Black vine weevil

Near Adult emergence

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

Order
Coleoptera
Type
root-feeder
Host Plants
1
GDD₃₂ Adult emergence
2,319
Indicator: American yellowwood first bloom
Peak Activity
Overwintered adults active 148-400 GDD50 (late March through May). New-genera...
Damage Severity
lethal
Now: Larval Feeding & PupationModerate Risk

Larvae resume active feeding as soil warms, accelerating root damage. Pupation begins in late spring. Plants stressed by winter larval feeding may show unexplained wilting. Check declining plants by pulling root balls.

Next: Adult Emergence

Black vine weevil Adult emergence typically begins around 2319 GDD₃₂. As of May 13, 2026, Puget Sound stations range from 1906.2 to 2098.2 GDD₃₂, approximately 221 units before the expected threshold.

Regional Season Tracker

GDD₃₂ accumulation across 7 Puget Sound stations · as of May 13, 2026
Station GDD₃₂ Current Stage Next To Go
Issaquah / East King 2,098 Pre-season Adult emergence 221
Kent / Auburn 2,089 Pre-season Adult emergence 230
Seattle / UW 2,063 Pre-season Adult emergence 256
Olympia / Tumwater 2,025 Pre-season Adult emergence 294
Tacoma / Puyallup 1,993 Pre-season Adult emergence 326
Bellingham / Whatcom 1,972 Pre-season Adult emergence 347
Sequim / Rain Shadow 1,906 Pre-season Adult emergence 413
Regional estimate

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. Wisconsin Extension: Wisconsin Extension (landscape pest table). About GDD₃₂ →

Monitoring & Action

How to Monitor

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.

When to Act

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-nursery infestations are typically managed preventatively once black vine weevil is established.

What Damage Looks Like

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

Host Plants (1)

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.

— Chris Welch, ISA Certified Arborist

Sources & References

Primary: PNW Insect Management Handbook

Data Maturity
Structured Multiple sources. Expert review underway.