The tell-tale signs are there: C- shaped notches in leaves; stunted growth; and even yellow, wilting leaves – a black vine weevil infestation! This insect infects over 200 plants but the most common include rhododendrons, azaleas and yew. There are few garden pests that are as difficult to get rid of as this weevil but by targeting the young and vulnerable larvae and using a few cultural practices, gardeners can get good control of this nuisance.
The adult weevil is a dark, oval-shaped insect, ½” in length with a blunt snout and distinct antennae. In North America, all of the adults are females so every insect has the ability to cause and infestation. Feeding occurs at night; the adults do not fly but instead need to crawl up plants. The larvae are off-white in color with dark heads and can also do extensive damage as they feed on plant roots.
The adult weevils emerge from the soil in late May or early June through mid-July. These adults feed for 4-5 weeks in order to produce the 300+ eggs that are dropped into the ground under the plant. The eggs hatch within 2 weeks and the larvae then tunnel into the soil where they feed on plant roots. They then tunnel deeper into the soil to protect themselves from frost and pupate in the spring.
Understanding the lifecycle and feeding practices of the weevil is the key to keeping the pest under control. Since adult weevils feed on leaves during the night and look for dark, moist spots during the day to rest, trapping them in these areas can be quite effective.
With the aid of a flashlight, pick-off the adults in the evening as they come out to feed.
*Simply placing a white drop cloth under your plant in the evening or early morning and shaking the leaves will help to catch many of the adults.
*During the day, place a board under the plant. Check the board for hiding adults and scrape them off into a bucket of soapy water.
*Place a wrapper around the trunk of the plant and coat it with a sticky substance such as Tanglefoot. The wrapper should be at least 6″ wide.
The eggs and larvae of the black vine weevil require moisture to survive. If your plants are heavily mulched, pull back this mulch to allow the surface of the soil to dry out and do not water plants unless necessary.
Rather than attacking the adults, beneficial nematodes go after the larvae in the soil and are a safe & natural method of controlling black vine weevil. A few things are critical in order to get good control:
Timing & Temperature – Beneficial nematodes require a soil temperature of at least 60 F to work. Gardeners often miss the critical period in the spring since the weevil larvae pupate fairly early, before the soil warms. The late summer and early fall is the best time to apply nematodes.
Moisture – The root zone around the plant must be moist since nematodes don’t swim and require water to carry them through the soil. Water the area before and after application.
Rutgers Cooperative Extension Fact Sheet – http://www.rce.rutgers.edu/pubs/pdfs/fs667.pdf
Ohio State University Fact Sheet – http://ohioline.osu.edu/hyg-fact/2000/2016.html – Black Vine Weevil information
Terra Viva Organics – http://www.tvorganics.com – source of beneficial nematodes
Arzeena is an agronomist and garden writer.
Photo. Ian Lindsay