Whiteflies are white insects with pale yellow bodies that are approximately 2 mm long. They belong to the order Homoptera and are close relatives of aphids, scales, mealy bugs, hoppers and cicadas.
The life cycle consists of an egg, 4 nymphal instars, a pupal and an adult stage. Depending on the species and environmental conditions, eggs require 10-12 days to hatch, and completion of life cycle from egg to adult takes 30-40 days. Nymphal instars behave in a manner similar to scale insects. The first nymphal instars are active and they are sometimes called crawlers. The remaining nymphal instars are sedentary and may mimic immature scales.
Whitefly feed by extracting plant fluids with sucking mouth parts. Feeding damage appears as yellow, stunted growth and, in severe cases, honeydew and sooty mold can develop. Sooty mold is a black fungus that grows on honeydew. Honeydew and sooty mold can reduce photosynthesis and crop value. Plant death can occur if large populations of whitefly are left untreated.
Whitefly species identification can be made most easily on the pupal stage. The pupae have species-specific shape, color pattern and wax filament arrangement. Adults can be identified, but identifying species according to pupal stages is more accurate.
When infested plant material is installed in the landscape, whitefly species that infest greenhouse crops can become pests in the landscape. Some examples of susceptible greenhouse crops include gerbera daisy, poinsettia, lantana, fuschia, geraniums (Pelargonium), rue and belles of Ireland. Three common species found in greenhouse environments:
Typically, whitefly infestations are most serious in greenhouse environments, but there are some whitefly species that can be pests in the landscape. Unlike infestations in greenhouses, most species that occur in the landscape are usually not major pests. In the landscape, beneficial insects usually keep whitefly populations below damaging levels. However, certain species are more problematic than others.
Monitoring: Whitefly adults can be monitored by placing yellow sticky cards in greenhouse environments. Sticky cards should be placed near doors and vents to monitor incoming adults from the outdoors. This is especially important in fall when outside air temperatures begin to cool. Gently tapping plants will disturb whitefly resting on the leaves which will allow you to see them. Periodically inspect the undersides of leaves for the presence of pupae and adults. Look for yellowing or stunted plants. Counting adults weekly on sticky cards will help you monitor outbreaks. Thresholds vary depending on the crop.
Suppressing whitefly populations before they reach large numbers is essential to prevent plant damage. Once high-density populations become established they are difficult to suppress.
Marathon (Imidacloprid), Orthene (Acephate), Diazinon, pyrethroids and other labeled products can be used to control populations. Complete coverage is essential for suppression and it is better to use a systemic or a product with a long residual in environments with high-density populations.
Horticultural oil and insecticidal soaps may provide control of low-density populations in the landscape, but they may not provide adequate control in greenhouse situations if beneficial insects may be absent. Insect growth regulators (IGR) can be used for suppression of populations, but they will not kill adults. IGR pesticides must be applied more frequently, especially in situations where adults are abundant.
Suppression of greenhouse whitefly populations with beneficial insects has been very successful in greenhouses fitted with insect screening on the vents. Insect screening keeps beneficial insects in and pest insects out of the greenhouse. Periodic releases of the parasitic wasp Encarsia formosa have suppressed greenhouse whitefly very well for many growers. Eretmocerus californicus is another parasitic wasp that is known to parasitize sweet potato and greenhouse whitefly, but it has not been as successful as Encarsia. Finally, there is a predatory beetle, Delphastus pusillus, which is commercially available. Delphastus has been used less frequently by growers.
A microbial insecticide that contains the entomopathogenic fungus, Beauvaria bassiana, is commercially available for whitefly suppression. Two products that contain Beauvaria are BotaniGuard and Naturalis-O. These products require a moderate humidity level to infect insects and they must be applied more frequently than conventional insecticides. It is important to date the product container because these products have a shorter shelf life than most conventional insecticides.
Insect screening can be retro-fitted over greenhouses to prevent whiteflies from entering. Note: Installing screening over greenhouse vents changes static pressure and ventilation capacity. Ventilation openings must be adjusted to ensure adequate air circulation and prevent damage to ventilation equipment. Weeds can often harbor whitefies and they should be removed from greenhouse floors and outside greenhouses near vents and doors.