Bean leaf beetles are a significant pest of snap beans in Ontario. The adults are 5 mm in length with four black spots on the wing covers. A small, black triangle is visible at the base of the head (Figure 1). The wing covers also have a black border. The colour of the bean leaf beetle varies from yellow to tan or red and the spots are not always present.
Bean Leaf Beetles over-winter as adults in grassy fence rows, leaf litter and wooded areas. They emerge in late April and begin feeding in alfalfa fields. Snap bean fields planted close to alfalfa are at the greatest risk of early bean leaf beetle damage.
Female adults lay lemon-shaped, bright orange eggs in clusters at the base of the bean plant. Egg laying continues until late June. The larvae feed on roots and soil debris for up to 30 days prior to pupating. The adults emerge in mid-July becoming a potential pest of late-season snap bean fields. These adults are active for up to one month while they feed and lay eggs. The overwintering generation of adults emerge in late August.
The beetles often feed in clusters. Damage starts as large holes in the leaves (Figure 2). As feeding continues, they consume the entire leaf expect for the veins, leaving a skeletonized appearance. Bean leaf beetles will feed on leaves, blossoms and pods.
Systemic insecticide seed treatments used to control wireworm and leafhoppers in snap beans, should also adequately control beetle feeding through the seedling and pre-bloom stages. A small amount of feeding may be evident in the crop before the beetles ingest enough insecticide to kill them. The efficacy of certain seed treatments may be reduced under prolonged wet soil conditions.
Examine 2 ft of row at each of 10 locations across the field. Apply a control if beetles exceed 1 beetle per foot of row OR if defoliation exceeds 20%. Defoliation prior to bloom does not have as strong an impact on yield. During pod set, the control threshold is 10% defoliation.