Gardening Harmful insects

Dealing With Black Vine Weevil

Rhododendron leaves showing black vine weevil damage.

By Larry Hodgson

Here’s a pest you rarely see, but whose damage is quite obvious: notches cut into the leaf margin of such plants as rhododendrons, euonymus and bergenia. Unlike leafcutter bee damage (see What’s Cutting Round Holes in My Plant’s Leaves), the notches are not round, but irregular. On some plants (yews, for example), the leaf is not just notched, the tip of the leaf or even the entire leaf is sometimes clipped off. 

The culprit is a weevil (Otiorhynchus sulcatus), called the vine weevil since it attacks grape vines or black vine weevil because the adult is black, but also yew-root or taxus-root weevil, as the larvae often feed on yew (Taxus) roots. It actually has a very wide host range of several hundred plants. Here are a few genera it is known to feed on:

  1. Acer (maple)
  2. Arisaema (jack-in-the-pulpit)
  3. Aster, Eurybia, Symphyotrichum, etc. (aster)
  4. Astilbe (astilbe)
  5. Bergenia (bergenia)
  6. Camellia (camellia) 
  7. Cyclamen (cyclamen)
  8. Echinacea (echinacea, purple coneflower)
  9. Euonymus (euonymus or wintercreeper)
  10. Fragaria (strawberry)
  11. Heuchera (heuchera, coral bells)
  12. Hosta (hosta)
  13. Humulus (hops)
  14. Hydrangea (hydrangea, hortensia)
  15. Impatiens (impatiens)
  16. Kalmia (mountain laurel)
  17. Lilium (lily)
  18. Mentha (mint)
  19. Phlox (phlox)
  20. Primula (primrose)
  21. Rhododendron (rhododendron and azalea)
  22. Rubus (raspberry)
  23. Saxifraga (saxifrage)
  24. Solanum (potato)
  25. Spiraea (spirea)
  26. Syringa (lilac)
  27. Taxus (yew)
  28. Tsuga (yew)
  29. Vitis (grape vine)
  30. Wisteria (wisteria)

Recognizing a Vine Weevil

Adult black vine weevil. Photo: Udo Schmidt, Wikimedia Commons

The adult is a rather curious-looking beetle, with a snout and an oval shape, including distinctly grooved and pitted elytra (wing cases) and measuring about 3/8 to ½ inches (7.5 to 11 mm) in length. It’s black to slate gray in color and peppered with small yellow marks. Also, it has distinctly hinged antennae (they’re said to have elbows). Oddly, the elytra are fused together, so the weevil can’t fly, although it walks very quickly. Largely nocturnal, it is usually seen on leaves and stems.

All specimens found so far have been females: it apparently reproduces uniquely through parthenogenesis.

Black vine weevil larvae. Photo: Wikimedia Commons

The larva is found underground feeding on plant roots. Legless, C-shaped and creamy white, up to 1/3 inch (1 cm long), it has a light brown head.

Panicle hydrangea leaves damaged by black vine weevils. Photo: Salix, Wikimedia Commons

That said, you’re more likely to recognize a black vine weevil by the damage it causes than by seeing the insect in either phase. The notched leaves created by feeding adults are very obvious, although more esthetically disturbing than truly harmful to the plant, as most damaged leaves remain on the plant and continue to carry out photosynthesis. They also damage some flowers, cutting notches into them like they do to leaves.

The real damage is underground, though, caused by the larvae feeding on roots, crowns and stem cambium. They can girdle stems and kill roots, weakening their host plant and even, although quite rarely, killing it. 

Distribution

Native to Europe, this pest is now widely distributed in temperate areas all over the world, including Argentina, Japan, Tasmania and New Zealand. It was first seen in the United States in the early 1830s and has since spread throughout most of the US and Canada as well as the highlands of Mexico. 

It often spreads not only on its own (and adults can cover dozens of yards [meters] per day), but also hitches rides on vehicles and is carried from place to place on nursery stock, as larvae are often found in container plants. 

Life Cycle

All black vine weevils are females. Photo: Martin Cooper, Wikimedia Commons

There is one generation a year. 

Females emerge from their overwintering spots in the soil in the spring and begin feeding on leaves at night. After a few weeks, they start laying round, pale orange eggs in the ground or in leaf litter at the base of host plants. Each can lay up to 800 eggs over her 3-year life span. 

The larvae soon hatch and begin feeding on plant roots, rootlets, crowns and cambium. This damage can lead to root rot diseases more serious than the original infestation. The larvae go through 6 instars, increasing in size as they go, overwintering as late-stage larvae or white pupae. These become adults in the spring and the cycle repeats.

In the fall, these weevils sometimes move indoors, either traveling on plants brought in for the winter (see Bring Your Plants Indoors… Without the Bugs to learn how to prevent that) or, seeking warmth, through open doors, windows or other openings. They can even hitch a ride on your clothing when you come in from the garden in the late fall. They’re pretty much harmless indoors (unless they feed on some of your houseplants), but their presence is annoying and is best prevented by sealing any cracks or spaces they can crawl in through, as that remains their main way of getting inside your home.

Lookalike

Strawberry root weevil. Photo: Line Sabroe, Wikimedia Commons

If you’re finding similar damage, but the weevil is smaller and brown to black, plus a bit fuzzier, it’s probably the strawberry root weevil (Otiorhynchus ovatus), also widespread. It’s damaging to strawberries, as the name suggests, but also raspberries, rhododendrons, grapes and mints, among others. Much of the information about the black vine beetle applies to this pest as well.

There are other similar weevil species: you don’t have to make a definite identification to learn how to treat them

Control

In the home garden, learning to tolerate moderate black vine weevil damage it is actually far easier than trying to treat them. Even among theoretically susceptible host plants, some suffer less damage than others and should be preferred in order to reduce the population.

Given their nocturnal habit, adults are rarely seen. Try going out with a candle at night and inspecting plants showing recent leaf damage. (Avoid bright light like a flashlight or mobile phone light, as that causes them to drop instantly to the ground and hide.) You can then handpick and destroy them (you could drop them in soapy water, for example, or crush them under your shoe).

Another treatment method is to place a white sheet or rag (white so you can better see them) under an infested plant at night and shake its branches. They’ll react by dropping off and you can then just pick up the sheet and dispose of them.

Tanglefoot glue being applied to a tree trunk. Photo: amazon.ca

You can also trap adults by wrapping the base of infected plants in burlap or tape, then painting it with Tanglefoot or some other sticky, non-drying glue. Since the adults can’t fly, but must crawl between their food source (leaves) at night and their hiding places (soil or litter at the base of the host plant) during the day, then climb back up the plant at night, such a sticky trap can stop them in their tracks.

You can also try spraying plants at night with neem, insecticidal soap or another insecticide product. You’ll certainly need to repeat: one treatment will not get all of them. More persistent insecticides with a longer period of effectiveness as well as systemic ones (that are absorbed by the plant and cause it to become poisonous) used to be available, but have largely been removed from the market for safety reasons.

Where larval damage is severe, you may find it necessary to treat with beneficial (parasitic) nematodes, usually Heterorhabditis spp. or Steinernema spp., available online and in garden centers. They’re applied by watering them into the soil in midsummer or fall when the soil is at an appropriate temperature (at least 60°F/16˚C). Soil must be moist but not soggy before applying nematodes and also for 2 weeks following the application. Follow instructions on the product label for further details.

Personally, I solved what at a certain point seemed to be becoming quite a serious weevil infestation by removing all severely damaged plants from my property and putting in plants the weevils didn’t like. Two years later, there was no longer any damage worth noting, not even on plants that had previously shown some signs of damage.

Take away an insect’s main food supply and it will go somewhere else!

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

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