Corn Wireworms

Introduction

Wireworms are the larvae of click beetles (Coleoptera: Elateridae), which include several agricultural pest species. The name “corn wireworm” refers to the larvae of multiple species, including those in the genus Melanotus. Melanotus communis (Gyllenhal) is the most common wireworm pest of corn in the US and the most abundant wireworm species in the southeastern states, including Virginia. They are especially common in fields of newly planted sod or those that follow no-till practices. They are found in many cropping systems, including maize, potatoes, sweetpotatoes, sugarcane, wheat, and other grasses and small grains. Most of their feeding damage

is to underground portions of the plant, such as the roots, shoots, and crowns. Feeding damage to germinating seeds and small seedlings leads to weakened plants and noticeable gaps in field rows due to reduced stands. In potato, sweetpotato, and carrot, wireworm feeding and tunneling forms holes up to ½ inch deep, which impacts the marketability and/or grade of the produce (Figure 1).

Identification

Eggs are white, spherical, and laid singly in batches of 50 to 130. Corn wireworms are 13 to 38 mm long, with slender, cylindrical bodies and a smooth, tough cuticle (Figure 2). First instars have a brown head and a pale-yellow thorax and abdomen that turn reddish brown with age. The subterranean larval stage can last two to several years with one or two moltings each year. Second instar larvae will overwinter in the soil. Behavior, including the duration of the larval stage and vertical movement through the soil, depends on temperature and moisture. Migration towards the surface occurs around 70 °F, and deeper burrowing is associated with extreme hot or cold.

Pupae are white or cream-colored and found in cells 15 to 30 cm deep in the soil for around 2 weeks (Figure 3). Adults will emerge in late March- early April. They are reddish brown, elongated, and around 13 mm long (Figure 4). They fly approximately 3 meters, and females prefer laying their eggs in weedy or grassy fields. Adults are nocturnal and the only life stage that occurs above ground.

Monitoring

Wireworm scouting can involve physical removal near a damaged plant, but it typically involves bait traps. Traps can be placed two to three weeks before planting, and baits can consist of a variety of soaked seeds, with oatmeal-cornflakes and rolled oats being particularly effective. In corn, four traps per acre is suggested, but a general recommendation is 10 traps per field. The economic threshold in corn is more than one wireworm per trap per week, but it varies in other crops.

The sex pheromone for the corn wireworm has been described and can be used in traps to target male click beetles. Their range can be limited, but they are a useful tool for monitoring populations, crop damage predictions, and timing insecticide applications.

Methods of Control

There are limited sources of biological control for wireworms because they are subterranean. Entomopathogenic nematodes like Steinernema feltiae, S. riobrave, or Heterorhabditis megidis may be effective, and the application of entomopathogenic fungi like Metarhizium anisopliae as granular formulations of their spores can diminish wireworm damage in potatoes. Tilling and disking a field before planting can lower M. communis populations by physically injuring larvae and allowing them to desiccate or be eaten by predators at the surface. Delayed planting can avoid peak larval feeding activity, and resistant crop varieties are available.

Insecticide treatments are not available for this pest after planting, so prevention relies on seed- applied or in-furrow insecticides. Cyclodienes (IRAC Group 2A), carbamates (IRAC Group 1A), and organophosphates (IRAC Group 1B) have been used previously. However, their use has been restricted due to impacts on non-target organisms, high soil residues, and human health concerns. Current insecticides in use include various neonicotinoid compounds (Group 4A) such as clothianidin, imidacloprid, and thiamethoxam, pyrethroids (Group 3A) such as bifenthrin and tefluthrin, and the phenylpyrazole insecticide fipronil (Group 2B). Neonicotinoid and pyrethroid insecticides can lower wireworm damage but may not cause significant mortality. Wireworms can be intoxicated by these chemistries, halting feeding activity temporarily, but recover later on.

Meta-diamides and isoxazolines (Group 30) were first used in agriculture to control insect and mite pests, including those with resistance to other pesticides. Broflanilide has been found to be effective against pests resistant to cyclodienes and fipronil. As of December 2021, there have been no known cases of resistance to Group 30 insecticides. Currently, investigations into their effectiveness as wireworm control are ongoing in both Canada and the US.