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Section 3: Pests and humans

• Arthropods (insects and near relatives) longassociated with crop damage, and sicknessin humans and animals.

• Direct effects (feeding damage).

• Indirect effects (disease transmission).

• Insects alone are damaging, but vectorcapacity makes them particularly deadly.

Examples of plant diseasestransmitted by insects

N. America,Mediterranean

Beet curly topVirusBeet leafhopper

AfricaMaize streakVirusMaize leafhopper

AsiaRice TungroVirusGreen riceleafhopper

WorldwidePotato leaf rollVirus

WorldwideBeet yellowsVirusGreen peach(peach-potato)aphid

WorldwideBarley-yellowdwarf

VirusBird-cherry aphid

DistributionDiseaseParasiteVector

An example of an insect-transmitted plant disease

• Aster yellows, aleafhopper-transmitteddisease, initiallycauses reddiscoloration of thefoliage, hairiness ofthe roots, andeventually affects cropgrowth and survival.

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More diseases

N. America, Asia,Europe

Dutch elm diseaseFungusBark beetles

N. & S. America,Europe, NewZealand

FireblightBacteriumBees

AfricaNematospora taintof coffee

FungusAntestia bug

AfricaSwollen shootVirusCocoa capsid

AsiaRice stripeVirusMaize planthopper

Africa, AsiaRice grassy stuntVirusBrownplanthopper

DistributionDiseaseParasiteVector

More insect-transmitted diseases

• The deformed tomatofoliage (above) signifyingtransmission of tomatospotted wilt by thrips is notdramatic injury, but whenfruit is harvested (below) itis discolored and uneven intexture, making the fruitunacceptable for sale(photos by Joe Funderburk).

• Mountain pine beetle transmits blue stain fungi. This deadlycombination of insect and fungus modifies entire westernecosystems as vast tracts of trees die.

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What patterns do you observe?

• Several taxa of pathogens involved

• Several taxa of vectors involved

(but most having piercing-sucking mouthparts)

• Distribution of pathogens variable

• Some vectors carry more than one disease

Examples of animal and human diseasestransmitted by arthropods

C. & S. AmericaChagas diseaseProtozoaTriatomine bugs

Tropics, subtropicsLeishmaniasisProtozoaSand flies

Africa, S. AmericaOnchocerciasisFilariaBlack flies

AfricaTrypanosomiasisProtozoaTsetse flies

S. America, AfricaYellow feverArbovirusesCulicinemosquitoes

TropicsFilariasisFilaria

Tropics, subtropicsMalariaProtozoaAnophelinemosquitoes

DistributionDiseaseParasiteVector

More diseases

WorldwideBabesiosisProtozoa

WorldwideLyme diseaseSpirochetes

N. AmericaColorado tickfever

ArbovirusesIxodid ticks

Tropics, subtropicsTick-bornerelapsing fever

SpirochetesArgasid ticks

Tropics, subtropicsEpidemicrelapsing fever

Spirochetes

WorldwideEpidemic typhusRickettsiaeBody lice

WorldwidePlagueBacteriaFleas

DistributionDiseaseParasiteVector

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Some medically importantinsects

Black salt-marsh mosquito

Deer flySand fly

Head louse

How do plant and animal diseasescompare?

• Sometimes the same pathogen type isinvolved

• Different insect taxa vectoring animal andplant diseases

• Similar mouthparts

Blood cells infected with the malariapathogen Plasmodium vivax

Some particularly importantgroups of insects

• Locusts and grasshoppers

• Aphids

• Armyworms and cutworms

• Mosquitoes

• Tsetse flies

• [rat] fleas

• [body] liceDo you know which of these groups are disease vectors?

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The vectors of disease are italicized;* indicates plant disease,

** human disease

• Locusts and grasshoppers

• Aphids*

• Armyworms and cutworms

• Mosquitoes**

• Tsetse flies**

• [rat] fleas**

• [body] lice**

Pea aphid, a vector of plant diseases

Effects of insectson humans

• Many interesting stories• Insect-transmitted diseases often kill more

people during wartime than die during fighting

• Your text cites Mormon cricket as an exampleof insect impact• Attributes arrival of predators (sea gulls) todivine intervention• Actual scenario slightly different, but quiteinteresting

Fleas vector plague

Adult male Mormoncricket. These insects areflightless, but form bandsof marching insects thatremain together for life.

Band of cricketsdispersing acrossroadway.

Mormon Cricket, Anabrussimplex

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The Mormon cricket story

• Mormon cricket (actuallya katydid) threatenedexistence of initialsettlement at Salt LakeCity, Utah

• Mormon crickets weredevouring crops essentialto settler’s survival

• Huge flock of gullsarrived and consumedcrickets

• Statue was erected tocommemorate thismiraculous event.

Mormon cricket:the real story

• The gull event really happened, but there isno need to invoke divine intervention as anexplanation.

• Insect-eating gulls are common in theRocky Mountain west, and are commonlyobserved flocking to insect populations, andfollowing farmer’s tractors as fields aretilled, where they feed on exposed insects.

More on crickets

• To this day, Mormon cricket populations increaseperiodically, plaguing farmers and ranchers.

• Because the crickets are so abundant, so mobile,and cover such a large areas, they are beyond theability of individuals to cope effectively.

• Area-wide suppression programs, coordinated bygovernment agencies, are used to combat thethreat.

• This is a good example of the value of area-widemanagement; this subject is treated later in thecourse.

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What makes an insect a pest

• An insect in the wrongplace

• Insects being toonumerous

Examples: termites inforest versus infesting abuilding; single honey beepollinating flowers versusswarm

Formosan termites

What makes an insect a pest,continued:

• Consume food,fiber, or shelter

• Biting or stinging

• Sheer abundance

• Vector plant oranimal diseases

Bee stings can result in swelling, which ispotentially dangerous if it involves the neckarea. People differ in their sensitivity tostings.

Mosquitoes are, for mostpeople, just a nuisance.However, when they aresufficiently abundant (seethe leg at right) they canliterally cause mortality topeople or livestockthrough excessive bloodremoval (exsanguination).They also are importantvectors of disease.

Welcome to summer in theEverglades! (photo, CollierCounty Mosquito Control)

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Examples of insect damage to crops

• Yellow-margined leafbeetles have turned thesecollard plants into trash.

• Russian wheat aphidshave invaded the edge ofthis wheat field and arekilling or impairing thegrowth of young plants.

Another example of crop damage

• Here you can see howmelon thrips havedefoliated bean plantson the edge of thissouth Florida field.Despite their smallsize, they can have agreat impact.

More damageWestern corn rootworm damage to corn roots (left) versus healthyroots (right). Though not always apparent because it is happeningbelow-ground, the impact can be serious.

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Can you identify the cause of thewhite spots on this carnation?

Carnation spotting

• Spots and speckles are caused by piercing-sucking arthropods such as leafhoppers,plant bugs, thrips and mites.

• In this case, the small number of spotscaused by thrips earlier in the developmentof the blossom is considered seriousbecause of the cosmetic high standards forflowers.

Insect feeding canproduce a variety ofdeformities thatdecrease the value ofornamental plants .Here you see gallsproduced by sprucegall aphids.

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The cause of damage isnot always obvious.This ear of corn wasoriginally damaged bybirds. Adult cornrootworms and sapbeetles will be attracted,and feed, but the realissue is birds.

Insects will never cease to amaze you on how they affect humans.This is a potato being consumed by termites. Generally we think oftermites as feeding on wood, but they can be serious pests of cropsand living trees. In Florida they affect newly planted trees in citrusgroves; in New Orleans they are destroying old street trees.

Vectors of disease

• Disease vectors are a special problem

• Although the importance is related to the numberof vectors and proportion capable of vectoring,even a single vector can be damaging

Examples: certified seed, race horse, someone inyour family!

Vectored disease greatly magnifies the importance ofthe insect and need for control.

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Yellow fever, malaria,and other diseases havekilled millions in thepast, and in some areasof the world continueto take a high toll ofhuman lives. Withouteffective vector control,the threat of diseaseoutbreak remains.

Jacksonville, and many othereastern US cities, were repeatedlyaffected by yellow fever. As late as1888, nearly 1/3 of the Jacksonvillepopulation was killed by thisdisease.

Body lice (also calledcooties, above) but nothead lice (called hobolice andindistinguishable frombody lice) or crab lice(called pubic lice, below)are important vectors ofdisease. Body licetransmit epidemictyphus, trench fever, andlouse-borne relapsingfever. All lice can be aproblem, however, asthey bite even if they aredisease-free.

This palm tree is dying froma disease called lethalyellowing of palm. It iscaused by a phytoplasma(mollicute) transmitted byplanthoppers, generallyMyndus crudus. This isconsidered by many to bethe most serious disease oftropical agriculture.

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Not all pests are equivalent inimportance

• Major pests and vectors

• Minor pests and vectors

• Occasional pests vectors

• Migrant pests and vectors

• Potential pests and vectors

Note: There may be some overlap in categories, orvariation among times and places.

Major pests and vectors

• The real “bad guys” or “key” pests

• Regularly abundant or widespread

• Influences economic production of crops orhealth of animals

Body louse, A. Shostak, Bio-DiTRL

• Management is critical

Examples of major pests

• Anopheles gambiae - malaria

• Locusts and grasshoppers - arid LDCs

• Leafhoppers of rice and corn - viruses

• Bollworms - cotton

• Diamondback moth - crucifers

• Termites - buildings

• Body lice - human disease

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Some majorpests

• Various species ofbollworms affectcotton throughout theworld.

• Japanese beetle is aserious pest of turf,ornamental plants andvegetables in theeastern U.S.A.

Minor pests and vectors

• Rarely abundant enough to cause damage

• Not well adapted to exploit host, adapted tothe climate, or held in check by naturalenemies

Corn leaf aphid, thoughabundant, rarely causesdamage to corn

Examples of minorpests

• Palestriped flea beetle on bean• False chinch bug on alfalfa• Tomato hornworm on eggplant• Spotted asparagus beetle on asparagus• Sweetpotato leaf beetle on sweet potato

False chinch bug

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If you found a paper product, such as this card, withevidence of feeding injury, to what would you attributethe damage?

Silverfish, minor pests, inflict this type of damage.

Why worry about minor pests?

• They can become serious, even if for brief period

• Some have gained notoriety by transitioning tomajor pests

Examples: twospotted spider mite, diamondbackmoth, fruittree leafrollers.

• Status change due to weather, use of disruptiveinsecticides, reduced use of insecticides, change inhost susceptibility

Occasional pests and vectors

• Intermediate between major and minor

• Something to watch for because they do causedamage, but not regularly

• Crop producers/health professionals monitor these,but don’t plan activities around their occurrence(as with major pests)

• Weather and host abundance are potential causes

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Examples of occasional pests

• Mosquitoes - heavy rainfall

• Tent caterpillars - regular population cycles

• Bluebottle flies - cadavers

• Various migrants - in areas of invasion• Flea beetles - ?

• Lovebug - ?

• Seedcorn maggot- ?

Lovebugs doing what they do best

Lovebugs arebest known forbecomingplastered onthe front ofvehicles in theGulf Coastarea of theU.S.A.

More occasional pestsPickleworm isan occasionalpest ofcucurbits in theeastern U.S.A.Its occurrenceis variable dueto its migratorybehavior,which isweatherdependent.

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Migrant pests and vectors

• Long range dispersal is possible for someinsects variable success

• Weather usually involved in movement, inaddition to flight

• Success varies

• When unexpected or in unusually largenumbers, migrants can be very damaging

Examples of migrant pests

• Desert locusts in North Africa and MiddleEast

• Pickleworm and black cutworm north fromGulf Coast

• Greenbug from TX and OK to northernGreat Plains

• Potato psyllid from Southwest to RockyMountain region

Pickleworms disperse northannually from the Gulf Coast area

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More migrants

• Not all pests move northward

• Army cutworms move west to RockyMountains, then east to great Plains

• Beet leafhopper moves betweenCalifornia’s valleys and foothills

Other forms of dispersal

• Movement of people, produce, and animals fosterdispersal of insects because they “hitch-hike”

• Increasinginternationaltrade, tourism,and rapidtransit increaserisk oftransport

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Examples of ‘hitch-hiking”insects

• Formosan termite from Asia to Southeast USA

• European corn borer from Europe to several areasof USA

• Asian tiger mosquito from Asia to Europe andUSA

• Mosquito Anopheles gambiae from Africa toBrazil

• Colorado potato beetle from USA to Europe

Questions about “hitch-hikers”

• Can you name 5 additional insects that havebeen accidentally introduced to the USA inrecent years?

• Can you identify the primary route oftransport?

• What can be done to reduce the movementof pests?

Potential pests and vectors

• Potential for additional transport remains

• Inspection and fumigation procedures atports and airports unpopular

• Some products not allowed

• How long barriers will remain effective isuncertain

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Medfly: a potential pest• Although Mediterranean

fruit fly occurs widely insome areas (Hawaii, forexample) it remainsabsent from other (e.g.,mainland U.S.A.). It is aterrible pest of fruit, andquarantine proceduresexist to keep it fromestablishing in new areas.

Smuggled products that couldcontain insects

• These items, includingfruit, vegetables, herbsand a home-madesausage, are just a fewexamples of theproducts that areremoved from theluggage of travelers asthe enter the U.S.A inMiami despite theprohibition againstsuch importation.

Examples from California

• California has worked hard to preventinvasion

• Gypsy moth and Japanese beetle eliminatedafter entry

• Were not able to prevent some insects suchas imported red fire ant, glassywingedsharpshooter, and Africanized bee fromestablishing.

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Not all insects arepests

• Scavengers and decomposers -termites break down wood,flies and beetles consumeanimal carcasses

• Pollinators - very important forsome fruit, vegetable andflower crops, but not grains

Colony of bumblebees forpollination of zinnias

(S. Bauer, ARS)

Without decomposers like flies the landscape would belittered with “roadkill.” Here you see some maggots feedingon a cadaver only a few days after the animal has died.

After 2 weeks the cadaver is reduced to skin and bone, withthe remaining maggots desperately seeking more food.

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More benefits from insects

• Natural enemies - predators, parasitoids,herbivores of pest plants (weeds)

• Insect products - honey and wax from Apismellifera, silk from Bombyx mori, shellac fromLaccifer lacca scales, cochineal dye fromDatylopus coccus mealybugs, maggot therapyusing greenbottle (Lucilia) and bluebottleCalliphora) flies

Insect-pollinated crops

• Do you know 10 cropsthat require, or greatlybenefit from, insectpollination?

• Can you find sourcesof information on thissubject?

Honey bee inside squashblossom.

Crops that benefit or requirepollination by insects

Fruit crops: apple, apricot, blueberry, cherry, peach,pear, plum, mango, papaya, raspberry, some citrusfruits

Vegetables: cucumber, melons, squash, pepper,eggplant

Other crops: almond, coconut, olive, cashew, cacao,coffee

Seed production: most vegetables, oilseed, and someforage crops

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Insects as food

• Consumption by humans an oddity indeveloped countries

• Sometimes and important source of protein andfat, especially in LDCs

Examples: chironomid (formerly in USA) andchaoborid (in Africa) flies; termites and locustsin Africa; caterpillars and grasshoppers inMexico; many types in SE Asia

Insects as food, continued

• Insects are important food for wildlife.

Examples include:

• Small mammals such as fox, opossum, skunk,raccoon, shrews, bats and many rodents.

• Songbirds and other birds, including some that arenormally thought of as granivorous (grain feeding),especially when feeding nestlings.

• Fresh water fish - a critical food source.

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Horned toads depend on insects for food. Areinsects important for most reptiles and amphibians?

Dietary habits of birds

• Based on stomach contentanalysis is is apparent thatinsects comprise 2/3 of ashrike’s diet.

• Chipping sparrows, normallythought of as seed-feeders,also consume many insects,over 1/3 of their diet.

When a hungry fox is looking for a meal, evengrasshoppers look good!

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Questions:

• Can you distinguish between direct andindirect pests?

• Name some plant and animal diseasestransmitted by arthropods?

• Describe what makes an insect a pest?

• Name some beneficial insects and explainwhy they are important?

Questions from supplementaryreadings

• Reading 1, History– Can you name 5 important insects that transmit diseases to

humans?– Can you name 5 of the great insect-vectored diseases, and the

important effects they had on human history?

• Reading 2, Decomposition– What are some roles of insects in trophic processes?– What arthropod taxa are important in decomposition of plant

remains? Excrement? Corpses?– Colonization of corpses occurs in “waves.” What taxa are

involved in the waves, and what is the significance of knowingthis?

More supplementary reading• Reading 3, Plant disease

– Are all plant diseases caused by pathogenicorganisms?

– What are 3 ways that pathogens are transmitted byinsects?

– What plant disease-causing organisms are transmittedby insects?

– How do bacteria casue plant wiliting?– What is a mollicute? What plant tissues initially are

invaded/infected?– What is sooty mold?