Superweeds and pesticide resistance in Hawai`i

The diamondback moth, a major pest of Hawaiian watercress, cabbages and other crucifers, has repeatedly developed resistance to pesticides used to control it.

There has been a lot of discussion in the Islands about weeds gaining resistance to pesticides used on genetically modified crops, but pesticide resistance has been an issue for farmers since long before genetic modification became prevalent. 

(Image: Diamondback moth larvae feeding on cabbage. Credit: Doug Wilson, USDA.)

Almost every pest control method—organic and synthetic pesticides, and even cultural techniques like crop rotation, have been defeated by persistent pests. 

The message of pesticide resistance is that there’s no magic bullet. Farmers need to use a range of techniques to control their pests—they can’t simply count on a single chemical spray to solve all the problems.

In the case of the diamondback moth, Plutella xylostella, it became resistant to pyrethroid pesticides in Hawai`i in 1992, and when farmers switched to using another natural pesticide product, Bacillus thuringiensis, the bugs began developing resistance to it in three years.

And not just in Hawai`i—the resistance appears everywhere. 

Farmers have switched to other pesticides and have fine tuned their applications to try to defeat it, but are increasingly depending on biological controls—other insects that feed on the various life stages of the moth. Researchers are looking at increasing the effectiveness of viruses that attack the moths, and there's some genetic modification of moths underway in hopes of improving control.

But with respect to pesticide use, there’s still a concern, as consumers insist on food crops that look nice: “In food crops where damage to vegetables must be minimal, chemical control has not been eliminated by biological control,” wrote Ronald F.L. Mau and Jayma L. Martin Kessing of the University of Hawai`i at Mānoa.

How pesticide resistance develops is pretty simple to explain. In every population, there are a few individuals that will be tolerant of a new pesticide—will survive it. The few who survive will be the only ones to reproduce—and their offspring are far more likely to be tolerant. Soon, if you keep using the pesticide, most of the population will be resistant to it and your bug spray will no longer be effective.

Resistance is found for fungicides, insecticides and herbicides as well as for some cultural techniques. There is a bug that responded to farmers switching to a different crop every other year, by going dormant for the intervening year so they’d be ready when the original crop was again available.

But while pests are resilient, so are farmers. There are numerous techniques to overcome pesticide resistance.

“Growers can help delay the development of resistance by applying pesticides only when they are needed, by rotating between different chemical classes, and by using rates of pesticides within the labeled range. Integrating non-chemical approaches such as pheromone mating disruption and cultural controls can also help delay resistance,” wrote the authors of Fruit Crop Ecology and Management, a publication of Michigan State University. 

So, some of the techniques to combat resistance:

Use pesticides that biodegrade rapidly, so subsequent generations of pests aren’t exposed and don’t further increase resistance.

Use combinations of pesticides—some may be resistant to one or the other, but few to both.

Plant “sacrificial” crops—either in nearby fields or even every other row—so there are always non-resistant bugs for resistant bugs to mate with, reducing the overall resistance. Alternatively, leave some areas untreated.

Learn more about the pests and use a tighter target—in the case of a bug, attack only a specific life stage rather than trying to kill the larvae, the adults and the rest all at once.

Experiment with non-pesticide techniques to reduce bug numbers—remove plants they require to complete their life cycle, rotate crops to reduce buildup of crop-specific pests, use baits and attractants to draw pests away from the crops.

And there are other techniques. 

One that doesn’t work well is a homeowner’s solution of simply using more pesticide in hopes of getting ALL the pests.

“Deceptively reasonable on first inspection, this approach rarely works in practice. The reason is that pesticide residues are usually deposited very unevenly in most field situations, even when very high rates are used. Uneven deposition of pesticides allows resistant pests to survive in greater proportions than susceptible pests, thereby increasing resistance,” says Washington State University’s online pamphlet on pesticide resistance. 

The popular press likes to write about Roundup-resistant "superweeds." But they are, of course, not super in any way other than Roundup resistance. They're still susceptible to mulching, to other herbicides, to hoes and mowers, and to techniques that restore their Roundup sensitivity.

Pesticide resistance is not the end of the world—but its management is a significant and normal part of modern farming. 

The Washington State paper says integrated pest management, including moderate, targeted pesticide use, has been shown to work. Key missions for the farmer, Washington State says:

They must monitor pests regularly, carefully manage (not overuse) their treatments, and “make full use of nonpesticidal methods, such as biological and cultural control, sanitation and host plant resistance.”

The whole issue of resistance is complex—stacks of scientific papers and many very dense books have been written about it. Included above are just a few insights into the issue.

Missouri farmer Blake Hurst was quoted in the New York Times:

"The war between man and weed goes on. No different than it has since the beginning of time,"

© Jan TenBruggencate 2015