Root-feeding weevil larvae

Overview

Root weevils feed on a variety of plants, including needled and broad-leaved evergreens, deciduous and herbaceous plants, and several important food crops. Adults feed on the leaves of the host plant and larvae feed on the roots. Host plants differ, but similarities in habits, life cycles, and management practices permit them to be addressed as a group.

Representative root weevil species:

Black vine weevil (Otiorhynchus sulcatus) syn: taxus weevil. Host plants: yews, hemlocks, rhododendrons, several other broad-leaved evergreens, and greenhouse plants such as asters, cyclamens, and impatiens. (see IPM sheet: Black Vine Weevil)

Carrot weevil (Listronatus oregonensis) Host plants: carrots and other vegetables. More common in the East.

Strawberry root weevil (Otiorhynchus ovatus) Host plant: strawberries.

New York weevil (Ithycerus noveboracensis) Eastern U.S., west to Nebraska and Texas. Host plants: Hickory, oak, beech.

Sweet potato weevil (Cylas formicarius elegantulus) syn: sweet potato root borer. Introduced species occurring more commonly in the south U.S. Host plant: sweet potato. Probably not here yet, but with global warming, it's probably only a matter of time.

Symptoms and Diagnosis

Starting in spring, emerging root weevil adults feed on the leaves of host plants, notching the leaf edges quite distinctively. The adult weevils rest during the day and feed on the leaf margins after sundown. Conifer needle damage is less evident than that on broad-leafed plants. Needles may be severed and dropped from the tree. Adults feeding on new growth may result in sparse foliage.

Although top feeding of the adults is the most conspicuous damage, root feeding of the larvae does the most damage as they chew and girdle the roots of their various host plants. Root weevil larvae can be devastating to conifer seedlings. Woody plant seedlings and propagation cuttings are at risk from both root and top feeding. Injured plants may develop a greenish-yellow cast that does not respond to fertilization and watering. Container-grown plants may be severely stunted or killed. Injured plants may die suddenly.

Carrots and sweet potatoes affected by root feeders are not commercially viable and mostly not fit for the table. Damage to roots and crowns can weaken, stunt, and kill strawberry plants.

Life Cycle

Outdoors, there is one weevil generation per year. Greenhouse conditions may permit two generations annually. Many, if not most, root weevils dispense with the annoyance and inconvenience of males, producing the eggs of the next generation by means of parthenogenesis.  Females deposit eggs in soil and debris near the host plant, starting usually in mid-summer, and continuing well into the fall, laying hundreds of eggs. Eggs hatch in about 10 days and the larvae move into the soil, feeding on root hairs and roots. Older larvae may girdle entire stems. Larvae are legless, c-shaped, and cream-colored with brown heads. They spend the winter in the soil and continue their development in the spring as the soil warms. This second feeding period is the most destructive.  Later in the spring, the mature larvae pupate in the soil and soon emerge as adult weevils. After several weeks of feeding on the leaves of host plants, adults begin the egg-laying cycle again. Some adult weevils will not expire in the fall but will over-winter. These over-wintering adults will begin egg-laying earlier than those adults just emerging from the pupae state.

Integrated Pest Management Strategies

(Control efforts may be directed towards both larvae and adults.)

1. Try an organic solution. Weevils are attracted to water and may be trapped in shallow pans inset into the soil near affected host plants. Products that contain the organically derived insecticide spinosad are also labeled for leaf-feeding beetles on a wide variety of plants and may be worth a try. Diatomaceous earth dusted at the base of affected plants is useful. In serious infestations, the lower sides of stems and leaf undersides should also be dusted.

2. Use sticky substances. Adult weevils emerge from soil and debris at night and climb the plants to eat. Lower trunks of host plants may be wrapped with a four-inch band of masking tape and the product Tanglefoot applied to the taped surface. Check for buildup of dead insects regularly, and refresh as needed.

3. Nematodes can be applied as a soil drench to control root weevil larvae. Two commercially available cultures, Steinernema and Heterorhabditis are effective. Timing is critical. Apply nematodes when the soil temperature is 55°F and the soil is moist. Pull back any mulch, apply the nematodes, and replace the mulch.

4. Apply pesticides. Chemical stomach poisons applied to the foliage are effective. Pesticides registered for use include acephate, azadirachtin, imidacloprid, and cyfluthrin. Pesticides should be applied to the foliage thoroughly, allowing excess to run off into the soil beneath the plant. Repeat at least twice at two-week intervals. Pesticides appropriate for ornamentals may not be safe for vegetable and fruit crops. Read and follow the label instructions on all pesticides.

Organic Strategies

Strategies 2 and 3 are strictly organic approaches. For an organic approach to Strategy 2, consult the Organic Materials Review Institute (OMRI™) for appropriate spinosad and diatomaceous earth products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against weevils. Not following the pesticide label before usage is a violation of federal law.