By Harvey Black
As the effectiveness of the primary chemical weapon against the Colorado potato beetle (Leptinotarsa decemlineata) starts to wane, new ways to manage this pest are needed where potatoes are intensively grown, according to an article in the the Journal of Integrated Pest Management.
The beetle is a major problem in areas such as Wisconsin, Michigan, New York, and Maine. It attacks the foliage of the potato, thus interfering with photosynthesis and reducing energy that helps the potato grow. Both chemical and non-chemical methods can be used to deal with the pest, according to two of the authors — Anders Huseth, a postdoctoral associate at North Carolina State University and Russell Groves, a professor of entomology at the University of Wisconsin-Madison.
Neconicotinoid insecticides have been successfully used since 1995 to fight the beetle, but their effectiveness has been waning in some areas. While resistance is increasing, Huseth notes that it may not spread to all areas where potatoes are grown. Areas where potatoes are not grown year after year on the same soil are less likely to see insecticide-resistant potato beetles.
But to ward off resistance where it may become an issue, the researchers advocate moving away from broad-spectrum pesticides toward more highly targeted ones, as well as using non-chemical methods.
The beetle “has a long and decorated history of developing resistance to most of the chemical classes that have been used against it,” Groves said.
Hence, the researchers have devised a strategy of rotating various pesticides with different modes of action over the years in order to prevent, or at least significantly delay, the development of resistance.
For example, one strategy calls for using benzoylureas — an insect growth regulator that interferes with chitin synthesis — early in the season, followed by a late-season application of spinosyn, which interferes with the nervous system. The second year would begin with an early-season application of diamide, which affects muscle contraction.
The researchers also note that non-chemical, or cultural, means can be effective as well. For example, crop rotation can, in certain cases, be effective.
As Groves explained, the beetles are tired after emerging from under the ground after winter.
“It has used a lot of its energy to simply stay alive,” he said. “The vast majority have only enough energy reserves to walk about one quarter of a mile. If a grower can move the crop up to half a mile from last year’s [planting], it can have a significant effect.”
But that may be hard to do, given the location of farms and the nearby presence of housing, according to Amanda Gevens, assistant professor of plant pathology at the University of Wisconsin-Madison, who was not involved in the research.
One solution, said Groves, is what he calls area-wide pest management, in which various growers cooperate. But, he added, the “impetus to do that has never been great.”
Another non-chemical approach is called the spring trap crop, which is essentially a decoy for the beetle. According to Huseth, a small “trap” crop can be planted two weeks before the main commercial crop is planted.
“The idea is that the foliage is up and expanding before the primary crop emerges,” he said. “When the adults have colonized that trap crop, they are in a confined spot, so the crop can be destroyed with a mechanical method like a soil chopper (a device like a lawnmower) and you’re not using any insecticide, so there is no selection pressure, but you’re managing the adults before they get into the commercial crop, lay eggs, and attack the crop.”
However, Gevens cautions that this method may only be practical for small and moderate scale growers.
“When you move to larger fields that may be 80 acres or greater, fields are more highly concentrated and the trap crops won’t work as well, given the quantity of potato and attractive plant material for the insect pests,” she said.
While non-chemical means of controlling the beetle and other pests may be helpful, pesticides “used in a judicious way” will always be a factor in growing potatoes, according to Huseth.
“Pesticides in this system are important,” he said. “Pesticides are commonly used. That’s where growers are as far as their pest-management toolbox. A rapid transition away may not be in their best interests as far a profitable crop. To that end, we wanted to provide alternative recommendations that can help them manage resistance with cultural controls.”
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Harvey Black is a freelance science writer. A long-time resident of Madison, Wisconsin, he has written for numerous publications including Environmental Health Perspectives, Scientific American Mind, New Scientist, The Scientist, and the Milwaukee Journal Sentinel.