status of biological control of parthenium hysterophorus l ... · the wilting and drying of a few...

Insect Sci. Applic. Vol. 12, No. 4, pp. 347-359, 1991 0191-9040/91 $3.00 + 0.00Printed in Kenya. All rights reserved © 1991 ICIPE—ICIPE Science Press

STATUS OF BIOLOGICAL CONTROL OF PARTHENIUMHYSTEROPHORUS L. IN INDIA: A REVIEW

J. SRIKANTH and N. A. PUSHPALATHADepartment of Entomology, University of Agricultural Sciences, G. K. V. K. Campus

Bangalore 560 065, India

(Received 15 August 1989; revised 7 May 1990)

Abstract—Biological control efforts on Parthenium hysterophorus L. (Asteraceae) in Indiahave gained momentum after the limitations of other methods were realized. Native surveysrevealed a large number of insects, but none of them was host specific. Although theintroduced beetle Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) hasestablished at the sites of initial releases, its real impact on the weed and performance indifferent parts of the country need further evaluation. Fungal pathogens of the weed holdpromise for classical as well as microherbicidal control. The use of parthenium phyllody MLOas a biocontrol agent requires establishment of host and vector specificity. Mycotoxins are apotential group of herbicides on which serious studies are yet to begin. Studies on control ofthe weed through interference and allelopathy by Cassia uniflora Mill. (= C. sericea Sw.)(Leguminosae) have produced promising results. Toxic leachates of C. uniflora and autotoxicprinciples of the weed deserve attention. Integrated biocontrol strategies envisaged forwastelands using introduced insects and pathogens, allelopathic plants, and agroecosystemsusing native pathogens, mycotoxins and autotoxic principles, will help combat this apparentlyinvincible weed.

Key Words: Parthenium hysterophorus, biological control, Zygogramma bicolorata, mycotoxins,allelopathy, Cassia uniflora

Resume*—Les efforts contrdle biologique sur Parthenium hysterophorus L. (Asteraceae) enInde se sont accel6res des qu'on s'est rendu compte des limites des autres methodes. Desrecherches locales ont revele un grand nombre d'insectes mais aucune d'el les n'etaient unnote splcifique. Malgre que le coleoptere Zygogramma bicolorata Pallister (Coleoptera:Chrysomelidae) recemment introduit se soit etabli sur des sites ou on l'a initialement e~leve~,son impact reel sur I'ivraie et le resultat obtenu dans d'autres regions du pays necessite uneplus ample evaluation. Les microbes pathogenes fongaux de I'ivraie donnent une lueurd'espoir tant pour le contrdle classique que pour le contrdle microherbicidal. L'utilisation dela phyllodie du parthenium MLO comme agent biocontrole exige la connaissance de laspecificity de l'hdte et du vecteur. Les mycotoxins sont un groupe potentiel d'herbicides surlesquels on doit commencer des etudes serieuses. Les etudes sur le contrdle des mauvaisesherbes au moyen d'interference et d'allelopathie par la plante Cassia uniflora Mill. (= C.sericea Sw.) (Leguminosae) ont donne1 des resultats prometteurs. Les extraits toxiques du C.uniflora et des principes autotoxiques de mauvaises herbes merite une attention particuliere.Les strategies inte'gre'es du biocontrole envisagees pour des terres en friche en utilisant desinsectes et des pathogenes introduits, des plantes alleiopathiques, et, peur des agro-ecosystemes, ('utilisation des pathogenes indigenes, des principes mycotoxins et autotoxiquesaideront a combattre cette mauvaise herbe apparemment invincible.

347

348 J. SRIKANTH and N. A. PUSHPALATHA

INTRODUCTION

Very few weeds in India have generated as muchalarm and concern among the scientificcommunity and common man as the weedParthenium hysterophorus L. (Asteraceae).Native to Mexico, the weed, within the last 100years, has found its way to Africa, Australia andAsia, and currently enjoys worldwide distribution(Haseler, 1976; Towers et al., 1977). Although itwas first noticed in India in the year 1956 (Rao,1956), its actual date of entry into the country wastraced back to 1810 by Bennett et al. (1978) andconfirmed by Maiti (1983). Also popularly calledbroom-bush, carrot weed, congress grass, whitetop, etc., its spread to different parts of the countryhas been very rapid. According to Krishnamurthyet al. (1976), about 2 M ha of land is covered bythis weed in the country. However, the weedremained in obscurity for about a decade after itsdiscovery and rose to prominence only after itshazardous effects on the lives of people reachedintolerable levels. The interference of this weed inagriculture (Vartak, 1968; Kanchan andJayachandra, 1976) and animal health(Narasimhan et al., 1977) has been welldemonstrated. Its distribution, chemistry, hazardsand control have been extensively reviewed(Krishnamurthy et al., 1977; Towers et al., 1977;Basak, 1984; Parihar and Kanodia, 1986; Joshi,1990).

Although several methods have been proposedfor the suppression of this weed, each has its owndisadvantages (Joshi, 1989). For instance,mowing the weed as soon as it flowers, thoughpreventing seed production, results in rapidregeneration of new shoots necessitating repeatedoperations (Gupta and Sharma, 1977).Mechanical uprooting is constrained by thedevelopment of dermatitis in workers engaged inthe operation (Krishnamurthy et al., 1977).Chemical control, though effective, is temporaryand needs repeated application, besides havingproblems of residues, selectivity and cost ofapplication. Moreover, it is rather impossible toadopt these methods in vast stretches ofwasteland. These disadvantages prompt us toinclude biological methods as a component ofparthenium control programme. In the presentpaper we review the status of biological control ofP. hysterophorus in India and discuss its future inlight of several available and potential tools forintegrated biological control programme.

The studies made so far in this regard rangedfrom surveys conducted for natural enemies and

employment of polyphagous insects, mycotoxinsand MLO diseases, to the more serious effortsmade to import and release exotic natural enemiesfor classical biological control.

Insects and mites

An examination of the list of insects and mitesreported on this weed in India (Table 1) revealsmany of them to be polyphagous pests ofagricultural crops. Although many were noticedas sporadic occurrences causing minor damage, afew occurred in serious proportions often killingthe plant.

Amongst the four species of scale insectsfound attacking the stems and branches,Parasaissetia nigra (Nietn.) and Saissetia coffeae(Wlk.) caused wilting of parthenium plants in theglasshouse, while P. nigra often killed groups ofthe weed in the field (Kumar et al., 1979).Similarly, the lantana scale, Orthezia insignisBrowne, attacked the stem, leaves and flowers ofthe weed in dense colonies in summer, resulting inthe wilting and drying of a few plants (Srikanth etal., 1988a).

Adults and nymphs of the membracidLeptocentrus taurus F. attacked the main stem,petioles and bases of midribs of about 20% of themore than 1000 plants examined (Thangavelu,1980).

Several species of mealy bugs were reportedon this weed. Ferrisia virgata (Ckll.) attackedespecially the roots of young plants which wiltedand dried (Char et al., 1975). Potted plants of theweed when infested with Planococcus sp. wiltedwithin a week without producing a single seed(Hegde and Patil, 1979). Similarly, Iceryaseychellarum (Westw.) killed the weed in theglasshouse, whereas F. virgata when reared andreleased on isolated patches of the weed failed tokill the latter (Kumar et al., 1979).

Larvae of the beetle Oberea sp., and grubs andadults of the scolytid Hypothenemus eruditus(Westw.) were found boring into the stemscausing wilting and death of the plants. As manyas 15-20 adults of the latter were seen in a singleplant besides many grubs, and up to 75% of theplants were bored in some areas (Kumar et al.,1979). Adults of the flea beetle Luperomorphavittata Duvivier caused characteristicskeletonized spots on leaves by scraping theepidermis. They also fed on stems by scraping theepidermis and scooping the cortex portion(Srikanth et al., 1991).Larvae of Spilosomaobliqua Wlk. voraciously fed on leaves and main

Biological control of Parthenium hysterophorus L. 349

Table 1. Insects and mites recorded on P. -hysterophorus in India

Order Family Species Reference

Heteroptera Pentatomidae Chrysocoris stollii (Wolff)Dolycoris indicus Stal

Homoptera Aphididae Aphis gossypii Glov.

Aphis spiraecola Patch

Cicadellidae Aconurella sp.Batracomorphus angustatus (Osborn)Batracomorphus indicus (Lethierry)Empoasca sp.Empoasca devastans DistantExitianus indicus (Distant)Hecalus arcuatus (Motschulsky)Hishimonus phycitis (Distant)

Leofa sp.Orosius albicinctus Distant

Thaia sp.

Coccidae Ceroplastes sp.Coccus longulus (Dgl.)Parasaissetia nigra (Nietn.)Saissetia coffeae (Wlk.)

Margarodidae Icerya seychellarum (Westw.)

Membracidae Coccosterphus minutus (F.). Leptocentrus taurus F.Oxyrhachis tarandus (F.)Telingana campbelli Dist.

Ortheziidae Orthezia insignis Browne

Kumar et al. (1979)Kumar et al. (1979)

Rajendran (1976),Krishnamurthy et al. (1977),Kumar et al. (1979), Raodeoand Tayade (1979), Keshwal(1982)Rajendran (1976)

Mathur (1989)Ravi (1983), Mathur (1989)Mathur (1989)Mathur (1989)Padmanabhan (1982)Mathur (1989)Mathur (1989)Padmanabhan (1982),Mathur (1989)Mathur (1989)Keshwal (1982), AnanthaMurthy (1984), Mathur(1989)Mathur (1989)

Kumar etal. (1979)Kumar etal. (1979)Kumar etal. (1979)Kumar etal. (1979)

Kumar etal. (1979)

Kumar et al.( 1979)Thangavelu (1980)Kumar etal. (1979)Kumar etal. (1979)

Srikanth etal. (1988a)

Pseudococcidae Ferrisia virgata (Ckll.)

Nipaecoccus corymbatus (Newst.)Planococcus sp.

Orthoptera

Coleoptera

Acrididae

Cerambycidae

Chrysomelidae

Neorthacris simulans (Bol.)

Oberea sp.

Luperomorpha vittata DuvivierMetriona circumdata (Hbst.)Monolepta signata (01.)Metriona circumdata (Hbst.)Monolepta signata (01.)

Char etal. (1975), Kumaret al. (1979)Kumar etal. (1979)Hegde and Patil (1979)

Kumar et al. (1979), Raodeoand Tayade (1979)

Kumar etal. (1979)

Srikanth etal. (1991)Krishnamurthy et al. (1977)Kumar etal. (1979)Krishnamurthy et al. (1977)Kumar etal. (1979)


Table l.Contd.

Curculionidae

Scolytidae

Lepidoptera Arctiidae

Noctuidae

Acari Tetranychidae

Ptochus ovulum Faust

Hypothenemus eruditus (Westw.)

Spilosoma obliqua Wlk.

Trichoplusia orichalcea F.Heliothis armigera (Hub.)

Kumar et al. (1979)

Kumar et al. (1979)

Krishnamurthy et al. (1977),Vaidya and Vartak (1977)

Krishnamurthy et al. (1977)Raodeo and Tayade (1979)

Tetranychus cinnabarinus (Boisd.) Kumar et al. (1979)Tetranychus ludeni Zacher Harish Kumar et al. (1990)Tetranychus macfarlanei Baker & Pritchard Harish Kumar et al. (1990)Tetranychus neocaledonicus Andre- Puttaswamy et al. (1976)(= cucurbitae Rahman & Sapra) •

Tenuipalpidae Brevipalpus phoenicis (Geijskes) Dagar and Singh (1979)

stem, completely destroying the plant above theground level (Vaidya and Vartak, 1977).

Four species of spider mites, namelyTetranychus neocaledonicus Andre (= cucurbitaeRahman & Sapra) (Puttaswamy et al., 1976),Tetranychus cinnabarinus (Boisd.) (Kumar et al.,1979) and Tetranychus ludeni Zacher andTetranychus macfarlanei Baker & Pritchard(Harish Kumar et al., 1990) were recorded on thisweed. The false spidermite Brevipalpus phoenicis(Geijskes) often destroyed the attacked plants ofthe weed and suppressed its growth whenartificially infested at various growth stages of theplant (Dagar and Singh, 1979).

Sundara Rajulu and Gowri (1976) employed atrandom various phytophagous pests found oneconomic plants for the control of the weed. Aphisfabae Scopoli, Pseudococcus sp. and Tetranychussp. reduced the growth of potted plants greatlywhich often resulted in their wilting when infestedartificially. When all these pests were introducedinto vast stretches of parthenium fields, theyestablished in 1 month and in another 2 monthsplants in about 30-40 acres wilted completely.

The first systematic biological control attemptto tackle the problem weed by using exotic insectswas initiated at the Indian Institute ofHorticultural Research (IIHR), Bangalore, withthe importation of Zygogramma bicolorataPallister (Coleoptera: Chrysomelidae) fromMexico in 1983. The grubs fed on terminal andaxillary buds, and peripheral leaves leading tostunted growth and reduced flower production.Host-specificity tests with 40 species of plantsshowed no adult feeding, oviposition or larval

feeding on all the hosts except jasmine and nigeron which slight nibbling by adults was observed(Jayanth and Nagarkatti, 1987). After the insectwas found safe for field liberation, releases weremade in 500 m2 area of a 10 ha parthenium plot.The beetles established in about a month and alsodispersed to about 2 ha. It was active in the fieldduring heavy rainfall, diapaused in soil duringsummer and emerged with the rains (Jayanth,1987a). Further studies conducted in the field todetermine the number of insects required to bringabout the control of the weed showed that apopulation level of 3-5 adults per plant resulted incomplete defoliation of the weed in 6 weeks. Mostof the adults dispersed after defoliation leavingbehind a residual population, which preventedregeneration of the weed and also suppressednewly emerging plants before flowering (Jayanthand Geetha Bali, 1990).

A stem borer, Epiblema strenuana (Walker)(Lepidoptera: Tortricidae) originating fromMexico, that prevents growth by forming stemgalls and also reduces flower production, wasimported from Australia in 1985. Amongst 49species of plants tested for host specificity, normalcompletion of life cycle, egg laying and hatchingobserved in niger prompted the IIHR to terminatethe culture (Jayanth, 1987b).

Pathogens and mycotoxins

Several fungal, two bacterial and onemycoplasmal disease have been reported on P.hysterophorus (Table 2).


Table 2. Fungal, bacterial and mycoplasmal diseases of P. hysterophorus in India

Pathogen

Rhizoctonia solani KuhnSclerotium rolfsii Sacc.

Colletotrichum capsici (Syd.) Butl. & BisbyColletotrichum gloeosporioid.es Penz.

Exserohilum rostratum (Drechsl.)Leonard & Suggs

Fusarium semitectum Berk, and Rav.Alternaria tenuis Auct.Alternaria zinniae PapeOidium sp.

Oidium parthenii Satyaprasad and Usharani

Pseudomonas solanacearum Smith

Xanthomonas parthenicolaMLO

Disease

WiltCollar rot

Leaf spotLeaf spot

Leaf spot

Leaf spotBlightBlightPowdery mildew

Powdery mildew

Wilt

BlightPhyllody

Reference

Kumar et al. (1979)Siddaramaiah et al.(1984)Rao and Rao (1979)Vijayakumar and Rao(1977)Rao and Rao (1987)

Rao and Rao (1987)Rao (1965), Datar (1981)Rao(1965)Mhaskar (1972), Datar(1981)Satyaprasad andUsharani (1981)Ram Kishun and RameshChand(1987)Ghosh and Singh (1979)Anupam Varma et al.(1974), Phatak et al.(1975), Hegde andPatil (1976), Kumar et al.(1979), Mali andVyanjane (1979),Deshpande et al. (1982),Padmanabhan (1982),Mathur (1989)

The root rot fungus, Rhizoctonia solani Kuhncaused wilting in a few plants of the weed (Kumaret al., 1979). Similarly, infection by Sclerotiumrolfsii Sacc. resulted in wilting and death ofseedlings. The pathogen caused pre-emergencerot and post-emergence death of seedlings inpathogenicity tests (Siddaramaiah et al., 1984).

The leaf spot disease produced byColletotrichum gloeosporioides Penz. wascharacterized by necrotic lesions which graduallyincreased in size and covered the entire lamina.Pathogenicity tests with spore suspensionsproduced symptoms in 5-6 days and reisolationsyielded the fungus (Vijayakumar and Rao, 1977).A similar leaf spot caused by Colletotrichumcapsici (Syd.) Butl. & Bisby showed no diseasesymptoms in some local varieties of chillies whencross inoculated (Rao and Rao, 1979).

Alternaria tenuis Auct. and Alternaria zinniaePape were found causing a very serious blight inthis weed (Rao, 1965). A. tenuis, which waspathogenic when inoculated, produced grey

coloured spots on the lower leaves which enlargedand coalesced turning the leaves dark brown. Theprevalence of the disease was up to 23% (Datar,1981).

A powdery mildew caused by Oidium sp.produced white floury patches on leaves whichwithered and dropped in later stages. The diseaseincidence was found to be up to 10% (Datar,1981). Satyaprasad and Usharani (1981) alsonoticed widespread incidence of a similarpowdery mildew caused by Oidium partheniiSatyaprasad and Usharani.

The bacterial blight caused by Xanthomonasparthenicola produced dark brown water soakedlesions on usually lower leaves which coalescedto form larger patches resulting in completeblighting of leaves. Purplish to light brown streaksappeared on the stem close to the ground level,turning darker in due course of time (Ghosh andSingh, 1979). The wilt caused by Pseudomonassolanacearum Smith led to drooping of leavesfollowed by wilting and death of plants. Browning


of stem at soil level became prominent with theadvancement of disease (Ram Kishun and RameshChand, 1987).

The first observations on the occurrence ofphyllody in parthenium seem to be those ofAnupam Varma et al. (1974) and Phatak et al.(1975). Typical witches' broom symptoms,reduction in leaf blades, floral abnormalities,sterility and failure to produce seeds wereobserved in infected plants (Anupam Varma et al.,1974; Phatak et al., 1975; Hegde and Patil, 1976;Deshpande et al., 1982; Padmanabhan, 1982;Mathur, 1989). The incidence of phyllody wasfound to be about 10% (Hegde and Patil, 1976;Ravi, 1983), while it varied from 15.5-100% inplots near agricultural fields and 8-73% in vacantareas (Mathur, 1989). The incidence wasmaximum from September-December, while itwas low from January-April. The infected plantsshowed a 25.9% reduction in number of flowersper plant and 96.3-100% sterile flowers per plant(Keshwal, 1982).

The causal organism was identified as amycoplasma like organism (MLO) on the basis ofelectron microscope studies (Anupam Varma etal., 1974), and transmission studies and antibiotictherapy (Mali and Vyanjane, 1979). AlthoughPhatak et al. (1975) suggested that partheniumphyllody may be the same as sesamum phyllodysince association of strikingly similar organismswith sesamum phyllody was recorded earlier, thepathogen was not found infecting sesamum or anyother test host (Mali and Vyanjane, 1979).

Mechanical sap inoculation failed to transmitthe disease, but it was successfully transmitted bygrafting (Keshwal, 1982; Padmanabhan, 1982).An unidentified species of aphid that was foundinfesting the diseased plants was inferred to havetransmitted the disease (Hegde and Patil, 1976),whereas the disease was not transmitted by theaphid, Aphis gossypii Glover (Keshwal, 1982).The leafhopper, Orosius albicinctus Distanttransmitted the disease to parthenium andThchodesma sp. (Keshwal, 1982), and toparthenium and 10 other host plants (Mathur,1989). The leafhopper Hishimonus phycitis(Distant) was found transmitting the disease toparthenium (Padmanabhan, 1982), Sesamumorientalis L. and Gomphrena globosa L.(Keshwal, 1982), whereas the leafhopperEmpoasca devastans Dist. failed to transmit thedisease to parthenium (Padmanabhan, 1982).

Deshpande (1981) tested the efficacy of toxicmetabolites of seven species of fungi as weedkilling agents. Metabolites of Fusarium

moniliforme Sheldon produced typical wiltsymptoms in 5-6 hr whereas those of otherpathogens needed 36 hr to produce symptoms intreated plants.

Parasitic plants

Orobanche aegyptiaca Pers. was recordedparasitizing the roots of parthenium in addition tomany other hosts (Dagar, 1981). Similarly, theparasitic plant Cuscuta reflexa Roxb. was foundgrowing extensively on this weed oftenweakening smaller herbs (Deshpande et al.,1982).

Competitive replacement and allelopathy

The work of Desai and Bhoi (1981) appears tobe the first attempt made to control the weed bycompetitive replacement. Broadcasting seeds ofstylo, Stylosanthes hamata (L.) Taub.(Leguminosae), after uprooting parthenium,followed by light and complete cultivation of theplots resulted in significant control of the weed inthe second year. Similarly, replacement of theweed in its natural habitat by Cassia uniflora Mill.(= C. sericea Sw.) (Leguminosae) was observedby Singh (1983), Syamasundar and Mahadevappa(1986) and Mamatha and Mahadevappa (1988).

The allelopathic properties of C. uniflora andthe weed were demonstrated in the laboratory byJayakumar (1985). Seed germination andseedling growth of the weed were inhibited byaqueous extracts of various parts as well as higherseed density of C. uniflora, the latter beingattributed to phenolic compounds leaching fromthe seed (Syamasundar and Mahadevappa, 1987;Joshi, 1991a). The inhibitory effects of C. unifloraon parthenium were also observed in field plotexperiments (Singla et al., 1990). On the contrary,undiluted extracts and higher seed density of theweed significantly promoted the growth of C.uniflora. In addition to the allelopathic effects onthe weed, undiluted extracts of C. uniflorainhibited seed germination, and root and shootgrowth of certain cultivated crops in petri platesand pot culture (Vasudevan, 1986).

Studies conducted in wastelands for 3 yearsindicated that C. uniflora was capable of replacingparthenium gradually and the replacement wouldbe faster if parthenium plants are pulled out in theinitial stages of competition (Mahadevappa andRamaiah, 1988). Long-term field trials currentlyin progress indicate that C. uniflora can reduce theweed by over 90% in 4-5 years with substantial


monetary returns (Joshi, 1989; Joshi, 1991b). Apackage of practices for growing C. uniflora tocontrol the weed and its seed multiplication hasbeen recommended (Mahadevappa and Joshi,1985; Joshi, 1990).

Besides C. uniflora, several other plants havealso been shown to possess allelopathicprinciples. Aqueous extracts of fresh leaves, leaflitter and bark of Alstonia scholaris R.Br.(Apocynaceae) inhibited the germinationpercentage, early seedling growth and emergenceof first leaf of parthenium (Ghildiyal, 1987).Similarly, aqueous extracts of aerial parts ofHyptis suaveolens Poit. (Lamiaceae) inhibitedseed germination of the weed completely whereasthe seeds of five crop plants were unaffected (Raoet al., 1987). Aqueous and organic leachates, andmethanol, water and chloroform fractions ofEucalyptus sp. (Myrtaceae) reduced cell survival,chlorophyll content, soluble proteins, RNA andcarbohydrates of leaves of the weed to variouslevels when sprayed on 2 months old seedlings(Kohli et al., 1988). Amongst 20 species of plantsexamined, Cassia tor a L. (Leguminosae) andStylosanthes fruticosa Alston. (Leguminosae)brought about 40-50% suppression of parthenium(Joshi, 1990).

Parthenium is vulnerable not only to theallelopathic activity of other species of plants butalso to its own leachates. Such autotoxicity toleachates from leaves and inflorescencemanifested in the form of reduction in percentagecell survival and chlorophyll content, preventionof rooting and sprouting, and reduction inregeneration potential in treated plants (Kumariand Kohli, 1987).

DISCUSSION

The search for natural enemies, particularlyinsects of P. hysterophorus in India appears tohave begun many years after its discovery in1956. Lack of information on the occurrence ofpests or diseases on the weed (Jayachandra, 1971),except for a blight (Rao, 1965), could be due to thefact that no serious surveys were conducted priorto 1971. Asystematic survey (Kumaretal., 1979),in addition to several casual observations revealedan array of insects and mites attacking the weed.However, majority of them were sporadic thoughsome appeared to be endemic like the membracidL. taurus (Thangavelu, 1980) and some otherswere found in high intensity, such as the scolytidH. eruditus (Kumar et al., 1979). Most of theinsects lacked host specificity and attacked the

weed only during the absence of their primaryhosts. For instance, the scale O. insignis heavilycolonized the weed in summer when its primaryhost Lantana camara L. collapsed under thepressure of a heavy population buildup of theinsect (Srikanth et al., 1988a, b). All the abovecharacteristics render these insects and mitesunsuitable candidates for biological controlnotwithstanding the apparently impossiblesuggestion to produce host specific strains byrecurrent selection (Vaidya and Vartak, 1977).Yet Sundara Rajulu and Gowri (1976) claimingcomplete control of the weed with a highly illconceived and futile exercise of using thepolyphagous pests A.fabae, Planococcus sp. andTetranychus sp., disregarded all principles andprocedures of biological weed control. In a latersurvey in the same area, Thangavelu (1980),however, not only failed to record A.fabae, butalso pointed out the need for confirmation of itsoccurrence in tropical India.

Concerted efforts for classical biologicalcontrol of parthenium commenced in the countrywith the introduction and subsequent studies onhost specificity, field releases and recovery of Z.bicolorata (Jayanth, 1987a; Jayanth andNagarkatti, 1987). Although the beetleestablished at the sites of release and showedslight dispersal, its activity appears to be limitedby the distribution of rainfall as it was observed toundergo diapause in the dry months of the year(Jayanth, 1987a). This perhaps necessitatesdetailed studies on the extent of survival andcarryover of the beetle to the next season toascertain the need for supplementary releases atthe beginning of the rainy season coinciding withthe emergence of diapausing individuals. It willalso be worthwhile to ascertain the possibleimpact of other flea beetles, such as L. vittata thatattack parthenium, on the populations of Z.bicolorata in the event of delayed or inadequate orerratic rainfall for a couple of years (Srikanth etal., 1991). As the beetle is likely to perform betterin well distributed heavy rainfall areas (Jayanth,1987a), its introduction and evaluation indifferent parts of the country need attention. Atthe same time, the possibilities of importing otherpotential natural enemies from Mexico, its placeof origin, as well as from other countries wherethey have been found to be promising, should beexplored. For instance, the host specific delphacidStobaera concinna (Stal) (Homoptera:Delphacidae) (McClay, 1983), the narrowlyoligophagous defoliator Bucculatrix parthenicaBradley (Lepidoptera: Buccutatricidae) (McClay


et al., 1990) and other insects (McClay, 1981)found in Mexico are likely candidates. As thephenomenon of diapause and a possible lag phasein the buildup of populations of Z. bicolorata arelikely to result in rapid growth of partheniumpopulations with the onset of monsoon, there is aneed for other insects which can attack the weedearly in its growth stages. Stem borers that destroyplants before flowering, such as the weevilListronotus setosipennis (Hustache) (Coleoptera:Curculionidae) (McFadyen, 1985) and flowerfeeders, such as the weevil Smicronyx lutulentusDietz (Coleoptera: Curculionidae) (McClay,1981) should be accorded priority. The stem borerE. strenuana showed considerable impact on theweed in Australia (McFadyen, 1985) after it wasapproved for field release due to its restricted hostrange (McClay, 1987). The same insect, however,was not approved for field release in India as it fedand reproduced on niger in host specificity tests(Jayanth, 1987b). Such failures should not hinderfurther natural enemy exploration.

The fact that fungal pathogens have receivedless attention than insects as biocontrol agents ofparthenium in India is evident from theconsiderably fewer diseases reported on this weed(Table 2). None of these diseases caused anyappreciable damage even though some incidenceof powdery mildew (Satyaprasad and Usharani,1981) and blight (Datar, 1981) was reported. Thusthe status of these diseases appears to beanalogous to that of insects. Fungal pathogens, byvirtue of their abundance, destructive nature,amenability to mass culture and formulation, andability to actively penetrate the host (Charudattan,1985), constitute a very promising group of weedkilling agents. Of the three approaches, viz.classical, augmentative and microbial herbicidestrategies, classical strategy is best suited forcontrolling weeds in undisturbed areas andrangelands (Charudattan, 1985). The rust causedby Puccinia abrupta var. partheniicola (Jacks.)Parmelee (McClay, 1985) and other pathogens(Evans, 1987) found in Mexico are worthy ofconsideration for the control of parthenium inwastelands. The microbial herbicide methodwhich relies on native, endemic diseases that areadapted to local conditions (Charudattan, 1985),is best suited for the control of the weed inagricultural ecosystems. Such diseases have beensuccessfully utilized commercially for the controlof milkweed vine and northern jointvetch(TeBeest and Templeton, 1985). Intensivesurveys for the identification of host specificspecies or strains of pathogens of parthenium in

the country will pave the way for their use asmicrobial herbicides. For example, the powderymildew fungus O. parthenii (Satyaprasad andUsharani, 1981) and the bacterium X.parthenicola (Ghosh and Singh, 1979), recordedas new species, may be specific to the weed.Similarly, the failure of C. capsici to producesymptoms in some varieties of chillies when crossinoculated (Rao and Rao, 1979) suggested thepossible presence of host specific strains. Even ifhost specific strains are absent in naturalpopulations, such strains can be created by geneticimprovement using conventional mutation andstrain selection process or by the new recombinantgenetic techniques (see Charudattan, 1985).Besides host specificity, genetic alterations canalso induce adaptability to adverse environmentalconditions.

In addition to pathogenic fungi, parthenium isknown to harbour a great variety of floral(Padmabai Luke, 1974), rhizosphere andrhizoplane (Padmabai Luke, 1976), phyllosphere(Ghosh and Singh, 1979) and seed (Bharathi andRenuka Rao, 1983) mycoflora. Despite the factthat some of these rhizosphere and rhizoplanemycoflora, such as A. tenuis and Fusarium sp. arepotentially pathogenic, they did not seem toproduce any symptoms in the plants. Changes inthe composition or amount of root exudates weresuggested to be responsible for the differentialgrowth of fungal populations in the rhizospheresoil of pre- and post-flowering stages of the weed(Padmabai Luke, 1976). Although thisphenomenon may impose a limitation on the useof fungal pathogens, it can be easily surmountedby synchronizing their application time with themost susceptible stage of the weed. Similarly, awater soluble spore germination inhibitor waslocated in the leaf surface wax of the weed, butaddition of glucose to sprays allowed germinationof certain fungal spores. Thus, altered nutritionalstatus of the foliage rendered it a more suitablehabitat for the pathogen and it was suggested thatselective activation of leaf or root microflora maytrigger the mechanism for initiation of successfulinfection (Deshpande, 1981).

Yet another method that can be convenientlyincorporated into the microbial herbicidalapproach for parthenium control is theemployment of mycotoxins. Although toxicmetabolites of F. moniliforme which producedrapid wilt symptoms in the weed in preliminarytrials are promising, partially purified toxin of thefungus Helminthosporium tritici-repentis Diedwas inactive due to its binding by leaf surface


wax, but produced symptoms in dewaxed leaves(Deshpande, 1981). While further studies on thehost selectivity in laboratory trials, andeffectiveness and stability under field conditionsof F. moniliforme should be continued, a carefulstudy of leaf components of the weed prior to theutilization of mycotoxins of H. tritici-repentisand other similar fungi, will facilitate theformulation of foliar sprays with dewaxingagents. In addition, large scale screening ofmetabolites of other fungi may yield severalpotential mycotoxins.

MLOs have not yet been seriously consideredweed killers in any part of the world. Although theMLO that causes phyllody in parthenium cannotbe projected as a promising weed killer at thepresent, its present status and future potential as abiocontrol agent merit consideration. Thepathogen is capable of causing 100% incidence(Mathur, 1989) resulting in significant reductionin flowering and complete sterility of flowers(Keshwal, 1982). Despite the fact that the diseaseintensity is dependent on variations in localclimatic conditions, activity of leafhoppers andpresence or absence of agricultural crops (Mathur,1989), the MLO is widely distributed in manyparts of the country unlike fungal and bacterialdiseases, which occurred in isolated patchessuggesting that it is well adapted to a wide rangeof climatic conditions. The absence of the diseasein sesamum or any other test host (Mali andVyanjane, 1979) stresses the need forconfirmatory studies with vectors, such as H.phycitis which transmitted the disease to sesamum(Keshwal, 1982), to establish the host specificityof the pathogen. Any attempts to use MLO forbiological control of parthenium primarily dependon its host specificity. This assumes greatersignificance in the light of the polyphagous natureof established vectors, the rapidity with whichthey can transmit the disease, and thesusceptibility of various cultivated and wildplants, the latter possessing the potential to formreservoirs of the disease.

A new approach that is being projected as aneffective alternative to conventional biologicalcontrol methods is the employment of dominantplant species to replace parthenium by theirvigorous growth or allelopathic activity. Abeginning has been made in this direction byDesai and Bhoi (1981) and Mahadevappa andJoshi (1985) whose attempts with S. hamata andC. uniflora, respectively have producedencouraging results. However, a major limitation

to the use of this strategy is the removal of theweed prior to sowing the competitor plants. Thisoperation becomes impracticable in large tracts ofwasteland in addition to the problem of allergy.Ongoing field trials with C. uniflora indicate thatbroadcasting or sowing the seeds in 1-2 cm deepfurrows in standing parthenium, also results ingood germination and establishment (Joshi,1990). Another constraint in the use of this methodis the differential rates of natural propagation ofthe weed and competitor plant owing to thedifferences in their seed size and growth rate.Extensive human efforts are required to manageand manipulate seed dispersal as in the case of C.uniflora (Joshi, 1990) to overcome thisdisadvantage. Although undiluted extracts of theweed inhibited seed germination and seedlinggrowth of C. uniflora (Jayakumar, 1985) andextracts of C. uniflora affected seed germinationof a few cultivated crops (Vasudevan, 1986) in thelaboratory, these leachates are unlikely to occur insuch high amounts in the field and hence, may notconstrain the use of this method. However, studiesin the field are necessary to establish the safety ofC. uniflora to various cultivated crops. Inaddition, its safety to human beings in terms of theallergenic properties of pollen should beascertained before it is recommended for largescale use in the field. The nutritional value of theseed (Ramachandra and Monteiro, 1990) andother uses suggested by Joshi (1990) should bestudied in greater detail as a large quantity ofbiomass is produced in the field. Studies onidentification of plants with allelopathicprinciples, such as A. scholaris (Ghildiyal, 1987),H. suaveolens (Rao et al., 1987), Eucalyptus(Kohli et al., 1988) and other plants (Joshi, 1990)should give due consideration to the aspectsdiscussed above. While employment of C.uniflora which requires a fairly long time toestablish and replace parthenium is best suited forwastelands, the extracted leachates of C. unifloraand autotoxic principles found in the weed(Kumari and Kohli, 1987) seem to hold promisefor its control in agricultural lands as pre-emergence bioherbicides. The exploitation ofthese toxic principles depends on several factorsincluding selectivity and residues.

CONCLUSION

Although biological control is the most viableand practical solution to the problem ofparthenium, serious efforts to use this method


have been far from satisfactory. A surprisinglylarge array of; tools, namely insects, pathogens,mycotoxins, allelopathic plants and autotoxicprinciples await exploration by biological weedcontrol specialists. Barring the trials on theintroduced chrysomelid Z. bicolorata and theallelopathic plant C, uniflora, no serious attemptsare underway to use the other components.Integrated use of these tools improves the overallefficiency of biological control attempts. Thecombined action of Z. bicolorata and anintroduced or native pathogen may have adevastating effect on the weed since several leaffeeding beetles are known to serve as mechanicalvectors or create points of entry for bacterial,fungal and viral pathogens. For example, fieldpopulations of water hyacinth infested by theweevil Neochetina eichhorniae Warner and themite Orthogalumna terebrantis Wallworkshowed higher incidence of the fungusAcremonium zonatum (Saw.) Gams and othernecrotic leaf spot diseases while in laboratorytests, water hyacinth could be killed by thecombined effects of the weevil and A. zonatum(Charudattan et al. , 1978). The almost completecontrol of water hyacinth within one growingseason when arthropod attacks were combinedwith diseases, such as Cercospora rodmaniiConway (Charudattan, 1985) also gives credenceto this approach. The reduction in parthenin andphenol content of parthenium plants attacked byMLO is likely to increase the susceptibility of theweed to insects and diseases as evidenced by therapid germination of spores of Helminthosporiumsp. in the root zone of diseased plants (Deshpandeet al., 1982). While introduced insects andpathogens, and allelopathic plants are best suitedfor undisturbed ecosystems like wastelands,others, namely native pathogens, mycotoxins andautotoxic principles can be used as bioherbicidesin unstable agroecosystems. An integratedbiological control approach will be the forerunnerfor a lasting solution to the poisonous weedparthenium.

Acknowledgements—We wish to thank Mr. S.Ramani, Department of Entomology, Universityof Agricultural Sciences, Bangalore, for hiscritical review of an earlier draft and usefulsuggestions.

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status of biological control of parthenium hysterophorus l ... · the wilting and drying of a few...

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