Black Vine Weevil beetle (Otiorhynchus sulcatus) aka: Taxus weevil is a very serious pest both landscape plantings and in nursery and greenhouse settings.
Hailing from Europe, this pest was first seen in the United States in the early 1900s.
Today it is one of the most widely distributed and destructive pests in the US. The larvae feed on the roots of plants and are especially fond of hemlock types of plants.
There are over 100 species of this pest, and they lay waste to wild and cultivated plants alike.
Highly susceptible annual and perennial plants include:
- Physostegia virginiana
- Heart-shaped bergenia
- Alumroot or Coral Bells
- Barren strawberry
- Lily of the valley
- Heuchera hybrid
- Maidenhair fern
- Peony Bushes and Trees
Slightly susceptible annuals and perennials include:
- Creeping Jenny
- Garden balsam
- Toad lily
Highly susceptible woody plants include:
- Japanese holly
- Rhododendron Bushes and Trees
- Ilex crenata
Slightly susceptible woody plants include:
- Sweet gum
- Euonymus (burning bush plant)
Black Vine Weevil Life Cycle
Adult beetles cannot fly. They range in color from light gray to deep black.
Most are about a third of an inch to a half-inch long, but the strawberry root weevil is about half this size.
They have crooked antennae and a short snout. The front wings have tiny indentations all over and are covered with very short, golden hairs.
Black Vine Weevil Larvae
The larvae are a bit bigger than the mature adults. As a grub or larva, the pest is C-shaped, legless and white with a small brown head.
They dwell in the soil through the winter months and create pupal cells early in the springtime.
They emerge as adults throughout the late spring and early summer months (May-June).
The adult females must feed for two or three weeks before they can mate and lay eggs.
Females can lay up to five-hundred eggs in this time period. They lay the eggs at the base of host plants in the soil.
It takes a couple of weeks before the eggs hatch into small larvae which immediately begin to eat the roots of the plant and grow larger.
The larval stage is when they are naturally most destructive during the late autumn and early months of spring.
The adults are most destructive during the evenings of the summer and early months of autumn.
They feed at night and hide on the ground in mulch and leaf litter during the day.
When the weather turns cold, the larvae burrow down deep to overwinter.
Adult pests usually die when the cold weather comes, but if they are able to find a place to hide indoors, they may last the winter and reproduce again.
What Kind Of Damage Does The Black Vine Weevil Cause?
The larvae cause a great deal of damage to plant roots from the time they hatch (mid-summer) through the autumn months.
They resume this destruction when the weather warms up in the springtime.
As small larvae, they consume the smaller, more tender plant roots.
As large larvae, they eat large roots, stems and tree bark. They can completely girdle a plant or tree.
The damage caused may not be noticed when a plant is at the nursery, but when the plant is put into a garden or landscape.
Where it will quickly die and the pests will go their merry way to other plants.
Adult black vine weevils feed on the leaves of broad-leafed evergreen plants.
This damage is unsightly but not really harmful. Still, you must deal with the adults to reduce the numbers of the larvae and vice-versa.
Controlling Black Vine Weevil What Can You Do?
There are many options for eradicating the adult insect pests.
You can carefully use an insecticide on plant foliage during the months of May and June.
Here’s one technique that works well for timing your application correctly:
Early in the month of May, place squares of wood (6″x6″) on the mulch at the base of potential host plants.
Burlap can also be used for this purpose. In the heat of the day lift the burlap or turn the boards.
If you see adult black vine beetles underneath, you know that you have a problem.
You can go ahead and pick out the adults and drop them into a bucket of hot soapy water when you see them. Naturally, this will not get rid of all of them.
Two or three weeks from your first sighting is when the females will lay eggs.
Make your first pesticide application at this time.
It’s a good idea to apply your treatment at dusk for maximum effect on night feeding beetles and minimum effect on beneficial insects (especially bees). [source]
Between the months of July and October, apply a registered pesticide as a soil drench to eliminate larvae.
In the past, the best way to do this was to drench the soil or dip entire young plants and their pots in a solution of the organophosphate insecticide called Dursban (chlorpyrifos).
This is a dangerous pesticide that not only harms people and wildlife but also harms the roots of the plants being treated.
Today a pyrethroid known as Talstar (bifenthrin) is preferred for dealing with both larvae and adult pests.
Pyrethroids break down rapidly in the environment, so they have become a preferred pest control substance in both home and agricultural settings.
Keep in mind, however, that the rapid breakdown of this chemical is due mostly to sunlight exposure.
When used as a soil drench, pyrethroids linger quite a bit longer.
Furthermore, if used in low concentrations, some black vine weevil larvae can survive soil drenching with bifenthrin and will naturally build up a resistance to it, which they will pass on to their future offspring.
Another reason why this kind of drench may not be entirely effective against black vine weevil larvae is that they may tend to remain in one place if they find a good food source.
This behavior would limit their contact with the pesticide. For this (and many other good reasons) use of nematodes to deal with larvae is preferable to pesticide soil drench.
What Are Nematodes?
Entomopathogenic nematodes are a type of roundworm that is nearly microscopic. They live in the soil and hunt down susceptible insects to attack.
Once they locate a likely subject, they enter its body and release symbiotic bacteria which multiply rapidly and kill off the hapless creature.
The bacteria continue to live in the carcass of the dead insect and reproduce for a couple of generations.
Once that source of food has run out, the nematodes emerge and seek out new victims. [source]
There are several species of beneficial nematodes commercially available. They are:
- Heterorhabditis bacterio-phora
- Steinernema carpocapsae
- S. feltiae
The best times to add nematodes are during the month of May or at the end of the summer, in late August.
When you add nematodes to your garden, be sure to water first. Keep the soil moist for several weeks after adding them. This will help them thrive and survive.
Remember that the larvae like to eat the roots of all kinds of broad-leafed perennial plants, as well as those of spruce trees, arborvitae, hydrangeas and ferns.
Be sure to add nematodes to the soil around all of the potential host plants.
Nemasys Vine Weevil Killer provides a handy way to introduce nematodes to your yard and garden as demonstrated in this video:
Build And Replenish Your Nematode Population
While this method of Black vine weevil control may take a while to become completely effective.
Once you have established a healthy population of beneficial nematodes in your soil, you should have little trouble with this sort of pest and many others.
It was once thought that nematodes could not survive in the cold soil of the northern United States or Canada, but naturally occurring populations of them have been found there.
If you live in one of these cold areas, be sure to introduce your nematodes in May so that they will have the whole spring, summer, and fall to settle in, acclimate and reproduce.
Be aware that the cold temperatures of these far north locations may interfere with natural nematode / black vine weevil interaction.
Nematode reapplication during the time periods when black vine weevil larvae are most active and destructive.
It may be a bit expensive to get started with nematodes; however, once established, they will continue to reproduce and spread throughout the soil of your garden.
Of course, if your nematodes are entirely successful in killing off all of your pests, they will also die as they will have nothing left to eat.
This is unlikely, however, since nematodes eat lots of different kinds of earth dwelling pests.
How To Avoid Introducing Black Vine Weevils To Your Yard And Garden
Because so many good treatments for black vine weevil are available now, you should not ever get an infested plant from a reputable nursery.
Nonetheless, just to be on the safe side, when you bring plants home, quarantine them in a dark place for few hours.
If there are any adult beetles, they will come out in the darkness so that you can find them when you check on the plants.
It is also a good idea to give new plants a soil drench with a pesticide or neem oil preparation before introducing them to your garden or landscape.
Keep the plant separate (quarantined) for a week or two to be sure any pests that have hitched a ride are dead.
Use of a bifenthrin soil drench, while the plant is quarantined, along with the addition of nematodes when it is transplanted, will provide lasting benefits and Black Vine Weevil control.
An Integrated Pest Management Plan (IPM) To Control Black Vine Weevil
Black vine weevil and many other types of grubs and weevils are very common in the soil of yards, gardens, and fields all around the world.
You are unlikely to ever be fully rid of them, but a good Integrated Pest Management (IPM) plan includes careful use of:
- Natural alternatives
- Beneficial insects and other organisms
… can go a long way toward keeping these pests under control.
Remember you must control both the weevils and the larvae, so the application of topical pesticide treatments to plant foliage, along with soil drenches and nematodes must be combined.
For the best effect, apply foliar treatments monthly during the spring and summer months to prevent adult females from being able to attain sexual maturity and begin egg laying.
Keep in mind that if you repeatedly use any single foliar product, the weevils will build up a resistance to it, so mix it up.
Use pyrethroid-based products monthly (especially if dealing with severe infestation). Use neem oil based products in between times.
Learn more about the IPM concept here:
In addition to the use of pesticides and natural solutions, add these four techniques to your arsenal:
#1 – Physically remove any grubs or beetles that you see. Keep a bucket of soapy water beside you when you work in your garden. This is an excellent receptacle for all sorts of pests.
#2 – Physical barriers and obstacles can help control the pests’ movements. You can keep adult BVW away from plants by encircling the stems of plants or trunks of trees with unclimbable materials such as plastic that has been coated with grease.
#3 – You should also keep vegetation pruned so that limbs from one plant are not touching another. This prevents the bugs from climbing from plant-to-plant.
#4 – To hamper the movements of larvae and mature pests, you can bury aluminum flashing all around the plants in such a way that it protrudes from the earth a couple of inches and extends below the soil surface by 5 or 6 inches.
Coat the protruding part of the metal with auto mechanic’s (lithium) grease. This grease will stay in place well even in moderate rain. You will need to refresh it from time-to-time, especially after a very heavy rain.
Of course, if there are already weevils around the plant, you will be shutting them in rather than out, so turn the soil and remove any grubs and beetles you encounter before constructing your bug fence.
Remember that you must protect your beneficial insects because they will help you by eating both larvae and adults.
Predatory insects may be more susceptible to pesticides than Black Vine Weevil, which easily builds up resistance.
This is a good reason to keep pesticide use light and employ a robust and varied IPM plan.
A pitfall trap survey of BVW to predatory insect ratio conducted in a yew nursery where insecticides were in heavy use found that there were only 200 predator insects in the test area and 1500 Black Vine Weevil.
By contrast, the same sort of survey conducted in a nursery where pesticides were not used yielded 950 predator insects and only 3 Black Vine Weevil. [source]
You can conduct this sort of experiment in your own garden by burying a 16-ounce plastic glass in your garden soil up to the level of the rim.
Be sure to place it in an area where Black vine weevil is likely to be found. If you have a rhododendron bush, that’s an ideal spot.
Coat the lip of the cup (about 1″) with motor oil so that bugs cannot climb out.
Check your trap every few days. Your catch of black vine weevil and predatory insects will give you an idea of the ratio of prey-to-predator in your location.
Welcome Carabid Beetles
Provide your carabids with a welcoming habitat by providing a good layer of mulch for them to hide in during the heat and light of the day.
It is especially important, in far Northern locations, that you protect your carabidae ground beetle population as they are your best natural allies against black vine weevil.
To encourage a healthy population of Carabidae beetles avoid turning the earth unnecessarily.
You should also avoid using slug and snail bait as it can be deadly to beneficial beetles.
Even if these predator insects greatly outnumber black vine weevil they can survive and thrive because they are generalists when it comes to eating.
They will basically eat any insect they can grab and consume. So during periods, the black vine weevil is not present, carabids will help you out by eating lots of other pests.
You Don’t Have To Eradicate Black Vine Weevil To Be Successful
Be willing to accept a small amount of weevil feeding on the leaves of your plants as this lets you know that you still have enough weevils to keep your natural predator friends well fed.
This is the sign of a healthy, diverse, natural and self-sustaining ecosystem.
Remember that a good IPM is key to controlling black vine weevil larvae and adults.
Use pesticides sparingly and in a very targeted manner. Assemble an arsenal of natural barriers, substances and insect allies to help keep these pests under control.
Keep a vigilant watch for the adults and the larvae, and deal with them immediately when you see them.