Weevils

Weevils are beetles that love to eat plants.

Diaprepes citrus root weevil (Keith Weller, USDA) Public Domain

Weevil identification​

Most weevils are small (less than 1/4” to 1/2” long), somewhat triangular in shape, dark gray, brown, or black, with some markings, elbow-jointed antenna, and they all feature the telltale snout that distinguishes them from other insects. Most weevil larva are white or pink, C-shaped, only 1/3” long and legless, with a brown head. The Fuller rose beetle larva has a white head and the vegetable weevil larva is green.

Strawberry root weevil (Janet Graham) CC BY 2.0

The weevil family

There are over 60,000 different types of weevils worldwide. Being so large, the weevil family is divided between true weevils (Curculionidae) and primitive weevils (Orthoceri). The division is unimportant from our standpoint, but it is important to understand the damage weevils can cause in the garden and in your pantry, how they live, and how to avoid infestation. Some of the more common weevils, and their foods of choice, include:

• Bean weevil (Acanthoscelides obtectus) - beans
• Black vine weevil (Otiorhynchus cribricollis) - olives, raspberries, blackberries, grapes, azaleas, euonymus, rhododendron, and liquidambar
• Boll weevil (Anthonomus grandis) - has cost cotton growers over $15 billion
• Broad bean weevil (Burchus rufimanus) - black, kidney, navy. and other broad beans 
• Cowpea weevil (Callosobruchus maculatus) - cowpeas and other legumes
• Cribrate weevil (Otiorhychus cibricollis) - artichokes, olives, raspberries, blackberries, cherries, apple
• Diaprepes root weevil (Diaprepes abbreviatus) - citrus, roses, guava, potatoes, loquat, avocado, palm, and birch 
• Fuller rose beetle (Pantomorus cervinus, aka Asynonychus godmani)
• Obscure root weevil (Sciopithes obscures) strawberries, rhododendron, pine
• Pepper weevil (Anthonomus eugenii) - sweet and hot peppers
• Rough strawberry root weevil (Otiorhynchus rugosostriatus) - strawberries
• Strawberry root weevil (Otiorhynchus ovals) - strawberries
• Vegetable weevil (Listroderes costirostris obliquus) - potatoes, lettuce and other greens, carrots, tomatoes 
• Woods weevil (Nemocestes incomptus) - raspberries, blackberries

Weevil lifecycle

​Adult weevils hide in the soil during the day. Weevils generally attach their eggs to a food source or lay them in cracks provided by pods, stems, and other plant structures. Some species lay their eggs in the soil near host plants. When the eggs hatch, the larva burrow into the nearby food source, where they gorge themselves before metamorphosing into a pupal stage, and then into adulthood. Adults emerge June through September, leaving small round holes in seeds as the only sign of infestation. Weevils can destroy crops in the field or harvested produce while in storage. Not only do weevils feed on your produce, they also leave behind frass. Most weevils are flightless and nocturnal, so all we may see are signs of infestation.

Weevil damage

Weevils are known for burrowing into beans, cotton bolls, and cereal grains, but they also feed on roots, stems, buds, flowers, leaves, and fruit. Often, the first sign of infestation is leaf wilting, early fruit drop, scalloped leaf edges, and damaged seeds.

Weevil controls

Controlling weevils is mostly a matter of prevention and sanitation. First, only install plants that are weevil-free. Then, make sure that old beans, pods, seeds, and other plant debris from previously attacked crops are not left in the field. Crop rotation can help a lot, since many weevils are host specific. Infested areas should be plowed or dug and allowed to go fallow (unused) for 2 or 3 months to starve any weevils in the soil. Sticky material can be used around tree trunks to protect against soil borne weevils, but it will do nothing against plants that are already infested. Insecticides are marginally effective, but fumigants and parasitic nematodes are used commercially with some success. Nettle, mustard, cheeseweed (little mallow), and oxalis are also host plants, so you may want to keep them away from your garden if weevils have been a problem.

For those of you who can't hear the word "weevil" without thinking of the blues song, here's your link.