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Whiteflies cause problems forsouthern California growersMarshall W. J ohnson 0 Nick C. Toscano 0 Harold T. ReynoldsEdward S . Sylvester Ken Kid0 Eric T. Natwick

A task force of scientists is searchingfor effective whitefly controls.

D u r i n g 1981, unusually high whiteflypopulations on cotton and vegetable cropsbegan causing multiple problems in the desertareas of southern California and westernArizona. The problems included direct plantinjury by whitefly feeding, reduced quali ty ofproduce and fiber because of honeydew accu-mulation, and lower crop yields asaresult ofvirus or viruslike diseases carried by thewhiteflies. Total estimated economic damageranged in the millions of dollars: cotton, su-garbeets, squash, melons, and iceberg ettucewere the most seriously injured crops.Several species of whiteflies are present inCali fornias Imperial and Coachella valleys.Among these are the sweetpotato whitefly(also known as the cotton and tobaccowhitefly), Bemisia tabaci (Gennadius);banded-wing whitefly, Trialeurodes abu-tilonea (Haldeman); and iris whitefly,Aleyrodes spiraeoides Quaintance. Thebanded-wing whitefly is predominantlyfound in cotton from early spring to July. I nJuly, populations of the sweetpotato whiteflysurpass those of the banded-wing whitefly.The iris whitefly is rarely found and is not aproblem on agricultural crops.Whiteflies are generally found in the trop-ics and subtropics, roughly corresponding tothe area between the 30th parallels. I n thetropics they occupy the general ecologicalniche held by aphids in temperate areas.Sweetpotato whitefly has been reported inpartsof Africa, southern Europe, the M iddleEast, India, Sumatra, Formosa, Brazil, andthe southwestern United States. The banded-

wing whitefly is found in 33 of the50 UnitedStates, partsof Mexico, and the West Indies.Biology

Whiteflies are small plant-sucking insects(1 to 3 mm) found on leaf undersurfaces.They are not true flies, but belong to theorder Homoptera, which includes aphids,scale insects, and psyllids.Eggs are deposited on the undersides ofleaves and in5 to 12days, during the summer,hatch into the crawler stage. T he six-leggedcrawler moves about until it finds a suitablefeeding site, usually on the leaf undersurface.Once the crawler inserts its mouthparts (sty-lets) to feed, it remains at the site until theadult stage. The stylets usually penetratebetween the epidermal and parenchyma cellsof the leaf to the phloem (path of sugar andnutrient movement).After feeding begins, the crawler molts andappears scalelike. It continues to feed andpasses through two more molts. A fter thethird molt, the whitefly pupates and stopsfeeding until emergence as an adult.During the summer, the whitefly developsfrom egg to adult within 16 to 35 days, de-pending on the temperature. After emergingfrom the pupa, adult whiteflies mate, and thefemales begin to deposit their eggs. Eggsfrom unmated females produce males only.The female lays approximately 300 eggs inher l ifetime. Whiteflies can fly only for shortdistances, but winds disperse them severalmiles becauseof their minute size.Both of the major whitefly pests in the

southern California desert have a wide hostplant range. The sweetpotato whitefly report-edly feeds on as many as 56 plant speciesworldwide. In the Imperial and Coachellavalleys, cultivated host plants include cotton,lettuce, squash, cucumbers, and melons,although the whitefly is not known to re-produce on lettuce in the field. Wild hostsinclude bindweed (Lantanaspp.), prickly let-tuce(Lactuca serriola),andMalvaparvrforaL . The banded-wing whitefly has a largerworldwide host range of approximately 140plant species, severalof which grow in south-ern California.Crop damage

Whiteflies may injure a crop in a variety ofways. High populations feeding on nutrientsmay affect the plants physiological process-es, ultimately causing leaf shedding and areduced growth rate in some crops. Chloroticspots appear at feeding sites on leaf surfaces.Vast amounts of honeydew produced bylarvae may discolor leaves. In cotton, honey-dew falling onto lint in open bolls frequentlysupports the growth of sooty molds (Alter-naria spp.). When abundant in the lint, thesticky excretion interferes with picking andginning. Honeydew Contamination results inlower quali ty fiber that is difficult to spin andthus reduces its market value.The ability to spread virus or viruslikeplant diseases makes high whitefly popula-tions a menace to southern California melon,lettuce, and cotton. Larvae and adults pickup vi rus particles when they feed on infected

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B J ack Kelly ClarkA J ack Kelly Clark

C

Thesweetpotato whitefly (A ) feeds on 56 plant species, includingcotton, lettuce, and melon. It caused major damage in southernCalifornia n 1981.Thebanded-wingwhitefly (B) has a host rangeof 140species. A tiny wasp, Encarsia formo sa, (C)isan effectivenatural enemy of the whitefly in desert areas.

plants. Adults moving and subsequentlyfeeding on susceptible healthy plants spreadthe disease. A cquisitionof a pathogen fromadiseased host plant may take fromafew min-utes to several hours, depending on the path-ogen and whitefly species. Usually, thepathogen may be transmitted over a long dis-tance and period of time, once it has been ac-quired by a whitefly adult.The major diseases thought to be spread bywhiteflies in the desert areas are cotton leafcrumple, infectious yellows, and cucurbit leafcurl . Several less prominent diseases havealso appeared on many vegetable crops.Unfortunately, the viruslike pathogen orpathogens responsiblefor the three most im-portant diseases have not been positivelyidentified, although several plant pathologylaboratories are investigating the problem.Some infested melon crops suffered almost

total yield losses in the 1981-82 growing sea-son. Of the three diseases, infectious yellowshad the most impact, reducing early-seasonlettuce production as much as 50 percent.Population increase

The increased whitefly populations andsubsequent spreadof viruslike plant diseasesin southern California were probably pro-duced by several simultaneously occurringfactors. Examination of the typical popula-tion cycle of the sweetpotato whitefly in theImperial and Coachella valleys may provideinsight. Basically, whiteflies undergo similarpopulation cycles annually, although largedifferences in their peak density levels mayoccur among individual years. The whitefliesoverwinter on wild plants such asMavaspp.and on available volunteer and stub cotton(regrowth from cut stalks). Stub cotton is not

Max B adgley

permitted in California, but is found in A ri-zona. L ow winter temperatures inhibit rapidwhitefly development. During May, after thespring cotton crop has germinated, whiteflyadults move onto young cotton seedlings thathave developed their first true leaves.Unusually warm winters without killingfrosts experienced in previous years mayhave allowed large numbers of whiteflies tooverwinter, producing abnormally high pop-ulations in the spring. Higher initial springpopulations may have springboarded fromcotton onto susceptible vegetable crops. Thepresence of cotton not destroyed after har-vest, along with wild host plants harboringdisease pathogens, may have been respon-sible for the unexpected epidemic of plantdiseases spread by the high numbers ofwhiteflies.Whiteflies develop fastest during August

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and September when daytime desert temper-atures are greater than 100F.Whitefly pop-ulation growth, normally checked by naturalenemies, was rapid, apparently becausepyrethroid-based nsecticides used in the areareduced parasite numbers. Records from1975 to 1981show an increase in whitefly den-sities in Imperial Valley experimental cottonplots with the more frequent use of pyre-throids to control the tobacco budworm,Heliothis virescens (F.), and the pink boll-worm, Pectinophora gossypiella(Saunders).Similar outbreaks were reported in Thailandand Sudan in 1980 and 1981, respectively,when pyrethroids were used to treat cottonpests.After whitefly densities peak in A ugust orSeptember, they decline to a relatively low,stable density until the cotton is harvested orcotton leaves are killed by frost. At this time,winter vegetable crops and wild plants be-come important overwintering hosts. A ftervegetable crops are harvested, whiteflies anddisease pathogens survive on perennial wildhosts.Control

Conventional pesticide applications do notcontrol whiteflies on cotton. Insecticides mayreduce large adult whitefly populations butusually do not affect immatures. The longegg incubation period allows crawlers toemerge long after insecticides have reducedadult populations. The immobile larval andpupal stages have a waxy covering, whichgenerally protects them from insecticides.Materials that kill whitefly mmatures cannotbe applied effectively by air because of poorpesticide delivery to leaf undersurfaces.Fortunately, whiteflies in desert areas haveseveral effective natural enemies, such as thetiny parasitic waspsEncarsia formosaGahanandEretmocerus haldemani Howard. A dultfemale wasps deposit their eggs in the bodiesof whitefly larvae and pupae, and parasitelarvae feed on the body fluids of the imma-ture whiteflies.Encarsia formosa requires 15days to develop from egg to adult at 75O F.The female parasite lays up to32eggs duringher li fetime. The immature form of Erermo-cerus haldemani feeds only on the pupal stageof the whitefly and takes 22 days to developfrom egg to adult at 75O F. The valueof thewhiteflies' natural enemies has been demon-strated many times in various parts of theworld. When insecticides such as DDT wereapplied to control other pests, whitefly para-sites were reduced, and whiteflies increased.Until the major factors contributing to theincreasein whiteflies and disease spread canbe established, no tested preventive or con-

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'7 8d79 ' 80Survey date (1964-81)Populations of sweetpotato whitefly have ballooned in imperial Valey cotton.trol measures can be recommended. How-ever, some guidelines exist for decreasingpotential whitefly problems. To reducesourcesof disease, destructionof all stub cot-ton and management of weed hosts alongroadsides, ditchbanks, and fencerows, and inproduction acreage is advisable. To conservenatural enemiesof whitefly, insecticidal con-trol of budworm and pink bollworm in cot-ton fields with potential whitefly problemsshould be conducted with pyrethroids onlywhen absolutely necessary. Immediately afterharvest, plant stands should be destroyed toprevent regrowth and the production ofweeds that could harbor diseases. Cottonshould be terminated early. Lettuce or cucur-bit crops should be planted away from ma-ture or obviously disease-infected crops toreduce chancesof virus infection by naturallydispersing whiteflies.Researchers at the University of Califor-nia, University of Arizona, and the U.S.Department of Agriculture (USDA) arestudying the whitefly problem. A whiteflytask forceof 17 University of California andUSDA scientists has proposed several re-search objectives to determine the causesofand solution to this problem. Plant patholo-gists are attempting to isolate and characterize

the causal agent(s) of whitefly-transmitteddiseases in cucurbit and lettuce crops andto develop serological methods for rapiddetection of the agents. Lettuce and squashvarieties and the "world's collection" ofmuskmelon varieties are being screened forresistance to whitefly and disease pathogens.In-depth studiesofwhitefly biology and pop-ulation dynamics may reveal weak links in theinsect's life cycle that can be exploited to con-trol the pest. I nvestigations into biologicaland chemical controls of whiteflies and theirinteractions on cotton and lettuce may pro-duce more efficient management practices.Field reports indicate that overwinteringwhitefly population densities are quite low. Itis hoped that the whitefly problem in the1982-83growing season will be minimal. For-tunately, growers facing the upcoming seasonwill be better prepared to face the whiteflythreat if it materializes.Marshall W. J ohnson is Post-graduate ResearchAssistant, Department of E ntomology, Nick C .Toscano is Pest Management Program Director.CooperativeExtension, and Harold i ? ReynoldsisProfessor of E ntomology, all at University of Cal-ifornia,Riverside; Edward S. Sylvester is Professorof E ntomology, U .C., Berkeley; Ken Kid0 isStoffResearch Associate, Cooperative Extension, U.C.,Riverside; and Eric i ? Natwick is Farm Advisor,Cooperative Extension, I mperial County.

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