Western Bean Cutworm

Bryan Jensen, UW Extension and IPM Program

Hopefully, most of you have seen DATCP’s Pest Survey Report (subscribe) which indicates some Western bean cutworm have emerged in Wisconsin. I know some of you have because it has prompted a few calls regarding the potential mismatch between western bean cutworm activity and corn growth and development.

Although corn appears behind “normal” I am not sure at this point if we will have a large scale mismatch.  Let me explain. Yes, adults have started to emerge, however, this is just the start of a flight which will not peak until approximately 1400 degree days (base 500 F). Once the adults do emerge, time is needed for these adults to reach sexual maturity, to find a mate and for those eggs to develop within a female. Furthermore, once the eggs have been deposited an additional 5-7 days are required before those eggs will hatch. That will help close the gap between egg hatch and susceptible corn.

Likely, there will be geographical areas where WBC eggs will be laid and hatched on vegetative corn. From the references I have seen, it is probable that they will feed on newly emerging leaves until tassels and ears emerge. I do not know if extensive feeding on corn leaves will affect survival. Could other hosts be attractive in the event that only vegetative corn is available?  The references aren’t clear but soybeans may/may not be a host and snap beans are a possibility. Either way, I would not expect significant egg laying in other crops.

Be mindful that delayed planting may make the earliest planted fields more attractive to WBC egg laying. These would be fields to watch early and may serve as a sentinel fields to gauge WBC activity.

Scouting

Although we don’t know for sure what the outcome will be, scouting will help determine the need for treatment. Concentrate your efforts on the earliest planted corn fields as we approach the peak flight period.

  • Examine 20 consecutive plants in each of five locations to get a representative sample.
  • Observe the upper 3-4 leaves for larvae and/or eggs.
  • Consider treating if 5% of the plants are infested with eggs and/or larvae.
  • Once larvae have entered the ear they cannot be controlled.

Continue scouting for at least 7-10 days after peak flight. Infestations may be patchy w/in fields and this pattern may be accentuated by variability in corn development. Scout fields with above ground Bt proteins. These hybrids may have significant feeding depending on the protein used.

WBC eggs are dome-shaped and laid in clusters on the upper leaf surface. Initially they are cream-colored but will turn purple prior to hatch. Larvae will consume their egg shells after hatching making late scouting more difficult. Early instar larvae are dark colored w/ black heads.  Initially larvae will migrate to the tassel to feed on anthers and/or pollen and are capable of dispersing several feet.

Western bean cutworm egg mass near/at hatch. Egg mass turns from cream colored to purple 24-48 hours before hatch. Newly hatched larvae consume the egg shell, so egg masses are most evident before or immediately after hatch.
(Photo: E. Cullen, Univ. of Wisconsin-Madison)

In addition to egg masses, look for early instar larvae on plant surfaces (leaves, tassels, leaf axils, silks) before larvae enter the ear. (Photo: E. Cullen, Univ. of Wisconsin-Madison).

 

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