Battle for the Belt: Season 2 Episode 8- Bean Leaf Beetle

Episode 8 of Battle for the Belt is now available: https://www.youtube.com/watch?v=BupYuRu30BI

In Episode 8, Dr. Maggie Lewis, a research scientist for the Department of Entomology at The Ohio State University, walks us through scouting for bean leaf beetle in soybean.

Bean Leaf Beetle Lifecycle

Figure 1. Bean leaf beetle. Picture credit: Amy Raudenbush

Bean leaf beetle (Figure 1) is an insect pest of soybean that we see in Ohio. They overwinter as adults and emerge in the early spring. When they first emerge they feed on a broad range of hosts including various leguminous species and alfalfa. But once the soybeans begin to emerge the beetles will move into the soybean fields quickly, as soybean seems to be their preferred host plant. Early planted soybeans therefore have a higher risk for feeding damage from overwintering bean leaf beetle adults, particularly if that field is the first to emerge in the area.  The adults will feed on the soybean and lay eggs in the soil.

Scouting and Management

Early season, our main concern with bean leaf beetle is adult feeding on above-ground plant tissue, which causes defoliation damage, although we rarely see economic injury due to early-season bean leaf beetle activity. Soybeans are fairly resilient to defoliation and can undergo quite a bit of damage before we start to see yield losses.

To scout for soybean damage, we recommend evaluating defoliation levels in your field. When soybeans are in the vegetative growth stage, defoliation levels at 30% or more economically justify an insecticide application when bean leaf beetles (or other defoliators) are also present in the field.

Figure 2. Defoliation of soybean leaflet at 30%. Image sourced from: Leaf Defoliators PDF_0.pdf (osu.edu)

In terms of determining when you have reached the economic threshold there are established scouting protocols we recommend using (access here). Defoliation is extremely easy to overestimate, and if you make an insecticide spray before you hit the threshold, you will not see any yield benefit. We do have scouting protocols that are available on the agronomic crops insect website (Home | Agronomic Crops Insects (osu.edu))

There are two components, using a defoliation guide and comprehensive sampling. Overestimating defoliation levels is very common and easy to do, but the defoliation guide gives a visual respresentation of defoliation levels ranging from 5% to 50% (Soybean defoliation Final.pdf (osu.edu)). Comprehensivley sampling the field is necessary because most insect defoliators will infests the borders of fields more heavily than the interior. Therefore evaluating 40 plants from different locations throughout the field will give an accurate respresentation of damage. Looking at all parts of the plant is important as well, evaluate a trifoliate from the upper, middle, and lower canopy to effectively evaluate the whole plant. Average the defoliation that you collect together and that is your overall defoliation level for the field.

What’s happening in the field?

Crops at the Western location are progressing rapidly with the warmer temperatures. Bean leaf beetle damage is still visible on planting date one in soybean (March 25); however, limited defoliation is present in planting date two soybeans (April 16) (Figure 3). Corn had a herbicide application to take care of giant ragweed plant populations.

Figure 3. Bean leaf beetle defoliation on March 25 planted soybeans compared to April 16 planted soybeans.

At the Wooster location, there have been no pests. However, rainfall has prevented completion of the third planting date. The crops are progressing well at this location, other than some weeds presence.

At the Northwest location, the first planting occurred on May 16. This is later than we planned for but frequent rainfalls prevented earlier planting. This has been a wet year around the state but Northwest Ohio has struggled to progress in planting due to the rain, heavy clay soils, and poorly-drained fields.

Table 1. The planting date conditions for planting date one at the Northwest Location, 2024.

Table 2. Weekly weather conditions for planting dates one, two, and three at the Western location and planting date one and two at the Wooster Campus, with day of planting, soil, air temperature averages, and growing degree days (GDDS) from May 13 to May 19. Information from CFAES Weather System (https://weather.cfaes.osu.edu/).