Wireworms as pests of maize
Date: 21 December 2016
A healthy uniform stand is the prerequisite of a successful crop. The first step in attaining this is to arm seeds with protection against yield-robbing pests. Greasy cutworm, Argentine stem weevil and black beetle can be effectively controlled by a combination of Poncho® (clothianidin) seed treatment and sound management practices. In recent times, wireworm (Melanotus communis) has been reported as an emerging pest of maize. Wireworms are unpredictable and can completely disappear into the soil depths for long periods, only to reappear later.
During early crop growth stages wireworms damage plants by either eating the seed or cutting off the seedlings below the ground level. With bigger plants they bore into the stem (underground), causing the plant to wilt and die. They are also capable of either tunneling into or damaging the roots of much larger plants.
Wireworm larva Adult (click beetle)
Photos by J. Obermeyer
Cold temperatures in winter drive wireworms down the soil profile. Warmer spring soil temperatures (≥10°C) coupled with damp conditions encourage the pest to move closer to the soil surface. Hot, dry weather ≥25° C will send larvae deeper into the soil rendering them harmless to the growing plants. Wireworms can survive on nothing but humus for up to two years. Mated females prefer weedy or grassy paddocks for egg laying. Paddocks with a long history of grass pasture have a greater chance of wireworm infestation.
More damage is expected when maize is planted in cold, wet conditions especially where sod exists or when maize is planted after a weedy winter crop. Low lying areas and high organic matter also promote wireworm populations. Heavily tilled soil pushes organic matter deeper, creating CO2 as it decomposes which attracts wireworms to the surface. Infestations are usually more common in areas that stay moist for longer.
Once established, wireworms cannot be eradicated, but their effects can be mitigated by establishing strong plants capable of negating significant damage. Because wireworms live underground, there are no practical or effective ways to control the pest after the crop is planted. An integrated management approach that encompasses chemical and cultural control options before or at planting is the best way to manage wireworms.
Though not registered for wireworm in maize locally, in the US Poncho® is registered as a seed treatment control measure for maize. As a systemic insecticide, Poncho works by eliminating the pest when it feeds on the plant. When wireworm populations are very high plant losses will still occur, but they will be less than for untreated seed.
Cultural practices that may minimise the impact of wireworm damage include:
A level, uniform, well packed and warm seedbed allows good seed-to-soil contact for quick, uniform emergence to establish a strong stand.
Planting in warm soils is preferable as cool soils can potentially delay emergence reducing vigour and exposing crops to insect attack for a longer time period.
If a paddock has potential for wireworm infestation consider increasing the seeding rate by a small percentage. Low planting populations result in thinner stands, leaving no room for recovery should the crop suffer wireworm damage.
The optimum seeding depth should be 1.5 – 2″ (38-51 cm). The goal is to plant into moisture, but too shallow or too deep especially if the growing conditions are cool and the aim is to uniform emergence and strong plants which reduces vulnerability to wireworm attack.
Pre-plant discing of fallow paddocks can reduce wireworm populations by physically injuring the larvae and bringing them to the soil surface where they can be desiccated by sunlight or eaten by birds.
Prior to planting scouting to establish a threshold for economic injury is important for wireworm management. Given their ability to survive in the soil for up to five years, multiple generations can be present in one paddock at any given time giving the larvae the potential to damage several successive crops. If you have a wireworm issue this season it would be best to assess the level using bait balls made from maize flour and determine whether a preventative measure is required at or before planting the next crop.