Wireworms are the immature, damaging phase of click beetles (Coleoptera: Elateridae). The following species can be found in the PNW region:
- Flat Wireworm (Aeolus mellillus)
- Exotic Click Beetle (Agriotes sputator)
- Great Basin Wireworm (Ctenicera pruinina)
- Sugarbeet Wireworm (Limonius californicus)
- Pacific Coast Wireworm (Limonius canus)
- Western Field Wireworm (Limonius infuscatus)
Wireworms feed underground and are mostly known for the considerable damage they can cause to potatoes, sugarbeets, and other mature root crops. Spring-seeded cereal grains are often affected and wireworm damage can also occur to germinating or seedling beans, brassicas, corn, and many other vegetables.
Identification and Life Cycle
Adult click beetles are 1/4- to 1/2″ long beetles with elongated bodies. The name comes from a behavioral adaptation: beetles are able to right themselves (and possibly escape predation) by ‘clicking’ an appendage located between the thorax and the abdomen.
Larvae (wireworms) live exclusively in the soil and have:
- Hardened, cylindrical bodies. Coloration is yellow to dark orange, and both ends may appear darker.
- The head is slightly tapered and the posterior end has claspers that form a ‘keyhole’, the shape of which aids in identifying to species.
- 3 pairs of legs are present on the thorax.
The life cycle of this insect is important to understand because it may aid in monitoring and management.
Larvae (wireworms) prefer cool, wet conditions (except C.pruinina), so they migrate vertically within the soil column twice a year. When soil temperatures warm to about 50°F(10°C), wireworms can be found actively feeding in the top 6″ of soil. During the summer (>80°F, 27°C), they move back down, sometimes to extreme depths of 5 ft or more. When soil cools in the fall, a similar movement occurs. Adults are nocturnal and feed on foliage and pollen.
Prior to planting in the spring is the best time to sample for wireworms . Populations tend to be clumped throughout a field, so while the ‘shovel & sift’ method is easy to do, it may miss patches and therefore underestimate abundance. If this is your method of choice, be sure to take at least 20 samples throughout a field, and concentrate on low, wet areas.
The most effective way to estimate wireworm pressure is by using bait stations placed in any area suspected of wireworm activity:
- Fill 6+ nylon stockings (pantyhose toes) with a 50:50 mix of untreated wheat or corn seed that has been soaked in H2O for 24 hours.
- Dig shallow holes (4-6″) across the field and bury 1 stocking in each hole. Flag the locations. Covering with plastic to incr. the temperature will increase efficacy. Predator proofing may be necessary to prevent birds, raccoons, etc. from digging up the bait.
- Return in 7-10 days, open and sift the contents of the stocking, and count the number of wireworms. Damage potential varies, but a nominal threshold is that 1-2 wireworms per trap indicates elevated levels and the potential need for management. If 4+ wireworms are found per trap, the field may need to be abandoned, depending on the crop.
Cultural control: Although some propose that including brassicas in a crop rotation may help (due to potential toxicity of allyl-isocyanate to soil dwelling arthropods), rotation in general is not considered a useful strategy because larvae can persist for 3-6 years in the soil (?!). If plausible, deep tillage in mid-summer can help destroy pupae.
Biological control: Birds do consume the larvae, but usually do not have a large impact. Carabid beetles and rove beetles and other generalist predators should be encouraged. There is a Dipteran parasite but effects are unknown.
Chemical control: Seed treatments (imidacloprid, clothianidin, thiamethoxam) are approved for some crops and can help reduce damage, but efficacy is sometimes reduced at lower (recommended) label rates. Broadcast treatments may be more effective than banded applications because wireworms are highly mobile (moving 18″ (45 cm) in 2 weeks).