By Leslie Mertz
If there’s one word that describes the squash bug (Anasa tristis), it is frustrating, according to Hélène Doughty, the lead author of a new descriptive article that appears in the Journal of Integrated Pest Management.
A noted problem insect in the United States for more than a century, the squash bug continues to do considerable damage to crops, particularly in light of the trend away from conventional insecticides to newer strategies, including organic approaches.
“We’re really trying to find some answers, especially for organic growers, but it’s just a very difficult pest to control,” said Doughty, a research specialist senior at Virginia Tech’s Hampton Roads Agricultural Research and Extension Center. “It’s definitely been a frustrating pest and a frustrating area of study.”
A major issue with squash bugs is that they can transmit a bacterial infection called cucurbit yellow vine disease. This disease, which is most common in Oklahoma and Texas, has begun showing up in other states in the midwestern and eastern United States. The bugs spread disease-causing bacteria while feeding on plants within the genus Cucurbita. Squash bugs especially like zucchini and other summer squash, and pumpkins, and occasionally will snack on cucumbers, watermelons, and winter squash. Bacteria-infected plants experience rapid yellowing and wilting of the leaves, and subsequent decline.
Farmers have successfully used so-called broad-spectrum insecticides against squash bugs for decades, but many are now getting away from that practice, in part because such insecticides kill not only the target pest, but also many other insects, including beneficial ones. Pyrethroids, for instance, are insecticides that not only take out squash bugs, but also can destroy the natural enemies of aphids, therefore allowing aphid populations to flair up and cause problems of their own.
“Growers are attempting to use less harsh chemicals and are turning to integrated pest management, meaning they are using a variety of different techniques that don’t just involve spraying insecticides. This is due to public perception, cost, and simply trying to be more environmentally friendly,” she explained.
Unfortunately, no pest-management program has yet come close to matching the effectiveness of the broad-spectrum insecticides against squash bugs.
Researchers have looked into boosting the natural enemies of squash bugs. One that is often noted is the tachinid fly Trichopoda pennipes, which lays eggs in adult squash bugs. The eggs hatch into larvae inside the bug and continue to live there until they pupate.
“It takes a while for Trichopoda pennipes to actually kill its host, so the squash bug can go on feeding. When we’re talking about a disease vector, that’s problematic, because as that squash bug continues to feed, it can continue to transmit the disease,” Doughty said.
Another tactic to fight squash bugs is to clean up the area around plants. Debris and dead vegetation can serve as shelters for the insects, which like to hide in the darkness. She took that idea a step further in her research by testing whether she could use strategically placed cover boards to draw adult squash bugs away from plants, and then regularly flip over the cover boards to manually destroy them.
“For small plantings, this would eventually reduce the population in the field, but in large or commercial-sized settings with populations of squash bugs that are very heavy, that’s probably not a good strategy,” she said.
An additional option is to use row covers, which are installed at planting to prevent colonization by squash bugs and then removed at flowering to allow for proper pollination. Biological insecticides then follow.
“This option showed some promising results, particularly for fields with moderate pressure,” she said.
One successful but drastic tactic is to grow something other than squash, cucumber, or melons for at least a year until the squash bug numbers drop. For those die-hard growers who insist on having at least some Cucurbita crops, Doughty suggests trying a “trap crop” of the bug’s favored varieties — for example, zucchini — next to less-tasty varieties, such as watermelons. Here, the grower is effectively luring the bugs to the zucchini, which acts as a sacrificial planting to save the watermelon.
“Of course, there’s no way to use that strategy if you are growing summer squash, because there is nothing more attractive to squash bugs,” she added.
Despite the decades of growers’ experience and researchers’ experiments with this pest, options remain limited, Doughty admits.
“Under very high pressure, it will be difficult to use any other strategies aside from crop rotation or using conventional broad-spectrum insecticides,” she said. “As Metcalf et al. wrote in 1962, ‘There is no more vexatious pest of the garden than the squash bug.’”
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Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.