Cannabinoids, natural compounds found in hemp plants, may have evolved to deter pests from chewing on them, according to experiments that showed higher concentrations of cannabinoids in hemp leaves resulted in proportionately less damage from insect larvae.
The study opens the door to the potential development of pesticides from cannabinoid extracts, although such use would be limited to inedible plants, given the pharmacological properties of the compounds, which include CBDA, THCA and their precursor CBGA. These compounds are naturally produced by hemp plants and are converted into the better known CBD, THC and CBG when heated.
In the decades since scientists first identified cannabinoids, research has focused on their medicinal and intoxicating effects, but it was never clear why these plants developed cannabinoids in the first place. Researchers have hypothesized that cannabinoids may protect plants from ultraviolet radiation, pathogens, and herbivores.
These have been speculated to be defense compounds because they primarily accumulate in female flowers to protect seeds, a fairly common concept in plants, said Larry Smart, a plant breeder and professor in the School of Integrative Plant Sciences at Cornell AgriTech in the School of agriculture and natural sciences (CALS).
But no one has compiled a comprehensive set of experimental results to show a direct link between the accumulation of these cannabinoids and their harmful effects on insects, said Smart, who is senior author of the study, Cannabinoid Function in Chewing Herbivore Defense in Cannabis Sativa L., which was published in Oct. 13 in the journal Horticulture Research.
The study gives us insight into how cannabinoids work in natural systems and may help us develop new THC-matched hemp varieties that maintain this natural built-in defense against herbivores, said Dr. George Stack 19. 23, a postdoctoral researcher in the Smarts lab and first author of the papers.
Cornell’s hemp breeding program began in 2017 by evaluating different commercially available hemp varieties to see which are best suited to the local climate, soil and environment, so that recommendations can be made to farmers. Smart, Stack and colleagues observed that all cultivars originating from a breeding program in Ukraine were highly susceptible to Japanese beetles, while other cultivars were spared such predation.
At the end of the season, as we characterized the chemistry of those plants, we learned that the plants from the Ukrainian program did not produce any cannabinoids, Smart said.
Because cultivars that produced more cannabinoids experienced fewer predators, the team suspected that these compounds might act as a defense agent to protect hemp plants from insect damage, and designed experiments to test their hypothesis.
In tests using hemp plants with different concentrations of cannabinoids, researchers found that damage from leaf-chewing insects (cabbage looper larvae) was greater in leaves with lower levels of cannabinoids.
In the absence of cannabinoids, we saw severe insect damage, and in the presence of cannabinoids, we saw much less damage, Smart said.
In controlled laboratory feeding studies, researchers isolated CBDA and CBGA and added the extracts to artificial insect diets at various concentrations. Larvae grew smaller and had a lower survival rate as the concentration of cannabinoids increased, the paper said.
The Cornell program cannot work with plants high in THCA (the intoxicating compound found in marijuana) because of federal restrictions, so THCA as a pesticide was not tested in this research, Smart said.
The potential use of cannabinoids as pesticides is an exciting area for future research, but there will certainly be regulatory barriers due to the compounds’ pharmacological activity, and more studies are needed to understand which pests cannabinoids will be effective against, Stack said.
Future work will investigate whether cannabinoids inhibit sap-sucking insects such as aphids. Researchers are also investigating whether species from other cannabinoid-producing plant genera, such as the South African woolly umbrella plant (Helichrisum umbraculigerum), may also benefit from their insecticidal properties. If so, it would indicate an example of convergent evolution, where the same adaptation arose independently in different species at different times and in different locations.
Cornell co-authors, all at CALS, include Ping Wang, professor of entomology; Virginia Moore, assistant professor of plant breeding; Jocelin Rose, professor and chair of the Plant Biology Section in the School of Integrative Plant Sciences; Julie Hansen, senior research associate in the School of Integrative Plant Sciences; technicians Steven Snyder and Michael Quaid; postdoctoral researchers Glen Phillip and Jacob Toth; and Jamie Crawford, Research Support Specialist.
Other co-authors include John McKay, professor of plant genetics at Colorado State University; and Nick Jackovetz Ph.D. 11, former chief scientific officer at Cirona Labs.
The project was funded by the New York State Department of Agriculture and Markets through Empire State Development.
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