Mid– to Late-Season Soybean Insects

Soybean plants have a great capacity to compensate for leaf loss from defoliating insects. Soybean can lose large amounts of leaf area without losing yield potential if the remaining leaves are still intercepting at least 90 percent of the incident light.1

When making management decisions concerning defoliating insects, consider the size of the remaining soybean canopy. Obviously, small canopies cannot tolerate as much leaf loss as large canopies. Also, consider the growth stage when defoliations occurs. Defoliation occurring during reproductive growth stages (beginning bloom and later) may have more impact on yield potential compared to defoliation during vegetative growth stages. Lastly consider if existing soil moisture conditions are favorable for soybean growth and overall compensation for missing foliage.


Soybean Insects

Figure 1. Bean leaf beetle (left) and green cloverworm (right).

 

Various insects in the region feed on soybean leaves and may also feed on pods. Some examples include bean leaf beetle, green cloverworm, painted lady, soybean looper, yellow woollybear, and soybean podworm which is also known as corn earworm (Figures 1 and 2).


Soybean Insects 

Figure 2. Soybean podworm (left) and soybean looper (right).

 

General economic thresholds suggest that in pre-flowering soybeans consider an insecticide treatment if insects are present and defoliation exceeds 30 percent. However, in pod-forming and pod-filling growth stages consider an insecticide treatment if defoliation exceeds 20 percent.1

These economic thresholds can vary by 5 to 10 percent due to the stage or type of insects that are present, environmental conditions, soybean growth stage, and the size and condition of the canopy.

Some insects such as bean leaf beetles (BLB) and grasshoppers also may feed on or clip soybean pods. BLB are capable of transmitting several pathogens including bean pod mottle virus. A general recommendation when BLB are feeding on pods is to consider an insecticide application when 5 to 10% of the pods are damaged, leaves are still green, and there are 10 or more beetles per foot of row.2

The dectes stem borer larvae tunnel into the plant stem and may cause premature plant death. Later, windy conditions may cause girdled plants to lodge resulting in harvest difficulties and potential yield loss. Control with insecticides is not feasible as the larvae are protected within the stem. Timely harvest is the most effective method to reduce potential yield loss from dectes stem borer.3

Sources: 1 Ohnesorg, W.J. and Hunt, T.E. 2015. Managing soybean defoliators. NebGuide G2259. University of Nebraska—Lincoln Extension. http://extensionpublications.unl.edu/. 2 Bissonnette, S.M., Pataky, N.R., Nafziger, E.D., et al. 2010. Field crop scouting manual. University of Illinois Extension. 3 Zukoff, S., McCornack, B.P., Michaud, J.P., Whitworth, R.J., and Schwarting, H. 2018. Soybean insect management. K-State Research and Extension. Kansas State University. https://www.bookstore.ksre.ksu.edu/. Web sources verified 07/27/18. 180731222214