The European hemp borer, also known as the hemp borer or Eurasian hemp moth, is a highly destructive pest frustrating many hemp cultivators across the U.S. With so many farmers growing hemp for CBD oil, food products, or cosmetics, finding natural solutions to the hemp borer problem is necessary. Many chemical pesticides, which may be available to industrial and fiber hemp producers, are inappropriate for consumer goods like CBD oils and cooking oils.
The U.S. Environmental Protection Agency (EPA) approved ten additional hemp-appropriate pesticides in late 2019. But unfortunately, it’s not entirely known how these appear post-extraction. As there is zero-tolerance for pesticide residue post-processing, a process that naturally concentrates anything on or in the raw material is a risk.
Crop failure due to hemp borer infestation, and post-harvest rejection due to pesticide contamination, are not risks many cultivators are willing to take. For farmers growing hemp for consumables, natural and preventative measures are the only safe and viable ways to solve the hemp borer problem. Hemp cultivation continues to require significant upfront investment to get started.
About the European Hemp Borer
The Eurasian hemp borer (Grapholita delineana) is technically the hemp moth’s caterpillar (immature) stage. According to the Centre for Agriculture and Bioscience International (CABI), it is prevalent across Europe, Asia, and now North America. In The U.S., the hemp borer widespread in Illinois and most hemp-production states east of the Rocky Mountains. Other reports indicate it is also widespread in eastern Colorado.
Hemp borers are small, white caterpillars under one centimeter in size. They have a dark blackhead, and as they mature, their bodies transition into a noticeable orange-red hue.
The hemp borer larvae cause significant damage when they bore into the base of the hemp flower. As they move into the flower, it wilts and eventually dies. Hemp borers could additionally cause stunted and distorted growth along stems and branches. Thus, it’s a serious issue for farmers growing CBD and grain hemp cultivars and possibly a concern for those producing fiber.
Although an infestation may only become apparent post-harvest as caterpillars dislodge from the flowers during the curing process, they technically worm their way in the weeks leading up to harvest. If not dealt with, the larvae will overwinter in the stems and seed heads. By mid-spring, and the pupation stage complete, the moths emerge to mate. During the growing season, hemp moths may go through upwards of three regenerations, ending with the colder fall season.
Natural Solutions to the Hemp Borer Problem
Hemp cultivators must monitor harvests for signs of any hemp borer infestation from year to year. Even if the caterpillars did little to no damage to the current year’s flowers, careful assessment is warranted to prevent an explosion the next year.
During harvesting, curing, drying, and other processing, monitor the flowers for signs of larvae. As they may break free during handling, keep a close eye on machines, floors, and other surfaces that collect debris from processing raw hemp material.
Because caterpillars overwinter within the plants, cultivators need to remove all seed heads and flowers from the field. Most machine harvesting cuts the plants at the stalk base and leaves little material behind, so this step is typically easy to accomplish.
Colorado State recommends the placement of processing plants at least ½ mile away from cultivation sites. The distance will help reduce the risk of borers surviving on drying room floors, pupating, and infesting the local fields the following year.
There is also some speculation that hemp borers may survive in several localized weeds growing around and within hemp fields. Cultivators would be wise to mow all weeds before planting and again at harvest to ensure no field survival over the winter.
Ongoing Crop Monitoring
Because the hemp moth goes through several generations in a single season, ongoing field monitoring can help catch the problem well before the larvae begin munching their way through flowers and seed heads.
According to Colorado State, generally speaking, infestations begin at the edges of a field because the moths are poor fliers with the capacity to travel only short distances. Inspection around the edges of the fields regularly for moth activity and the subsequent destruction of any infested plants could limit the spread.
And while pheromone traps are not effective for hemp borer monitoring, sweep nets are of some use. Regular sweep net tests can collect samples of the current insect population and catch the hemp moth well before it infiltrates further.
Are There Safe Pesticides?
Unfortunately, hemp borers are a challenging pest to target with even conventional pesticides. Because the caterpillar lives within the stems, stalks, and flowers, spray-over pesticides often fail to do much damage. Even if these pesticides penetrate the flowers’ interior, they are not suitable for most growers working with cultivars destined for the CBD market.
Again, the EPA recently approved ten additional pesticides for hemp. A total of nine of these chemicals are bio-pesticides, including Bacillus thuringiensis varieties, which means derived from natural sources. Bacillus-type pesticides are pathogens harmful to insects like the hemp borer.
Are these bio-pesticides harmful to humans? Most research indicates no, but again, how the CBD extraction process affects this pesticide is unknown. Furthermore, should any hemp flower be destined for a final smokable or vape-ready form (flower and vape oils), there is absolutely no safety information available.
Preventative Measures Key to Naturally Combating the Hemp Borer
The cheapest and safest way to tackle hemp borer infestation for hemp is to tackle the issues as they appear at harvest time. These are low-tech solutions, but thus far, the only proven way. Stopping the lifecycle of the caterpillar between seasons is effective and simple. Combined with continued field checks in the spring and summer, post-harvest cleanup will catch localized infestations should they arise.
The hemp borer’s habits may protect it from chemical sprays (natural or not), so the hemp cultivator is best served by ongoing preventative measures and crop monitoring.