Cannabis plants may have evolved cannabinoids to prevent herbivores getting the munchies

George Stack and colleagues from Cornell University and Colorado State University have demonstrated that cannabinoids like CBD and CBG help defend cannabis plants against herbivores. The new study provides evidence that producing these compounds is evolutionarily beneficial for the plants. The article, published in Horticulture Research, showed that higher cannabinoid concentrations in hemp leaves led to proportionately less damage from insect larvae. This could lead to opportunities to develop better pesticides.

Why do plants create cannabinoids?

The compounds produced by cannabis plants, known as cannabinoids, have been widely studied for their effects on humans. However, their natural function and purpose for the plants themselves have been unclear. Possible explanations have been protection against herbivores, pathogens or even ultraviolet light.

“It has been speculated that they are defensive compounds, because they primarily accumulate in female flowers to protect seeds, which is a fairly common concept in plants,” said Larry Smart, a plant breeder and professor at Cornell University, in a press release.

“But no one has put together a comprehensive set of experimental results to show a direct relationship between the accumulation of these cannabinoids and their harmful effects on insects,” said Smart, senior author of the study.

These specialised metabolites are produced in glandular trichomes and stored in high concentrations in flower tissues. To test the hypothesis that cannabinoids help defend cannabis plants against insect herbivores, the researchers conducted field studies, controlled lab feeding trials, and microscopic analysis.

The team used a genetically diverse F₂ population of hemp segregating for cannabinoid type and concentration. This variation allowed them to directly compare the impacts of different cannabinoid profiles on herbivore damage and preference under the same environmental conditions. By combining real-world field observations with carefully controlled experiments, the researchers aimed to demonstrate whether cannabinoid production provides an evolutionary benefit to cannabis plants.

Cannabinoids Reduce Insect Feeding and Growth

The researchers found evidence from multiple experiments that higher cannabinoid concentrations protect cannabis plants from insect herbivores. In field studies, hemp plants with greater cannabinoid levels in their leaves sustained less feeding damage from beetles and other chewing pests over a growing season. The team also conducted controlled trials where cabbage looper caterpillars were allowed to feed on hemp leaves with either high or low cannabinoid concentrations. The larvae ate less leaf material and gained less weight when eating leaves containing cannabinoids.

The team identified that the cannabinoids were a problem for the herbivores. They did this by adding different doses of the compounds CBDA and CBGA to an artificial insect diet. The caterpillars showed decreased survival and growth rates in proportion to the dose of cannabinoids added to their food. This result indicates cannabinoids deter herbivores directly through post-ingestive effects, not just by making the plants taste bad. 

“The study gives us insight into how cannabinoids function in natural systems and can help us develop new THC-compliant hemp cultivars that maintain these natural built-in defenses against herbivores,” said George Stack, a postdoctoral researcher in Smart’s lab and the paper’s first author.

Examining how diverse cannabinoid levels affect defence

The researchers used a diverse hemp population with natural genetic variation in cannabinoid levels for their study. Using scanning electron microscopy, the team observed notable differences in glandular trichome structure between hemp plants with high versus low cannabinoid concentrations. The trichomes appeared deformed and collapsed in the low-cannabinoid chemotypes. This result is significant as the cannabinoids are produced in these glandular trichomes. 

In feeding trials, cabbage looper caterpillars were less likely to feed on the underside of leaves with higher cannabinoid levels. The bottom of the leaves is where glandular trichomes are most dense. This suggests that both cannabinoids and the physical structure of trichomes could contribute to defence against insect pests. Further research is needed to tease apart these factors. Nonetheless, the study convincingly showed reduced herbivory on high-cannabinoid hemp using a diverse genetic population.

Future research will have to work within federal limits

The Cornell program cannot work with high THC (the intoxicating compound found in marijuana) plants due to federal mandate, so THC as a pesticide was not tested in this research.

“The potential use of cannabinoids as a pesticide is an exciting area for future research, but there will certainly be regulatory barriers due to pharmacological activity of the compounds, and more studies are needed to understand what pests cannabinoids will be effective against,” Stack said, in the Cornell Chronicle.

Further research flagged in the paper is examining how cannabinoids impact other agricultural pests like aphids that pierce, rather than chew, leaves. The protective benefits may also depend on which plant tissues and developmental stages are targeted by different pests.

However, while there might be potential to develop a better pesticide, it will have limits. The press release states that any pesticide would have to be used only on non-edible crops, “given the pharmacological properties of the compounds, which include CBD, THC and their precursor CBG.”

READ THE ARTICLE

Stack, G.M., Snyder, S.I., Toth, J.A., Quade, M.A., Crawford, J.L., McKay, J.K., Jackowetz, J.N., Wang, P., Philippe, G., Hansen, J.L., Moore, V.M., Rose, J.K.C. and Smart, L.B. (2023) “Cannabinoids function in defense against chewing herbivores in Cannabis sativa L,” Horticulture Research. Available at: https://doi.org/10.1093/hr/uhad207.

Cover image: Canva.