Description
Whitefly is a serious pest in the greenhouse and garden. Two common species are the greenhouse whitefly (Trialeurodes vaporariorum) and the sweet potato or silver leaf whitefly (Bemisia tabaci). They belong in the order Hemiptera along with aphids, scale and mealybug. Adult whiteflies are 1mm long and a powdery white. Larvae are flattened, legless, translucent 'scales' 0.8mm long. Both adults and larvae are found on the undersides of leaves, the adults mostly on upper plant leaves and the larvae lower down on the plant. It is difficult to identify whiteflies trapped on sticky traps, therefore examine adults and immature whiteflies on leaves.
Whiteflies damage crops by sucking plant sap, which weakens plants and causes shoot and leaf distortion. A more serious problem is the large amount of honeydew they secrete onto leaves and fruit. The honey dew is colonized by sooty molds which reduces the quality of greenhouse vegetables and ornamentals.
Target Crops
Whiteflies target a variety of ornamental and tropical crops. Poinsettias are especially vulnerable to whitefly, along with begonia, aster, nicotiana, calendula, cucumber, lantana, tomato, grape, ageratum, bean and hibiscus plants.
Life Cycle
A complete greenhouse whitefly life cycle takes 28 days at 68°F. Females lay pinpoint size, black eggs in circular patterns on the undersides of leaves. Each female lays up to 200 eggs during her one week lifespan. The eggs hatch in 7 days and the mobile larvae (crawlers) move over the leaves for 5 days, then settle and molt to a sedentary 'scale' stage. After 7 days and another molt, the last stage larva remains where it was feeding and pupates. Adults emerge in 8 days.
The life cycle of the sweet potato whitefly is similar, but the developmental time is shorter and females lay more eggs.
Monitoring Tips
- Begin in the empty greenhouse, before seedlings are planted out.
- Hang up yellow sticky cards (1 trap/500-2,000 ft. sq.) Traps should be placed just above the plant canopy or within the canopy for sweet potato whitefly.
- Whiteflies tend to remain in one area and gradually move out from the initial site.
- Check traps weekly for adult whiteflies and replace every 3 weeks.
Note: Using higher densities of traps helps pinpoint infestation sites more accurately. If monitoring time is limited, it is more important to check and record counts on fewer traps and do it weekly than to check larger numbers of traps at longer intervals. With small plants, examining the foliage of 1% of the plants will give a more accurate count.
Beneficial Insect Control
Encarsia formosa is a tiny 1mm long wasp that parasitizes immature stages of whitefly. It is sold as parasitized scales glued to cards, from which the adult wasps emerge. As the wasp develops inside, greenhouse whitefly scales gradually turn black. Parasitized sweet potato whitefly scales turn a tan color.
If there is a history of whitefly problems, best results are achieved when Encarsia are introduced preventively, at low rates, before whiteflies are found on monitoring traps. They are usually released weekly, until 80% of whitefly pupae appear parasitized.
In warm regions or areas where a large number of greenhouses have whitefly infestations, the whiteflies may move onto outdoor plants. This makes them more difficult to control as whiteflies continually reinfect the commercial crop. Encarsia are less effective during cool weather and overcast periods, therefore whitefly populations must be monitored closely during these times.
Release 1-2 Encarsia/10 sq. ft. of canopy. For pupae on cards, make sure they are place in the lower foliage away from direct light. Encarsia fly upward. Reapply every week until infestation reduces, gradually reduce the number applied. Typically this takes at least 3 weeks.
Eretmocerus californicus is another tiny parasitic wasp that is used along with Encarsia, especially at higher temperatures. Eretmocerus must be released at higher rates than Encarsia because, unlike Encarsia which are all female, Eretmocerus has both sexes and females must mate to lay eggs.
Release 1-20 Eretmocerus/10 sq.ft. For pupae on cards, make sure they are placed in the lower foliage away from direct light. Reapply every week for at least 5 weeks for full control.
Delphastus catalinae is a small 1.4 mm black beetle in the lady beetle family. Both adults and larvae feed on whitefly eggs and immature stages of whitefly. Delphastus is sold as adults and should be applied as soon as whiteflies are detected. Delphastus works especially well with Encarsia in whitefly 'hot spots' because it avoids feeding on parasitized whitefly scale.
Release 0.5 - 1 Delphastus/1 sq. ft. of crop canopy. Simply walk through the operation gently tapping the container to release adults. Only release when whiteflies are present as they do not survive without a food source.
Insecticide Options
Any plant material brought in should be thoroughly inspected and treated with compatible chemicals if necessary to reduce whitefly numbers before using biological controls. Insecticidal soap is moderately toxic to developing Encarsia pupae and will kill the adults, but has no residual effect. It can be used as a spot spray in whitefly hot spots or to knock down whitefly numbers at the beginning of a new crop. Use a 1% solution, weekly, directing the spray onto new leaves to kill adult whiteflies and the crawler stages. Other biological pesticides can be found on our Whitefly Control Page.
Cultural Control Tips
- It is essential to start with a clean crop at the beginning of the season. Destroy all previous crop residues and dispose of them at a site remote from the greenhouse.
- If the previous crop was infested with whiteflies, leave the greenhouse entirely empty of plants for 5 days, with heat, to starve whiteflies, or kill them with cold by allowing the greenhouse to freeze to at least 5°F for a week.
- Keep the greenhouse weed free and maintain a 10ft. wide weed free border around the outside of the greenhouse.
- Do not keep ornamental plants in vegetable greenhouses as these are also whitefly hosts and may vector virus as well.
- If greenhouse whitefly numbers are high, hang yellow sticky traps at the top of the plant canopy to trap adult whiteflies. If sweet potato whitefly is present, hang traps about 1 meter below the top of the plants.
- If whiteflies are present on outdoor plants, screen al entry points with anti-virus screen or whitefly proof screen.