Western Bean Cutworm Scouting Procedures for Beans

The trap in Blyth which is set up at an edible bean field caught its first WBC moth late last week (July 24th). That makes it an appropriate time to discuss scouting and management of WBC in the edible bean crop.

As mentioned in previous articles, WBC prefers to lay eggs in the corn crop until it starts to tassel. Once the field is in tassel, they will look for younger corn fields not in tassel yet and neighbouring edible bean fields. They do not lay eggs or feed on soybeans.

Not as much is understood about WBC and managing them in beans. Unfortunately the eggs are not as easy to find as they are in the corn crop. And they change their behaviour in beans compared to when they are in corn. In the corn, they lay their eggs on the top of the plant on the top surface of the leaves. In beans, they lay their eggs on the underside of the leaves, typically in denser areas of the canopy. And once the larvae get larger, they prefer to feed at night in the bean crop unlike the corn crop which they feed on day and night. During the day, larger larvae hang out on the ground in the bean crop.

Newly hatched larvae can disperse very easily down and across the rows, by a good 10-12 feet. One egg mass can therefore contribute to several plants being damaged at one time. One egg mass can contain anywhere from 2 to 500 eggs. Damage will begin as leaf feeding but once the larvae get bigger, they will move to feed on and into the pods and seeds. Once they are in the pods, it is impossible to control them.

Thresholds are based on monitoring adults through pheromone traps first, to indicate if and when scouting is necessary. However, any edible bean field that is neighbouring a corn field that has reached threshold for WBC it is advised to be sprayed as well.

Monitoring with traps consist of placing two traps per bean field. Traps are placed on opposite sides of the field and are put up no later than the last week of June and are monitored through the growing season. Moth catch totals are accumulated over time until peak flight occurs. Peak flight is noticed when adult populations start to decline from last week's counts, indicating that the previous week was when the peak flight. This requires checking traps regularly to ensure all moths are counted. If the accumulated catch is less then 700 moths per trap, there is low risk of reaching damaging levels in that field. 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 based on the Ontario moth catches, we are well below threshold (less than 10 captured at all sites). But bean producers will want to consider purchasing traps for next year as what is taking place in Michigan is a good heads up for the future.

Figure 1. Photo of a WBC Trap.
Photo Credit: Marlin Rice, Iowa State University

Pheromone traps are pretty simple to make. Using a 2L plastic milk jug, cut four windows out on the sides of the jugs, leaving approx 2 inches of a basin along the bottom to hold the antifreeze mixture. Corn fields require one trap, bean fields require two. Traps are positioned on a post approx 4 feet up from the ground, close to the fields edge where corn or beans are. Place a mixture of 4:1 water to antifreeze. I prefer to use the non toxic antifreeze (specifically for winterizing the plumbing in RVs) in case animals take a drink from it. Also include a drop of dish soap. A pheromone lure is hung using a paperclip hung through a tiny hole made through the cap of the jug. Western Bean Cutworm Sentry® lures can be purchased through Gemplers (www.gemplers.com) or Great Lakes IPM (www.greatlakesipm.com). Lures are changed every 3 weeks. Traps need to be checked regularly so that fluids can be topped up and moths can be counted.