By: Heather Rhoades
Root weevils are a plant pest both indoors and outdoors. These destructive little insects will invade the root system of a healthy plant and then proceed to eat the plant from the roots up. Identifying and controlling root weevil in your garden and houseplants can keep your plants from suffering unnecessary damage.
Root weevils can be one of several kinds. The most common in the garden is the black vine root weevil or the strawberry root weevil. The black vine weevil attacks shrubs and strawberry weevils attack strawberries. While these are the most common, they are far from the only kind. All plants in your home or garden are susceptible to weevil infestation.
Larval root weevils will look like white grubs or worms and will be found in the soil. Adult weevils are beetle-like insects that can be black, brown or grey.
If root weevils are present in your garden or houseplants, there will be damage to both the roots and the leaves. The leaves of the plant will be irregular, as though someone has been taking bites out of the edges. This damage will appear in the night, as root weevils come out to feed at night.
Controlling root weevil is possible. Organic root weevil control methods include purchasing parasitic nematodes or predatory beetles, which can be bought to hunt down the weevils. You can also hand pick the adults off the plant at night while they are eating. Weevils are also attracted to moisture, so a shallow pan of water can be set out at night and the weevils will climb into it and drown.
Non-organic root weevil control methods are to spray the leaves of the plant with a pesticide and to thoroughly soak the soil with a liquid pesticide. But, remember, when you do this, you may also be killing off beneficial insects and small animals as well.
Finding these insects in the roots and leaves of your plants is unpleasant, but it can be fixed. As always, the best root weevil control is to make sure you never get any in the first place. Be sure to practice good garden hygiene and clean up dead plants and do not over mulch.
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Beneficial nematodes seek out and kill all stages of harmful soil-dwelling insects. They can be used to control a broad range of soil-inhabiting insects and above-ground insects in their soil-inhabiting stage of life. More than 200 species of insect pests from 100 insect families are susceptible to these insect predators.
They are a natural and effective alternative to chemical pesticides, and have no detrimental affect on non-target species such as ladybugs, earth worms and other helpful garden insects.Finally, there is no evidence that parasitic nematodes or their symbiotic bacteria can develop in vertebrates. This makes nematode use for insect pest control safe and environmentally friendly. The United States Environmental Protection Agency (EPA) has ruled that nematodes are exempt from registration because they occur naturally and require no genetic modification by man.
Beneficial nematodes can be applied anytime during the year when soil-dwelling insects are present and soil tempertures are above 52-F during the day. Beneficial nematodes seek out and kill over 200 pest insects in the soil. They are a natural effective alternative to chemical pesticides.
Are most effective against Japanese Beetles, Grubs, Weevils, and many other target pests in lawn and garden. They burrow down in the soil to a depth of 7", have shown superior host-seeking abilities in looking for deep soil- dwelling pests.
Target pests include: Cucumber Beetle, Grubs, Gall midge, Strawberry Rootweevil, May/June Beetle, Masked Chafer, Cranberry Rootworm, Flea, Scarab and Japanese beetles, Straw- berry Root and Black-vine Weevils, Chafer, Squash Bugs, Leaf Beetles, Termite, Cutworms, White Grubs, Algae Gnats, Black Fly, Potato Tubeworm, Meal Worm, Bark Beetle, Corn Root Weevil, Fire Ant, sting Bugs, Pine Beetle, Gall Gnats, Gypsy Moth, Corn Root Worm, Billbug, Colorado Potato Beetle, Thrips, Ants and termites (apply directly to mound and nest areas),and many other deep soil dwelling insects.
They are highly efficent when the pest is more widely dispersed in the soil because they have a "tooth" to rupture the insect's skin and enter through the insect's body wall and openings.
Kills pre-adult fleas in the yard, and pet run areas and soil. It's most effective against flea larvae and caterpillars in lawns, garden soil, and under trees where larvae pupate. They stay near the surface waiting to ambush surface dwelling pests. Steinernema is the most widely researched species for insect control. It is the most readily available for yard and Garden use because it is easier to rear and handle. In field applications, Steinernema carpocapsae tend to be most effective against caterpillar larvae. In laboratory and field trials, it has controlled sod webworms, cutworms and certain borers (raspberry crown borer, carpenter worm). It also has been effective against billbug larvae in Colorado State University trials. Other research indicates that adult billbugs may be controlled as well. Steinernema are less effective against white grubs, root maggots, rootworms and black vine weevil. Unfortunately, some commercial products make claims of effective control of some pest species based on research conducted solely in artificial environments these often do not reflect performance in the field.
Target pests include: Fleas, Dog and cat flea larvae, Codling Moth, Cutworm, Armyworm, Leafminer, Bluegrass billbugs, termites, ants, Sod Webworm, Mole Cricket, some caterpillar pests, Billbug, Flies, ArmyWorms, Loopers, European Crane Fly, Cranberry Girdler and many other surface dwellers.
Are the most effective against larval control of several fly species (sciaridae, phoridae, leaf miners, domestic fly and also of some moth larvae. They patrol the top 3" of the soil.
Target pests include: Fungus Gnat, Mushroom Flies, Fruit Flies, Flea Beetles, Saw Flies, Tachina Flies, Crane Flies, Shore Flies and fruit flies.
They are effective against some plant parasitic nematodes, particularly root-knot nematodes.
Many species of granary and seed weevils, which are often found in agricultural areas or as a common pantry pest, are not native to the United States. Scientists believe that these types of weevils were transported accidentally in stored grains on ships from Europe, and flourished once they reached land.
Facts about weevils often include how granary and seed weevils may be found in your stored dry goods, but weevils in your home might not be searching for food. Many of the weevils common to gardens are often found inside homes during the summer and early autumn months. Scientists are not sure why these insects enter homes in droves, but believe they are attracted to the moisture and cooler temperatures. If you are seeing weevils near sinks, in bathrooms or in other areas where water is present, they are likely vine or root weevils. These pests do not cause damage to your property and will typically leave on their own. However, you can vacuum them up and dispose of them outside if they are a bother.
Probably second only to slugs up here in the Northwest is the damage caused by a small, bright green caterpillar that devours plants in the brassica family. Called the imported cabbageworm, cabbage caterpillar, cabbage moth caterpillar or – incorrectly – the cabbage looper, this pest is the larval stage of a white butterfly with black spots on its wings called Pieris rapae.
In the Pacific Northwest this pest is particularly problematic because our climate is ideal for the brassica family plants, including broccoli, cabbage, and kale, that the cabbageworm prefers. Lots of food for this pest plus mild temperatures mean several generations of the cabbage moth can hatch out in a single summer.
Damage is dramatically more likely to occur on big, tender leaves of plants in the cabbage family. No brassica is safe but in my experience cabbage, Asian cabbage, broccoli, cauliflower, kale and collards are more likely to be targeted than radish, arugula, rutabaga or turnip.
The cabbage moth lays her eggs on the underside of leaves that are large enough to support her brood when they hatch out, so typically plants are “teenage sized” before they are targeted for destruction.
Look for chew holes right through the middle of brassica leaves. In heading plants like cabbages, the cabbage moth caterpillar will chew right through the middle of the cabbage, irritatingly maximizing the damage done to the plant.
Both eggs and the cabbage moth caterpillar itself hang out on the underside of leaves. Look at the bottom half of the image and you’ll see both pale yellow eggs and freshly hatched, tiny little cabbage worms on the underside of a broccoli leaf.
Prevention is very effective. If you’re willing to net all your brassicas with lightweight floating row cover from the minute they germinate or get transplanted out until you harvest them, I can guarantee you will not have a cabbage moth caterpillar problem. That’s too much work for me, but it’s a good option if you are passionate about no spray and you like a Casper the Friendly Ghost motif in your garden.
Hand-picking is very effective for a small cabbage worm infestation. Hand pick the caterpillars that are large enough to get your fingers around and toss them to your chickens. Squish any tiny caterpillars or egg clusters.
Larger infestations of cabbage worm can be managed extremely effectively with a targeted, biological, organically approved insecticide called Bacillus thuringiensis (Bt). Bt is the second of the two insecticides I use in my garden. (See above for the other: Sluggo.)
Bt (pronounced “Bee Tee”) is a special strain of bacteria that paralyses the gut of caterpillars who eat treated leaves. The caterpillars stop eating soon after ingesting the Bt-treated leaves, and die within a few days. It is very effective against caterpillars, but non-toxic to bees and other, non-caterpillar beneficials. It’s safe to use around kids, pets, etc. Just keep it away from any plant like milkweed that might be a host for beneficial butterflies.
I find two targeted sprays of Bt – once in the spring about a week after I first see the Cabbage White Butterfly, and one in mid-summer is usually enough to keep populations of the cabbage moth caterpillar low in my garden.
(Note: some of this information originally appeared in an article about the Cabbage Moth Caterpillar. See that article more additional information, including what beneficial insects to attract to your garden to help in the battle against the cabbage worm.)
Thought to be native to Central Mexico, the weevil migrated northwards to the United States sometime in the late 19th century. Within just twenty years, weevils infested nearly every cotton-growing area in the country.
Although weevils don’t produce nearly as much economic disruption nowadays, they are still a frustrating pest common to the South. Below, we’ll jump into the best ways to keep weevils from wreaking havoc on your property.
The best way to prevent a pantry weevil infestation is to be very cautious with the food you buy, prepare, and store. As always, prevention is often the best way to keep an infestation from occurring.
A root weevil is a type of “snout beetle” that develops on the roots of various plants. Adult stages produce more conspicuous plant damage, cutting angular notches along the edge of leaves when they feed at night. Adult root weevils also may attract attention when they wander into buildings, acting as a temporary “nuisance invader”.
The most common root weevils found in Colorado are strawberry root weevil (Otiorhynchus ovatus), rough strawberry root weevil (O. rugostriatus), black vine weevil (O. sulcatus) and lilac root weevil (O. meridionalis). Dyslobus decoratus is established in some areas and chews leaves of various shrubs. Two small root weevils that sometimes wander into buildings are hairy spider weevil (Barypeithes pellucidus), and Trachyphloeus asperatus. Several other species likely occur in the state as root weevils are easily moved in nursery stock.
The life history for black vine weevil and strawberry root weevil have been most studied and likely have life histories similar to that of other common root weevils. A one year life cycle is normal for all species. With few exceptions, winter will be spent as a larva, in the soil, feeding on roots when temperatures allow. Larvae of root weevils are legless grubs, with a cream-colored body and a pale orange-brown head. In late winter and early spring, larvae complete development and then transform to the pupal stage, which also occurs in the soil. Most adult root weevils emerge in mid to late spring. Some species (e.g., black vine weevil, strawberry root weevil) produce only females and reproduce asexually. Others, such as lilac root weevil, produce males and females.
None of the root weevils can fly and they are night active, hiding during the day around the base of host plants, usually under a bit of cover. About an hour after sunset they become active and crawl onto the plants to feed on leaves, producing their characteristic angular notches. If disturbed, root weevils will readily drop from plants and play dead.
Adults typically live for at least a couple of months, and some may be present into autumn. Most eggs are laid in late spring and early summer with females squeezing eggs into soil cracks. A few days after they are laid, eggs hatch and the larvae move to the roots where they feed.
Leaf notching by the adults is the only injury that is easily observed. Several kinds of plants may be fed upon by each of the different kinds of root weevils and two or more kinds of root weevils may be present in a garden. Searching the plants at night is the best means to determine what species is present.
Lilac root weevil is most often found feeding on lilac, privet, peony and euonymus. Euonymus, Heuchera, Astilbe, hosta, creeping jenny, peony and wisteria are among the common hosts of black vine weevil. Strawberry root weevil also has a very wide host range that includes strawberry, raspberry, rhubarb, white clover, dandelion, dahlia, and mint but it has also been reported to feed on many kinds of trees and shrubs, chewing on the roots and bark at the base of the plant. Rose, strawberry, raspberry, gooseberry, and cotoneaster are among the hosts of rough strawberry root weevil. Cherry and buckthorn are among the more common hosts of Dyslobus decoratus. Hairy spider weevil is reported to feed on several types of trees and shrubs (cherry, elm, hawthorn, oak) as well as many common herbaceous plants, including some weeds (aster, dandelion, ragwort, thistle). The host plants for Trachyphloeus asperatus are unknown.
As root weevil adults normally travel only short distances, it is likely that larvae are developing on plants that show injury by adults. However, that may not always be the case and larval injury can only be confirmed by digging plants to expose the larvae feeding on roots. This is best done in autumn and early spring when larvae are more full-grown and easily seen. Severe root pruning by larvae can potentially cause plants to decline and dieback, but this is rarely observed in landscape plantings.
Adult root weevils can be controlled with sprays of several kinds of pyrethroid insecticides such as permethrin, bifenthrin, cyfluthrin, or cyhalothrin. These are best applied on the lower trunk of the plants and around the base of the plant to kill the weevils as they climb. If the plant is flowering and attracting pollinators, sprays should not be made to leaves and flowers. If plants are not in bloom it may be useful to spray lower leaves as well as the lower trunk and branches. Sprays made late in the day, shortly before the insects become active may improve effectiveness.
“Beneficial nematodes” (entomopathogenic nematodes) can be used as a soil drench treatment to control root weevil larvae in soil. (See fact sheet 5.573, Insect Parasitic Nematodes for more details.) Several species of these nematodes are commercially available, all via mail order, but those in the genus Heterorhabditis (e.g., Heterorhabditis bacteriophora) are most effective for control of root weevil larvae. These would be most effectively applied after most eggs have hatched and when soil temperatures are warm. For most root weevils this would be during the summer months, typically between mid-July and mid-September. The nematodes are applied as a drench to the soil and the treated area should be watered to keep the soil moist.
Some root weevils will incidentally wander into buildings at certain times of the year. This most often happens with strawberry root weevil, which most frequently invades homes during periods of hot, dry weather in late June and July. The hairy spider weevil and Trachyphloeus asperatus also tend to wander into buildings in early summer. During late summer and early autumn black vine weevil and rough strawberry root weevil are more commonly observed indoors.
Inside homes, the root weevils cause no injury to humans or household furnishings. Their occurrence is solely as a temporary nuisance invader that will not reproduce indoors. When root weevils are present often the best course is to tolerate the occasional beetles, vacuuming them as they are observed. Left alone, root weevils will either die out on their own or migrate back outdoors and infestations of buildings tend to be short-lived.
Migrations indoors are best prevented by sealing openings of the building that allow root weevils (and other insects) access to the interior. Root weevil numbers may also be reduced by removing plants around the outside of the home on which the insects feed. Reducing watering around building foundations may also limit appearance of root weevils within buildings, as the adult insects appear attracted to shade and moisture.
There is no effective use of insecticides applied indoors for reducing root weevils. Insecticides applied on the building exterior that target cracks and other points of potential insect entry may further suppress the incidental migration of root weevils into buildings.