Know Your Friends

Trichogramma wasps

There are numerous species of Trichogramma wasps that attack the eggs of over 200 species of moths and butterflies. These almost microscopic wasps (0.5 mm; 1/50 inch) are very important in preventing crop damage because they kill their hosts before the insects can cause plant damage. The female Trichogramma lays an egg within a recently laid host egg, and as the wasp larva develops, the host egg turns black. Each female parasitizes about 100 eggs and may also destroy additional eggs by host feeding. The short life cycle of 8-10 days allows the wasp population to increase rapidly. These wasps are harmless to people, animals, and plants. Although Trichogramma occur naturally throughout the United States, they usually do not occur in high enough numbers to be effective at suppresing pest populations.

Life cycle of a Trichogramma wasp parasitizing a corn earworm egg. Adults are <0.05 mm (1/50") in length.

Illustration from Natural Enemies of Vegetable Insect Pests by Michael P. Hoffman and Anne C. Frodsham, Cornell University, (1993).

Trichogramma are readily available for augmentative releases in large quantities from commercial suppliers. There are several species and strains of Trichogramma, which vary considerably in their ability to control different insects and in their adaptation to different environmental conditions and crops. Determining the best species or strain to release may be difficult. Most suppliers provide detailed instructions for the strain selection, release, and rates to use of Trichogramma, but their recommendations may not always be accurate. Frequent releases made over several weeks result in better parasitism and control than a single large release. Whenever possible, releases should begin at the time of the first moth flight, before the pest population has built up. Pheromone traps, black light traps, or visual inspection are useful for monitoring adult flights. Regular scouting to determine the appearance of caterpillar eggs is a more accurate method to determine when hosts for Trichogramma are present.

The wasps are shipped as immatures inside moth eggs glued to small cards that can be attached by hand to infested plants. Aerial application is possible for large acreages. Keep the cards in a warm, humid place out of direct sunlight until the emerging adults can be seen as small dots moving around in the closed container. A few tiny caterpillars may also be found in the container because it is very difficult to obtain 100% parasitization of the moth eggs, but these are harmless in most crops. When most of the adults have emerged, place the containers in a shaded spot upwind of the areas where moths are suspected or egg laying is occurring. The adult wasps will fly onto the plants in search of new host eggs to attack. Do not put the cards out before the wasps have emerged because ants and other predators may eat them. The emerging wasps will have the best chance of finding and parasitizing eggs when the weather is moderate. The best time to release is early morning or evening when direct sunlight will not hit the cards. Avoid making releases under extremely hot, cold, rainy, or windy conditions.

- Susan Mahr, University of Wisconsin - Madison

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