Wireworms and corn
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Wireworms (multiple species of Coleoptera: Elateridae) the larval stage of click beetles.
They are wormlike with an elongated body and distinct heads. They have short but visible legs and a hard textured body. The wireworm’s chewing mouthparts project forward (Figure 1). They are very active when disturbed.
Species from multiple genera can attack corn and several species occur in MN. Species of Melanotus are perhaps most commonly associated with corn damage but other species have also been cited as corn pests. They include
Identification to the species level is difficult.
The larvae of most species are dark orange. They have a distinct head capsule and projecting mouthparts. True legs are small but present near the head.
A large Melanotus sp. wireworm is often associated with alkali rims in parts of southern MN. A small, cream-colored species also causes problems in southwestern Minnesota.
The adults are long, narrow beetles that are rounded on the front and back (Figure 2). They have a spine on the 1st thoracic segment with a notch on the 2nd thoracic segment.
This joint allows the beetle to propel themselves into the air with an audible clicking sound. The trait is where the click beetle gets its name. The trait allows the beetle to right itself if it becomes turned on its back and may also function as a defensive mechanism.
Where they live
Click beetles are distributed worldwide, depending on the species. Some European species have become established in North America.
Members of the click beetle family live in many habitats. The larvae of some species are plant feeders, while some are predaceous and others may be both.
In addition to corn, wireworms are pests of wheat, potato and several other crops in agricultural environments. Wireworms are listed as pests of soybean but damage to that crop has been rare in Minnesota.
The species that attack corn are associated with grass and fields that were formerly in sod. These fields are at higher risk for damage.
Wireworm pest species overwinter as larvae. Research suggests that females prefer particular soil types and soil moisture regimes for egg-laying.
Although Conoderus spp. completes its life cycle in a single year, most species can have multi-year life cycles depending on food supply and climate.
Many wireworms move vertically in the soil profile and are found shallow in spring and deeper later in the season. This behavior may depend on species and in response to time season or response to soil temperature.
Pupation occurs in the soil. The adults are nocturnal plant feeders but are not pests. They are common and attracted to lights.
Because they live in the soil, the natural enemies of wireworm are not well documented. Predators include certain nematode and fly larva species. They are also susceptible to fungal pathogens.
Wireworms and their feeding injury occur below ground. Corn seed is typically attacked at the embryo or germ. Injury to germinating seeds can kill plants before they emerge.
On emerging corn, the growing point is often targeted. Above ground symptoms on emerged, injured plants include wilting or stunting. Stunted plants are often purple or dark in color. Closer inspection reveals holes or tunnels in seed, roots and lower stems (Figure 3). Corn plants are more vulnerable when small.
Damage is often worse in cooler springs when corn development is slowed. Yield loss occurs as a result of reduced stand and stunted uncompetitive plants. Historically, wireworm problems have been associated with fields planted after sod but we have also seen considerable injury to fields in a corn-soybean rotation associated with previous year’s grassy weeds.
Wireworm can often be observed when digging in fields in early spring. However, if you find them, it does not mean that stand loss is inevitable.
If you want to sample for wireworms before planting in the spring, use bait stations buried with wheat and covered with black plastic. However, this requires considerable effort a during a short sample window and action thresholds are somewhat subjective.
Scout for damaged plants from emergence until the V5 stage. Scouting should be combined with evaluation of corn emergence, weeds and other early season corn insect pests such as cutworms, white grubs and seed corn maggot.
While economic problems generally haven’t been widespread or common, wireworm problems have been reported even less frequently in recent years. This may be due to increased use of neonicotinoid insecticides (Cruiser, Gaucho, Poncho) on corn seed or simply unfavorable environmental conditions.
There are no rescue treatment options for corn wireworms. However, a labeled at-plant rootworm soil insecticide or neonicotinoid insecticide treated corn seed is recommended when corn is planted following CRP, pasture or alfalfa and in replant situations. In-furrow placement of liquid and granular insecticides will provide more consistent control.
Neonicitinoid insecticides and pyrethroid insecticides can prevent wireworm damage but may not kill wireworms. The wireworms may be only temporarily intoxicated and stop feeding but can recover to fight another day (or year).
Always read and follow the pesticide label.
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Reviewed in 2018