Know Your Friends

Encarsia formosa, Whitefly Parasite

Encarsia formosa was one of the first recorded parasites in greenhouses. The tiny females (about 0.6 mm long) are black with a yellow abdomen and opalescent wings. Males are somewhat larger than females and completely black, and are extremely rare. Females deposit 50-100 eggs individually inside the bodies of 3rd instar nymphs or pupae. The wasp larvae develop through four instars in about two weeks at optimum temperatures. As the larvae grow they kill the whiteflies. Parasitized greenhouse whitefly pupae turn black in about 10 days, while parasitized sweetpotato whiteflies turn amber brown. Both are easily distinguished from unparasitized pupae. Wasp pupation occurs within the whitefly body. Adults emerge about 10 days later. The adult wasps also kill whitefly nymphs by feeding on them through holes made with the ovipositor. They are most active in sunny weather. E. formosa readily attacks greenhouse and bandedwing whiteflies. It will parasitize sweetpotato whitefly in greenhouses, but this whitefly is not a good host for this wasp, so control is not as effective as with greenhouse whitefly. This species is widely available commercially, although the taxonomy of these wasps is currently very confused, and the true identify of many species or strains is questionable.

Encarsia formosa has been used since the 1920s throughout the world in commercial vegetable greenhouses. It can successfully control whiteflies on vegetable crops, including cucumber and tomatoes, and several ornamental crops, such as poinsettia. There are also many other promising natural enemies that can be used alone or in conjunction with Encarsia wasps. The best time to introduce Encarsia is when the whitefly population is low. Wasp efficiency is seriously impaired when whitefly nymphs are too numerous (5-10/cm2). The wasps spend more time cleaning the excessive honeydew from themselves than searching for and parasitizing whiteflies.

This wasp does best at between 61 and 83F. A daytime air temperature around 75F is the optimum for greenhouse whitefly control. Encarsia is not very efficient under cool, cloudy conditions. Higher release rates, or auxiliary natural enemies, may be required to maintain control under these conditions. Yellow sticky traps can also be used along with Encarsia to reduce whitefly populations. The whiteflies are strongly attracted to the yellow sticky traps, while the parasites are not, as long as sufficient whitefly nymphs are present.

Encarsia wasps are shipped in the pupal stage, on leaves or glued to small cards that can be hung directly on the plants. Release rates vary considerably depending on the whitefly density, the crop, and the time of year. Most suppliers provide detailed instructions for the release and use of Encarsia and can make recommendations about the number of wasps to release based on specific situations. In general, for very low initial infestations (less than 1 whitefly per 50 to 100 plants) rates of 1-5 parasite/ft2) are recommended for each release. Release rates are better established for vegetable crops than for ornamentals.

Susan Mahr, Universityof Wisconsin - Madison

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