It should come as no surprise to Prairie potato growers that Colorado Potato Beetle (CPB) insecticide resistance is increasing.
The trend of resistance to neonicotinoids (the Group 4 class of insecticides which includes Admire, Actara, Poncho and Calypso) has long been noted in Eastern Canada, and is now spreading west.
Ian Scott, a research scientist with Agriculture and Agri-Food Canada based in London, Ont., recently participated in a study monitoring CPB populations for resistance and cross-resistance with other insecticides. His team, he says, recently published a survey of neonicotinoid resistance in Canadian CPB populations in which they concluded that resistance to imidacloprid (Admire), the first neonic to be registered for CPB control, was present “in a significant number of populations.”
The study also notes decreasing susceptibility to one of the recently-registered products, thiamethoxam (Actara). “Resistance to the more recently registered neonicotinoids, thiamethoxam and clothianidin (Titan), is likely if the products are continuously used without proper insecticide resistance management (IRM) practices being applied,” Scott explains.
Perhaps most troublingly, the survey also demonstrated that cross-resistance is on the rise. Using comparisons of CPB mortality generated from lab bioassays with Admire, Actara and Titan, a positive, and significant, correlation indicated that there is potential for cross-resistance between these three neonics.
“This correlation was weaker between Admire and Coragen (chlorantraniliprole), an anthranilic diamide class (Group 28) insecticide, indicating the cross-resistance between these classes is less likely to develop, but not impossible, as has been the case in other insect populations,” Scott explains.
In Manitoba, says Vikram Bisht, the provincial plant pathologist for Manitoba Agriculture, Food and Rural Development, resistance has been noted against neonics. Some CPB populations collected from treated fields in 2012 and 2013 and tested by Scott were found to be imidacloprid resistant (with less than 30 per cent mortality in lab bioassays) or showed reduced sensitivity to clothianidin or thiamethoxam (with 30 to 70 per cent mortality in lab bioassays).
Bisht is careful to note that the survival of beetles after insecticide application could potentially be due to a variety of factors, including improper coverage, failure to apply the full rate of insecticide, or precipitation soon after application.
But genuine insecticide resistance develops because the beetle, he says, has a very high sexual multiplication rate, which lends itself to high mutation rates and thus can lead to increased resistance in a given population.
It’s clear that constant use of one class of insecticides will be harmful long-term. “Use of any chemical to kill insects leads to a selection pressure on the insect population, which could give rise to resistance types,” Bisht says. Additionally, repeated use of similar mode-of-action insecticides, he says, “may often lead to development of cross-resistance against newer insecticides (even though they are less used). It is for this reason that resistance against clothianidin and thiamethoxam may be appearing.”
For this reason, rotation of chemical classes is increasingly crucial.
“It is extremely important to rotate chemistries against CPB,” says Bisht. “This could reduce selection pressure and slow development of resistance in CPB populations.”
Judicious use of neonics will prolong their use, but Scott says that once resistance has developed, as in some U.S. potato growing regions, CPB populations have shown neonic-resistance years after the products were no longer applied. “IRM practices, if followed, will definitely ensure a longer life-time for each product. The history of CPB-insecticide use contains many examples of products that will eventually fail given a long enough use in the field,” he says.
Thus, a combination of chemical and non-chemical management techniques is a grower’s best bet to ensuring CPB protection long-term. The combination, Scott explains, will slow resistance development and help maintain currently effective insecticides for as long as possible.
On the list of non-chemical management strategies is crop rotation, as well as, Scott says, “increasing the amount and variety of landscape features between potato fields to reduce CPB abundance and migration, where possible.”
“The closer fields are to each other the greater chance of CPB movement between fields, and thus the movement of insecticide-resistant populations,” he says. “Increasing field borders, such as grass areas, has been shown to reduce CPB movement, in part by creating a physical barrier, and possibly providing more habitat for egg predators of CPB, that act as natural biological control agents.”
Other techniques, such as constant monitoring and scouting and the use of crop refuges, are important tools in the grower’s toolbox.
Manitoba growers, for the time being, are doing all they can to prolong use of neonics. Marlon Kuhl, vice president of Fresh and Chipping Potatoes for Southern Potato Co., says the company has so far observed no resistance on their operation.
This is partly due to careful management — when treating seed, Kuhl says, they always use proper rates of application. If neonics are used as a seed treatment, they are not used as foliar insecticides. Scouting begins at emergence and continues until harvest.
“If products are not managed properly, resistance will be inevitable,” says Kuhl. “It’s just a matter of time. As farmers we need to ensure that we do not cut rates when applying either on seed or foliar. If foliar application is needed following seed treatment, the use of a different group of chemistry will help delay resistance.”