Putting Mother Nature to Work

August 9, 2002 - 09:37

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 new
developments in biological control are with natural enemies such as
parasitoids, predators and pathogens, though there are also a number of oils,
soaps and other biological agents (see Table 1, left for a listing of controls
and manufacturers).

Parasitoids

Several species of whitefly
parasitoids occur naturally in the United States and may migrate into
greenhouses and attack whiteflies. However, the degree of control is usually
insufficient. Augmentative releases of commercially reared parasitoids are
typically more effective.

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

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

Eretmocerus eremicus is an
equally tiny wasp but differs from Encarsia formosa in that the adult is
entirely yellow with green eyes and clubbed antennae. Males have longer, more
prominent antennae than females. Parasitized whitefly nymphs appear beige in
color. Release rates are 2-3 per sq. ft. of growing area. Repeated releases at
7-14 intervals are often necessary. On most occasions, Eretmocerus will not
reproduce in a greenhouse environment, so repeated applications are necessary
until the whitefly population is reduced to a desired level. Unfortunately,
this parasite is relatively expensive and costs significantly more than
applications of pesticides to control whiteflies.

Predators

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

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

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

Spiders are the best-adapted
arthropod predators to successfully prey on mobile, more visually acute insects
such as adult whiteflies. Spiders have had an effect on Bemisia sp. adults in
field crop situations. In outdoor growing beds, where herbaceous perennials are
grown, preserving the spider populations by avoiding broad-spectrum, long
residual pesticides will aid in naturally occurring biological control.

Pathogens

Microbial insecticides offer an
alternative to traditional chemical sprays and drenches. One potential
candidate that has received federal registration is a microbial insecticide
containing the entomopathogenic fungus, Beauveria bassiana. This fungus is a
naturally occurring insect pathogen and has been found to be effective in
controlling whiteflies, certain aphid species, mites and thrips.

Two different strains of the
fungus are commercially available. BotaniGard (GHA strain) is formulated as a
wettable powder and an emusifiable suspension. The B. bassiana spores in
Naturalis-T&O are formulated to mix readily in water and are applied using
standard, high-volume spray equipment. The fungus kills insects either by direct
contact from the spray equipment or through secondary contact with spores on
foliage. When spores come in contact with an acceptable host, a germ tube
penetrates the insect’s cuticle and feeds from the host body, resulting
in death of the host. In most cases, 8-10 fungal spores are needed to cause
fungal infection and insect death. The warm temperatures and relatively high
humidity in greenhouses are ideal environments for using this fungal pathogen.
Because fungal spores kill insects through direct contact, good spray coverage
is 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 spider
mites in Southern states and in the greenhouse where humidity levels are
relatively high.

About The Author

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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