Eurasian hemp borer, Grapholita delineana
Order: Lepidoptera
Family: Tortricidae
Description
Eurasian hemp borer is a species of moth widespread in eastern Colorado. The moths have white lines on dull, brown, and gray wings and a wingspan of roughly 1.3 cm (0.5 inches). At rest, the wings are held in a tent-like position over the back of the abdomen. The eggs are small, light in color, and laid singly on hemp.
Young caterpillars are small, cream colored and have a dark head. Late-stage caterpillars are red and orange. These caterpillars reach lengths of 6-8 mm (0.2-0.3 inches) when fully grown. Older caterpillars are much easier to see than young caterpillars due to their larger size and color.
Quick Facts
- Eurasian hemp borer is a species of moth. Caterpillars are the damaging life stage of this pest; they tunnel into developing buds of hemp plants which can reduce yield.
- In addition to hemp, this pest has been reported developing in hops and knotweed. Although alternative hosts of Eurasian hemp borer have not yet been identified in Colorado, the presence of alternative hosts near hemp fields seems to sustain populations of this pest.
- Effective cultural control is an important component of Eurasian hemp borer management. Chemical control is challenging due to the pest’s life history traits.
Larva of Eurasian hemp borer inside hemp plant. Eurasian hemp borer is a species of moth that is only present east of the Rocky Mountains in Colorado. The caterpillars bore into hemp plants where they feed and develop. There are several symptoms of feeding injury plants including stem swelling, wilting, and dieback. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult of Eurasian hemp borer. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Late-stage larva within hemp plant. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Pupa of Eurasian hemp borer in hemp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Stem swelling and protruding frass on hemp stem. Both symptoms can be used as indicators of a Eurasian hemp borer infestation. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Leaf wilting due to feeding of Eurasian hemp borer. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Eurasian hemp borer overwinters as fully grown caterpillars in stems near the top of the plant and leaf folds around seed heads. Pupation begins in the spring, and adults emerge later in the spring. After mating, female moths lay several hundred eggs over a couple of weeks.
Newly emerged caterpillars are extremely small and feed on the leaf surface. The caterpillars then bore into plant stems where they remain until maturing into moths. There are multiple generations of this pest each growing season, and the most injurious generation of caterpillars are produced later in the summer. It is this generation of Eurasian hemp borer that overwinters in the hemp plants.
Alternative hosts of the borer have not yet been identified but appear to be important in sustaining populations of this pest when in proximity to hemp fields. The moths are also not strong flyers, and therefore heavy infestations are expected to occur close to wild hemp or in fields where hemp was grown in previous years.
Injury
Caterpillars bore into the base of developing buds which causes stems to wilt, swell, and eventually die. In mid to late summer the larvae tunnel into newly emerged flower buds and stems, causing stunted growth, additional dieback, splitting of stems, abnormal seed production, and possibly death of the plant. In addition to the physical symptoms described above, feeding injury of Eurasian hemp borer can reduce yields.
Monitoring
The best way to monitor Eurasian hemp borer is to scout fields for moths or by sampling with a sweep net. Black lights can also be used to monitor adult activity; however, they can be costly and will also attract many other moth species.
Cultural control
All crop debris, especially stems and stalks, should be removed and destroyed to eliminate overwintering larvae. Removal of wild hosts is likely to be important, however they have not yet been identified in eastern Colorado. Drying sheds can also serve as overwintering sites and should therefore be positioned at least ½ mile away from crops. Lastly, removing wild hemp and planting new crops a significant distance from the previous year’s crop can also help limit densities of Eurasian hemp borer.
Chemical control
The use of foliar insecticides for managing this pest is challenging because larvae feed and develop within the plant. The only time larvae are exposed to foliar insecticides is briefly after hatching when newly emerged larvae feed on the surface of leaves. Therefore, timing applications with egg hatch is important.
When insecticides are used, only the edges of fields need to be treated since Eurasian hemp borer is a weak flier and infestations tend to begin at field edges.
References
Bolt, M. 2020. Be on the Lookout for Borers. Purdue University: Entomology Extension Newsletter. Available https://extension.entm.purdue.edu/newsletters/pestandcrop/article/be-on-the-lookout-for-borers/
Colorado State University. 2018. Eurasian Hemp Borer. Colorado State University. Available https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Eurasian%20hemp%20borer%20September%202018%20rewrite(1).pdf
Groves, R. and Jensen, B. 2020. Insect and Mite Pests of Field Grown Hemp in Wisconsin. University of Wisconsin-Madison: College of Agricultural Life Sciences. Available at ipcm.wisc.edu
Utah State University. (n.d.). Eurasian Hemp Borer. Utah State University: Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/hemp-eurasian-hemp-borer