In the agricultural muck soil area of southwestern Quebec, two native parasitoids, Peristenus pallipes and P. pseudopallipes (Hymenoptera: Braconidae) were found to parasitize nymphs of the tarnished plant bug Lygus lineolaris (Hemiptera: Miridae), on various flowering plants and on cultivated lettuce. Parasitism levels (ranging from 1.3% to 92.3%) varied with host plant species sampled, and with methods used for the calculation of percentage parasitism. The percentage parasitism of tarnished plant bug populations was expressed as pooled values from all samples in the season (method 1), and by the Southwood and Jepson's graphical method (method 2). Phenology of parasitism, defined as the time during which female parasitoids are active in the field, was evaluated for both native parasitoid species. The cold-hardiness of P. digoneutis, a European nymph parasitoid introduced in northern United States, was evaluated in the laboratory by measuring the supercooling point and cumulative cold damage at sub-zero temperatures. When compared to the native P. pallipes and P. pseudopallipes the exotic parasitoid showed the same capacity to withstand cold temperatures. Prior to introduction, the evaluation of the impact of P. digoneutis on indigenous species will require a rapid identification to species level. The development of immature stages of P. digoneutis was studied to find taxonomic and developmental differences between the three species. At $21 pm1 sp circ$C, the egg matures in five days, and there are three larval instars, the first two molting inside the host while the third emerges from the host, three to four weeks after egg deposition. Adult characters of the pre-imaginal stage can be used to distinguish the exotic P. digoneutis from the two native species since mouth sclerites of the final instar larvae of P. digoneutis, P. pallipes and P. pseudopallipes do not possess reliable distinguishing characters. (Abstract shortened by UMI.)

No description available.