Distribution and abundance of nymphal parasitoids of Lygus rugulipennis and Adelphocoris lineolatus in northwestern Italy

The European tarnished plant bug Lygus rugulipennis Poppius and the alfalfa plant bug Adelphocoris lineolatus (Goeze) (Rhynchota Miridae) are widespread in Italy and in many other European countries, where they are often noxious to several crops. Since the chemical control of these plant bugs is difficult, a three-year study was carried out in Piedmont (NW-Italy), to assess the presence of nymphal parasitoids, and evaluate their efficacy in controlling pest infestations. From May to September in 2001, 2002 and 2003, plant bug populations were sampled on different crops (alfalfa, meadow habitat, peach orchard groundcover, strawberry, sunflower, wheat). Overall 4092 and 1529 nymphs of L. rugulipennis and of A. lineolatus, espectively, were field-collected and reared in the laboratory to allow parasitoid development. Parasitism levels varied in relation to the year and the crop, but it was generally lower than 10%; higher percentages, between 10 and 30%, were found for L. rugulipennis nymphs collected on alfalfa, meadow habitat and wheat. Peristenus digoneutis Loan (86.0%) and Peristenus relictus (Ruthe) (9.9%) (Hymenoptera Braconidae) emerged from parasitized plant bug nymphs. In the surveyed agroecosystems P. digoneutis completed three generations as did its host L. rugulipennis: after overwintering in the cocoons in the soil, adults emerged in late March. In spring they attacked plant bug nymphs mainly on winter cereals; adults of the new generation emerged in late May-early June, and migrated to other plants in search of hosts. Subsequent adult emergences overlapped over the summer.