Putting Mother Nature to Work By Stanton Gill

Biological control methods

With restricted re-entry intervals, and fear of resistance development,many growers are looking for alternative methods for controlling whiteflies,and some of them are turning toward biologicals. The most interesting newdevelopments in biological control are with natural enemies such asparasitoids, predators and pathogens, though there are also a number of oils,soaps and other biological agents (see Table 1, left for a listing of controlsand manufacturers).

Parasitoids

Several species of whiteflyparasitoids occur naturally in the United States and may migrate intogreenhouses and attack whiteflies. However, the degree of control is usuallyinsufficient. Augmentative releases of commercially reared parasitoids aretypically more effective.

Parasitoids that have been usedsuccessfully in greenhouse crops include tiny wasps of several Encarsia andEretmocerus species. These wasps attack whitefly nymphs, killing them in one oftwo ways. First, the female wasp uses her needle-like ovipositor to lay an eggwithin or beneath a whitefly nymph. Encarsia sp. prefers the third to fourthinstar whiteflies. The egg hatches, and the parasitoid maggot feeds on thenymph. Pupation occurs within the nymph. When the adult wasp emerges from thewhitefly pupa, it chews a round exit hole through the cuticle at one end of thewhitefly pupa. Second, the female wasp punctures the whitefly nymph with herovipositor, killing the nymph, and feeds from the fluids that exude from thewound. The wasps do not attack adult whiteflies. Á

Encarsia formosa is a very tinywasp (0.6 mm), with a black head and thora, a pale yellow abdomen andtransparent wings. Females give rise to females; males are rare. Greenhousewhitefly pupae that have been parasitized by Encarsia formosa turn black;silverleaf whitefly pupae turn amber-brown. The adult wasps are rarely noticedand should not be a deterrent in sale of the plants. This parasitoid is widelyused for biological control of greenhouse whitefly on greenhouse vegetables.Release rates vary from 3-6 wasps per square foot of growing area with repeatedreleases at 7- to 14-day intervals. Encarsia will reproduce on many greenhousecrops once populations are established. The cost of Encarsia formosa can beequal to foliar pesticide applications or slightly higher.

Eretmocerus eremicus is anequally tiny wasp but differs from Encarsia formosa in that the adult isentirely yellow with green eyes and clubbed antennae. Males have longer, moreprominent antennae than females. Parasitized whitefly nymphs appear beige incolor. Release rates are 2-3 per sq. ft. of growing area. Repeated releases at7-14 intervals are often necessary. On most occasions, Eretmocerus will notreproduce in a greenhouse environment, so repeated applications are necessaryuntil the whitefly population is reduced to a desired level. Unfortunately,this parasite is relatively expensive and costs significantly more thanapplications of pesticides to control whiteflies.

Predators

Delphastus pusillus is a tiny,black ladybird beetle that is a voracious predator of whiteflies. This beetlereproduces quickly, lays many eggs and lives a relatively long time

(1 1/2-2 months). Larval andadult beetles eat all stages of the whitefly but concentrate on eggs andimmature whiteflies. Adult beetles eat Á

150-600 eggs or 10-12 fourthinstar whitefly larvae per day. Individual beetles can consume as many as10,000 whitefly eggs or 700 fourth instars during a typical lifetime.Delphastus lady beetles also have potential for integration in control programswith parasitoids because they do not compete. Though Delphastus will eatparasitized whitefly nymphs, they prefer to eat unparasitizied nymphs.

Spiders are the best-adaptedarthropod predators to successfully prey on mobile, more visually acute insectssuch as adult whiteflies. Spiders have had an effect on Bemisia sp. adults infield crop situations. In outdoor growing beds, where herbaceous perennials aregrown, preserving the spider populations by avoiding broad-spectrum, longresidual pesticides will aid in naturally occurring biological control.

Pathogens

Microbial insecticides offer analternative to traditional chemical sprays and drenches. One potentialcandidate that has received federal registration is a microbial insecticidecontaining the entomopathogenic fungus, Beauveria bassiana. This fungus is anaturally occurring insect pathogen and has been found to be effective incontrolling whiteflies, certain aphid species, mites and thrips.

Two different strains of thefungus are commercially available. BotaniGard (GHA strain) is formulated as awettable powder and an emusifiable suspension. The B. bassiana spores inNaturalis-T&O are formulated to mix readily in water and are applied usingstandard, high-volume spray equipment. The fungus kills insects either by directcontact from the spray equipment or through secondary contact with spores onfoliage. When spores come in contact with an acceptable host, a germ tubepenetrates the insect’s cuticle and feeds from the host body, resultingin death of the host. In most cases, 8-10 fungal spores are needed to causefungal infection and insect death. The warm temperatures and relatively highhumidity in greenhouses are ideal environments for using this fungal pathogen.Because fungal spores kill insects through direct contact, good spray coverageis essential for achieving adequate control.

The entompathogenic fungus,Paecilomyces fumosoroseus, is being marketed as PFR-97 (Olympic Chemical Co.).The fungus has been shown to give control of whiteflies, aphids and spidermites in Southern states and in the greenhouse where humidity levels arerelatively high.



Stanton Gill

Stanton Gill is a regional specialist at the CMREC, University of Maryland Cooperative Extension and adjuct professor at Montgomery College, Montgomery County, Md. He can be reached by phone at (301) 596-9413 or E-mail at sg10@umail.umd.edu.



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