Peristenus digoneutis is a braconid wasp parasite of tarnished plant bugs (Lygus spp.). In its native Europe it parasitizes L. rugulipennis, but in North America it parasitizes L. lineolaris.
Beginning about 1978, entomologists at the USDA-ARS European Biological Control Laboratory, now located in Montpellier, France, collected Peristenus wasps in Europe and shipped them to the USDA-ARS Beneficial Insects Research Lab in Newark, Delaware. Reared wasps were first released in northern New Jersey in 1979, with continued introductions through 1987, for control of tarnished plant bug on alfalfa. By 1983 Peristenus was recovered in low numbers and was determined to be established there by 1988. Within a few years parasitism levels began to rise, reaching 50% by 1990-92. By 1988-90 it had become the dominant parasite species in northern New Jersey and was spreading to the northeast. Since then this parasite has been found in six new states (Pennsylvania, Massachusetts, New Hampshire, Vermont, Connecticut and New York).
P. digoneutis is a 2-3 mm long brown wasp. The adults emerge from overwintering cocoons in May-June. The adults live 2-4 weeks. The female wasp lays eggs singly in the abdomen of lygus bug nymphs. The eggs hatch in 5-7 days. After the wasp larva completes its development in about 7-10 days, the mature larva emerges from the dying host to spin a cocoon just under the soil surface. The second generation of adults emerge in July to attack the next generation of lygus bug. There may also be a partial third generation in August, with each generation synchronized with the host's major generations. In each generation an increasing proportion of individuals enter diapause (delay emergence from their cocoons until the following spring) to carry the species through possible periods of host scarcity and the winter.
P. digoneutis has reduced tarnished plant bug numbers in New Jersey alfalfa. When parasitism rates reached 55%, tarnished plant bug numbers in alfalfa promptly declined. Now that pest numbers have decreased by 75%, parasitism averages about 30%.
This reduction in plant bug numbers in alfalfa benefits not only that crop, but may indirectly benefit valuable fruit and vegetable crops by reducing the number of tarnished plant bug adults which fly into and damage them, especially when nearby alfalfa is harvested. Researchers in New York and New Hampshire are working with the USDA Lab in Newark to determine if P. digoneutis will fly into strawberries and parasitize bugs there or if alfalfa must be nearby. If P. digoneutis is found to disperse to and remain in perennial fruit crops such as peaches, strawberries, apples, and raspberries, or annual crops such as lima and dry beans and several seed crops, even greater biological control of tarnished plant bug should be realized.
P. digoneutis persists well in alfalfa fields, despite 3-4 harvests per year. Weeds around fields may also be a suitable habitat for the adult parasites, especially if they harbor tarnished plant bug. Flowers such as the composites Erigeron spp. and goldenrod, crucifers, and umbelliferous flowers such as Queen Anne's lace provide nectar as food for the wasps. These plants may be important in the conservation of this wasp so it can become an effective biological control agent in other crops.
It is very difficult to rear P. digoneutis, so commercial availability of this parasitoid is unlikely. However, it is dispersing naturally, so most suitable areas will eventually have self-sustaining, natural populations of this parasitoid. It may eventually move into the Midwest on its own, or be could possibly be established from releases of wasps collected in the East.
- Bill Day, USDA-ARS Beneficial Insects Research Lab, Newark, DE and Susan Mahr, University of Wisconsin
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