Rally Against Pesticide Resistance

Insecticide resistance was first documented in 1914, followed by herbicide resistance in the 1950s. The ag community has battled both ever since. Scientists are continually developing products to fill the gaps left by those rendered ineffective due to over-use, but because of the time it takes to bring a new product to market, preserving effective chemistries through strategic application and supportive cultural practices is crucial.

Kevin Langdon, Ph.D., technical product lead for insecticides at Syngenta, says it’s important for growers to understand that resistance to pest control products is an evolutionary process.

“A lot of folks think if they spray the same insect generation multiple times, it’s only those who become resistant,” Langdon says. “They don’t think of resistance as a trait that's conferred to the next generation.”

According to Langdon, mature insects that aren’t killed by an insecticide can develop metabolic or structural/target site changes that render them immune to the product’s mode of action. Some of their offspring may inherit the immunity-conferring trait, which enlarges the resistant community. Offspring that don’t inherit such traits may develop them if sprayed with the same product or mode of action. The key to preventing this cycle of resistance is to stagger sprays using different modes of action.

“For example, the first and third generation would be sprayed with one mode of action, then the second and fourth generation, if there are that many in a single crop, would be sprayed with a different mode of action group of chemistry,” Langdon says.

In plants, resistance builds over the course of the growing season, and just as with insects, it only takes a few resistant plants to leave a legacy for the following year.

“If you spray a million weed plants and three survive, you never notice that in your field. You killed 99% and the field looks awesome,” says David Belles, Ph.D., technical product lead for herbicides at Syngenta.

But, Belles says, those three that survive can produce up to 250,000 seeds per plant and cross with other plants. Soon, the only ones left are resistant, and subsequent sprays with the same mode of action will only speed up the process of resistance development.

Stewarding efficacy

Good stewardship of effective pest control products takes a variety of forms according to Fred Musser, Ph.D., professor of row crop entomology at Mississippi State University. Musser conducts research on integrated pest management (IPM), which aims to look at pest management in a more holistic way. IPM includes a variety of practices like cultural controls, biological controls, host-plant resistance and insecticides to manage field health.

Corn-soybean crop rotation is a common cultural practice in the Midwest, which, among other benefits, can help control corn rootworm. However, when the same crop rotation is used consistently in a broad geography, resistance to the crop rotation itself can develop, according to Musser.

“The reason it worked against rootworm is the adults would lay their eggs in the fall in corn,” Musser says. “But when they hatched in the spring, the field had been planted to soybeans, so those worms couldn’t feed and didn’t make it.”

A lot of folks think if they spray the same insect generation multiple times, it’s only those who become resistant. They don’t think of resistance as a trait that's conferred to the next generation.

At first the practice was virtually 100% effective, Musser says, but since then, two resistance mechanisms have been discovered that undermine it. One is that the adults started laying their eggs in soybeans, which hatched when the field was corn again. The other is an extended diapause, which is a spontaneous interruption in the larvae’s development that delays the hatch.

“So, if the eggs were laid in corn in the fall of ’22, the worms wouldn’t emerge until the spring of ’24, when the field’s back to corn again,” he says.

As a result, rotation is no longer enough to control corn rootworm in pockets where resistance has developed, even in first-year corn. “In those cases, it’s not even pesticide resistance; it’s crop rotation resistance,” Musser says.

Crop by crop, season by season

Geography and climate can also be factors in resistance management. John Palumbo, Ph.D., professor of entomology and an extension specialist at the University of Arizona, works in Yuma, Arizona, where the state’s river valleys and temperate climate mean pest management is done on a year-round, crop-by-crop basis, versus the Midwest’s single growing season.

“Through irrigation, we can grow crops 365 days a year because of our weather,” Palumbo says. “Every month we’re planting something, and every month we’re harvesting something.”

Every acre gets an average of two or three crops in a calendar year, Palumbo explains. While crop-specific pests will move out after harvest, growers are conscientious about managing them while they last.

“We always look for multiple modes of action,” Palumbo says. “We do best when we have a minimum of three to four, and we’re fortunate that whether they’re vegetables or field crops, we do have several available.”

But there are plenty of pests that hang around, Palumbo says. Beet armyworm is a good example. It’s in the Yuma fields year-round and isn’t picky about its host crop.

“It'll go from lettuce to broccoli to spinach to cauliflower during the winter, then to alfalfa, cotton and melons,” he says, “so we’re very conscious of resistance.” Beet armyworm resistance has been building since the mid-1990s.

“We had organophosphates, carbamate and pyrethroids,” Palumbo says. “We put on too much selection pressure using those chemistries, and then we had problems.”

Fortunately, developments with various product chemistries helped get the problem under control, and educators like Palumbo continue to stress the importance of resistance management: rotating new chemistry, never spraying the same mode of action more than once and rotating to an alternative mode of action.

“When the worms move from the winter vegetables into crops like alfalfa and cotton, we recommend completely different modes of action,” Palumbo says. “So, even if resistance does build during the eight months in the winter, it's going to get hit with something else in the summer before our next produce season starts.”

Mode of action rotation

To effectively rotate modes of action, Langdon recommends growers start with the IRAC, FRAC or HRAC codes, boldly displayed on the front of product labels, to ensure they’re getting the variety of chemistry they need. He notes that proper application and rate are important, too.

“Make sure you’re getting really good coverage,” he says. “If aerial application is your only option, keep in mind the optimal spray coverage, so the carrier volume is high enough that you're essentially spraying the entire plant and the bug doesn’t have refuge.”

Likewise, Langdon adds, a sub-lethal dose of pesticide can be as bad as none, since surviving the spray is where resistance can start.

“Some folks cut the rate just to save money, but you've got a chemical that's already being metabolized by the plant, or UV-degraded, so you end up exposing the insect to an even lower dose than you meant to,” Langdon says. “What we really can’t afford is to lose a product to resistance from mismanagement or misuse.”