Perhaps the lone weevil you have ever heard of is the boll weevil (Anthonomus grandis), infamous for devastating cotton fields in the United States in the early 20th century, but you neglect another beetle only at your own risk: the black vine weevil. In fact, historical interests aside, the average gardener is well-advised to pay more attention to the black vine weevil (Otiorhynchus sulcatus), a native of Europe, than to its better-known relative. This weevil uses as hosts several plants popular in landscaping, to which it can cause unsightly, albeit not life-threatening damage. Worse yet, the attacks of its larvae occur at the root level, sometimes inflicting lethal damage. The pest is found throughout much of southern Canada and the northern United States.
Find out what black vine weevils look like, which plants they are most likely to attack, and how to control them.
What Do Black Vine Weevils Look Like?
The black vine weevil is an insect that has a gray or dull black color and is 3/8 inch long. It has a pear-shaped body covered in pit marks, and it has six legs. Two antennae protrude far out from the head, and the snout is also prominent. The black vine weevil does not fly: It gets around by walking.
These pests start out as larvae (or "grubs") that resemble the grubs of better-known beetles such as June bugs (Phyllophaga longispina) and Japanese beetles (Popillia japonica). The larvae are worm-like (no legs), 1/2 inch long, and take the shape of the letter, C; their bodies are white, with reddish-brown heads.
4 Ways to Get Rid of Black Vine Weevils
The best approach to controlling black vine weevils is to know the causes behind infestations and take preventive action, accordingly. However, if it is too late for that, there are a few ways to get rid of the pest.
Since adult black vine weevils are nocturnal, looking for the pests, themselves is not the easiest way to detect an infestation. But it is one option if you do not mind wielding a flashlight and inspecting the leaves of known host plants on a few nights in May and June. If you find any weevils, pick them off the foliage and kill them.
If you do not mind using pesticides in the garden, a systemic kind is your best bet. A systemic pesticide effective against both black vine weevil adults and their larvae is imidacloprid. It is applied at ground level with water so that the soil around the plant becomes drenched.
Follow label instructions and mix the imidacloprid in a bucket with water. The amount of imidacloprid to use is based on plant height.
There are many traps that are effective for getting rid of black vine weevil adults. Some you can make, yourself; others are available at home improvement stores.
An example of a homemade trap you can make puts to good use that old piece of burlap lying in a corner of your garage. At the end of the day, wrap the trunks of evergreen plants that the pests attack (such as the hemlock tree) with the burlap, in anticipation of nighttime feeding. The beetles will often use such material for shelter. In the morning, inspect the burlap for the pests and kill any black vine weevils you find in it.
An example of a store-bought trap is Tanglefoot, which, again, you wrap around the trunk of a host tree or shrub. This product works on the same principle as the flypaper that you use indoors.
Another way to avoid a major infestation while staying organic (in addition to handpicking and traps) is to use predators. The predators can kill the larvae before any further plant damage takes place (if the application occurs in a timely manner).
The predators, in this case, are parasitic nematodes (Heterorhabditis bacteriophora). They are available through online biological control businesses. Nematodes can be good or bad; in this case, they help you fight a pest.
Like a systemic pesticide, this method "gets to the root of the problem": The nematodes are drenched right into the soil beneath the host plants. Make sure your soil is sufficiently well-drained to receive them properly. The best application time is when black vine weevil larvae are first expected to be present, which is in mid-summer. Apply the drench in the early morning or in the evening. The ground in the area should be watered prior to the application to make it evenly moist; keep it evenly moist for 2 weeks after application.
Signs of a Black Vine Weevil Infestation
Adult black vine weevils are active from mid-spring to late summer. Their grubs stay around for a longer time: from mid-summer to the following spring. Adult black vine weevils eat the leaves of plants during the night; they leave behind a crescent-shaped hole along the leaf margin. This damage mars the appearance of the plant, but it is unlikely to result in serious health consequences.
The larvae, by contrast, eat the roots of plants and can girdle their lower stems. This damage can cut off necessary water and nutrients moving from their source to the plants' above-ground growth. The damage they do is more serious than that done by the adults. The damage they inflict will cause the plant to wither and turn brown; it can even result in the death of the plant. Because of this destructive activity in the larval stage, an alternate common name for the species is "root weevil."
During May and June, look for the damage caused by adult black vine weevils on known host plants. For example, grapevines (Vitis spp.) are a well-known target of these pests; thus the "vine" in the common name.
Since the larvae, too produce visible signs of damage on affected plants, you may wonder why we do not let the detection of their damage serve as an indicator of a black vine weevil infestation. The reason is two-fold: grub activity takes place underground, so you do not see them; and by the time the impact of larval feeding becomes visible at the surface, much of the damage to the plant will have already occurred and is irreversible for that year's garden.
However, if you inspect plants carefully going forward and detect, in a timely manner, the adults that develop from the larvae, act promptly to eradicate them, thereby keeping them from laying eggs. Doing so will prevent the emergence of another generation and save the following year's garden.
What Causes a Black Vine Weevil Infestation?
Since black vine weevils do not fly, they do not cover much territory on their own. But they can exploit human activity to travel many miles surreptitiously. That potted plant you buy at a garden center can be a Trojan Horse, with larvae present in the root ball. When the larvae mature, they can spread to other plants on your property.
Try to buy your potted plants only from reputable nurseries. You may pay more, but it is worth it: Reputable nurseries are more likely to treat the soil that they use to prevent black vine weevil infestations. If you are going to repot a plant that you have just bought (or plant it in the ground), inspect the root area for grubs first and kill any that you find.
How to Prevent a Black Vine Weevil Infestation
But even if black vine weevil larvae were to find their way onto your property, there are ways to thwart (or, at least, impede) the adults that develop from them.
The best way is to deprive them of the host plants that they feed on for sustenance. Plants commonly attacked by black vine weevils include the following:
- Hosta (Hosta spp.)
- Euonymus (Euonymus spp.)
- Hemlock (Tsuga canadensis)
- Mountain laurel (Kalmia latifolia)
- Azalea (Rhododendron spp.)
- Yew (Taxus spp.)
If you love one or more of these known host plants and decide that you must take the risk and grow them in your yard, there is still a way to slow down the attacks that will come from black vine weevils. This tactic takes advantage of the fact that black vine weevils can't fly. Simply prune the branches of a known host shrub (such as mountain laurel) growing in mass or with other plants, creating spaces between vegetation. Doing so eliminates the "bridges" that black vine weevils need to move from one plant to another. Likewise, as much as you may like the look of a branch arching down to the ground, prune off such branches on known host plants so that black vine weevils can't climb them to access your plant.
Can a black vine weevil bite you?
While somewhat threatening-looking with its sturdy proboscis, no, this insect does not bite or sting humans.
Is the black vine weevil just an outdoor pest or does it also enter the home?
Sometimes a black vine weevil will be accidentally brought into the house when you bring a potted plant indoors. Once indoors, it can do damage to popular houseplants such as Begonia.
Outside of the plant damage it causes, is the black vine weevil harmful in any other way.
No. This insect is not harmful to pets, nor does it damage houses.