Mar 13, 2023

Weed, pest resistance a universal crop concern

Weeds and insects forming resistance to pesticides that haven’t even been developed yet sound like science fiction story plot, but that possibility exists, according to Jed Colquhoun, professor and IPM director at the Department of Horticulture at the University of Wisconsin-Madison.

Speaking at the 2023 Wisconsin Cranberry School, Colquhoun discussed metabolic pesticide resistance, a phenomenon in which weeds and insects metabolize – or break down – a pesticide causing it to be ineffective.

“As a scientist, I find it completely fascinating what organisms are able to do to overcome toxins,” Colquhoun said during the Jan. 19 session, on the second day of the two-day event in Wisconsin Dells. “In my nearly three decades as a weed scientist, this is the biggest agronomic challenge in pest management that I have ever seen.” 

Also referred to as “creeping resistance” because of the way it evolves in plant and insect populations, metabolic resistance has been happening slowly over time. “But in the past three or four years we’ve seen amazingly rapid selection pressure for metabolic pesticide resistance,” Colquhoun said.

With few pesticides registered for use in cranberries, Colquhoun called metabolic resistance a frightening prospect for growers of the fruit.

The first documented case of resistance to an insecticide was in Washington State in apples in 1908. By 1944, researchers had observed 44 insect species with resistance to insecticides. That number increased to 853 insects by 2017.

In the plant world, Colquhoun said there are 580 weed species identified as being resistant to herbicides currently used in agricultural production.

“The difference now is that in individual weed populations, we’re starting to see cross resistances to multiple sites of action within a population,” he said.

Colquhoun cited a recent case in Illinois where waterhemp resistant to dicamba was identified, yet the field had never been treated with dicamba or 2,4-D. The waterhemp was also shown to be resistant to five other herbicide sites of action.

Along with being the No. 1 producer of cranberries, Wisconsin is also the leading grower of table beets. As is the case with cranberries, few pesticides are registered for use in beets.

“If waterhemp were to infest a beet field, none of the chemistries currently available for use in beets will control waterhemp. And we’re not too far off in cranberries,” Colquhoun said. 

Herbicide resistant weeds can be divided into two general groups: those with target site resistance and those with non-target site resistance.

In target site resistant weeds, the specific enzymes that the herbicide targets are either mutated so that the herbicide can’t bind to it or the target enzyme is overproduced and the herbicide can’t effectively bind to all the sites.

“Think of it like the pieces of a puzzle and those pieces can’t fit together,” Colquhoun said.

In the case of non-target resistance in weeds, the herbicide may not be absorbed or translocated as well. It may get sequestered in plant parts away from the target site, or the herbicide may be metabolized by the plant.

Herbicide metabolism involves the breakdown of the herbicide’s active ingredient into metabolites that are less mobile – and less toxic – to the plant. The metabolites are then sequestered or “dumped” into plant parts where they are no longer harmful to the plant.

Colquhoun noted that metabolic resistance is more challenging to research than target site resistance. It’s also harder to observe in the field.

“For years growers and scouts have been told to keep an eye out for living target plants that normally would have been killed,” Colquhoun said.” That stark contrast of living versus dead weeds was often the smoking gun of resistance. In metabolic resistance, the selection pressure creeps along where target weeds may be injured but eventually recover and produce viable seeds.”

So what can be done to reduce the risk of metabolic resistance developing?

Colquhoun advises against reducing herbicide rates so much that not all weeds are killed, saying it may have the inadvertent consequence of increasing resistance.

“Remember that resistance is heritable,” he said. “Dead weeds that can’t reproduce can’t become resistant because they’re not putting seeds into the soil.” 

Colquhoun recommends cranberry growers make a three- to four-year plan to diversify herbicide use across modes and sites of action.

“Remember there is waterhemp out there with resistance to more sites of action than you have in your herbicide toolbox,” he said.

Colquhoun also recommends using pesticides only when truly needed, based on economic and action thresholds. 

“The prophylactic days are done where we just go out and spray something because we might potentially have a problem or someone else down the road has a problem,” he said.

Research is underway to gain a better understanding of the complex metabolic interactions among pesticides and pests, and how that affects practical management decisions, Colquhoun said.

In the longer term, Colquhoun said researchers need to develop reasonable alternatives to pesticides.

“And by reasonable, I mean something that you would be willing to adopt, something that is economically feasible and is at least equal to what you are doing now,” he said.

— Lorry Erickson, FGN Correspondent