Insecticidal control of late-season plant bug (Hemiptera: Miridae) infestations in Manitoba has no effect on alfalfa seed quantity and quality

In Manitoba, plant bugs (Lygus spp. and Adelphocoris spp.) can be numerous in seed alfalfa crops in late August and September and are often the target of insecticide applications. From 2002 to 2004 in field plots in commercial fields, one or two insecticide applications in late August and September were made to reduce plant bug numbers relative to those in untreated check plots. Despite significant reductions in plant bug numbers in sweep net samples in sprayed plots, there were no significant effects of treatments on total seed yield per unit area, seed weight, percentage germination or percentage of hard seed. Relative to the check plots, reductions in plant bug numbers in insecticide-treated plots were not associated with increases in seed yield. Also, seed yield increases in insecticide-treated plots were not correlated with numbers of plant bugs immediately before insecticide application, and so it was not possible to estimate an economic injury level. The practice of applying an insecticide in August to control late-season plant bugs is economically unjustified within the range of plant bug numbers encountered in this study.