RNAi insecticide offers promise for managing Colorado potato beetle

By John P. Roche, Ph.D.

The Colorado potato beetle (Leptinotarsa decemlineata), a pest of potatoes and other members of the nightshade family, causes about half a billion dollars in crop losses globally each year. Chemical insecticides can be used to try to manage this pest, but with continued use the beetles often develop insecticide resistance to these chemicals. A new insecticide has been developed using a process called RNA interference that suppresses elimination of a damaged protein in the beetles and offers promise to help provide control.

Graduate student Samuel Pallis and principal investigator Andrei Alyokhin, Ph.D., at the University of Maine at Orono and Brian Manley, Ph.D., Thais Rodrigues, Ph.D., Ethann Barnes, Ph.D., and Kenneth Narva, Ph.D., of GreenLight Biosciences in North Carolina tested the effects of this RNA insecticide, known as ledprona, on mobility and reproduction in Colorado potato beetles, reporting their findings in a study published in March in the Journal of Economic Entomology.

A study evaluating one of the first insecticides developed to use RNA interference as a mode of action finds significant effects on mobility, pupation, and reproduction of the target insect, the Colorado potato beetle (Leptinotarsa decemlineata). The biopesticide, known as ledprona, inhibits the expression of an enzyme in the beetles that facilitates the breakdown of proteins. With this enzyme inhibited, metabolites accumulate, which, if allowed to continue, eventually leads to mortality. Researchers say the new insecticide could be a useful component of a combined integrated pest management program for this species. (Photo by Peggy Greb, USDA Agricultural Research Service, Bugwood.org)

Ledprona is what’s termed a ″biopesticide″ because it is based on double-stranded RNA (dsRNA), a biological molecule. It is manufactured by GreenLight Biosciences, under the brand name Calantha. The dsRNA in ledprona uses a gene-silencing process called RNA interference (RNAi), and ledprona is one of the first insecticides on the market using RNAi. Ledprona inhibits the expression of an enzyme in Colorado potato beetles that facilitates the breakdown of proteins. With this enzyme inhibited, metabolites accumulate, which, if allowed to continue, eventually leads to mortality. ″Ledprona has a totally new mode of action unlike any other insecticide,″ Alyokhin says. ″The majority of insecticides target some kind of a protein inside their target pests. Ledprona, on the other hand, prevents a protein from being synthesized by targeting mRNA.″

Previous studies tested ledprona in the field and established its lethality to Colorado potato beetles. Pallas and colleagues focused on testing potential effects of ledprona on adult mobility, survival during pupation, and reproduction. The Colorado potato beetles used for the research originated at the University of Maine’s Aroostook Research Farm in Presque Isle, Maine.

In the test of adult mobility in the beetles, the investigators used wind tunnels that led to potato foliage used as an attractant. They drew a line within the tunnels and measured how often beetles placed in the tunnels crossed the line and how long it took beetles to cross the line. Treatment-group beetles in the mobility experiments were fed foliage treated with ledprona; control beetles were fed foliage treated with distilled water.

To test survival until pupation, the investigators placed late-stage fourth instar larvae into plastic containers and fed them according to one of three conditions: foliage treated with a low concentration of ledprona, foliage treated with a high concentration of ledprona, or foliage treated with distilled water. They checked containers every day until all individuals had pupated or died.

Measures of reproduction in the study included the number of eggs laid, the proportion of eggs hatching into larvae, the number of egg-laying events within 24 hours, and the time until egg masses started hatching. In all reproduction tests, treatment-group beetles were given foliage treated with a low concentration of ledprona, and control beetles were given foliage treated with distilled water.

In their mobility experiment, Pallis and colleagues found that there was no significant treatment effect on the number of beetles crossing the line in the wind tunnels after three days of feeding, but there was a significant reduction in the number of beetles crossing the line after an additional four days.

In the survival during pupation experiment, the results were nuanced. ″Fewer treated beetles survived through pupation,″ Alyokhin says. ″The ones that did survive, however, completed their pupation as quickly as the untreated beetles.″

The effects ledprona had on Colorado potato beetle reproduction, however, were pronounced. The group of sexually immature females that fed on ledprona-treated foliage had a significantly lower proportion laying eggs when they reached maturity, a significantly lower proportion of eggs hatching, and a significantly lower number of egg-laying events. In addition, treatment-group females had a significantly longer incubation time required for eggs to hatch. In sexually mature females that received ledprona-treated foliage, a significantly lower proportion laid eggs.

In sum, significant negative impacts to the potato beetles were observed in terms of mobility, pupation, and reproduction. Together, these results are encouraging, but they don’t mean ledprona can be the sole answer to controlling the Colorado potato beetle. ″In the field of pest management,″ Alyokhin says, ″there should be a switch in mentality from a perennial quest for a silver bullet solution that is supposed to take care of a problem once and for all, yet never does, to a quest for good components of integrated pest management systems. I think that ledprona can be such a component.″

Alyokhin’s team will begin testing the effects of ledprona on Colorado potato beetles in commercial potato farms during the 2023 field season.

Read More

″Effects of Low Doses of a Novel dsRNA-based Biopesticide (Calantha) on the Colorado Potato Beetle″

Journal of Economic Entomology