Wireworms are the larvae of beetles in the family Elateridae. The adult beetles are commonly known as click beetles. When these beetles are placed on their back, they click loudly and jump up into the air, giving them the name click beetles. There are thousands of species of click beetles worldwide, and close to 1000 in North America alone. While the adults do little to no damage to crops, the larval wireworms are a destructive soil pest and can plague root vegetables and other plants. They seem to be most problematic with corn, potato, and sugar beet. The larvae can destroy germinating seeds, bore into roots, and even tunnel into the stems of certain plants. Root feeding on young plants can cause serious damage and even crop failure. While wireworms have a wide host range, they appear to be most prevalent on grasses and grains. Presence of nearby grasslands increases the likelihood of a farm having an issue with wireworms.

Photo of a larval wireworm. Image from Flickr.

After hatching from the egg, wireworms are as small as 2 mm and white. As they mature, the worms darken to a tan or reddish brown color and can reach lengths of up to 1½ inches depending on the species. Their bodies are segmented and appear similar to mealworms. The larvae live in the soil, consuming plant material, and descend deeper into the soil (up to 5 feet) when temperatures drop during the winter. The larvae overwinter there and return to shallower soil in the spring. It can take 2-5 years for wireworms to fully mature and metamorphose into an adult beetle, depending on the species.

In the spring and summer, larvae begin to pupate. The pupal stage lasts about three weeks. Once the adults emerge, they tend to remain in the same general area as they developed, even though they can fly. The adults live in grassy areas and moist soils, and mate from mid-April to early June. Females can lay 50-350 eggs, either singly or in clusters. They lay these eggs 1-6 inches deep in moist soil, where they will hatch 3-4 weeks later. After laying eggs, the adult beetles continue to live in the soil and can overwinter deep in soils, similar to the larvae. Due to their ability to overwinter both as larvae and adults, it can be difficult to rid infested soils of wireworms and click beetles.

Photo of an adult click beetle. Image from Flickr.

Wireworms were historically managed with synthetic insecticides. Many of the insecticides that were originally used to control these insects are now banned, but newer ones have come to market and are used by many conventional growers. Regardless, organic growers do not have the option of using chemical pesticides. This is unfortunate, because the current strategies for organic management of wireworms can be insufficient for serious infestations.

There is some evidence that certain nutrient imbalances in crops can increase wireworm damage. As with most pest insects, maintaining healthy plants should be a priority. Soil pH seems to have an effect on the development of wireworms. When soil pH is below 5.5, there is a significantly greater risk of wireworm damage, whereas soils with pH above 6.5 are safest. Multiple studies have shown that the soil temperature at time of planting has a significant effect on damage caused by wireworms during that growing season. If the soil temperature is above 54 ℉ when seeds are sown, there seems to be much less herbivory damage. Wet conditions in the spring before sowing seem to favor wireworm infestation as well. Soils with organic matter content above 5.5% may promote wireworm damage, although the mechanism is unclear. It is possible that the additional moisture retention capacity of the organic matter creates favorable conditions for the larvae.

When managing any pest organism on an organic farm, supporting a diverse community of predators is a foundational step in controlling the pest. Predatory nematodes and ground beetles are both known to prey on larval wireworms and can help keep their populations in check. While precise strategies for the biological control of wireworms are still in their infancy, there is evidence that supporting a diverse fungal community can help prevent wireworm outbreaks. Given that wireworms and click beetles both have exoskeletons of chitin, some fungi which produce the chitinase enzyme are able to digest these pest insects. Organic amendments which are high in chitin can be added to the soil to promote bacteria and fungi which produce chitinase. Fungi in the genus Metarhizium are known to infect and kill wireworms and adult click beetles, and there have been multiple studies on the applications of these fungi as biological control agents. A study showed that Metarhizium strains with higher chitinase activity were more virulent to wireworms in the genus Agriotes. It has been thoroughly demonstrated that Metarhizium fungi can infect and control these insects, but there is some doubt as to whether soils can be practically inoculated on a large scale. Some authors have suggested that this could be partially solved by placing fungi-inoculated bait throughout fields, in order to attract the wireworms directly to the fungus. It has also been shown that Metarhizium fungi and Spinosad interact synergistically: When wireworm larvae infected with Metarhizium are exposed to 1.5 to 3 ppm Spinosad, their fatality rate increases significantly higher than Metarhizium or Spinosad alone. This indicates that Spinosad somehow makes the wireworms more susceptible to the pathogenic fungi. While more research is needed, applying Spinosad with Metarhizium inoculants could be a useful tool to reduce wireworm damage in infested fields.