By SUSAN BLOWER Indiana Correspondent WEST LAFAYETTE, Ind. — Those who grow vegetables in high tunnels – or hoop houses – may be growing a bountiful crop of pests, as well. Recent studies have shown that high tunnels do not keep out insects and may even attract a larger-than-normal population. Rick Foster, Purdue University professor of entomology, tested tomatoes, broccoli and cucumbers and their common pests. Over two years of testing, Foster and his collaborators found larger numbers of the pests in the high tunnels when compared with adjacent open fields. He tested three different plant families and the pests most often associated with them: hornworms (tomatoes); cabbage looper, imported cabbageworm and diamondback moth (broccoli); and cucumber beetle (cucumber). Tomatoes, especially, were affected. “In field situations, hornworms are kind of an anomaly. They’re there, but not in great numbers. We found that in high tunnels, they just exploded,” Foster explained. “There was just nothing green left on the plants. There were just stems hanging on them.” He thinks insects are getting into the tunnels on warm days when the hoop house must be ventilated. The sides are rolled up to keep temperatures from reaching 120 degrees Fahrenheit or more. Often these insects leave the plants they’ve fed on by flying straight up, but in a high tunnel they are being trapped. Laura Ingwell, a postdoctoral researcher at Purdue, has been working to find a solution for four years. She has found success along two avenues: the use of flowers along rows of crops, and insect exclusion screens. “In one study we had exciting results. We replaced a portion of the crop with extra floral resources. We were recruiting and retaining beneficial predator insects by providing extra floral pollen and nectar,” Ingwell explained. “We saw a higher diversity and abundance of predators and less pest outbreaks on the focal produce crops. The flowers offer alternative food for a lot of the beneficial insects. They’ll feed on the pest insects, but they also need pollen and nectar for their diets.” Flowers like zinnias and gomphrena, which growers could sell, drew more spiders, microscopic wasps (enemies of aphids) and stilt bugs (enemies of caterpillars). These insects are natural enemies of pests that eat vegetable plants. Ingwell found even more effective results when combining the flowers with biocontrol – this involves early, multiple releases of natural enemies of pest insects. She tried purchasing and releasing lacewing, lady beetles, minute pirate bugs and predatory stink bugs. Of those, the insect with the highest potential was lacewing eggs and larva and, second, minute pirate bugs. These insects are less mobile and therefore more likely to stay in the high tunnel and build up a population, she said. “If using this strategy, growers should commit to releasing them early, before an outbreak of harmful insects, and having multiple releases,” she noted. Ingwell said she found the ideal pore size of insect exclusion screens – one that would keep out harmful bugs while allowing natural enemies and breezes into the hoop house. Having experimented for two years with replicated data, she determined the ideal pore size to be 0.7 x 0.9 millimeters. She said this screen is widely available. If the pore size is not listed on the packaging, growers can call the company to ask. This size is large enough to allow air circulation, while small enough to keep out most damaging insects. To watch a video on how to install exclusion screens, visit https://youtu.be/U1BYxOXc-RM For growers who are supervising crops in a high tunnel, Ingwell said the most important thing is to stay ahead of the game: “They should scout their crops early and often, keep records and anticipate problems.” Ingwell continues to study other options for high tunnel growers. USDA’s National Institute of Food and Agriculture funded her and Foster’s research. |
