Wireworms & Click Beetles

Order: Coleoptera
Family: Elateridae
Species: Agriotes obscurus & A. Lineatus

Description and Life History: Most gardeners are familiar with wireworms. We generally associate them as pests on potatoes, sugar beets and other tubers. However, most wireworms play an important role in our environment. Many wireworm species are recyclers and decomposers of forest litter and wood. Only a few species give us grief in our gardens. But, guess what? We have two new species of wireworms in Whatcom County. A survey done by Eric LaGasa (our most awesome Chief Entomologist-WSDA) found established populations of Agriotes obscurus and A. lineatus in Whatcom County (to read more about his survey, visit http://whatcom.wsu.edu/pestsurvey/Wireworm.htm.

Click beetles in general (adult wireworms) are elongate, parallel-sided, compact, streamlined beetles with serrate (saw blade like) antennae. When looking at the beetles from the top, you will recognize that the segment behind the head (pronotum) has corners that are pointed and fit snuggly around the corners of the wing covers (elytra). The joint between these two regions is extremely flexible. This is a distinguishing character of adult click beetles, in the family Elateridae. An easier way to ID a click beetle is to roll it over onto its back. If the beetle arches its back and produces a notable clicking sound as it catapults through the air, then you have a click beetle. If not, you just have an upset beetle of another sort on its back. The power conducted by this clicking mechanism is one of the strongest forces produced by a land animal and is used to escape bad situations. The two new Agriotes click beetles are a dusty brown/gray color and look like many of our other click beetles.

Wireworm larvae look like tubular, caramelized noodles that are about an inch long when fully grown. Wireworms resemble mealworms that you buy at the pet store to feed your critters. These new wireworms have a single-pointed rear end, instead of multiple points or bumps and ridges. Wireworms (along with mealworms) can be a tasty, crunchy treat when honey roasted (a common dish served at WSU’s annual Bug-Luau)!

The life cycle of wireworms can take up to 5 - 6 years to complete. This is why they are hard pests to get rid of. Adult beetles lay eggs in the soil, near suitable food sources in the soil. Eggs hatch in 2 - 7 days and larvae start to search for food by wiggling through the soil. Larvae enjoy soil temperatures between 50-70oF and will move up in the soil as it warms in the spring to find your new plants, and then move down if it becomes too hot or cold. Larvae continually move as much as 2 feet down into the soil depending on soil temperatures and food sources. The larvae also pupate in the soil in the late summertime. Adults will emerge from the pupal stage as soil temperatures warm to 50oF during the following spring. Again, adults emerge 5 years into the lifecycle; at any given time, you can have different life stages in the soil with overlapping generations.

Damage: The larvae of these two species prefer to feed on grass seeds and roots but also have been found in strawberries (roots and fruit), vegetables and tubers. The preferred habitat is pasture, sod and grassland. Even though these species of wireworms are serious agricultural pests in Europe, we haven’t seen major problems in Whatcom County. So Todd, why is this our pest of the month?

Last year, after I became aware of our new beetles, I noticed that a few customers (even some Master Gardeners) complained of planting their veggies from seed and nothing sprouting. After investigation we found Agriotes wireworms to be the culprit. Larvae use the carbon dioxide (CO2) produced from germinating seeds and growing root tips to find their food and can detect CO2 from up to 20cm (~8”) away in the soil!! Wireworms will feed on and destroy the germinating seeds. Additionally, wireworms feed on the developing root systems and tubers of garden grains, corn and vegetables. The adults do not cause any important damage; it’s only the larval stages that are damaging.

Canadian growers have found that heavy wireworm populations cause problems via an unusual secondary pest, the crow. Crows have been seen tearing up strawberry plants to get to the tasty wireworms in the soil.

Monitoring: It is good to check your soil for wireworms prior to planting. During tillage of the soil, sift through the dirt to inspect for wireworm larvae. We have noticed that during fallow times, larvae are easy to see underneath rocks or garden borders. Inspect these regularly for wireworms as you would for slugs. Baited traps can be constructed by placing 3-4 tablespoons of coarse, whole-wheat flour inside a fine mesh bag, such as panty hose (panty hose seem to be an important tool for many entomologists, almost as useful as duct tape). Place the bait in a hole about 6 inches deep into the soil.

Place a sheet of plastic (like a trash bag) over the bait and cover the plastic with soil. You can then come back and lift the plastic layer and the topsoil to inspect the bait for wireworms a week later. In agriculture, the threshold for pre-plant insecticide treatment is an average of .5 to 1 wireworms per bait station. After planting, monitor the germination of your seeds and scout for stunted plants.

Management: We don’t have many management tools available to the home gardener yet. This is due to the sporadic nature of the pest and our lack of knowledge about these new pests. In all the problems encountered by the home gardener, there is a single commonality; wireworms become an issue when a new garden is planted in a spot that was previously occupied by grassland pasture or sod. Once the pasture or sod is removed, the wireworms get hungry and ravaged the germinating seeds. Do not plant your garden on land previously occupied by turf or pasture. Keep the ground fallow for the majority of the growing season to drive out the wireworms from your garden area. Although we haven’t documented this scenario, these tactics may be important when growing your lawn from seed.

Tilling the soil may help reduce wireworm populations in the early spring. Wireworms are very vulnerable to mechanical damage during the pupal stage. Canadian small fruit growers have responded to the wireworm infestation by using the worms’ taste to the growers’ advantage. Farmers have planted grain in between the rows of strawberries to draw wireworm populations away from the strawberry plants in the spring. Growers then till or treat the soil with a pesticide. This method of pest management is called “trap cropping.” Canadian entomologists have been testing different seeds treated with insecticides already; however, gardeners do not have any insecticides registered for this purpose. As with all pest management, predicting and monitoring your pest problems is key; so grab a handful of whole wheat flour , a pair of pantyhose and start digging.