Lygus lineolaris (Palisot), the tarnished plant bug, a native species, has a broad host range. It damages many fruit and vegetable crops over most of North America, and injures cotton and seed crops in some areas. In 1990, we reported the establishment of the European parasite Peristenus digoneutis (Loan) in New Jersey, its initial impact, and its spread into adjacent New York. This article contains observations made in alfalfa during 13 consecutive yr,before and after the parasite established and became abundant. Mean peak parasitism of Lygus nymphs averaged 15% by 2 native species [P. pallipes (Curtis) and Leiophon uniformis (Gahan)] before the establishment of P. digoneutis. Within a few years, total parasitism of Lygus nymphs increased to 40-50%, and Lygus numbers in alfalfa decreased by 75%. Parasitism by the 2 native braconids, which prefer other mirid species, remained about the same. L. lineolaris was the preferred host of P. digoneutis (80% were reared from this mirid), but some alfalfa plant bugs, Adelphocoris lineolatus (Goeze), were also attacked. A number of other factors were found to also influence Lygus numbers, especially plant density, which coincidentally increased, obscuring the beneficial effects of the new parasite for several years. The successful biocontrol of a native Lygus in alfalfa by P. digoneutis, a foreign parasite, indicates that this natural enemy has the potential to reduce damage by native Lygus spp, to other, high value crops, and additional research is warranted.
