Severe Colorado potato beetle larval damage to potatoes. Photo by Vikram Bisht, MAFRD.
Colorado potato beetle (CPB) has been a challenging pest for potato growers for more than a century, and today it continues to be the most damaging insect defoliator of potatoes across Canada and the U.S. If uncontrolled, growers suffer yield losses and potential crop failure in potato crops and other field vegetable crops such as tomatoes and eggplant. Growers continue to rely on insecticides for control, however CPB is very adept and successful at developing resistance, creating additional challenges for researchers, industry and growers.
With increasing potato farm size and repeated use of effective insecticides, there has been high selection pressure on CPB to survive. Insecticide resistance develops largely from the overuse and repeated application of chemicals with similar modes of action that increase the selection of the resistant individuals in the population. “Colorado potato beetles resistant to DDT were first reported in 1955, and have since developed resistance to over 51 different insecticides, including imidacloprid and eight other neonicotinoids in the U.S.,” Ian Scott, research scientist with Agriculture and Agri-Food Canada (AAFC) in London, Ont., says. “Colorado potato beetle resistance in Canada has developed more slowly, with the first findings of resistance in 2003, to imidacloprid (Admire), the first neonicotinoid registered in Canada, and increasing reports of problems and crop failures in Ontario and Quebec by 2007.”
In 2008, Scott and colleagues initiated a four-year project to survey CPB neonicotinoid resistance in Canadian potato fields. Growers and agronomists from across Canada sent live samples to AAFC every summer. “We partnered with different chemical companies on the project and, depending on which products were surveyed in a particular year, we conducted bioassays in the lab,” Scott says. “We were able to screen quite a few populations of Colorado potato beetle through the bioassays. We were trying to confirm what growers and extension people were seeing related to resistance concerns with registered neonicotinoid uses. Also, some companies were planning to introduce new products of the same class or different groups and wanted to see if there was any potential cross-resistance with the older products.”
The results of the four-year survey confirmed previous studies showing that many CPB populations have become less sensitive to imidacloprid and that cross-resistance with the second-generation neonicotinoids thiamethoxam and clothianidin is a growing concern. Scott adds the survey also demonstrated cross-resistance is on the rise.
“Using comparisons of Colorado potato beetle mortality generated from lab bioassays with Admire, Actara and Titan, a strong positive correlation indicated there is potential for cross-resistance between these three neonics,” Scott notes. “However, cross-reaction correlation was weaker between neonic Admire (Group 4A) and diamide insecticide Coragen (chlorantraniliprole, Group 28), indicating the cross-resistance between these classes is less likely to develop.”
According to Vikram Bisht, plant pathologist with Manitoba Agriculture, Food and Rural Development, the Atlantic provinces, and Quebec and Ontario have had a greater problem with CPB resistance to Group 4 insecticides. “However, Manitoba has seen some population shifts to resistance,” he notes. “Alberta currently does not have serious issues and so far even older chemistries appear to still be effective there. Resistance to the newer insecticides is highly likely if the products are continuously used without proper insecticide resistance management practices being applied. It is important to rotate chemistries to prevent multiplication of insecticide resistant populations.”
Vikram adds CPB populations collected from treated fields in Manitoba in 2012 and 2013 and tested by Scott were also found to be imidacloprid resistant (with less than 30 per cent mortality) or showed reduced sensitivity to clothianidin or thiamethoxam (with 30 to 70 per cent mortality). Repeated use of similar mode-of-action insecticides may often lead to development of cross-resistance against newer insecticides, even though they had not been previously applied. Therefore, resistance against clothianidin and thiamethoxam has a greater chance of appearing in imidacloprid-resistant CPB populations.
“Generally, every commercial grower treats seed potato for Colorado potato beetle and in most cases control is quite effective,” Bisht says. “In areas where resistance is developing and in situations where growers also need to use a foliar application for in-crop control, they must make sure to use a different chemistry in-crop than was used for seed treatment. There are at least five different seed treatment or in-furrow chemistries and a few other chemistries for in-crop, so growers do have options available. Growers should make rotating insecticide chemistries a priority whether or not they have resistance concerns, as it is good practice to ensure long-term use of the good tools available.”
Bisht adds crop rotations of two years or longer are needed to be effective. However, fields must not be adjacent to last year’s crop. “Short distances from previous potato crops reduces the advantage of crop rotation, since the overwintering Colorado potato beetle adults from last year’s field can easily crawl to adjacent fields. Also, Colorado potato beetles can fly short distances during warm days and can easily move 100 to 200 metres.”
New research and management options
“We are working on other projects focused on understanding the mechanisms responsible for resistance of Colorado potato beetle through molecular and biochemical analyses,” Scott says. “We have maintained a number of Colorado potato beetle colonies in the lab for use in various projects; for example, screening alternative biopesticide products and the use of natural plant compounds as insecticide synergists.”
One recent project studied the potential use of dillapiol, the main constituent in Indian dill (Anethum sowa), as a synergist to help improve insecticide activity and longevity, and control of resistant pest populations. Synergists have a long history of use, one of the most frequently used ones being piperonyl butoxide (PBO) in combination with the natural insecticide pyrethrum. PBO is a very effective synergist but has related toxicological concerns.
“Our laboratory trials testing of pyrethrum alone versus pyrethrum combined with dillapiol indicated that the synergist increased the activity of pyrethrum against insecticide-susceptible and -resistant Colorado potato beetle,” Scott explains. “In field trials, both the PBO and dillapiol synergized pyrethrum had 10 times the efficacy of pyrethrum alone. We continue to work on dillapiol and other natural plant-derived synergists that may offer compounds with improved health and environmental safety and potential for organic certification.”
Research continues on other strategies to help growers manage the challenges of CPB. “There continues to be a focus on the development of new varieties with Colorado potato beetle resistance, however transgenic varieties have not yet been accepted by the market,” Bisht says. “Plant breeders are also using conventional breeding to introduce natural resistance genes from wild species, and hopefully in the future varieties will become available.
“There are other cultural practices that can be options for organic or smaller growers, but so far most are not practical for larger commercial growers. There is a new biological insecticide available – Novodor (Bacillus thuringiensis subsp. tenebrionis, strain NB-176) – but it is not currently used by commercial potato growers. Plastic-lined trenches on field edges could be effective for small farms, which trap Colorado potato beetle and prevent movement into the field.”
Considerable efforts continue to be focused on addressing resistance. Scott and Bisht remind growers everyone must do their part to be vigilant in monitoring and scouting for CPB resistance, follow proper insecticide resistant management practices and alternate classes of insecticides available.
“The survival of beetles after an insecticide application could be due to a variety of factors, including improper coverage, failure to apply the full rate of insecticide, or precipitation soon after application and existence of actual insensitivity (tolerance/resistance) to the insecticide,” Scott says. “It is very important for growers and industry to practice good stewardship and maintain the use of these products as long as possible. It takes a considerable [amount of] resources, time and funds to develop new chemistries, so protecting the tools we have as long as possible is a best practice.”
Burn, beetle, burn
In February 1994, Potatoes in Canada magazine, a sister publication to Top Crop Manager magazine, published a story with an eye-catching headline: “Burn, beetle, burn.”
The story explains that in the summer of 1992, field trials began on an eight-row flamer designed to control Colorado potato beetles (CPB) that had recently demonstrated resistance to traditional chemical treatments. The flamer tests were initiated by ICG Propane, in co-operation with the Ontario Ministry of Agriculture, the University of Guelph, and growers in Alliston and Leamington, Ont.
The trials showed that flaming had several mechanisms for limiting beetle damage. The most obvious was that it could kill the beetle adults outright. But the trials also shows there were three secondary controls: First, flaming caused sufficient damage to the CPB so that it died in the near term; second, if eggs and antennae were damaged, the beetles became disoriented and, thus, unable to feed or mate; and third, the heat from the flaming resulted in a high mortality for the eggs already laid.
The trials showed that using the propane flamer in the spring for overwintering adults gave control of 70 to 80 per cent of the adult beetles and also killed nearly 50 per cent of the eggs. Late season control, the time of top killing, gave 95 to 100 per cent control of adult CPB. In addition, the 1994 story stated growers were “astonished at the level of weed control that came as a result of the flaming activity.”
Even more interesting was the low level of plant damage from the flaming. The story states that in almost all locations of the study, slight plant injury similar to that observed when plants are hit by a late frost did occur. However, the growing tip of the plant was not affected, and most injury was unnoticeable after about a week. “Potato yields taken at four of the test sites showed no difference in yield between flamed and unflamed areas.”
At the end of the day, physical methods to control Colorado potato beetle and other pests are seeing resurgence, as the very real threat of insecticide resistance increases.