by Sonja Heyck-Merlin
Are winter cutworms wreaking havoc in your high tunnels?
“We have been hearing a lot of people say they have problems with cutworms in their high tunnels,” said Anna Wallingford, a University of New Hampshire Extension state specialist of entomology and IPM. Wallingford was one of five panelists participating in the webinar discussion “Cutworms in Tunnel Vegetables and Other Cool-Season Production Issues,” sponsored by UNH Extension.
Wallingford laid out three anecdotal scenarios as reported by Northeast tunnel growers indicative of cutworm damage. First, growers are observing leaf damage to autumn high tunnel plantings and high concentrations of larvae during the winter months. Second, growers are experiencing the death of transplants in spring plantings. Third, in the summer they report seeing damage to foliage and fruit.
According to panelist Dan Gilrein, a Cornell Cooperative Extension entomologist, the term cutworm is a catch-all for certain moth caterpillars that feed on low-growing plants, sometimes cutting them off at or below the soil level. Cutworms pupate in the soil. “Many cutworms look the same,” Gilrein said, “until you get close and start to look at the details. Some even look similar when you look at the details, so for some you may want to submit a sample for identification.”
Although a few other species can be active in winter, the panelists agreed that the winter cutworm is the likely culprit responsible for the damage reported by Northeast tunnel growers. According to Gilrein, you’ll notice rows of dark dashes on either side of the back that fade out toward the front of the winter caterpillar. Their colors can vary greatly.
Another diagnostic characteristic is that the caterpillars have dark vertical bands on the head. The adult moth, referred to as the large yellow underwing, lives 20 – 40 days, laying 1,000 – 2,000 eggs. All panelists stressed that it’s critical to obtain a positive identification to help control the pest. “They are often active at night, so checking tunnels with a flashlight can help catch the cutworms in the act,” said Gilrein.
The winter cutworm, originally from Europe, was first found in North America in 1979 in Nova Scotia. The entomology collection at UNH includes a specimen from the 1990s. The adult moths are effective fliers and the species is now ubiquitous across the northern U.S. They feed primarily at night and are tolerant to cold temperatures.
Gilrein referenced data from a blacklight trap study that took place in Nova Scotia from 1980-85. The study showed there were two distinct flight periods for the adults – a small peak that occurs in June and July and then a much larger peak in August and September. Growers, including panelist Becky Sideman, a UNH Extension sustainable horticulture specialist, report finding a concentration of both small and large larvae in high tunnels. Gilrein extrapolated that the large larvae might be from the earlier flight period and the smaller larvae from the latter. There is no evidence, however, that these larvae are maturing and starting successive generations during the winter.
According to Gilrein, winter cutworm feeds on many different plants including lettuce, tomatoes, potatoes, dandelions, grapes, carrots, beets and strawberries. “The winter cutworm is also a really aggressive feeder in grasses and can build up in this area,” Gilrein said. If high tunnels are located close to grasslands, caterpillars can easily migrate into them.
Cultural practices should be at the forefront of controlling winter cutworm. If weeds are well managed, the high tunnels will be less attractive to the egg-laying adults. “There still may be threats by caterpillars moving in from nearby, but at least they aren’t going to be encouraged to be there because there is a food supply before the crop is put in,” Gilrein said.
There is little academic research on the use of insecticides to control winter cutworm in the high tunnels because according to Wallingford, “This is a super sporadic pest, and we don’t get good populations to actually test products on specific problems.” Foliar applications of insecticides may be ineffective if larvae are inactive or spending part of their time in the soil. Gilrein suggested, however, that insecticides labeled for caterpillar larvae in the crop, such as Bt, may be effective controls. Pre-formulated bait products may also be effective. Wallingford stressed that growers should pay attention to what products are permitted by their state in high tunnel production.
Exclusion netting around the base of high tunnels or on crops is also a possible control for winter cutworm. “Keep in mind there are always going to be tradeoffs. The smaller the holes are, the more things you can exclude, but that’s going to impact your passive and active ventilation. The smaller you get, you might be excluding beneficials,” Wallingford said.
Entomopathogenic nematodes (EPNs), commonly called beneficial nematodes, are another control option for winter cutworm. EPNs are microscopic worms that carry deadly bacteria with them. They have a broad host range, including most cutworm species. Meeting participants reported success using EPNs to control winter cutworm larvae in their tunnels.
Environmental conditions should be favorable for EPNs. Wallingford said that soil moisture is the most important part of using EPNs. “Use water to move EPNs to where they need to be. Consistent moisture aids in their movements. As long as they can move around, they can get to their targets,” she said. “Maintaining soil moisture is more important than the species you select.” Gilrein cautioned that cold temperatures may be a limitation to the efficacy of EPNs, especially if the soil temperature is below 55º F.
Finally, Sideman encouraged growers not to overlook hand control, even in larger tunnels. At dusk or later, she dons a headlamp and hand removes the larvae. “It’s a serious focused effort over a very small amount of time – a couple of weeks, short visits every night. It has been very effective and brought numbers that were very high down to zero two years in a row,” Sideman said.