Resistant Colorado potato beetle populations are a reality – maybe even in your fields. But it’s possible to effectively manage the pest’s increasing resistance to Group 4 (neonicotinoid) insecticides.
The first sign can be reduced control
Growers are reporting that the residual control of Colorado potato beetle (CPB) achieved by some of the old, Group-4 legacy products, is slipping. Be careful, that could be the first sign of resistance.
“It could very well be an early warning sign of insecticide resistance if you notice that some beetles seem to be coming back sooner after an application of a Group 4 insecticide than before”, says Dr. Jeff Tolman, an integrated pest management research scientist based at Agriculture and Agri-Food Canada’s Southern Crop Protection and Food Research Centre in London, ON.
How to reduce risk of resistance
Resistance can, of course, also occur at time of spraying when some individuals from the beetle population are able to withstand the chemical being used better than others. The chance of resistance developing is much less when everyone uses recommended application rates. “Don’t cut rates,” Tolman stresses.
Both diligent scouting and following economic thresholds are also important tools for effective management of resistance to Group 4 insecticides. Also extremely critical is the rotational application of crop protection products with different modes of action.
Use a Group-28 insecticide
“A Group 28 insecticide has a completely different mode of action than a Group 4 insecticide, and is a very valuable resistance- management tool,” says Tolman.
Group 28 insecticides, the diamides, include DuPont™ Coragen®, which is an anthranilic diamide. Due to its different mode of action, Coragen® does not demonstrate cross resistance to existing crop protection products. “Coragen® is very important to the grower in preventing Group 4 resistance,” says Philippe Parent, who conducts research at Les Patates Dolbec Inc. in St-Ubalde near Quebec City. “It is also effective because it lasts a long time, and causes insects to rapidly stop feeding.”
Coragen® also controls European corn borer. It has a wide window of application and longer residual, which means the plant is protected for a longer period than with contact-only insecticides.
Rynaxypyr® (the active ingredient in Coragen®) moves quickly into the leaf tissue where it is protected from wash-off and can be ingested by pests feeding on both surfaces of the leaf.
Coragen® should be applied to coincide with peak CPB egg hatch and is rainfast once the spray is completely dry. Although Coragen® controls hatching insects all the way through to adult stages of development, applications made at early larval stages provide optimal control. “Young larvae are generally the life stage most susceptible to insecticides,” notes Tolman.
Using any insecticide during the CPB larval stage may coincide with (or just precede) a period of rapid plant growth. “Coragen® moves quickly into the plant and has a long persistence, lasting as long as three to four weeks on the leaf,” says Tolman.
“However, since Coragen® is not systemic, if you’ve got very vigorous growth occurring, that new growth will not be protected. In this situation, another application may be necessary.”
Coverage at the highest recommended rate is essential for maximum effectiveness. Tolman also stresses that “growers may not have to apply over the entire field. Look for ‘hot spots’ when scouting and target those.”
Resistance is best managed using a multi-pronged strategy. Rotate potato fields as far away as possible from where potatoes or another host crop were last grown. Since beetles are unable to fly if the early season is chilly, planting new potato fields even a half a kilometer away with a barrier such as a field of winter wheat in between can make all the difference. Non-host crop barriers work well because the beetles may not have the physical resources to walk through the barrier to the new potato field.
In addition, if you haven’t already, have a discussion with your neighbours to start coordinating a rotational community (also called area-wide rotation).