The Japanese beetle (Popillia japonica) is potentially significant defoliator of hemp and also feeds readily on hemp flowers. A new profile in the open-access Journal of Integrated Pest Management offers a current synopsis of existing research on insect and arthropod pests of hemp and notes where future research is needed. (Image originally published in Cransaw et al 2019, Journal of Integrated Pest Management)

By Marguerite Bolt and Amanda Skidmore, Ph.D.

Hemp (Cannabis sativa) is a novel crop recently legalized for commercial production in the United States via the 2018 Farm Bill. Reasons for hemp production can be separated into three different categories: grain, fiber, and the compound cannabidiol (CBD). Each production system needs to be evaluated to decide which pests will require management strategies to keep damage below economic thresholds.

In particular, one important need is to determine if insect feeding can increase tetrahydrocannabinol (THC) production, which is a very real concern, because the THC content in a hemp crop must remain below a threshold set at 0.3 percent on a dry-weight basis.

As expected with a new crop, gaps exist in the literature surrounding all aspects of hemp production. Our colleagues in this field recognized the need for an updated synopsis of arthropod pests that have been observed on hemp during various research trials. A group led by Whitney Cranshaw, Ph.D., professor and extension specialist at Colorado State University, with colleagues at CSU, Virginia Tech, the University of Vermont, and the University of Tennessee, have assembled such a profile, published last week in the open-access Journal of Integrated Pest Management.

Integrated pest management (IPM) plans for pests of hemp are still being developed because arthropod pests are still being identified and described. Meanwhile, for insects already recognized as hemp pests, research on cultural, mechanical, and biological control strategies are still needed. Not all the arthropods in the hemp agroecosystem are causing apparent economic impacts, and therefore many would not require any management considerations. Corn earworm (Helicoverpa zea), Eurasian hemp borer (Grapholita delineana), hemp russet mite (Aculops cannibicola), and cannabis aphid (Phorodon cannabis) are pests that can cause economic loss for growers in some regions of the U.S. Pest management options include natural enemies, removing volunteer hemp plants, and rotating hemp with other crops. Some states have allowed the use of biopesticides (such as Bacillus thuringiensis var. aizawi or the Helicoverpa armigera nucleopolyhedrovirus), but growers should review their state’s regulatory rules to determine if this is a permissible option.

The passage of the 2014 Farm Bill in the United States paved the way for the return of commercial hemp farming. (Image originally published in Cransaw et al 2019, Journal of Integrated Pest Management)

Corn earworm (Helicoverpa zea) is a highly damaging pest of hemp, particularly in the large flowers of cannabidiol cultivars. (Image originally published in Cransaw et al 2019, Journal of Integrated Pest Management)

Various species of stink bugs have been observed in hemp, such as the redshouldered stink bug (Thyanta custator). (Image originally published in Cransaw et al 2019, Journal of Integrated Pest Management)

Various species of Lygus bugs are commonly found feeding in hemp. (Image originally published in Cransaw et al 2019, Journal of Integrated Pest Management)

Cannabis aphid (Phorodon cannabis is the most common aphid found in hemp crops. (Image originally published in Cransaw et al 2019, Journal of Integrated Pest Management)

However, until repeated economic-loss-valuation studies are conducted and economic thresholds and injury levels are developed, IPM recommendations for hemp pests are still in their infancy. This article in JIPM is important for the scientific community to gain an understanding of the current knowledge of hemp agroecosystem research.

The long gap in large-scale industrial hemp production followed by a surge in production over the last five years has resulted in a lack of knowledge that puts hemp growers in a difficult position when developing new pest management practices. Although some states have a list of allowable pesticides for use in hemp, currently no pesticides are broadly labeled for hemp pests in the U.S. Thus, a unique opportunity exists to focus on cultural, mechanical, and biological control methods as pest pressure increases, which is especially important for hemp that is grown for consumable and medical use (specifically hemp oil and grain cultivars and CBD cultivars).

Some current and prospective hemp growers are new farmers and may be alarmed by the number of insects associated with hemp. This paper debunks the misconception that arthropods are not a potential threat to hemp production. Not all arthropods found in hemp fields are likely to cause economic damage, but monitoring for pest populations, damage, and yield is necessary. Some hemp farmers (especially CBD growers) may panic when they observe damage to their plants—these growers have invested a lot of time and money into their crops, and insect damage can potentially cause crop losses. The more research that can be done to identify which arthropods are causing measurable yield loss, the more we can help growers feel confident they will not suffer a loss when minor feeding damage is observed on their plants.

Developing action and economic thresholds and explaining their value to producers will be a key step in hemp IPM. Additionally, because this crop will be produced across the country, it will be important to adapt such hemp IPM materials for varying regional conditions. Some arthropod pests may not be as concerning in some regions but could cause serious damage in other regions.

Marguerite Bolt, M.S., is the first-ever hemp extension specialist at Purdue University in West Lafayette, Indiana. She is working on connecting current and prospective hemp growers with researchers and processors to help build the hemp industry in Indiana while providing fact-based information on hemp production. Instagram: @margueritebolt. Twitter: @HempMarguerite, @PurdueHemp. Website: www.purduehemp.org. Email: hemp@purdue.edu.

Amanda Skidmore, Ph.D., is an assistant professor at New Mexico State University in Las Cruces, New Mexico. She is currently the NCB Representative to the ESA Diversity and Inclusion Committee. Instagram: @Dr_Skidmore. Twitter: @Dr_Skidmore. Email: amanda.skidmore@gmail.com.