Problem and Description of Damage
The Euroasian hemp borer (EHB), Grapholita delineana, was found in Lexington in mid-October 2020 while scouting hemp pests in Lexington. This is the second EHB sighting in Kentucky conducted by Raul Villanueva; the previous report was from Oldham County in June 2019; however, it may have spread further in Kentucky.
The larva of this invasive moth bores into the stem of hemp plants, making tunnels (Figure 1). It can also feed on leaves and, later in the season, attack seeds and flowers. The Eurasian hemp borer has been documented as a pest of economic importance in hemp grown for fiber. One larva may consume on average of 16 seeds. Furthermore, it is estimated that 10 larvae per plant can cripple growth and seed production.
Distinctive characteristics of the EHB are pinkish white to pale brown larvae with several barely visible pale bristles (setae) per body segment. Eurasian hemp borer caterpillars are small (7 to 8 mm) compared with corn earworm (20 mm). Earlier larval instars are difficult to distinguish from the background; they are whitish or creamy colored (Figure 2a). It may have 2 or 3 generations in Kentucky. The head is yellowish to brown and can be dark (Figure 2b) with black ocelli (light detecting simple eyes). The late larval instar in the fall is the overwintering stage and can have an orange reddish coloration (Figure 2b). This stage passes the winter in well sheltered spaces (folded leaves or tunneled stems). Tunneling may cause some stunting and distortion of stems and stalks (Figure 3). Adults emerge during spring, and after mating, they start to lay eggs. In the absence of hemp, they can feed on hops and knotweed (Polygonum) and probably other weeds. Adults (Figure 4) are small moths, displaying white stripes on their front edges and four chevron-like stripes near the center.
European hemp borer could pose a great risk to the hemp industry; previous reports showed that this insect caused large amounts of damage to hemp fields in Southeastern Europe in the 1960s. They are commonly spread to new locations via infested hemp seeds or debris. It has also been noted that wild hemp in Vermont seems to have innate resistance to the pest compared to that of wild hemp in the midwestern U.S. This makes the latter interesting progenitors in any hemp breeding program to develop new cultivars with improved resistant to this pest.
Pruning, cleaning, and disposing of infested plants should be the first steps to manage EAB as this is an insect that lives in stems. This is especially important if hemp is replanted in the same field the following year. Removal of wild weeds is also recommended.
Insecticide applications are challenging as EAB immature stages live inside stems. Furthermore, systemic insecticides are not registered in Kentucky for hemp. Bacillus thuringiensis var. aizawai (Agree®) and nuclear polyhedrosis virus are registered for control of caterpillar in hemp; however, applications of these products will not reach EAB larvae.
Eurasian Hemp Borer, Colorado State University
Insects in industrial hemp production in Michigan, Michigan State University
By Raul T. Villanueva, Entomology Extension Specialist and Caleb Whitney, Entomology Laboratory Technician