As part of the race to combat global insecticide resistance, new research shows that the same CBD people use to treat a variety of ailments is also extremely effective at killing mosquito larvae.
The study, published in the journal Insects, found that hemp leaf extract - which contains the active ingredient cannabidiol, or CBD - kills mosquito larvae from two different strains of the yellow fever mosquito within 48 hours, one that was resistant to typical insecticides and another that was not.
"Mosquitoes are one of the deadliest animals in the world, mainly because as adults they serve as vectors of disease," said Erick Martinez Rodriguez, lead author of the study and a graduate student in entomology at The Ohio State University. "It's very important to be able to control these pests at an early stage, when they are at the most vulnerable."
Unlike adult mosquitoes that can fly across great distances and transmit disease by biting, mosquito larvae are aquatic organisms often found in standing water. Conventional synthetic insecticides are critical chemical tools for killing mosquito larvae populations, but their rampant overuse has led to both insecticide resistance and other detrimental effects on the environment.
Inspired by previous Ohio State research that discovered that the bark properties of a plant native to Madagascar worked as a natural mosquito insecticide and repellent, Martinez Rodriguez sought to determine if hemp could act as a viable and safe alternative to current biopesticides.
The hemp plant is related to marijuana, but does not contain its levels of the ingredient that makes people feel "high." CBD from hemp is used in a variety of products from gummies and oils to treat anxiety, pain, insomnia and other conditions.
To test hemp's toxic effects against mosquito larvae, the team took air-dried hemp leaves, pulverized them into a fine powder and soaked the material in methanol for a few weeks to reach the desired CBD concentrations.
The methanol was later removed from the solution to make it easier to chemically analyze, resulting in an extract that was eventually given to the larvae with their food.
Depending on the concentration of hemp extract used, the team discovered that the hemp leaf was potent enough to be equally toxic to both strains of mosquito larvae. What was surprising, though, said Martinez Rodriguez, was the small amount needed to be so deadly.
"If you compare the amount of hemp extract needed to kill 50% of the population to other synthetic conventional insecticides, it is on the high side, but when you compare it side-by-side to other natural extracts we have tested in our lab, only a relatively low amount is required to produce high mortality values in larvae," said Martinez Rodriguez. While CBD eventually led to 100% mortality for the larvae, different concentrations of the hemp extract caused different mortality rates in the hours leading up to that time.
The mosquito larvae were considered dead if no movement was recorded after scientists gently touched their abdomen with a needle or pipette tip. Although it's still unclear how the extract affects them biologically, the team's findings showed that CBD was the primary active ingredient in the extract and that it can successfully bypass the larvae's metabolic resistance to other insecticides, which aligns with past work that found CBD has notable growth-inhibiting properties against other insects.
Since hemp is also a more sustainable crop than many other plant alternatives, insecticide products that use it could potentially be produced relatively cheaply, said Martinez Rodriguez.
While this discovery bodes well for developing future natural-based mosquito control products, said Peter Piermarini, co-author of the study and a professor of entomology at Ohio State, further research would have to be done to investigate how safe hemp and CBD would be for non-target organisms, like honey bees or other pollinating insects that scientists don't want to kill or disrupt by introducing potentially harmful additives to the environment.
"CBD is a compound that appears to be safe for people and our companion animals to ingest," said Piermarini. "It'll be interesting to learn more about how CBD interacts with various proteins in mammals and insects to understand why it's safe for people but not insects."
This work was supported by the Infectious Diseases Institute and the College of Food, Agricultural, and Environmental Sciences at Ohio State. Other Ohio State co-authors include Larry Phelan, Luis Canas, Nuris Acosta and Harinantenaina L. Rakotondraibe.
