There have been several discussions in COUNTRYSIDE magazine regarding the resistance to certain pesticides from Colorado potato bugs. Fact is, potato growers in some regions report a great deal of resistance, while others notice little or none.
While taking pains to point out that COUNTRYSIDE does not promote the use of biocides, at least part of the reason for the difference in this resistance has come to light.
Jeff Wyman is an entomologist who works with commercial potato growers in Wisconsin. While the Colorado potato bug (aka Colorado potato beetle) has developed resistance to most insecticides in many areas of the country where growers have relied primarily on chemical controls, Wisconsin growers don’t face that problem because they use both chemical and non-chemical practices to battle the insect, Wyman says.
“Wisconsin still has Colorado potato beetle populations relatively susceptible to insecticides,” according to the entomologist. “This has occurred because growers are mindful of problems that may develop when only insecticides are used to control the insect.”
Left uncontrolled, the Colorado potato beetle can devastate potato fields. In the past, Wisconsin growers controlled the insect primarily with insecticides, the most effective being Aldicarb. However, Aldicarb has been detected in a number of Wisconsin wells and was banned in that state in 1988. This year the manufacturer removed Aldicarb from use on potatoes nationwide.
Like many insects, the Colorado potato beetle goes through a number of larval stages before becoming an adult. Researchers at the University of Wisconsin have discovered this Colorado potato bug fact: The insect inflicts the most damage beginning at the third larval growth stage. Growers can determine when the insect reaches this stage by scouting fields to find out when egg laying begins and by monitoring daily temperatures to determine when they will hatch.
Growers then apply an insecticide only when the Colorado potato beetle reaches the third larval stage, and they make a second application to kill any late-developing larvae. Previously, growers made up to six insecticide applications during the growing season.
“We’ve found that these two applications, if timed correctly, are all we need to control Colorado potato beetles,” Wyman says. Growers can further reduce the possibility of insects developing resistance by alternating insecticides.
Bacillus thuringiensis (Bt), a natural insecticide made from a strain of bacteria, long-used by organic gardeners, is also being investigated by commercial growers. Bt products work only on specific insects, so they’re safe to beneficial insects, wildlife and humans. It breaks down rapidly in sunlight…one of its good points, so far as environmentally concerned gardeners and farmers are concerned. However, researchers are now studying ways to make it last longer. (They’re also trying to find ways to make the insects eat more of it.) This is a specific strain, no the one used to control destructive caterpillars such as cabbage worms.
Researchers have found that the Colorado potato beetle overwinters close to potato fields, in plant residue or mulch, and moves to new fields the following spring by walking. So they suggest that planting a trap crop around new potato fields might enable growers to kill the insects before they reach the potatoes and lay their eggs.
Wisconsin’s bitter winters also help, but this involves another two-edged sword. The absence of deep mulch gardening or any mulch for that matter exposes the beetles to frigid temperatures, but the mulch helps prevent soil erosion. (What is soil erosion?)
There is a demand for organically grown potatoes, and Wyman and other University of Wisconsin researchers are working with growers interested in producing for that market. One technique being studied is growing early maturing varieties to avoid insect and disease problems.
If these combined techniques control Colorado potato beetles on thousands of acres without increasing their resistance to insecticides, they also benefit the gardener who wants to use less insecticide..or none at all, favoring natural pest control for garden and farm use.
I hope this helps bring to light some important Colorado potato bug facts.
For more information about garden pest control, turn to Countryside Network for stories on aphid control, bagworms in trees and cutworms in gardens.
Originally published in September/October 1990