Floral Biology, Psychophily, Anemochory and Zoochory inChromolaena odorata (L.) King and H.E. Robins (Asteraceae)

Pakkurti Vara Lakshmi, Aluri Jacob Solomon Raju*, Dandangi Jeevan Ramand Kunuku Venkata Ramana

Department of Environmental Sciences, Andhra University, Visakhapatnam 530 003, India

(received August 25, 2010; revised January 6, 2011; accepted January 18, 2011)

IntroductionPlant-insect interactions are important to understandthe invasiveness of certain exotic weeds around theworld. Invasive species threaten the native biologicaldiversity of any ecosystem (Dupont et al., 2002) byinteracting with native species in several ways. Manyintroduced plant species compete with natives forresources (Daehler, 2003) or benefit from mutualisticinteractions with native resident species, often tothe detriment of native mutualism (Traveset andRichardson, 2006; Richardson et al., 2000). Severalstudies have described the disruption of plant-animalinteractions caused by competition for pollinatorsbetween invasive exotics and native plants (Moraguesand Traveset, 2005). On the contrary, Larson (2008)reported that invasive plants often occur in monotypicstands and provide a dense source of sustenance forinsects that rely on floral resources. A dense stand offlowering invasive plant may attract more pollinatorsto the area, and in the process enhances pollinationof neighbouring natives also. It is in this context, thestudy has been contemplated to understand howChromolaena odorata, an exotic species in India, isable to thrive well and attract a guild of insect speciesduring its flowering season in order to achieve greatsuccess in sexual reproduction and in seed dispersalto expand its distribution range in the world. This paperdescribes the pollination biology of C. odorata and

the importance of this plant as forage source forinsects, especially butterflies. The notable finding inthis work is that C. odorata is an important nectarhost plant for local butterflies and at the same timebutterflies are contributing to the expansion of thedistribution range through fertilization of flowers andsubsequent seed set.

Materials and MethodsChromolaena odorata is a noxious perennial weed.It is a free standing shrub and grows up to 1 m. Itis maintained by a system of abundant, yellowish,fine lateral roots. The opposite, three-nerved leavesare ovate-lanceolate, usually with a dentate marginand a long pointed tip. Seed germination starts inMay-June and active growth occurs during June-October. Their shoots root when touching theground. The plant possesses underground organswhich ensure the plant survival during drought ormechanical damage or in case of fire. Floweringoccurs once in a year. Flowers are very small, whiteto light purple, bisexual and zygomorphic. Calyxis reduced to pappus and represented by hairs.Corolla is tubular with five teeth at the tip. Stamensare 5, epipetalous, arise from the base of the corolla,with filaments free but anthers united, representingsyngenesious condition. The anthers are dithecous,have their connective prolonged into hood and theirbases produce hairy outgrowths, which in turn forma protective envelope for the nectary. The ovary is

Abstract. The study investigates the pollination biology of Chromolaena odorata an exotic species ofIndia, and also its importance as forage source for insects especially butterflies. The plant possessesfloral characteristics typifying psychophily. It is an important nectar source for insects and attractsbutterflies, hawk moths, bees, wasps, flies and other insects, which act as facultative pollinators whilecollecting the forage. Among the butterflies, Nymphalids play a prime role in the pollination. Seed setrate is very high in each head inflorescence in open pollination. Seed dispersal is by both anemochorousand zoochorous modes but former is the principal one.

Keywords: Chromolaena odorata, nectar, psychophily, insects, anemochory, zoochory, pollination,butterflies

*Author for correspondence; E-mail: solomonraju@gmail.com

Pak. J. Sci. Ind. Res. Ser. B: Biol. Sci. 2011 54 (1) 1-8

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bicarpellary and syncarpous, unilocular with asingle basal ovule (Fig. 1i,j). The style is surroundedby the nectary at the base and it is forked into twoparts.

The the present study, C. odorata was found at anelevation of over 700 m in the Seshachalam Hills ofsouthern eastern ghats (lat. 17° 43′ N; long. 83°90′ E) of Andhra Pradesh, India in open, sunny areasof forest margins and open gaps in the forest and wasused for the study during the summer season of 2009.C. odorata grows profusely during the active growthperiod and suppresses the growth of native lowground herbaceous flora. Twenty five tagged maturebuds, five each from five individuals were followedfor recording the time of anthesis and anther de-hiscence; the mode of anther dehiscence was alsonoted by using a 10× hand lens. Pollen was collectedfrom ten mature but undehisced anthers of staminateflowers of five different plants. Each anther wasdabbed with a needle in a drop of lactophenol-aniline-blue. The anther tissue was then observedunder compound microscope and the total number ofpollen grains was counted. The mean pollen outputper anther was multiplied by the number of anthersof a flower for obtaining the mean number ofpollen grains per flower. The pollen-ovule ratio wasdetermined as per Dafni et al. (2005). Twenty fiveflowers, five each from five individuals were used totest stigma receptivity with hydrogen peroxide frommature bud stage to flower drop (Dafni et al., 2005).Regular observations were made on the insectspecies visiting the flowers for forage. The insectswere observed for their foraging behaviour such asmode of approach, landing, probing behaviour, thetype of forage they collect, contact with essentialorgans to result in pollination and inter-plantforaging activity in terms of cross-pollination. Theobservation days for these aspects were thirty andthe time spent each day was five to eight hours.Twenty flowers each from fifty plants were selectedfor collecting information on insect activity. Threeto five specimens of each butterfly species collectedat different times of the day were brought tolaboratory for examining their proboscis undermicroscope for the presence of pollen grains inorder to assess their role in pollen transfer andpollination. Fruit and seed aspects were recordedbased on fifty sample heads collected from tenplants. A sample of ten tagged cylindrical headsconsisting of 489 flowers was followed for fruit set

rate in open-pollination. Field studies were carriedout also for seedling aspects.

Results and DiscussionFloral biology. Flowering occured during October-December. The inflorescences consisted of corymbs ofcylindrical heads, emerging at the terminal part of thebranches (Fig. 1a). A head consisted of 21 to 28 tubularflorets (Fig. 1b,c). The florets opened during 0600-0800 h (Fig. 1d-g). The anthers dehisced by longitudinalslits prior to anthesis. The pollen grains were 196 ± 14.6per anther and the total pollen output per floret was980 ± 11.8 (Fig. 1h). The pollen-ovule ratio was 980:1.The narrow anthers form a hollow space and pollen isliberated into this space. The receptive surfaces of thestylar branches stayed in closed state and as theygrew through the hollow space, brushed out the pollenliberated in the hollow space. Further, the style stretchedout beyond the anthers, and spread out its brancheswhich then became receptive and ready to receivepollen from the foragers. The receptivity remained sountil the noon of the 2nd day and the stylar brancheswithered away gradually. The nectar was secreted intraces only. The florets remained in place for about3-4 days and later fell off if not pollinated.

Flower visitors and pollination. The florets werevisited during day time by butterflies for nectarcollection and by other insects for pollen and/or nectarcollection; and by diurnal hawk moths, during dawnand dusk hours for nectar collection. The butterfliesincluded 23 species representing Papilionidae, Pieridae,Nymphalidae, Lycaenidae and Hesperiidae (Table 1;Fig. 1l-p; Fig. 2a-i). Foraging activity of nymphalidbutterflies showed a general trend characterized bygradual increase from morning to noon and gradualdecrease towards the evening (Fig. 4). This foragingtrend was observed almost throughout the floweringseason for butterflies of all families during this study.The data collected on the foraging visits of butterfliesof each family showed that Nymphalids made 61%,Papilionids 17%, Pierids 11%, Lycaenids 8% andHesperiids 3% of the total visits. The cylindrical headsequipped with a cluster of florets provided convenientlanding place for butterflies and this arrangement alsoenabled them to probe several flowers during each visitin succession for nectar before their departure. Theflowering heads borne terminally stood out prominentlyand the butterflies were found to be attracted to themeven from a long distance. The butterfly proboscides

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3Floral Biology of Chromolaena odorata

Fig. 1. Chromolaena odorata: a. Flowering inflorescences; b. Individual cylindrical head; c. Mature head;d. & e. Mature florets; f. Open floret; g. L.S. floret; h. Pollen grain; i. Ovary; j. Ovule; k. Mature seedsready for dispersal; l. Pachliopta hector; m. Papilio polytes; n. Acraea violea; o. Ariadne ariadne;p. Junonia lemonia.

Fig. 2. Chromolaena odorata: a. Euploea core; b. Phalanta phalantha; c. Danaus genutia; d. Danauschrysippus; e. Tirumala limniace; f. Delias eucharis; g. Catopsilia pyranthe; h. Tarucus nara;i. Ypthima asterope; j. Macroglossum gyrans; k. Cephonodes hylas; l. Apis dorsata; m. Apis cerana;n. Xylocopa sp.

4 Aluri Jacob Solomon Raju et al.

invariably contained pollen grains ranging from 15-35in Papilionids, 14-23 in Pierids, 29-189 in Nymphalids,8-17 in Lycaenids and 9-21 in Hesperiids. All thesebutterflies stretched out their proboscis to reach thefloret base for accessing nectar; while doing so theproboscis invariably contacted the stylar surfaces andhence affected pollination. They frequently movedbetween individual plants of C. odorata which hadpatchy distribution along the forest margins and in theopen gaps of the forest interior.

Hawk moths (Fig. 2j,k) showed intense foragingactivity during dawn hours, when the florets offeredfresh nectar, while reduced activity during dusk hourswhen the florets offered only the left-over nectarwhich was not fresh; they rarely foraged during

daytime. They hovered over the floret heads, insertedthe proboscis, and collected nectar quickly from asmany florets as possible from several flowering headsin quick succession. Their proboscis gained contactwith the stylar surfaces of each floret they visitedand this contact was considered to be resulting inpollination. They collected nectar from a number ofplants within a patch and in different patchescontributing to cross-pollination.

Other insect foragers included Hymenoptera andDiptera (Table 1; Fig. 2 l-n, Fig. 3 a-d). Of these, beesand wasps were frequent foragers during floweringpeak season in November; other foragers visited theflorets/heads occasionally. They approached theflowering plants in upright position and landed on thehead inflorescences prior to probing the florets. Beeswere found to probe the florets for nectar as well aspollen; it was not clear whether they were successfulin getting the nectar with their short proboscides butthey voraciously collected pollen. The wasps and otherinsects probed the flowers to collect nectar only; theirsuccess depended on the force with which theystretched out their tongue into the tubular corolla fornectar collection.

Table 1. List of insect foragers on Chromolaena odorata

Family Scientific name Common name

LepidopteraPapilionidae Pachliopta hector Crimson Rose

Papilio polytes Common MormonPapilio demoleus Lime Butterfly

Pieridae Catopsilia pyranthe Mottled EmigrantAnaphaeis aurota PioneerDelias eucharis Common Jezebel

Nymphalidae Ariadne ariadne Angled CastorJunonia lemonias Lemon PansyJunonia hierta Yellow PansyPrecis iphita Chocolate PansyAcraea violae Tawny CosterEuploea core Common Indian CrowPhalanta phalantha Common LeopardDanaus genutia Striped TigerDanaus chrysippus Plain TigerYpthima asterope Common Three- RingMelanitis leda Common Evening BrownTirumala limniace Blue TigerParantica aglea Glassy TigerNeptis hylas Common Sailer

Lycaenidae Everes lacturnus Indian CupidTarucus nara Rounded Pierrot

Hesperiidae Borbo cinnara Rice SwiftSphingidae Cephonodes hylas Coffee Hawk moth

Macroglossum gyrans Diurnal Hawk mothHymenopteraApidae Apis dorsata Rock Honey Bee

Apis cerana Indian Honey BeeA. florea Dwarf Honey BeeXylocopa sp. Large Carpenter Bee

Crabronidae Stizus sp. Digger WaspScoliidae Scolia quadripustulata Blue-winged Wasp

Scolia sp. Blue-winged WaspDipteraSyrphidae Helophilus sp. Hover Fly

5Floral Biology of Chromolaena odorata

Fig. 3. Chromolaea odorata: a. Stizus sp.;b. Scolia guadripustulata; c. Scolia sp.;d. Helophilus sp.

Fruiting ecology. Natural fruit set was 89%. The fruitswere small and one-seeded. The seeds were dry, lightand 4 mm long, brownish grey to black achene with asmall hook and tipped with pale brown 5-6 mm long

Asteraceae, Tridax procumbens Danaid and Pieridbutterflies have been shown to be the main pollinators(Balasubramaniam, 1989). Therefore, psychophily isadaptive and advantageous for the plant to maximizepollination with specialized florets and nectarivore ofbutterflies.

The diurnal hawk moths, Macroglossum gyrans andCephonodes hylas, also visited C. odorata floweringheads during dawn and dusk hours. The dawn foragingactivity is energetically profitable for them while theforaging at dusk period may not be profitable as theflorets by that time are most likely to be devoid ofnectar due to butterflies and other insects during daytime. They collected nectar on clear sunny and rainydays and utilized this floral source until exhausted.Balasubramaniam (1989) reported that hawk mothsvisited the florets of Tridax procumbens during rainyweather when most butterflies took shelter amongfoliage in nearby bushes.

Balasubramaniam (1989) reported that honey bees,flies, ants and thrips acted as facultative pollinatorsof Tridax procumbens. In the present study, beesvisited the flowering heads to probe both nectar andpollen but it was not clear whether they accessed thenectar; however, they collected pollen voraciouslyand easily. The wasps and flies also visited the flow-ering heads; the former collected nectar easily withtheir tongue lengths exceeding the corolla tube whilethe latter attempted to collect nectar but it was notclear whether they were successful in nectar collec-tion. Nevertheless, bees, wasps and flies affectedpollination while collecting the forage; they may bedesignated as facultative pollinators since theyvisited the florets/heads during peak floweringseason or occasionally. Further, the pollen collectingactivity of bees can contribute to the unavailabilityof pollen for pollination purpose and hence theyacted primarily as pollen thieves. The study showsthat C. odorata being an exotic plant is psychophilousand its prevalence in the forest areas is a potentialsource of nectar for butterflies for a period of threemonths. Although it is a menace due to its prevalenceand suppressive activity on the growth of certainnative low ground herbs, its flowers are most attrac-tive to butterflies throughout its distribution rangeand act as a provisioning post for the butterflies.

In C. odorata, seed production is prolific in each headinflorescence in open pollination. The fruit producesa single seed due to the presence of a single ovule ineach floret (Fig 1 i, j). Anemochory is the principalmode for seed dispersal; it is highly effective due toseed dispersal during summer season when ambienttemperature is high and relative humidity is very low.Further, zoochory is also functional due to the presenceof small hooks on the seeds; animals especiallydomestic animals as well as humans disperse seeds.Therefore, the dual modes of seed dispersal areinstrumental in spreading the plant in the tropicallatitudes rapidly (Kushwaha et al., 1981).

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