (cockroaches) areexamples of families of insects.

Families are further divided into genera and spe-cies. These are the most finite levels of our classifica-tion system. The housefly, Musca domestica, serveshere as an example of classification:

Kingdom: Animalia

Phylum: ArthropodaClass: InsectaOrder: DipteraFamily: MuscidaeGenus: MuscaSpecies: domesticaCommon name: housefly

The most commonly found insects also acquirecommon names and sometimes one species willhave several common names. For example, Heliothiszea, when found on corn, is called the corn ear-worm, but when it is found on tomatoes it is calledthe tomato fruitworm. Common names are oftenused to refer to large groups of insects, such asfamilies or orders. The term beetle refers to theentire order Coleoptera, which includes thousandsof different species. The term moth refers to thou-sands of species in the order Lepidoptera.

Insect Form and Structure -MorphologyAll adult members of the class Insecta possess thefollowing characteristics: three body regions; threepairs of legs; one pair of antennae; and zero to twopairs of wings. Legs and other appendages areoften greatly modified to suit the insect’s environ-ment; the form of its appendages is often used toclassify an insect.

Head, Thorax and AbdomenThe adult insect’s body is made up of three regions:head, thorax, and abdomen, but the division is notalways obvious between thorax and abdomen. Aninsect’s body is not supported by a bony skeleton,but by a tough body wall, or exoskeleton. Thetough covering of skin is referred to as the cuticle.


The cuticle contains a layer of wax which deter-mines its permeability to water and prevents desic-cation or drying. The cuticle of each segment isformed into several hardened plates called sclerites,which are separated by infolds or sutures to givethem flexibility. The cuticle of the immature stage isnot usually as hardened as that of the adult.

The thorax is made up of three segments: prothorax,mesothorax, and metathorax. Each of these seg-ments bears a pair of legs. The wings are attached tothe mesothorax and metathorax, never to the pro-thorax.

The abdomen may have 11 or 12 segments, but inmost cases they are difficult to distinguish. Someinsects have a pair of appendages at the tip of theabdomen. They may be short, as in grasshoppers,termites, and cockroaches; extremely long, as inmayflies; or curved, as in earwigs.

LegsThe most important characteristic of an insect is thepresence of three pairs of jointed legs. These arealmost always present on adult insects and aregenerally present in the other stages as well. Inaddition to walking and jumping, insects often usetheir legs for digging, grasping, feeling, swimming,carrying loads, building nests, and cleaning parts ofthe body. The legs of insects vary greatly in size andform and are used in classification.

Leg adaptations of some insects (left to right):jumping (grasshopper), running (beetle), digging(mole cricket), grasping (praying mantis), swim-ming (diving beetle).

WingsVenation (the arrangement of veins in wings) isdifferent for each species of insect; thus, it serves asa means of identification. Systems have been de-vised to designate the venation for descriptivepurposes. Wing surfaces are covered with fine hairsor scales, or they may be bare. Note that the namesof many insect orders end in “-ptera,” which comesfrom the Greek word meaning “with wings.” Thus,each of these names denotes some feature of thewings. Hemiptera means half-winged; Hy-menoptera means membrane-winged; Dipterameans two-winged; Isoptera means equal wings.


AntennaeThe main features of the insect’s head are the eyes,antennae, and mouthparts. The antennae are aprominent and distinctive feature of insects. Adultinsects have one pair of antennae located on thehead usually between or in front of the eyes. Anten-nae are segmented, vary greatly in form and com-plexity, and are often referred to as horns or “feel-ers.” They are primarily organs of smell, but serveother functions in some insects.

MouthpartsThe most remarkable and complicated structuralfeature of an insect is the mouth. Great variationsexist in form and function of insect mouthparts.And although insect mouthparts differ considerablyin appearance, the same basic parts are found in alltypes. Most insects are divided into two broadcategories by the type of mouthparts they possess— those with mouthparts adapted for chewing andthose with mouthparts adapted for sucking.

There are intermediate types of mouthparts: rasp-ing-sucking, as found in thrips; and chewing-lapping, as found in honey bees, wasps, and bumblebees. Sucking types are greatly varied. Piercing-sucking mouthparts are typical of the Hemiptera(bugs), Homoptera (aphids, scales, mealybugs),

blood-sucking lice, fleas, mosquitoes, and the so-called biting flies. In the siphoning types, as seen inbutterflies and moths, the mandibles are absent andthe labial and maxillary palpi are greatly reduced.Houseflies have sponging mouthparts.

Some types of insect mouthparts: A. Chewing-lapping (honey bee); B. Piercing-sucking (plantbug); C. Sponging (housefly); D. Siphoning, coiled(butterfly)

The mouthparts of immature insects tend to bemore varied than those of the adults, althoughnymphs have mouthparts similar to those of theadults. Larval forms generally have the chewingtype regardless of the kind possessed by the adults.In some adult insects, the mouthparts are vestigial(no longer used for feeding).

Insect Development— MetamorphosisIn higher animals, the most important developmenttakes place before birth (in the embryonic stage); ininsects, it occurs after birth or egg hatch. The imma-ture period of an insect is primarily one of growth,feeding, and storing up food for the pupal and adultstages which follow. Many insects feed very little ornot at all during their adult lives.


One of the distinctive features of insects is thephenomenon called metamorphosis. The term is acombination of two Greek words: meta, meaningchange, and morphe, meaning form. It is commonlydefined as a marked or abrupt change in form orstructure, and refers to all stages of development.Insects undergo one of four types of metamorpho-sis.

Some insects do not go through a metamorphosis,but rather gradually increase in size while maintain-ing the same characteristics. Others experience agradual metamorphosis, going through a nymphstage.

In the case of gradual metamorphosis, the stagesare: egg, nymph, and adult. In some insects, fertili-zation of the egg by sperm is not necessary forreproduction. This type of reproduction is known asparthenogenesis. Aphids are notable examples ofinsects that can reproduce by parthenogenesis.

Insects that undergo complete metamorphosis gothrough the following stages: egg, larva, pupa, andadult.

The immature insect sheds its outer skeleton (molts)at various stages of growth, since it outgrows thehard covering or cuticle more than once. Mostinsects do not grow gradually as many other ani-mals do. They grow by stages. When their skeletongets too tight, it splits open and the insect crawlsout, protected by a new and larger skeleton that hasformed underneath the old one. The stage of lifebetween each molt is called an instar. Followingeach molt, the insect increases its feeding. Thenumber of instars, or frequency of molts, variesconsiderably between species and to some extentwith food supply, temperature, and moisture.

The pupal stage is one of profound change. It is aperiod of transformation from larva to adult. Manytissues and structures, such as prolegs, are com-pletely broken down and true legs, antennae, wings,and other structures of the adult are formed.

The adult insect does not grow in the usual sense.The adult period is primarily one of reproductionand is sometimes of short duration. Their food isoften entirely different from that of the larval stage.

Identifying InsectsMost home gardeners can classify an insect by thecommon name of its order, identifying it as a beetle,wasp, or butterfly. The ability to classify an insect tothe order level gives the gardener access to muchvaluable information. This information wouldinclude the type of mouthparts the insect has (thistells us how it feeds and gives clues towards meth-ods of control), its life cycle (and proper timing forbest control), and type of habitation.

Specific Insect Orders. For your reference, theinsect orders have been divided into three sections:those containing insects important to the gardener;those containing insects of lesser importance to thegardener; and common “non-insect” pests in NewEngland. The orders containing insects of impor-tance to home gardeners will be considered indetail.


Insect Orders Important to theGardener:

Coleoptera - Beetles, Weevils

• Adults have hardened, horny, outer skeleton

• Adults have two pairs of wings, the outer pairhardened and the inner pair membranous

• Chewing mouthparts

• Adults usually have noticeable antennae

• Larvae with head capsule, three pairs of legs onthe thorax, no legs on the abdomen. Weevillarvae lack legs.

• Complete metamorphosis

Dermaptera - Earwigs

• Adults are moderate-sized insects


• Gradual metamorphosis

• Elongate, flattened insects with strong, movableforceps on the abdomen

• Short, hardened outer wings; folded, membra-nous, “ear-shaped” inner wings

• Adults and nymphs similar in appearance

Diptera - Flies, Mosquitoes, Gnats, Midges

• Adults have only one pair of wings, are rathersoft-bodied, and are often hairy

• Adults have sponging (housefly) or piercing(mosquito) mouthparts

• Larvae may have mouth hooks or chewingmouthparts

• Most larvae are legless

• Larvae of advanced forms, housefly and rela-tives, have no head capsule, possess mouthhooks, and are called maggots; lower forms,such as mosquito larvae and relatives, have ahead capsule


Hemiptera - Stink Bug, Plant Bug, SquashBug, Boxelder Bug

• Have gradual metamorphosis; stages are egg,nymph, adult

• Have two pairs of wings; second pair is membra-nous, the first pair are “half-wings” -- membra-nous with thickening on basal half

• Adults and nymphs usually resemble oneanother

• Have piercing-sucking mouthparts

• Adults and nymphs are both damaging stages


Homoptera - Scale Insects, Mealybugs,Whiteflies, Aphids, Cicadas, Leafhoppers.

• Generally small, soft bodied-insects; cicadas maybe large and hard-bodied

• Winged and unwinged forms

• All stages have sucking mouthparts

• Have gradual metamorphosis

• Many are carriers of plant pathogens

Hymenoptera - Bees, Ants, Wasps, Saw-flies, Horntails

• Adults have two pairs of membranous wings

• Larvae have no legs (wasps, bees, ants) or threepairs of legs on thorax and more than four pair oflegs on abdomen (some sawflies)

• Generally have chewing mouthparts

• Rather soft-bodied or slightly hard-bodied adults


Lepidoptera - Butterflies, Moths

• Adults are soft-bodied, with four well-developedmembranous wings covered with small scales

• Larvae have chewing mouthparts

• Adult mouthparts are a coiled, sucking tube; feedon nectar

• Larvae are caterpillars; worm-like, variable incolor, voracious feeders

• Larvae generally have legs on the abdomen aswell as the thorax


Neuroptera - Lacewings, Antlions,Snakeflies, Mantispids, Dobsonfly,Dustywing, Alderfly

• Insect predators, many are aquatic

• Two pairs of membranous wings




Orthoptera - Grasshopper, Cricket,Praying Mantid

• Adults are moderate to large, often rather hard-bodied

• Gradual metamorphosis

• Adults usually have two pairs of wings. Fore-wings are elongated, narrow, and hardened;hindwings are membranous with extensivefolded area

• Chewing mouthparts; both adults and nymphsare damaging

• Hind legs of many forms are enlarged for jump-ing

• Immature stages are called nymphs and resembleadults, but are wingless

Thysanoptera - Thrips

• Adults are small, soft-bodied insects

• Mouthparts are rasping-sucking

• Varied metamorphosis (a mixture of completeand gradual)

• Found on flowers or leaves of plants

• Wings in two pairs, slender, feather-like, withfringed hairs

Insect Orders of Lesser Importance tothe Gardener:Order Examples

Anoplura sucking lice

Collembola springtails

Diplura no common examples

Ephemeroptera Mayflies

Embioptera webspinners

Isoptera termites

Mallophaga chewing lice

Mecoptera scorpionflies

Odonata dragonflies and damselflies

Plecoptera stoneflies

Protura no common examples

Psocoptera booklice, barklice

Siphonaptera fleas

Strepsiptera no common examples

Thysanura silverfish and bristletails

Trichoptera caddisflies

Zoraptera no common examples

Common “Non-Insect” PestsFound in New England:

Arachnida - Spiders, Spider Mites, Ticks

a. Spider mites: tiny, soft-bodied animals with twobody regions, thick waists, four pairs of legs, noantennae.Common species:• Two-spotted mites and near relatives - two

spots on the back, may be clear, green, orange,or reddish; usually hard to see without amagnifying glass.

• European red mite - carmine red with whitespines.

• Clover mites - brown or gray, flat, very longfront legs.

b. Spiders: resemble mites except that most arelarger, and the two body regions are more clearlydistinct from one another (thin waist). Mostspiders are beneficial predators.


c. Ticks: resemble large mites and are importantagriculturally and medically in that they areparasites of animals and humans.

Diplopoda - Millipedes

These are elongated invertebrates with twovisible body regions: the head and body. Theygenerally have a round cross section, and all butthe first four or five body segments possess twopairs of legs. Millipedes are generally inoffensivecreatures that feed on fungus and decaying plantmaterial, but at times, they can be fairly destruc-tive to vegetables or plants in greenhouses.

Chilopoda - Centipedes

Centipedes strongly resemble millipedes, exceptthat they have longer antennae, a flat cross-section, and only one pair of legs on each bodysegment. They are beneficial predators of otherarthropods.

Crustacea - Sowbugs, Pillbugs

These are oval with a hard, convex, outer shellmade up of a number of plates. Sowbugs arehighly dependent on moisture. Generally, theyfeed on decaying plant material, but they willsometimes attack young plants.


Types of Insect Injury

Injury by Chewing InsectsInsects take their food in a variety of ways. Onemethod is by chewing off the external parts of aplant. Such insects are called chewing insects. It iseasy to see examples of this injury. Perhaps the bestway to gain an idea of the prevalence of this type ofinsect damage is to try to find leaves of plantswhich have no sign of injury from chewing insects.Cabbageworms, armyworms, grasshoppers, Colo-rado potato beetles, and fall webworms are com-mon examples of insects that cause injury by chew-ing.

Injury by Piercing-Sucking InsectsA second important way insects feed on growingplants is by piercing the epidermis (skin) andsucking sap from plant cells. In this case, onlyinternal liquid portions of the plant are swallowed,even though the insect feeds externally on the plant.These insects have a slender, sharp, pointed portionof the mouthparts which are thrust into the plantand through which sap is sucked. This results in avery different, but nonetheless severe injury. Thehole made in this way is so small that it cannot beseen with the unaided eye, but the withdrawal ofthe sap results in minute spotting of white, brown,or red on leaves, fruits, or twigs; curling leaves;deformed fruit; or general wilting, browning, anddying of the entire plant. Aphids, scale insects,squash bugs, leafhoppers, and plant bugs are well-known examples of piercing-sucking insects.

Injury by Internal FeedersMany insects feed within plant tissues during partor all of their destructive stages. They gain entranceto plants either in the egg stage, when their mothersdeposit eggs into the plant tissue, or after they hatchfrom the eggs, by eating their way into the plant. Ineither case, the hole of entry is almost alwaysminute and often invisible. A large hole in a fruit,seed, nut, twig, or trunk generally indicates wherethe insect has come out, not where it entered.

The chief groups of internal feeders are indicated bytheir common group names: borers in wood or pith;worms or weevils in fruits, nuts, or seeds; leafminers; and gall insects. Each group, except thethird, contains some of the foremost insect pests ofthe world. Nearly all of the internal feeding insectslive inside the plant during only part of their lives,and emerge usually as adults. Control measures are

most effective when aimed at emerging adults orthe immature stages prior to entrance into the plant.

Leaf miners are small enough to find comfortablequarters and an abundance of food between theupper and lower epidermis of a leaf.

Injury by Subterranean InsectsAlmost as secure from human attack as the internalfeeders are those insects that attack plants below thesurface of the soil. These include chewers, sapsuckers, root borers, and gall insects. The attacksdiffer from the above-ground forms only in theirposition with reference to the soil surface. Somesubterranean insects spend their entire life cyclebelow ground. For example, the woolly appleaphid, as both nymph and adult, sucks sap fromroots of apple trees causing the development oftumors and subsequent decay of the tree’s roots. Inother subterranean insects, there is at least one lifestage that has not taken up subterranean habit.Examples include wireworms, root maggots,pillbugs, strawberry root weevils, and grape andcorn rootworms. The larvae are root feeders, whilethe adults live above ground.

Injury by Laying EggsProbably 95% of insect injury to plants is caused byfeeding in the various ways just described. Inaddition, insects may damage plants by laying eggsin critical plant tissues. The periodical cicadadeposits eggs in one-year-old growth of fruit andforest trees, splitting the wood so severely that theentire twig often dies. As soon as the young hatch,they desert the twigs and injure the plant no further.

Gall insects sting plants and cause them to producea structure of deformed tissue. The insect then findsshelter and abundant food inside this plant growth.It is not known exactly what makes the plants formthese elaborate structures when attacked by theinsects. However, it is clear that the growth of thegall is initiated by the oviposition of the adult(laying eggs inside plant tissue), and its continueddevelopment results from secretions of the develop-ing larva. The same species of insect on differentplants causes galls that are similar, while severalspecies of insects attacking the same plant causegalls that are greatly different in appearance.Although the gall is entirely plant tissue, the insectcontrols and directs the form and shape it takes as itgrows.


Use of Plants for Nest MaterialsBesides laying eggs in plants, insects sometimesremove parts of plants for the construction of nestsor for provisioning nests. Leaf-cutter bees nip outrather neat, circular pieces of rose and other foliage,which are carried away and fashioned together toform thimble-shaped cells.

Ways in Which Insects Injure Plants

• Chewing - devouring or notchingleaves; eating wood, bark, roots,stems, fruit, seeds; mining in leaves.

Symptoms: ragged leaves, holes inwood and bark or fruit and seed,serpentine mines or blotches, wiltedor dead plants, or presence of“worms.”

• Sucking - removing sap and cellcontents and injecting toxinsinto plant.

Symptom: usually off-color, mis-shapen foliage and fruit.

• Vectors of diseases - carryingpathogens from plant to plant, e.g.,elm bark beetle - Dutch elm disease,various aphids - virus diseases.

Symptoms: wilt; dwarf,off-colorfoliage.

• Excretions - honeydew depositslead to the growth of sooty mold,and the leaves cannot perform theirmanufacturing functions. A weak-ened plant results.

Symptoms: sooty black leaves,twigs, branches, and fruit.

• Gall formation - galls may form onleaves, twigs, buds, and roots. Theydisfigure plants, and twig gallsoften cause serious injury.

• Oviposition scars - scars formed onstems, twigs, bark, or fruit.

Symptoms: scarring, splitting,breaking of stems and twigs,misshapen and sometimes infestedfruit.

• Injection of toxic substances -

Symptoms: scorch, hopper burn.

Examples of insect injury to plants.


Insects as Disseminators ofPlant DiseasesIn 1892, it was discovered that a plant disease (fireblight of fruit trees) was spread by an insect (thehoneybee). At present, there is evidence that morethan 200 plant diseases are disseminated by insects.The majority of them, about 150, belong to thegroup known as viruses, 25 or more are due toparasitic fungi, 15 or more are bacterial diseases,and a few are caused by protozoa or mycoplasms.

Insects may spread plant diseases in the followingways:

• by feeding, laying eggs, or boring into plants,they create an entrance point for a disease that isnot actually transported by them;

• they carry and disseminate the causative agentsof the disease on or in their bodies from oneplant to a susceptible surface of another plant;

• they carry pathogens on the outside or inside oftheir bodies and inject plants hypodermically asthey feed;

• the insect may serve as an essential host for somepart of the pathogen’s life cycle, and the diseasecould not complete its life cycle without theinsect host.

Examples of insect-vectored (insect-carried) plantdiseases, their causative agents, and vectors include:

Disease Vector

Dutch Elm Disease (fungus) Small Beetle

Fireblight (bacterial) Pollinating Insects

Tomato spotted wilt (virus) Thrips

Cucumber Mosaic (virus) Aphids

X-Disease of Peach (mycoplasm) Leafhoppers

Benefits and Value of InsectsInsects must be studied carefully to distinguish thebeneficial from the harmful. People have often goneto great trouble and expense to destroy insects, onlyto learn later that the insect destroyed was not onlyharmless, but was actually saving their crops byeating destructive insects.

Insects are Beneficial to the Gardener inSeveral Ways:

• Insects aid in the production of fruits, seeds,vegetables, and flowers by pollinating the blos-soms. Most common fruits are pollinated byinsects. Melons, squash, and many other veg-etables require insects to carry their pollen beforefruit set. Many ornamental plants, both in thegreenhouse and outdoors, are pollinated byinsects (chrysanthemums, iris, orchids, andyucca).

• Insects destroy various weeds in the same waysthat they injure crop plants.

• Insects improve the physical condition of the soiland promote its fertility by burrowing through-out the surface layer. Also, the dead bodies anddroppings of insects serve as fertilizer.

• Insects perform a valuable service as scavengersby devouring the bodies of dead animals andplants and by burying carcasses and dung.

Beneficial Insects

Common species that causeno damage in the garden andhelp control other injuriousinsects.


Many of the benefits from insects enumeratedabove, although genuine, are insignificant com-pared with the good that insects do fighting amongthemselves. There is no doubt that the greatestsingle factor in keeping plant-feeding insects fromoverwhelming the rest of the world is that they arefed upon by other insects. Insects that eat otherinsects are considered in two groups known aspredators and parasites.

Predators are insects (or other animals) that catchand devour other creatures (called the prey), usu-ally killing and consuming them in a single meal.The prey is generally smaller and weaker than thepredator.

Parasites are forms of living organisms that live onor in the bodies of living organisms (called thehosts) from which they get their food, during atleast one stage of their existence. The hosts areusually larger and stronger than the parasites, andare not killed promptly. Some continue to live inclose association with the parasite, rather than bekilled.

Soil PreparationFor most fruits and vegetable crops, maintain aslightly acidic soil (around pH 6.5). If in doubt, havea soil analysis done through your local Extensionoffice. The appropriate pH allows vegetable plantsto have access to all the necessary soil nutrients andprovides a suitable environment for earthwormsand microorganisms. Follow recommended fertil-izer practices. Supplement chemical fertilizers withorganic material or compost to help assure that alltrace elements and major nutrients are available.Feed the soil, not just the plants; providing anappropriate environment for all soil life will resultin healthy plants which can resist pests and dis-eases.

When using manure and compost, be sure they areworked into the soil. Otherwise, millipedes, whitegrubs, and other pests may be encouraged. If theseinsects become a problem, you may be using toomuch; consider other means of adding organicmatter, such as cover-cropping or mulching.

When diseased plant material is added to compostto be used on the garden, delay using the compostuntil all material has decayed beyond recognition.Compost piles must be hot (160 degrees F.) to killdisease organisms, insect eggs, and weed seeds.

Till the soil in the fall to expose those stages of pestswhich live near the surface of the soil to naturalenemies and weather, and to destroy insects in cropresidues. If you do not till in the fall, do so earlyenough in the spring to give remaining vegetationtime to degrade before planting time.

Plant SelectionPlant crops that are suited to the soil and climate ofyour area. If you do plant vegetables or fruits thatare not normally grown in your area, do your bestto provide necessary conditions. For example,watermelons prefer a light, warm, well-drained soil;don’t try to plant in heavy clay without first addingcopious amounts of compost or other soil-lighteningmaterial, and allow the soil to warm up beforeseeding or setting plants out.

Use disease-free, insect-free, certified seed if avail-able. Select disease/insect-resistant or tolerantspecies and varieties. Resistance in plants is likely tobe interpreted as meaning immune to damage. Inreality, it distinguishes plant varieties that exhibitless insect or disease damage when compared withother varieties under similar circumstances. Somevarieties may not taste as good to the pest. Somemay possess certain physical or chemical propertieswhich repel or discourage insect feeding or egglaying. Some may be able to support insect popula-tions with no appreciable damage or alteration inquality or yield.

Select plants that are sturdy and have well-devel-oped root systems. Diseases and insects in youngseedlings may start in greenhouses or plant bedsand cause heavy losses in the garden. Buy plantsfrom a reputable grower who can assure you thatthey are disease/insect-free, or grow your own fromseed.

Avoid accepting plants from friends if there is anychance of also getting free insects or diseases!

Cultural PracticesThe most effective and most important of all prac-tices is to observe what is going on in the garden.Many serious disease or insect problems can behalted or slowed early by the gardener who knowswhat to look for and regularly visits the garden forthe purpose of trouble-shooting.


Rotation. Do not grow the same kind of produce inthe same place each year. Use related crops in onesite only once every three or four years. Somerelated crops are as follows: (a) chives, garlic, leeks,onions, shallots; (b) beets, Swiss chard, spinach; (c)cabbage, cauliflower, kale, Brussels sprouts, broc-coli, kohlrabi, turnips, rutabagas, Chinese cabbage;(d) peas, broad beans, snap beans; (e) carrots,parsley, celery, celeriac, parsnips; (f) potatoes,eggplant, tomatoes, peppers; (g) pumpkins, squash,watermelons, cucumbers, muskmelons; (h) endive,salsify, lettuce.

Interplantings. Avoid placing all plants of onekind together; alternate groups of different plantswithin rows or patches. If an insect lays eggs orotherwise attacks a specific species, the presence ofother species in the area can interrupt progress ofthe attack by diluting the odor of the preferredplants. This can also slow the spread of diseases andpests, giving the gardener more time to deal withthem.

Thinning. Thin young plants to a proper stand.Overcrowding causes weak growth and subsequentinsect and disease problems.

Watering. Water in the morning, so plants havetime to dry before the cool evening when fungusinfection is most likely. Drip irrigation preventsfoliage from getting wet at all when watering. Forplants susceptible to fungus infections, such astomatoes, leave extra space between plants to allowgood air flow and orient rows so that prevailingwinds will help foliage dry quickly after a rain orwatering. While this may reduce the number ofplants per square foot, yields may still be higherdue to reduced disease problems. To preventspreading diseases, stay out of the garden when theplants are wet with rain or dew.

Time Planting. Time plantings in such a way thatthe majority of the crop will avoid the peak of insectinfestations. For example, carrot rust fly problemscan be avoided by delaying planting until June 1and harvesting by late August. Keep a record of thedates insect problems occur. Also, by plantingwarm-weather crops after the soil has warmed,problems with seed and root rots will be avoided,and growth will be more vigorous.

Sanitation. Do not use tobacco products such ascigarettes or cigars when working in the garden.Tomato, pepper, and eggplant are susceptible to amosaic virus disease which is common in tobaccoand may be spread by your hands. Remove infectedleaves from diseased plants as soon as you observethem. Dispose of severely diseased plants beforethey contaminate others. Clean up crop refuse assoon as harvesting is finished. Old sacks, baskets,decaying vegetables, and other rubbish which mayharbor insects and diseases should be kept out ofthe garden.

Staking plants. Staking or planting them in wirecages prevents the fruit from touching the soil. Thisalso helps prevent fruit rots. Caging helps reducesun scald often seen in staked tomatoes, since cagedplants do not require as much pruning, leaving aheavier foliage cover. Boards or a light, open mulchsuch as straw, placed beneath melons, will preventrotting.

Avoid injury to plants. Cuts, bruises, cracks, andinsect damage are often sites for infection by dis-ease-causing organisms. In cases where fruit isdifficult to remove, such as with cucumbers andwatermelons, cut them instead of pulling them offthe plant. If you cultivate your garden, avoidcutting into the plant roots.

Mulching. Use a mulch to reduce soil splash, whichbrings soil-borne pathogens into contact with lowerleaves.

Weed control. Control weeds and grass. They oftenharbor pests and compete for nutrients and water.They provide an alternate source of food and can beresponsible for pest build-up. They provide coverfor cutworms and slugs.

Mechanical ControlsHandpicking. Inspect plants for egg clusters, beanbeetles, caterpillars, and other insects as often aspossible. Handpick as many as possible. If youdon’t like squashing the pests, knock the insects andegg clusters into a coffee can or quart jar with asmall amount of water, and then pour boiling waterover them. Kerosene is often recommended, butposes a disposal problem once you have finished;besides, water is cheaper.


Traps. Use appropriate insect traps to reducecertain insect populations. A simple, effectiveJapanese beetle trap can be made from two milkjugs or a single milk jug and a plastic bag. The baitused to attract the beetles is available at most farmand garden supply centers. Place traps away fromdesirable plants. Most scent-based insect traps areused for monitoring populations, not for control ofpests.

Light traps, particularly blacklight or blue lighttraps (special bulbs that emit a higher proportion ofultraviolet light that is highly attractive to nocturnalinsects), are good insect-monitoring tools, butprovide little or no protection for the garden. Whilethey usually capture a tremendous number ofinsects, a close examination of light-trap collectionsshows that they attract both beneficial and harmfulinsects that would ordinarily not be found in thatarea. Those insects attracted but not capturedremain in the area, and the destructive ones maycause damage later. Also, some wingless species aswell as those species only active during the day(diurnal, as opposed to nocturnal) are not caught inthese traps. Consequently, the use of a light trap inprotecting the home garden is generally of nobenefit and, in some instances, detrimental.

Upturned flower pots, boards, newspaper, etc. willtrap earwigs, sowbugs, and slugs; collect themevery morning, and feed them to pet frogs, toads,turtles, and fish, or destroy them with boiling water.Indoors, white flies can be caught with yellowsticky traps, made with boards painted yellow andlightly coated with oil or grease. There are alsocommercial sticky traps available through somecatalogs.

Barriers. Aluminum foil and other reflective mulchhas been shown to repel aphids. However, theenvironmental impact and energy consumptioninvolved in making aluminum foil deserves consid-eration. Spread crushed eggshells or hydrated limearound plants to discourage slugs. While heavymulch is good for weed control, it gives slugs aplace to hide.

Exclusion. Various materials can be used to physi-cally block or repel insects and keep them fromdamaging plants. Place wood ash, cardboard tubes,or orange juice cans around seedlings to keepcutworms away from plant stems. Use paper bagsover ears of corn to keep birds and insects out; donot cover until pollination is complete. Net-coveredcages over young seedlings will help prevent insect,bird, and rabbit damage. Cheesecloth screens for

cold frames and hot beds will prevent insect egg-laying; sticky barriers on the trunks of trees andwoody shrubs will prevent damage by crawlinginsects. Floating row covers of spun bonded poly-ethylene are a little more expensive, but theireffectiveness in excluding insects is proven by thenumber of commercial growers that use them,particularly on cole crops and strawberries. Remem-ber that such materials can exclude pollinatinginsects.

Biological ControlsPredators, Parasites, and Pathogens. The gardenand its surrounding environment are alive withmany beneficial organisms that are present natu-rally; however, they may not be numerous enoughto control a pest before damage is done. Actually,parasites and predators (usually other types ofinsects) are most effective when pest populationshave stabilized or are relatively low. Their influenceon increasing pest populations is usually minimalsince any increase in parasite and predator numbersdepends on an even greater increase in pest num-bers. Disease pathogens, however, seem to be mosteffective when pest populations are large.

Take advantage of the biological control alreadytaking place in your garden by encouraging naturalpredators, such as preying mantids, ladybugs,lacewings, ground beetles, and others. Purchasednatural predators are often effective for only a shortperiod, however, since they tend not to remain inthe place where they are put. Research the likes anddislikes of these helpers as to foods, habitat, etc.Provide these conditions where possible; somebeneficial insect suppliers now offer a formulationfor feeding/attracting the beneficials to keep themin the garden longer.

Learn to recognize the eggs and larvae of the benefi-cial insects, and avoid harming them. You can oftenfind preying mantid egg cases in weedy lots; justbring the twig with the cluster into the garden andset it in a place where it will not be disturbed.Spiders, toads, and dragonflies are also beneficial,and should not be a source of fright to the gardener;in most cases, they are harmless to people.

Learn to recognize parasites and their egg cases; forexample, the tomato hornworm is often seen with anumber of white cocoons, a little larger than a grainof rice, on its back. These were produced by para-sitic wasps. The hornworm will die and more waspswill emerge. You may wish to leave such parasit-ized caterpillars alone, rather than killing them.


Pesticides

Nonsynthetic Pesticides

Botanicals. Natural pesticidal products areavailable as an alternative to synthetic chemicalformulations. Some of the botanical pesticides aretoxic to fish and other cold-blooded creatures andshould be treated with care. Safety clothing shouldbe worn when spraying these, because some may bemore toxic than synthetics. Botanical insecticidesbreak down readily in soil and are not stored inplant or animal tissue. Often their effects are not aslong-lasting as those of synthetic pesticides.

Insecticide Use Against

Pyrethrum Aphids, leafhoppers, spidermites, cabbageworms.

Rotenone Spittlebugs, aphids, potatobeetles, chinch bugs, spidermites, carpenter ants.

Ryania Codling moths, Japanesebeetles, squash bugs, potatoaphids, onion thrips, cornearworms.

Sabadilla Grasshoppers, codlingmoths, moths, armyworms,aphids, cabbage loopers,blister beetles.

Some of these products may be very difficult tofind, expensive, and may not be registered for use inNew England.

In addition to botanical insecticides, some biologicalproducts can help in the battle against insects.Bacillus thuringiensis is an effective product com-monly used against caterpillars; B.T., as it is known,is a bacterium that gives the larvae a disease, and ismost effective on young larvae. Presently, there isresearch underway to develop strains that workagainst other types of insect larvae. Several formula-tions are available to the gardener under differenttrade names to provide effective control of severalcaterpillars without harming humans and domesticanimals. More than 400 insect species are known tobe affected by this important insect pathogen.Bacillus thuringiensis is quite slow in its action. Forexample, caterpillars that consume some of thespores will stop eating within 2 hours, but may

continue to live and move around until they die,which may be as long as 72 hours. When this occurs,the untrained gardener may assume the materialwas ineffective because of the continued pestactivity and impatiently apply a chemical pesticide.B.t. kurstaki is effective on caterpillars. B.t. israelensisis used for larvae of mosquitoes, black flies andfungus gnats. B.t. san diego is used for Coloradopotato beetle larvae. B.t. bui bui may soon be avail-able for Japanese beetle control.

Nosema locustae is a disease organism which showssome promise for controlling grasshoppers. Thereare claims that this parasite may be effective for upto five years after initial application. In some areas,this parasite is available commercially under differ-ent trade names. It is still too early to make exten-sive claims about its effectiveness in home gardens.

Enlist the aid of birds. In rural areas, chickens,guineas, and other domestic fowl can be released inunused areas of the garden to eat grubs and insects.Wild birds will also help, but they aren’t as control-lable. Provide appropriate conditions (i.e., shelter,nesting material, water) to encourage insect-eatingbirds.

Soaps. Commercial insecticidal soap (a specialformulation of fatty acids) has been proven effectiveagainst aphids, leafhoppers, mealybugs, mites, pearpsylla, thrips, and whiteflies. Homemade soapsprays also work to some extent: use three table-spoons of soap flakes (not detergent) per gallon ofwater and spray on plants until dripping. Repellentsprays, such as garlic sprays and bug sprays (madefrom a puree of bugs), have been found useful bysome gardeners, but their effectiveness is question-able. Some researchers believe that bug sprays maywork if a disease is present in the insect, which isspread through the spray to other insects. Be care-ful! Homemade soap sprays can injure some plants.

Synthetic PesticidesSynthetic pesticides, by their simplest definition, arethose pesticides made by humans in chemicallaboratories or factories. Examples of these includemalathion, diazinon, and sevin. The real surge ofdevelopment of synthetic pesticides began in WorldWar II with the discovery of DDT. For more infor-mation, refer to the Pesticides chapter.


SummaryInsects constitute one class of the phylum Arthro-poda, and yet they are one of the largest groups inthe animal kingdom. The insect world is made up ofindividuals that vary greatly in size, color, andshape. Although most insects are harmless or evenbeneficial to humans, the few that cause damagehave tremendous impact. Harmful species canusually be recognized with some basic knowledgeof their host, habits, life cycle, and the type ofdamage they inflict. Feeding damage varies due tothe type of mouthparts an insect possesses. Harmfulinsects can be controlled in many ways withoutresorting to the use of pesticides. Good culturalpractices and proper selection of plant varieties,coupled with mechanical and biological controls,will control insect populations. Use insecticidesjudiciously, wisely, and safely. Read and followlabel directions carefully when applying any pesti-cide.

chapter 3 basics of entomology - extension.unh.edu

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