The hemp borer, primarily the Eurasian hemp borer (Grapholita delineana), represents a serious challenge to hemp and cannabis cultivation across North America, particularly east of the Rocky Mountains. This pest’s larval stage tunnels into the plant’s stems and buds, causing significant structural damage and yield loss in crops grown for fiber, seed, or flower production. Managing this pest requires a multi-faceted approach combining timely identification with prevention and targeted eradication methods.
Recognizing the Hemp Borer and Its Damage
The damaging stage of the hemp borer is the larva, a small caterpillar that is difficult to spot before it has entered the plant. Newly hatched larvae are minute, cream-colored with a distinct dark, often black, head capsule. As the larvae mature, they typically reach a size of 6 to 8 millimeters and can develop a noticeable reddish-orange coloration, especially in their final overwintering stage.
The most recognizable symptom of an infestation is the physical damage the larvae inflict by tunneling into the plant’s vascular system. Larvae bore into the stalk, often entering near a leaf axil, causing swelling or a bulge in the stem. Growers may also observe an entry hole with sawdust-like excrement, known as frass, protruding from the stem.
Internal feeding disrupts nutrient and water flow, leading to wilting, dieback of terminal growth, and split stems. Later in the season, the borer targets developing flower buds, tunneling into the base of the bud, which can cause the entire bud to collapse or die. Since the larvae spend most of their life protected inside the stem, early and accurate diagnosis based on these symptoms is essential for control.
Proactive Strategies for Prevention
Prevention is the most effective approach to managing hemp borers, especially because the larvae are protected inside the stem for most of their life. Adult moths are not strong fliers, meaning infestations often begin at the edges of a field or near last season’s crop residue. This localized movement makes sanitation and field management essential for disrupting the pest’s life cycle.
A primary strategy involves meticulous sanitation post-harvest, as the full-grown larvae overwinter inside crop debris and stems. All plant stalks and stubble must be removed from the field and destroyed, either by deep plowing or burning, to eliminate these overwintering sites. This practice significantly reduces the number of moths that can emerge and infest the next season’s crop.
Weed control is another important cultural practice, as the hemp borer can utilize alternate host plants, such as knotweed and other weeds in the buckwheat family, to sustain its population. Removing wild hosts and maintaining a weed-free perimeter around the cultivation area helps to reduce the overall pest pressure near the crop. Furthermore, rotating hemp fields with non-host crops can help break the cycle of infestation that occurs when moths emerge directly into a new hemp crop planted in the same location.
Regular and timely scouting is necessary to catch the first generation of moths and newly hatched larvae. Monitoring moth activity with sweep nets or black light traps allows growers to estimate population levels and time control measures precisely to the egg-laying period.
Targeted Eradication Methods
Eradication efforts must be perfectly timed to target the brief period when the newly hatched larvae are exposed on the plant surface before they bore into the stalk. Once the larva is inside the stem, chemical sprays are generally ineffective due to the sheltered location. Therefore, a combination of biological and approved chemical treatments is necessary for active control.
Biological Controls
The most effective biological control involves the timely release of parasitic wasps, specifically Trichogramma species. These minute wasps are commercially available and act as seek-and-destroy agents by parasitizing the hemp borer eggs, preventing them from hatching into destructive larvae. Releases must be synchronized with the hemp borer moth’s egg-laying periods, which requires careful monitoring of adult moth activity.
These releases should begin proactively at the first sign of moth flight and continue every five to seven days throughout the moth’s active egg-laying cycle. Additional beneficial insects, such as assassin bugs, can also be encouraged in the growing area, as they prey on the eggs and young larvae. Using biological controls reduces reliance on sprays, which is particularly beneficial for flower-producing crops where residue is a concern.
Chemical/Approved Treatments
For active eradication, Bacillus thuringiensis (Bt) formulations are the most widely recommended and approved microbial insecticide for hemp. Bt is a naturally occurring soil bacterium that, when ingested, paralyzes the digestive system of the caterpillar, causing it to stop feeding and die. Specific subspecies, such as B.t. kurstaki and B.t. aizawai, are labeled for use against lepidopteran pests like the hemp borer.
The application of Bt must occur when the larvae are actively feeding on the leaf surface, immediately following egg hatch and before they bore into the stem—a very narrow window. Repeated applications on a five to seven-day schedule may be necessary during peak egg hatch, especially for later generations. Neem oil is another contact-and-ingestion treatment that can be alternated with Bt applications to manage early-stage larvae and prevent resistance. Targeting perimeter areas where infestations concentrate provides a more efficient use of treatments.