The Colorado potato beetle costs the agricultural industry billions of dollars per year and devours so many crops around the world that the insect has been branded an “international super pest.” Because the pest has become resistant to all major classes of insecticides and has few natural enemies, crop scientists are seeking a strategy to rein in the beetle’s feeding frenzies.
A team of researchers led by Ralph Bock at the Max Planck Institute for Molecular Plant Physiology, in Potsdam, Germany, now reports that it has found a way to protect crops from the Colorado potato beetle with a new insecticidal tool: RNA interference, or RNAi (Science 2015, DOI: 10.1126/science.1261680).
To use RNAi against the pest, the researchers first identified a gene the insect can’t do without—one that encodes a cytoskeleton protein vital to maintaining a cell’s shape. Researchers then engineered vulnerable plants to produce a custom double-stranded RNA. As the insect pest dines on the plant, the double-stranded RNA gets converted into small interfering RNA. These fragments prevent the insect’s ribosome from reading the messenger RNA for the essential protein. The obstruction blocks production of the essential protein, and the insect dies.
The inspiration to use RNAi to kill pests dates back nearly a decade, says Jiang Zhang, the study’s first author.
Although the RNAi strategy was implemented in plants years ago, it failed as a powerful insecticide because the pests didn’t all die, explains Steve Whyard, at the University of Manitoba, in Winnipeg, in an associated commentary (Science 2015, DOI: 10.1126/science.aaa7722). Bock, Zhang, and their colleagues, however, have now made a “clever modification” to the earlier, partially successful strategy, Whyard notes, by inserting the instructions to make the double-stranded insecticidal RNA into plant cells’ chloroplasts, instead of into their nuclei. The result of putting the insecticidal RNA into chloroplasts, a plant’s photosynthesis hot spot, was full crop protection from the Colorado potato beetle.
Previous attempts probably didn’t work well because the cytoplasm within plant cells has machinery that metabolizes double-stranded RNA before pests such as the Colorado potato beetle can consume it. Conversely, chloroplasts have no machinery to metabolize double-stranded RNA, allowing the insecticidal molecules to accumulate and be stored until a pest dines on the plant.
One general benefit of the RNAi approach, Zhang says, is that researchers can selectively target specific insect pests by targeting species-specific gene sequences; this avoids the blanket destruction of other insect species seen with many insecticides, he explains.
Whether the new approach will work on other insect pests is an open question, comments Niels Wynant, who studies pest control at KU Leuven, in Belgium. And it remains to be seen how quickly pests will develop resistance mechanisms to the RNAi insecticides. That being said, Wynant adds, the findings could have a “significant impact” on pest control strategies and should be further investigated by agricultural companies.