All – WBC is over threshold in some fields in NW Michigan. There is flight elsewhere. This week and next is likely our spray window. Additional info is below.

I will post pics of eggs on the CAT Alert this morning ( http://www.ipm.msu.edu/field-cat.htm). I have infested some corn and dry bean plot on campus yesterday to get addtional pics and some info on yield impacts.

Western bean cutworm over threshold in NW Michigan

Summary Points

• Western bean cutworm egg laying and hatch are confirmed;
• Scout NOW (starting the week of July 21), particularly in NW Michigan;
• Some fields in NW Michigan are over threshold for egg masses and small larvae;
• Populations are very variable from field-to-field;
• Application timing – based on scouting – is critical for caterpillar control.

Western bean cutworm trapping with milk jug pheromone traps began several weeks ago. Moths have been caught throughout the state. In general, the number of moths per trap is 1 to 2, but hot spots (for ex, Oceana County) report over 100 moths in a week. The catches confirm we now have a resident, overwintering population of WBC throughout western and central Michigan. Furthermore, egg-laying is confirmed in fields in the northwest part of the state; some fields were already over threshold for egg masses (30% infestation confirmed by me in Oceana, and similar levels reported in Benzie County) this week, the week of July 21.

WBC in corn

• Scouting for eggs and larvae begins when moths are detected, concentrating particularly on the weeks of peak catch. THAT is NOW, especially in NW MICHIGAN. Moths prefer to lay eggs in fields close to tasselling – this was very evident in Oceana County.
• Eggs are laid in the upper canopy, on the top 3-4 leaves. Eggs may even be on the leaf wrapped around the tassel. The eggs are on the UPPER leaf surface – on upright leaves, they are are on the leaf surface facing in to the tassel.
• Eggs are round (not flat and shingle-like, like corn borer egg masses), laid in in groups of a few to 200. The average sized mass I’ve seen was smaller than Lincoln’s head on a penny. They are bright white if freshly laid, turning tan to grayish in a few days, then purple just prior to hatch. Hatched egg masses are barely noticeable, since the larvae seem to eat the egg shells, leaving an iridescent circle.
• The tiny larvae emerge and sit on the egg mass, then crawl up the plant to the tassel.
  As second and third instars they move back down to the silks, then as larger larvae into the ear. Insecticide applications must be timed close to egg hatch and when small larvae are exposed on the tassel and leaf surface.
• The threshold for treatment is 5% of plants with egg masses or small larvae. Many bulletins and my MSU Insect Guide E-1582 give a threshold of 8%. However, in discussions this winter with my colleague Tom Hunt from the University of Nebraska, I reduced the threshold this summer to 5% (5 of 100 plants with eggs or larvae).
• Insecticides include pyrethoids – Warrior, ProAxis, Pounce/ Ambush, Mustang, Capture, Baythroid, Asana – and Lorsban. More critical than product is TIMING to kill exposed caterpillars.
• A special alert to people managing sweet corn – growers tend to have consecutive planting dates of sweet corn, which makes it likely that one or more of the plantings will be attractive to WBC. Beth Bishop in MSU Veg Program will develop a recommendation for sweet corn. Check the Vegetable CAT alert for her thoughts.

WBC in dry beans

• Dry beans are particularly attractive to WBC if neighboring corn fields have already tasseled.
• Less information is available for managing WBC on dry beans, and it is harder to scout for eggs on the plants. Eggs are laid on the underside of dry bean leaves and may be easily missed.
• On the plus side, if you are trapping, the trap can be used to time spray treatments on dry beans. Moth number is accumulated over time until peak flight. This requires checking traps regularly to detect the peak.
• Threshold: If the accumulated catch is between 700 and 1000 moths, damage risk to beans is moderate and beans must be scouted closely. Check fields for larvae and larval feeding 10-20 days after the peak, and spray if pod feeding is found. If the catch exceeds 1000 moths by the peak, risk is high for damage in dry bean is high and an insecticide application is most likely needed 10 to 20 days after peak flight.
• So far, we are considerably under the 700-1000 per trap required to treat dry beans.

Dr. Christina DiFonzo
Field Crops Entomology Program
Michigan State University