Some insects have become so resistant to insecticides that you can just about call them super bugs.
The green peach aphid, for example, is resistant to 100 different insecticides, more than any other bug out there. But insecticide resistance isn’t a new problem.
"We've had insecticide resistance documented as far back as 1914," said Bruce Steward, an entomologist and technical services manager for FMC, referring to San Jose scale in Washington state. “It's not new, and it's been around for a long, long time.”
The question is, what can growers do to help preserve the tools that are currently available? Answer: a little bit of everything.
The goal of insect resistance management, or IRM, is to prevent or delay resistance and to help maintain efficacy of insecticides. It matters, Steward said during a recent webinar put on by FMC, because it can take eight to 10 years to bring a new insecticide formulation to market.
Most resistant insects come from four groups: beetles, flies, sucking bugs, and moths and butterflies.
One of the keys to preventing resistance is understanding why it happens in the first place.
Resistance is different from tolerance. Tolerance is the ability of a particular insect population to withstand the effects of an insecticide right off the bat. An example are lacewings, which have a natural tolerance to pyrethroids, said Phillip Northover, a technical services manager for FMC and a trained plant pathologist.
Resistance, on the other hand, develops with time and over generations because of increased pressure from an insecticide and the pest’s ability to withstand it.
“From a functional point of view … it's when you see a product that you've used for years … you haven't changed your practices, but it’s just not that level of control or management that you're used to. That could be suggestive of insecticide resistance," Northover said.
And there are different types of resistance. Cross insecticide resistance happens when resistance to one insecticide leads to resistance to another insecticide through a common resistance mechanism.
Multiple resistance happens when an insect becomes resistant to two or more insecticides with unlike modes of action.
There also is behavioral resistance where an insect’s crawling or dropping allows it to avoid contact with an insecticide.
But look closer at what you’re doing because it might be something other than resistance that’s reducing your spray’s efficacy. It could be degradation of the insecticide due to improper water pH, or maybe you’re using the insecticide at the wrong insect growth stage.
Miscalibration of the sprayer is another reason your spray might not be as effective.
“This is the easiest to fix, but is often neglected,” Northover said. “If you go out spraying and don't know what you're putting on, this might waste money and get stuff over stuff that you don't want. You really want to do it, if possible, multiple times a season.”
The material’s rainfast is another thing to consider. “Basically, it’s the period of time applying the material to the point at which the material is adhering to those surfaces and cannot be removed,” Northover said.
You also want to be sure that you’re using the proper application rate.
“All of these should be considered before you jump to the conclusion that you have a resistance problem,” Northover said.
Lines of defense
One of the first lines of defense against resistance, Northover said, is the product label. Look at the group number, not the name of the product. The group number highlights the mode of action of the insecticide, which is different depending on what group it’s in.
Different modes of action will work on different parts of the insect.
“Have diversity in your approach,” Steward said.
That means making sure cultural controls — such as proper watering and fertilization, good sanitation and weed removal, and crop rotations — are being followed.
“Do a host-free period,” Steward said. “So within a field, make sure that you’re not allowing a pest to go from field to field.”
Biological controls — natural predators and parasitoids — are a good tool, but it requires looking at your cropping system to make sure you’re not applying anything that would kill a beneficial. Perhaps select an early-season chemistry that’s less disruptive to beneficials.
Using pheromones or other attractants can be a great way to disrupt mating and thus lessen future generations of an insect.
And then there’s genetic control. Look no further than western corn rootworm and its documented resistance to the cry protein in Bt corn. Planting Bt corn with the same cry protein year after year is asking for future resistance problems, so it’s important to rotate Bt proteins to reduce the chance of resistance.
If you’re going to mix sprays, Steward recommends mixing sprays with different modes of action and similar residual times.
“Don’t mix long and short residuals. This helps reduce selection pressure,” he said.
Be wary of premixes that bring two products together in a formulation, Steward said. The premixes often provide the wrong rate of active ingredients, reducing its overall effectiveness.