There are three common species of lygus bugs in Manitoba that all overwinter in crop residues or leaf litter in the area where they were feeding the previous summer. We know little about the species composition or seasonal patterns of occurrence of lygus bugs on edible beans in Manitoba. Loss of yield quantity occurs in other crops through abortion of buds, flowers, and pods, and it frequently overlooked unless a specific scientific study is performed to detect it. Yield quality loss occurs when lygus bug feeding causes blemishes on bean seeds.
A total of 17 navy bean, 10 pinto bean and 9 soybean commercial fields were surveyed between 2008 and 2010. In all years and crops, 83-92% of the plant bugs were tarnished plant bugs, 3-4 % were alfalfa plant bugs, and several other species made up the remainder. The nymphs moult to adults, but adult numbers are greatly augmented late in the season by immigration from early maturing crops. In none of the three seasons were we able to detect any negative effects on yield quality or quantity that could be linked to the plant bugs in the fields. In the laboratory, we have studied the effect of feeding by tarnished plant bug nymphs and adults navy beans at growth stages R1-2, R4-5 and R6-7. At R1-2, one insect per inflorescence significantly reduced total seed weight from the inflorescence. The major effect of feeding was the abortion of pods, and nymphs were more harmful than adults. At R4‐5, three insects per inflorescence were sufficient to cause seed weight loss, and many harvested seeds were shrivelled; again, nymphs were more damaging than adults. At R4‐5, most feeding occurs on the placental and funiculus regions of pods and injures the vessels conducting photosynthates to the filling seeds, resulting in them being shrivelled at harvest. At R6‐7, even five insects per inflorescence did not significantly reduce total seed weight. However, direct feeding on the seed reduced seed quality through surface pitting.