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CROP SCENE INVESTIGATION - 2
The solution: bean leaf beetles were the culprits
by DAVID TOWNSENDLast month, we posed the problem of what was causing the holes in the leaves of a Walkerton grower’s IP soybean crop. Joe, the grower in question, originally thought that ladybugs were the malefactors. It turns out that the culprits were bean leaf beetles.
Bean leaf beetles are typically orange-red in colour with four black spots on the wings, a black band on the border of the wings, and always a distinct black triangle behind the head. Because of the variation in colour and spot patterning, they are commonly mistaken for spotted cucumber beetles and ladybugs.
In mild Ontario winters, bean leaf beetles overwinter in wood lots, alfalfa fields, leaf litter and soil debris. They emerge in April and feed on alfalfa until soybeans emerge. They then move to soybean fields and lay eggs at the base of plants until June. The larvae feed underground on roots and tissues until they emerge in July as adults. The adults will feed on foliage and pods until harvest, when they migrate back to wood lots and alfalfa for the winter.
Bean leaf beetles can be a vector of bean pod mottle virus, and can cause infection and reduce quality if they feed on soybean pods. As a result, Joe decided to spot-spray an insecticide on the corner of his IP soybean field to control the beetles and reduce the risk of pod feeding and viral infection.
The best time to control bean leaf beetle is in spring, prior to egg laying. Spring threshold levels are 16 adult beetles per foot of soybean row. For July and August, the economic threshold is 30 per cent defoliation pre-bloom.
Roy Armstrong from Ruscom Ont. wins a wireless weather station for his response. At press time we had received 22 correct answers from other readers. BF
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