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Clover root borer

Hylastinus obscurus[4]

The clover root borer, Hylastinus obscurus, is a small bark beetle that lives in the roots of red clover rather than in trees. It is considered one of the most important causes of the decline of red clover stands, and no insecticide controls it.[1][2][3]

Group
Insect
Order
Coleoptera
Family
Curculionidae
US records
6 states
Sources
4 cited
Updated
2026-11-09

No licensed photo of this species is available yet; the identification notes below describe what to look for.

On this page
  1. Identification
  2. Where it lives
  3. Biology
  4. Damage and signs
  5. Health risk
  6. Control
  7. Related profiles
  8. Common questions
  9. Sources

How to identify it

  • Adults are tiny bark beetles, about 0.1 inch (2.5 mm) long and dark brown.[1]
  • The larvae are creamy white grubs found burrowing inside red clover roots.[1]

Where it lives

A pest of red clover that one research team describes as widely considered the most serious pest of the crop worldwide. It is covered in the Pacific Northwest Insect Management Handbook and is highly abundant in red clover plots at a research center in Chile's Araucanía region.[3][1][2]

US map: public records from 6 states since 1990 (GBIF).

Biology and life cycle

Adults become active as early as March and may disperse from March through August.[1]

The larvae burrow within the roots of red clover, and adults feed on the roots as well.[1][3]

Chemicals given off by the roots guide the beetles: in studies, a compound called E-2-hexenal from clover roots attracted them, while limonene repelled them.[2]

Damage and signs

  • Feeding inside the roots causes extensive damage, weakening and eventually killing plants and reducing stand density and longevity.[1][3]
  • Root damage also predisposes the plants to secondary diseases, particularly root rot fungi, which shorten the life of the stand further.[1]
  • Red clover is a perennial, but stands seldom last more than about three years, and the clover root borer is considered one of the main reasons. In Chile, red clover fields can reach infestation levels of about 70 to 100 percent after two or three years.[3][2]

Health risk

It is a root-feeding beetle of red clover. Neither the Pacific Northwest handbook nor the research papers describe any risk to people.[1][2]

Control

Prevention

Rotate crops. Rotational cropping is effective for keeping clover root borer down over time.[1]

Pesticides

No insecticides, registered or otherwise, are effective against this pest, and researchers report that chemical control has largely been unsuccessful.[1][3]

Common questions

Why is my red clover stand dying out after two or three years?
The clover root borer is a likely factor. Its larvae and adults tunnel in the roots, reducing stand density and opening the roots to root rot fungi.[1][3]
Can I spray for clover root borer?
No. The Pacific Northwest handbook states that no insecticides, registered or otherwise, are effective; rotating crops is the recommended approach.[1]

Sources

  1. Pacific Northwest Pest Management Handbooks (Oregon State, Washington State and University of Idaho Extension), “Clover hay - Clover root borer, Pacific Northwest Insect Management Handbook.” pnwhandbooks.org/insect/hay-pasture/clover/clover-hay-clover-root-borer (accessed 2026-11-09)
  2. Journal of Insect Science; PMC4014038, “Palma, Mutis, Isaacs and Quiroz 2013. Type and distribution of sensilla in the antennae of the red clover root borer, Hylastinus obscurus. Journal of Insect Science 13:133.” pmc.ncbi.nlm.nih.gov/articles/PMC4014038/ (accessed 2026-11-09)
  3. Molecules (MDPI); PMC6099530, “Espinoza, Urzúa, Bardehle, Quiroz, Echeverría and González-Teuber 2018. Antifeedant effects of essential oil, extracts, and isolated sesquiterpenes from Pilgerodendron uviferum on red clover borer Hylastinus obscurus. Molecules 23(6):1282.” pmc.ncbi.nlm.nih.gov/articles/PMC6099530/ (accessed 2026-11-09)
  4. iNaturalist, “Hylastinus obscurus taxon page (order and family).” www.inaturalist.org/taxa/221926 (accessed 2026-11-09)

Updated 2026-11-09. Built from sourced records. Spot an error? Tell us.