Hemp (Cannabis sativa) production is on the up worldwide, with many possible uses for the crop, beyond production of cannabinol products.
One novel idea is to repurpose some of the plant’s chemical components as organic biopesticides.
According to research, hemp-derived essential oils could be applied against a range of well-known pests across crop types, such as the Egyptian cotton leafworm (Spodoptera literalis), tobacco hornworm (Manduca sexta) and aphid species including the peach-potato aphid (Myzus persicae).
They have also been shown to be effective against insects posing health risks, such as mosquito species from the genus Anopheles and the housefly, Musca domestica, as well as pests of stored foods such as the Indian meal moth (Plodia interpunctella) and Callosobruchus chinensis, a type of bean weevil.
Not only that, but a study by international researchers has demonstrated the effectiveness of hemp essential oil against problem species affecting livestock, the poultry red mite (Dermanyssus gallinae) and Hyalomma dromedarii, a tick that primarily affects camels but also other domesticated mammals.
Evidence versus stigma
So, what’s holding back hemp-based crop protectants? Genís Oña from International Center for Ethnobotanical Education, Research and Service, who led a review of evidence on the use of C. sativa in pest control, thinks that resistance comes from a source outside science.
“While we were performing this systematic review, even some colleagues with which we shared this work stated that no one will be interested in a cannabis-based product for their vegetables. But this is supported from a moral standpoint, not technical nor scientific,” he said.
“We have robust evidence, as we showed with this review, about the strong efficacy of Cannabis sativa for pest control. There is an absence of investment for moral reasons and this is why we do not still have a cannabis-based product for agriculture.”
The work by Oña’s team narrowed down the most promising techniques for the extraction of pest-repelling cannabinoids and terpenoids: using upper clusters of flowers, in fresh, not, dried, form — while suggesting that extraction methods need to be improved through further exploration of novel approaches.
Whether this additional research gets done in a timely manner is another matter.
“Here we have a potentially-powerful product to fight against pests in a sustainable manner. In addition, the plant grows easily, produces high amounts of essential oil, and other parts of the plant not used for the extraction of essential oil can be used for many other purposes,” Oña said.
“I am not sure about the actual appetite of researchers for addressing these [outstanding] questions, but if one is clever enough, the appetite would be like post-cannabis munchies.”
More appeal in developing world?
There has been some interest from industrial hemp producers in making pesticide products from by-products of their extraction, says Murray Isman, PhD, of the University of British Columbia, but no major moves towards commercialisation.
He believes that such products, and botanical pesticides more generally, are more likely to be of interest to growers in the developing world, where higher crop losses and cosmetically-imperfect produce are more tolerated than in North America, for example. Subsistence farmers also suffer disproportionate numbers of poisonings through traditional pesticides, he noted.
“Some growers in North America and Europe may be reluctant to switch over to biopesticides because they are used to the consistency of conventional synthetic pesticides and their general ease of use,” he said.
“Biopesticides are sometimes less efficacious, or require more frequent application, and often have certain conditions around their use, so growers may need specific education or knowledge to use them optimally.”
Isman also believes a stigma against cannabis-derived products could exist, but that this is based on a misunderstanding of the chemistry involved.
“The types of extracts being proposed would not contain any THC nor CBD – there are much more lucrative markets for those derivatives in any case,” he continued.
“Pesticides based on hemp extracts would likely be based on terpenoids such as caryophyllenes that can be found in the essential oils obtained from many other types of plants.”
What’s eating hemp?
The boom in C. sativa cultivation has also highlighted that the crop has plenty of pests of its own. Clearly, ones for which the plant’s chemical components have little impact on heath or appetite.
The list of bugs found by researchers on hemp crops in the US alone is substantial, including the likes of thrips, aphids, beetles, moths and even fire ants. Canadian work showed that onion thrips (Thrips tabaci) alone can result in reduced yields of indoor-grown hemp.
Mites are a particularly notorious pest group affecting cannabis cultivation —a fact that could seem to be slightly running at odds with its reported potential as an agent against various mite species. The two-spotted spider mite (Tetranychus urticae) is perhaps the most famous, with its feeding affecting the chemicals expressed by hemp plants.
While generalist pests often find cannabis to their liking, there are a number of specialists too, such as the hemp russet mite (Aculops cannibicola) and cannabis aphid (Phorodon cannabis).
Beyond the invertebrates, there are also several fungal pathogens also posing major headaches for growers, such as powdery mildew and bud rot, as well as bacteria and viruses of concern. All in all, pests are costing C. sativa growers over a billions dollars a year in the US alone, suggests Matthew Gates, integrated pest management specialist for Zenthanol Consulting.
More understanding of threats needed
At present, engagement with pest management lags behind investment in the industry, Gates said.
“[Growers] might be very excited personally and professionally, they might be focusing on what what’s going to happen and what they want to happen, but they’re not focusing on the realities of biosecurity, and that’s partly because there hasn’t been historically a lot of infrastructural support, at least for the last several decades,” he said.
“There’s also a reticence and a mistrust of some of the organisations that would normally be interested in helping that along, like universities and government organisations, so this recalcitrance has perhaps caused a festering lack in an understanding of what the true threats could be.”
The good news for growers hoping for natural solutions to prevent losses is that the list of potential natural enemies confirmed through research efforts is similarly lengthy to those featuring the crop’s potential pests.
Scientists in Greece have also shown that in greenhouse cannabis production, the strategic release of commercially-reared natural enemies can provide very high levels of protection against spider mite, aphids, thrips and whitefly.
But while the use of biocontrols is the most well-known aspect of IPM to growers, there are other, lesser-used options available, such as biopesticides, insect-killing viruses, and the use of pheromone-releasing technologies to disrupt mating behaviours.
Flower focus
An industry-specific issue in regard to any pest control option is the need to not damage or foul the flowers of the crop in any way, given they are the main economically-valuable product.
Efforts to control an emerging pest of industrial hemp, the corn earworm (Helicoverpa zea) is illustrative of this centrality of the flower. IPM-friendly products such as Bacillus thuringiensis (Bt) or the nuclear polyhedrosis virus are available, but timing is critical to control the pests affordably, Gates explained.
In a recent case in Michigan, H. zea populations were appearing after the winter, but without pheromone traps to monitor their populations, timing preventative Bt applications was difficult, as the aim is to kill the pest before it gets in the flowers, he said. If it does, control becomes much harder, and even if successful might leave decaying dead insects right where you don’t want them to be.
Routes to improved practices
A fundamental grasp of what is likely to threaten crops, as well as a good biosecurity policy, are the bedrocks to minimising losses, Gates stressed. Then, effective crop scouting for the presence of pests should underpin any control efforts, with technologies for managing and visualising data having the potential to make this even more meaningful.
“When you’re in an offseason, you can then take all that information and assess what you improved upon and what you did not improve upon,” he added.
“That information is very crucial, and not only for cannabis, but also traditional crops. I feel like that is often collected and left without being assessed, and it doesn’t really matter if you take all that data if you never use it.”
