Sprayable RNA pesticide works best when potato beetles are small

The first sprayable insecticide made of RNA can target and kill ravenous Colorado potato beetles while sparing most beneficial insects, making it a promising environmentally friendly option.

Colorado potato beetles can devastate entire fields of potatoes, tomatoes and eggplants, leading to billions of dollars of annual crop damage globally. The pests have also developed resistance to many insecticides, which makes the need for a new control method especially important.

The bioinsecticide, marketed as Calantha, is highly effective when applied a few times once larvae first appear in the spring, according to a new study published Oct. 22 in Pest Management Sciencethat tested the spray's efficacy.

"Depending on where you are in the country, there are Colorado potato beetle populations that may be resistant to multiple active ingredients belonging to multiple classes of insecticides," said Brian Nault, professor of entomology at Cornell AgriTech in the College of Agriculture and Life Sciences and senior author of the study.

"This is a brand-new class with a novel mode of action and it's a benefit environmentally because it's not going to interfere with nontarget insects," he said.

Broad spectrum insecticides kill indiscriminately, affecting populations of beneficial insects such as bees, predatory wasps and ladybugs.

The sprayable RNA technology can also be tailored to address other pests. A new product for countering varroa mites that spread disease to honeybees is currently in the regulatory approval pipeline, and researchers are investigating using the tool to combat Asian citrus psyllids, bark beetles and even mosquitoes. Scientists have previously engineered corn plants that produce an RNA-based insecticide to suppress western corn rootworm, but this is the first sprayable product.

Calantha works by co-opting a genetic process called RNA interference. While DNA includes two strands of genetic code, RNA - which translates instructions in DNA for cells to produce specific proteins - is mostly a single strand. But, in some instances, such as with viruses, double-stranded RNA (dsRNA) occurs. In RNA interference, dsRNA silences a gene, which prevents it from making a specific protein.

In the case of Calantha, a tailored dsRNA is sprayed onto plants. Once eaten by Colorado potato beetle adults and larvae, the dsRNA enters into cells lining the gut. The cells' machinery acts on the dsRNA, following a pathway that ultimately silences a specific Colorado potato beetle gene called PSMB5, which plays a role in maintaining cellular health by removing defective proteins from cells. Without PSMB5, these defective proteins accumulate, cells die, and a few days later, so does the insect.

Colorado potato beetles have two generations, in spring and summer. In the first generation, other pests have yet to appear, making it the best time to apply the insecticide. By the time the second generation emerges, aphids and leafhoppers are also found, which may require coupling Calantha with other chemical sprays to kill the various pests.

To optimize Calantha, Nault and colleagues began applying it in spring when the first eggs hatch, followed by weekly treatments for the next two to three weeks, while populations and larvae are both small.

"It's highly effective when used at these times and intervals," Nault said. "With this particular product, it's much more effective against small larvae, so you want to make sure that you target the earlier stages of the insect's development."

A few caveats: in New York state, the Department of Environmental Conservation limits Calantha use to two applications per year to keep the insect from developing resistance. "In New York, it's going to work better to use it on a small to moderate size population," Nault said. "Two applications may be sufficient to control that spring generation."

Also, the product's manufacturing process doesn't align with guidelines for organic use, thereby depriving organic farmers of a potentially valuable tool, Nault said.

Marcelo Dimase, a postdoctoral researcher in Nault's lab, is the paper's first author. Co-authors include Brian Manley at Greenlight Biosciences, Andrei Alyokhin at the University of Maine and Russell Groves at the University of Wisconsin, Madison.

The study was funded by GreenLight Biosciences, which developed the dsRNA used in Calantha.