Black vine weevil
Otiorhynchus sulcatus
1 host plant
Last updated
Notched leaf margins on rhododendrons, yews, and other broadleaf evergreens are the calling card of black vine weevil adults feeding at night. The real damage happens underground, where white, C-shaped larvae devour roots from fall through spring. You will see plants wilting or declining despite adequate water, sometimes collapsing entirely when root systems are destroyed. This is one of the most damaging root-zone pests in home and landscape settings.
Apply entomopathogenic nematodes (Heterorhabditis bacteriophora) as a soil drench in September when larvae are small and soil temperatures are above 55 degrees. Sticky barriers around container rims trap flightless adults. Hand-pick adults at night with a flashlight. Avoid piling mulch against stems where adults hide during the day.
Quick Reference
Black vine weevil Adult emergence typically begins around 2319 GDD₃₂. As of April 3, 2026, Puget Sound stations range from 1070 to 1180.6 GDD₃₂, approximately 1138 units before the expected threshold.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,181 | Pre-season | Adult emergence | 1,138 |
| Seattle / UW | 1,171 | Pre-season | Adult emergence | 1,148 |
| Kent / Auburn | 1,111 | Pre-season | Adult emergence | 1,208 |
| Olympia / Tumwater | 1,106 | Pre-season | Adult emergence | 1,213 |
| Bellingham / Whatcom | 1,101 | Pre-season | Adult emergence | 1,218 |
| Tacoma / Puyallup | 1,075 | Pre-season | Adult emergence | 1,244 |
| Sequim / Rain Shadow | 1,070 | Pre-season | Adult emergence | 1,249 |
Source: UMass Extension / Cornell Cooperative Extension: 148-400 GDD50 (March 1 start) for overwintering adult activity. UMass Extension (Robert Childs): 1100-1665 GDD50 for new generation adults. UMD IPMnet catalog value of 607 GDD50 likely represents mid-range of new adult emergence (between overwintered and new gen). The 148 is the LOW END of overwintered adult activity range, not a point estimate. Verified 2026-03-20. Sources: UMass: UMass Extension; Cornell: ccetompkins.org. Additional sources: Rutgers: plant-pest-advisory.rutgers.edu (148-400 GDD50 confirms UMass/Cornell range); Penn State Extension: Penn State Extension. About GDD₃₂ →
Monitoring & Action
Scout for leaf notching on susceptible hosts from May onward. For confirmation, check at night with a flashlight (adults feed after dark). Place burlap or cardboard traps at the base of plants as daytime refugia; check in the morning for hiding adults. To assess larval populations, pull a declining plant and examine the root ball for white C-shaped grubs.
Adult leaf notching alone rarely warrants treatment. Intervention is triggered when plants show unexplained wilting, decline, or death, and root inspection reveals larval feeding. Container nurseries should treat preventatively when black vine weevil is established.
Adult feeding produces distinctive marginal notching on leaves, cutting irregular, crescent-shaped bites from the leaf edges. This foliar damage is primarily cosmetic and rarely threatens plant health. The real damage is underground: larvae feed on roots and cambium at the base of trunks and crowns, often girdling the root collar. Heavily infested plants wilt, decline, and may die without any above-ground cause being obvious until the plant is pulled and the destroyed root system is revealed. Container plants are especially vulnerable to sudden collapse from larval root feeding.
Cultural Controls
- Hand-pick adults at night with a flashlight
- Apply beneficial nematode soil drenches
- Reduce hiding places near susceptible plants
- Sticky barrier bands on trunks and container rims
- Inspect container stock carefully before purchase
Black vine weevil thrives in the Puget Sound lowlands. The mild, moist climate suits both the adult (which requires humidity) and the soil-dwelling larvae. Rhododendron, pieris, and yew are staple landscape plants here, providing abundant hosts. In Kent's clay soils, soil moisture remains high enough for nematode application through October. Apply Steinernema kraussei as a soil drench in late August through September when soil temperatures are above 55F for best results. The flightless nature of adults means infestations spread slowly between properties; new introductions almost always arrive via nursery stock.