Published on Tuesday, May 12, 2020
The larvae feed on corn kernels and the crown area of corn seedlings. Infested areas exhibit symptoms like random stunted or wilted plants from wireworm feeding on the root and crown portions of the plant. Random missing plants indicate where the kernel itself has been fed on.
The larvae of wireworms are slender and hard-bodied, measure between 0.5 and 1.5 in. in length, and vary in color from yellow to dark orange. Wireworms also have a distinctive head with protruding mouthparts as well as small (but visible) true legs near the head. Most species of wireworm have a multi-year life cycle, spending two to five years in the larvae stage. In fields with established wireworm populations, generations often overlap, making it difficult to eradicate the pest.
Control measures are limited to planter-applied insecticides and seed treatments labeled for wireworm control. Reducing vegetation ahead of planting and tillage to reduce residue can mitigate damage from wireworm.
Maggots feed directly on kernels which leads to missing plants or very small, weak plants that eventually die or fail to contribute to yield. Seedcorn maggots, unlike wireworms, tend to show damage throughout the field, not just in random areas.
Seedcorn maggots are small, white to yellow maggots that burrow into corn kernels and prevent emergence by damaging the embryo and hollowing out the seed. These maggots are often no longer than 0.25 in. in length, without legs, and lacking a segmented head. Seedcorn maggots overwinter in the soil in a puparium structure. Adults emerge in April or May as small flies, mate, and lay their eggs. The adults are attracted to areas with heavy decaying residue, adequate soil moisture, or manured areas. The eggs hatch in two to three days and then the larvae enter a feeding cycle, which lasts two to three weeks, begins. Multiple generations occur per year, but later generations pose no threat to corn crops.
There are no rescue treatments available for seedcorn maggots; thus, farmers must rely on seed treatments or planter-applied insecticides for control.
Most damage from white grubs occurs from root pruning and damage to the mesocotyl of the corn plant, causing plant death. White grub damage is typically loca lized in a field to areas that are attractive for egglaying, like those with vegetation or are close to a tree line.
These grubs typically range from 0.25 to 1 in. long with brown heads and six prominent legs. There are multiple species of white grubs – the generic term for the larvae of Japanese Beetles, May June Beetles, or chafer beetles, among others. Identification is important when diagnosing damage from white grubs as their lifecycles vary in length. The pattern of small hairs on the raster of the grub, as depicted in the image below, is useful for distinguishing species.
Control is limited to at-planting via seed treatments (which offer limited control) or through planter-applied insecticides.
Slug damage is limited primarily to the leaves, but stand loss can occur with extreme infestations. Slug feeding scars run parallel to the midrib and are often referred to as “window panes” since the leaf cuticle remains. Damage from slugs tends to be heavier in no-till fields, fields with cover crops, and areas that have experienced cooler, wetter weather.
Slugs are mollusks that can be early-season corn pests in wet conditions where there is a lot of residue.
Chemical control with slug baits has been somewhat effective, but can be very expensive. The best way to combat slugs is to remove vegetation that protects slugs and keeps the soil moist prior to planting. Row cleaners are beneficial because they remove plant residue in the row and disrupt the slug’s habitat. Waiting to plant until soils and air temperatures are warmer, which reduces slug populations and facilitates faster corn growth, can also be effective. Insect control traits in corn (i.e., Bt), planter-applied insecticides, and insecticidal seed treatments are not effective control measures.
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Author: Jon Skinner
Categories: Agronomy, Agronomy Talk