Bean Leaf Beetle: Two Generations of Attack

July 2, 2019

closeup of a bean leaf beetle

Could bean leaf beetles take a bite out of your soybean yields this season?

According to Marijke Vanderlaan, Syngenta Agronomic Sales Rep, growers are finding a resurgence in this damaging pest. Vanderlaan observes that over the last few years management practices have changed in that there has been a reduction in neonicotinoid seed treatments, which are effective in knocking the pest back.

“More and more, soybean acres are only being treated with fungicides. We’re seeing more survival of the overwintering bean leaf beetle population,” she says. Once that population emerges, they start feeding on seedlings and can have a devasting effect on a young soybean crop.

Feeding reduces yield and quality

Vanderlaan says growers are vulnerable to two generations of bean leaf beetles per year, not including the overwintering population that enters the soybean crop in spring.

Typically in mid-June, the overwintering population mates and females lay lemon-shaped, orange-coloured eggs in small clusters in the soil at the base of the soybean plants. The first-generation adult population emerges in mid-July to feed on soybean foliage and pods. They then mate and lay eggs that hatch to become the second generation and feed on soybeans in mid-to-late August.

“What this means is that we have a higher bean leaf beetle population at the pod fill stage and as the soybean plant is maturing, which can lead to reduced yield and quality,” says Vanderlaan.

While bean leaf beetle damage is primarily due to the direct attack by the foliar-feeding adults, the pests are also known vectors of the bean pod mottle, cowpea mosaic, and southern bean mosaic viruses.

Image: Adult bean leaf beetles vary in colour and may have four spots, but a small black triangle behind their head is a common feature.

Bean leaf beetle management

Vanderlaan advises growers to be proactive and use a seed treatment as a first line of defence.

“Early-season protection helps control the overwintering bean leaf beetle population, which will then reduce that first and second generation in season,” she says. For example, Fortenza® is a new non-neonicotinoid soybean seed treatment that provides early-season reduction of feeding from below-ground pests as well as above-ground feeding from bean leaf beetle. This time-lapse video shows the powerful impact of Fortenza when this pest is introduced in a young soybean crop.

Vanderlaan also encourages growers to scout in season for later generations of the pest. According to the Ontario Ministry of Agriculture, Food and Rural Affairs, the ideal scouting technique beyond the seedling stage is to:

sample 10 areas of the field;
pick trifoliate leaves that are fully expanded from the centre of the plant canopy from five plants;
discard the least and most damaged leaflets from each trifoliate collected; and
determine the percent defoliation that has occurred.

Damage thresholds

Well-timed foliar insecticides are beneficial when defoliation or pod feeding thresholds are reached.

Specific damage thresholds are posted in OMAFRA’s Agronomy Guide for Field Crops, Publication 811.

“Basically, for any late-season soybean pest, the need for a foliar insecticide depends on defoliation percentage,” says Vanderlaan. “If it’s early on, such as before flowering, you need to have quite a bit of defoliation, but once a soybean hits the reproductive phase and you get flowers and pod fill, then it’s just a 15 percent defoliation – that’s our damage threshold and you should apply an insecticide to control them.”

Foliar insecticide options for soybean growers include Endigo®, which gives fast knockdown and residual control of bean leaf beetles. Matador® 120EC insecticide also provides control of bean leaf beetles, and other labelled insect pests, that can rob a soybean field of yield and quality.