In western Canada, lygus bugs (Lygus spp., Heteroptera: Miridae) are parasitized by a native Braconid wasp, Peristenus pallipes Loan. To determine the phenology of parasitism of lygus bugs in alfalfa seed fields and to discern the effects of insecticide application on lygus and parasitoid populations, we monitored lygus populations in a conventionally managed and an organically managed seed field for three years. Research was conducted in a seven year old field of alfalfa cv. Rangelander, which was sprayed with an insecticide for lygus control shortly before leafcutting bees were placed in the field each spring, and a three year old field of alfalfa cv. Peace managed organically near Shellbrook, Saskatchewan (53°13'N Lat., 106°24'W Long.). Insect populations were monitored weekly from the middle of June until the end of August by taking five 180° walking sweeps in each of four sections of the respective fields with a standard 38 cm insect net. Lygus bugs were categorized into three groups: first, second, and third instar nymphs, fourth and fifth instar nymphs, and adults. Collected lygus nymphs were dissected shortly after capture and the levels of parasitism per age group were recorded. The numbers and percentages of parasitized lygus were compared using the T-test procedure of SAS (SAS 1989). In all three years in the conventionally managed seed field, which was sprayed with insecticide for plant bug control prior to leafcutting bee release, parasitoid population development was synchronous with that of its lygus host. Patterns of population growth of both species developed similarly and peaked at the same sampling date. Parasitism in the organically managed field, however, was not as closely aligned with lygus population growth. Similar population patterns were found between the two insect species in 1994, but lygus and wasp numbers were very low in that year. In 1995, levels of parasitism of small nymphs lagged behind lygus population growth, and in 1996 parasitism development in both nymphal categories was later than unparasitized lygus population development by about one week. On 22 of 25 sampling dates over all three years, absolute numbers of parasitized lygus were greater in the organic than in the conventionally managed field, with the differences being significantly higher on 10 of those dates. However, because of the associated larger numbers of unparasitized lygus in the organic field, on 19 of 25 dates the proportion of parasitised lygus was greater in the conventional than in the organic field (Figure), significantly so on nine of those dates.

Reference
SAS Inc. 1989. SAS/Stat User Guide Version 6.06. SAS Inst., Cary, NC.

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