An old pest is becoming a new problem, according to Tracy Shinners-Carnelley, director of research and quality enhancement for Peak of the Market.
Shinners-Carnelley was at Manitoba Potato Production Days in January to discuss Colorado potato beetle (CPB) control options as neonicotinoid seed treatments begin to lose effectiveness against the potato’s ancient enemy.
Since the 1950s, CPB has developed resistance to 52 different compounds belonging to all major insecticide classes, she said during her presentation, and resistance is growing to neonicotinoid chemistries. The first imidacloprid- (trade name Admire) resistant population was found in Manitoba in 2011.
Although field-level failures are uncommon and producers are still seeing benefit to neonics in south-central Manitoba, where she is based, Shinners-Carnelley said duration of control during the growing season has undergone a significant decline over time in the region.
Years ago, producers applying neonics as seed treatments or in furrow wouldn’t see much crop damage or feeding until late July or early August, “and at that point the damage was insignificant,” she said in an interview. But that time keeps creeping back, with producers now forced to consider using a foliar insecticide by early July.
“This past summer, there was an instance where there was heavy, heavy feeding early and other chemical interventions were needed,” she said. “But the more general response is to see adults and larvae more active earlier in the season than what we would typically see.”
Shinners-Carnelley is part of a team conducting a three-year study evaluating management strategies to control resistant CPB populations, including a combination of seed treatments, in-furrow and/or foliar insecticides.
In 2015 and 2016, Verimark, which can be applied in furrow or used as a seed treatment, appeared to break down by July, she said. “Verimark, and some other treatments with that active ingredient, was hoped for as an alternative to neonics, and we’re just not seeing the performance,” she said. “That’s important for growers to know.”
By contrast, the foliar insecticide Delegate showed good control in the study. But Shinners-Carnelley says producers using Delegate must respect the chemistry and avoid overuse.
“I don’t want to see somebody spray Delegate and then go into their field a few weeks later and see larvae and then get excited and spray it again,” she said. “There is a level of insect presence and defoliation that we can tolerate and we shouldn’t be too quick to go in and apply a product like Delegate.”
Neonics have been so effective for so long that many producers have got used to using an in-furrow or seed treatment without spraying a foliar at all. But Shinners-Carnelley said the paradigm might need to shift to allow for higher economic thresholds.
“We’re at a point where we have a new generation of field scouts and agronomists who haven’t had to manage beetles with foliar insecticides,” she said. “They’ve only been in this role in the neonic era, so they don’t understand what it’s like to assess foliar feeding damage and scout for larvae and be able to know how to respond to that. We’ve come full circle.”
Threshold levels haven’t been updated for at least a decade in Manitoba. Shinners-Carnelley cautions that different potato varieties have different thresholds, and producers and agronomists may need to relearn how to manage beetles by looking at foliar defoliation.
Neonics still have utility in Manitoba. Field observations from 2014 indicate that the use of a neonic plus other insecticides may still offer good control of mixed CPB populations. In 2016, only one of the study’s neonic treatments had CPB counts that triggered a foliar insecticide.
But best management practices recommend that neonics not be used where resistance is known to occur, Shinners-Carnelley pointed out in her presentation.
In addition, in late November 2016 Canada’s Pest Management Regulatory Authority published a proposal to re-evaluate imidacloprid, suggesting the cancellation of all agricultural uses in the next three to five years based on environmental risk assessments. The PMRA also intends to conduct a review of two other neonicotinoid insecticides (thiamethoxam and clothianidin). Producers and members of the public are invited to comment on the proposal by late March.
A scenario in the near future in which neonics stop working or cease to be available is likely, so producers should be prepared with backup plans.
“Beetles have always been a priority pest in Manitoba,” said Shinners-Carnelley. “In the last 15 to 20 years we’ve had some really good controls. With that changing, we really need to be paying attention to this pest and its impact, and be prepared with a strategy.”