Watch for bean leaf beetle feeding

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Fig. 1. Color and pattern variation in bean leaf beetle. Notice that this species range from yellow with no spots to red with spots and all combination in between; however each morph has a small black triangle just behind its “neck.” Photo from the University of Illinois. The past couple weeks have been pretty quiet on the insect front. With wind and flood damage, growers certainly have had other issues occupying their attention. Nevertheless, one insect to consider this time of year is bean leaf beetle. Bean leaf beetle can be a challenge to identify because of its variable coloration; it can be tan or red with spots or without, but the key is the black triangle at the top of its hard wing covers (Fig. 1).

Some parts of Pennsylvania, particularly central and southeastern counties, have seen large populations of bean leaf beetle this year. This insect species has two generations per year in Pennsylvania. The overwintering generation can be problematic in spring for the earliest emerging soybean fields. The summer generation feeds on leaves and can cause substantial defoliation. Fortunately, soybeans can tolerate upwards of 30% defoliation, so often bean leaf beetle populations often do not end up causing economic damage even though their numbers may be high. However, once soybean pods form, bean leaf beetle can have a slightly greater impact, causing damage by feeding on pods (Fig. 2).

Fig. 2. Bean leaf beetle damage to soybean pods. Photo from the University of Missouri. Bean leaf beetle damage results from the direct impact on the pod, but can also include indirect damage caused by transmission of bean pod mottle virus, which can be difficult to diagnose without a lab test. This disease can have a negative influence on yield and can be evident if soybean stems stay green late into the fall.

To keep an eye on this pest, inspect fields for feeding damage to the pods and consider sampling for beetles with a sweep net. Economic thresholds for bean leaf beetle in the soybean growth stages relevant for this time of year are as follows:

  • Bloom to Pod Fill - 20% of leaf area removed and there are 16 or more beetles per foot of row.
  • Seed Maturation - 5 to 10% of the pods are damaged, the leaves are green, and there are 10 or more beetles per foot of row.

See the Penn State Agronomy Guide for details on treatment options (http://extension.psu.edu/agronomy-guide/pm)


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