How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Greenhouse whitefly. The greenhouse whitefly adult is 0.9 (male) to 1.1 mm (female) long, with four wings, sucking mouthparts, a powdery waxy coating over the body, and wings that give the otherwise yellow body a white color. The wings are held nearly parallel to the leaf and cover the abdomen when the adult is at rest. There are seven life stages: egg, four nymphal instars, pupal stage, and the adult. Females occasionally lay eggs in circles on the undersides of leaves of plants with smooth leaves. Egglaying on plants with pubescent leaves results in random placement of the eggs. Eggs are partially inserted into the leaf, initially they are yellowish, but close to hatching they turn a purplish brown. The first nympal instar is called a crawler and has functional legs, while the remaining instars are attached to the underside of the leaf and do not move. The end of the fourth instar is called a pupa. The pupal stage is the most important for determining whitefly species identification. Greenhouse whitefly pupae are oval and have vertical sides, giving the pupa a cakelike appearance from the side. Along the perimeter of the upper surface is a fringe of filaments and relatively large wax filaments project above the upper surface of the body. The greenhouse whitefly can complete one generation in 21 to 26 days at 81°F.

Silverleaf whitefly. The silverleaf whitefly adult is 0.8 (male) to 1.0 mm (female) long, with four wings, and sucking mouthparts. The white, waxy coating covering its body and wings is not as thick as it is on the greenhouse whitefly and its yellow body has a whitish hue from the wax rather than a white color. The wings are held at the sides of the body, partially exposing the back of the abdomen when the adult is at rest. There are seven life stages: egg, four nymphal instars, pupa, and the adult. Eggs are not laid in a circular pattern, are partially inserted into the leaf, and remain yellowish until they hatch. The first nymphal instar is called a crawler and has functional legs, while the remaining instars are attached to the leaf and do not move. The end of the fourth instar is called a pupa. Silverleaf whitefly pupae are ovoid, but with a slightly pointed hind end and red eye spots easily visible from above. The pupa is fairly flat and does not have a marginal fringe of filaments. The length of wax filaments projecting above the upper surface of the body varies on different hosts. Silverleaf whiteflies can complete development in 16 (86°F) to 31 (68°F) days.

DAMAGE

Whitefly adults and immatures feed on sap. As they feed, they excrete honeydew, a sticky substance that causes unsightly glistening and supports the growth of black sooty mold. Very large populations of whiteflies cause stunting of plant growth, and leaves may senesce and die. Physiological abnormalities, such as white stem on poinsettia, may also occur. Usually populations are not high enough to stunt ornamentals, and damage is mostly caused by honeydew, sooty mold, and nuisance populations of flying adults.

BIOLOGICAL CONTROL

Encarsia formosa, a tiny, stingless parasitic wasp, is as an effective biological control for greenhouse whiteflies. Wasps are released once a week at a rate of two to five parasites per plant for 8 to 10 weeks of the growing season. This sort of release program can be effective if long residual insecticides have not been applied in advance of the parasite release, and where the initial population of whiteflies is quite low (only a few whiteflies per plant). Greenhouse whitefly pupae turn black when parasitized by Encarsia, which emerge as adults through circular exit holes. Delphastus pusillus, a whitefly predator, has been used against silverleaf whiteflies. Eretmocerus eremicus is a commercially available whitefly parasite for silverleaf whitefly. It will also attack greenhouse whitefly, whereas Encarsia is ineffective against silverleaf whitefly.

CULTURAL CONTROL

Because whiteflies feed on a large variety of plant species, keep production areas free of weeds, which can serve as hosts for whitefly populations. Exclusion of winged adults can be accomplished by covering openings to the greenhouse with screens that have a pore width of 405 microns or smaller.

Before starting a new crop, carefully inspect plants to ensure that they are free of whiteflies and other pests. Treat or discard any infested plants.

MONITORING and WHEN TO TREAT

Yellow sticky cards placed in greenhouses will capture adult whiteflies. However, traps need to be used at a greater density, one per 1,000 sq. ft., than for other pests. Trap monitoring should be supplemented with inspection of leaves for nymphs and pupae. When monitoring plant samples, it is imperative to look on the undersides of leaves for adults and nymphs.

Treatment thresholds vary with the crop. For example, cut flowers such as Gerberas can tolerate more whiteflies than poinsettias because only the flowers are harvested in the cut flower crop whereas the entire poinsettia plant is marketed.

TREATMENT

Selected Materials Registered for Use on Greenhouse or Nursery Ornamentals
Read and follow the instructions on the label before using any pesticide. Before using a pesticide for the first time or on a new crop or cultivar, treat a few plants and check for phytotoxicity.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Insects and Mites
K. L. Robb, UC Cooperative Extension, San Diego Co.
H. S. Costa, Entomology, UC Riverside
J. A. Bethke, Entomology, UC Riverside
M. P. Parrella, Entomology, UC Davis
Acknowledgment for contributions to the insects and mites section:
R. S. Cowles, Connecticut Agricultural Experiment Station, Windsor, CT

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