PSI: The case of the destroyed fruit

Whiteflies are one of the most destructive insect pests for greenhouse-grown tomatoes. The most common whitefly species encountered during greenhouse tomato production is the greenhouse whitefly, Trialeurodes vaporariorum. Greenhouse whiteflies are phloem-feeders and both the nymphs and adults have piercing-sucking mouthparts, which are used to withdraw plant fluids. The nymphs tend to cause greater plant damage because they feed more than adults. Direct feeding damage to tomatoes may result in leaf curling, leaf yellowing, chlorotic mottling, fruit spotting and stunting and wilting. Nymphs may also cause indirect damage by secreting copious amounts of honeydew, a sticky clear liquid that serves as a growing medium for certain black sooty mold fungi.
 

The breakdown

Greenhouse whitefly life stages including the egg, nymphs, pupa, and adult are located on the underside of tomato leaves. Adults are winged, approximately 1/6-inch (4.2 mm) long, with the body covered by a white, powdery wax. Adults hold their wings flat, parallel to the top of the body. A female greenhouse whitefly lays up to 20 eggs in small circles on the leaf underside. They can lay up to 300 eggs during their 30- to 45-day lifespan. Eggs are erect, spindle-shaped and attached to a short pedicel. The eggs eventually turn gray, hatching in four days. Nymphs move short distances on plants before locating a suitable place to settle down and start feeding. The pupa or fourth instar nymph does not feed and has distinct red eyes. Pupae possess elongated waxy filaments that encircle the outer periphery. The pupae are elevated in profile with vertical (perpendicular) sides, resembling “cakes” on the underside of leaves. Development from egg to adult takes 14 to 30 days to complete, although this development depends on the ambient air temperature, as warmer temperatures tend to shorten development time.
 

Prevention

Scouting tomato crops regularly by using yellow sticky cards for adults and visually inspecting leaf undersides for eggs, nymphs, and pupae will help avoid outbreaks of greenhouse whitefly populations. Removing weeds from both within and outside the greenhouse is important, many weed species, including annual sowthistle (Sonchus oleraceus) serve as reservoirs for greenhouse whitefly populations. It is important to avoid over-fertilizing tomato plants with nitrogen-based fertilizers, as whitefly adults tend to be more attracted to plants receiving excessive fertilizer, and the life cycle may be completed faster than normal because plants are a better food source (based on the higher concentrations of amino acids).
 

Containment and eradication

Contact insecticides may be used to suppress greenhouse whitefly populations; however, it is critical to obtain thorough coverage of leaf undersides and conduct multiple applications as both eggs and pupae are tolerant of many insecticides. Therefore, applications should be implemented frequently enough to kill both nymphs and/or adults that were previously eggs and/or pupae. It is also important to develop appropriate rotation programs by using insecticides with different modes of action in order to avoid the prospect of resistance developing in greenhouse whitefly populations. Consult insecticide labels for information regarding resistance management.
 

Natural enemies

The use of biological control agents has been successful against greenhouse whitefly in tomato production systems. The primary biological control agent used is the parasitoid, Encarsia formosa. Females lay eggs inside the nymphal stages of greenhouse whiteflies. Parasitized whiteflies eventually turn black during the pupal stage, with adult parasitoids emerging from these pupae. In addition to direct mortality through parasitism, parasitoid females may kill young nymphs by feeding on them, which is referred to as host-feeding. Encarsia formosa performs best at temperatures between 70 and 80º Fahrenheit (21 to 26º Celsius) with a relative humidity between 50 and 80 percent. This parasitoid is less effective on tomatoes with abundant trichomes or hairs because these obstructions consequently make it difficult for the parasitoid to detect greenhouse whiteflies and inhibits the ability of adult females to lay eggs in nymphs.

Before releasing any parasitoids into the greenhouse be sure to remove all yellow sticky cards because parasitoids are attracted to yellow and may be captured, reducing the effectiveness of the biological control program. It is recommended you wait three to four days before replacing yellow sticky cards. Consult with your biological control supplier to obtain information on the frequency of release and release rates of Encarsia formosa.

Conclusion

Greenhouse whitefly populations can be a serious problem on greenhouse-grown tomatoes. So, in order to avoid problems, it is important to act preventatively by routinely scouting tomato crops, applying insecticides accordingly, or releasing biological control agents early in the production cycle. Why is this important? Once greenhouse whitefly populations reach outbreak proportions then options for management are limited, which may result in an economic loss.

Raymond Cloyd is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.