There are several serious soil insect pests that we manage periodically in vegetable production, including wireworms, white grubs, and seedcorn maggot. Unfortunately, when symptoms of damage by these pests become apparent, there are no rescue treatments. Our management strategies are preventative, not reactionary. Two important factors that, in part, impact the risk of soil insect problems are rotation and field history. The most challenging rotation for soil pests is one following established sod where wireworms and white grubs can be common.
Most wireworm larvae are hard, chestnut brown, smooth, and vary from 1/2 to 1 1/2 inches in length when grown (Figure 1). Some species are soft and white or yellowish in color. The adults are click beetles. Wireworms are especially destructive to sweet corn, potatoes, sweet potatoes, root crops, cabbage, and beans. Eggs are laid around and near the bases of grass plants. Adults live 10 to 12 months spending most of the time in the soil. While the egg stage lasts only a few days to a few weeks, larvae require 2 to 6 years in the soil while feeding on roots of grasses and other plants to complete their development. Wireworm larvae migrate upward and downward in the soil depending on soil moisture, so it is often hard to find them in dry summer weather, even in severely infested fields.
Wireworms are most often a problem in fields that have been in sod for many years, particularly bluegrass sod. With wireworms, there is no effective rescue treatment once symptoms of damage are observed. In high risk situations, soil insecticides before planting or seed treatments can be used to manage wireworms.
White grubs (Figure 2) are commonly found with wireworms as they both frequently follow sod. White grubs are larvae of scarab beetles, including May beetles and Japanese beetle. They are root pruners; above ground symptoms include water or nutrient stress and stand loss.
As with wireworms, white grubs must be controlled preventively. Soil insecticides prior to planting and seed treatments can be used to lower white grub numbers. Once damage is apparent, there are no effective treatments.
Seedcorn maggots (Figure 3) feed on decaying organic matter, seeds, and seedlings. They attack a wide range of crops, including beans, peas, cucumber, melon, onion, corn, pepper, potato, and other vegetables. Seedcorn maggot eggs are laid just below the soil surface in tilled ground that is high in organic matter. This is a pest favored by early planting dates and heavy cover crops.
Crops planted early, when the weather is cool and wet for long periods of time, are potentially at greater risk to damaging infestations. Planting in well-prepared seedbeds and planting sufficiently late for quick germination are means of preventing injury from this insect.
Ric Bessin, Extension Entomologist