Wireworms and false wireworms in field crops
Garry McDonald, La Trobe University
Wireworms and false wireworms are common, soil-inhabiting pests of newly sown winter and summer crops. Wireworms are the larvae of several species of Australian native beetles which are commonly called "click" beetles, coming from the family Elateridae.
False wireworms are also the larval form of adult beetles, some of which are known as pie-dish beetles, which belong to another family (Tenebrionidae), but have distinctively different forms and behaviour. Both groups inhabit native grassland and improved pastures where they cause little damage. However, cultivation and fallow decimates their food supply, and hence any new seedlings that grow may be attacked and sometimes destroyed. They attack the pre- and post-emerging seedlings of all oilseeds, grain legumes and cereals, particularly in light, draining soils with a high organic content. Fine seedling crops like canola and linola are most susceptible.
The incidence of damage caused by wireworms and false wireworms is increasing with increasing use of minimum tillage and short fallow periods.
(family Tenebrionidae; numerous species)
These insects are the larvae of native beetles which normally live in grasslands or pastures and cause little or no damage in this situation. In crops, they are mostly found in paddocks with high stubble and crop litter contents. They may affect all winter sown crops.
There are a large and varied number of species, but the general characteristics of false wireworm are as follows. Larvae are cylindrical, hard bodied, fast moving, golden brown to black-brown or grey with pointed upturned tails or a pair of prominent spines on the last body segment. There are several common groups (genera) of false wireworms found in southeastern Australia:
- the grey or small false wireworm (Isopteron (Cestrinus) punctatissimus). The larvae grow to about 9 mm (3/8") in length. They are grey-green in colour, have two distinct protrusions from the last abdominal (tail) segment and tend to have a glossy or shiny exterior (Figure 1). Hence, they are most easily recognised in the soil on sunny days when their bodies are reflective. The adults are slender, dark brown and grow to about 8 mm in length. The eggs are less than 1 mm in diameter. There are several species of this pest genus, although I. punctatissimus appears to be the species most associated with damage.
- the large or eastern false wireworm (Pterohelaeus spp.). These are the largest group of false wireworms. They are the most conspicuous in the soil and grow up to 50 mm in length. They are light cream to tan in colour, with tan or brown rings around each body segment, giving the appearance of bands around each segment (Figure 1). The last abdominal segment has no obvious protrusions, although, under a microscope, there are a number of distinct hairs. Adults are large, conspicuous and often almost ovoid beetles with a black shiny bodies (Figure 2).
- the southern false wireworm (Gonocephalum spp.) grows to about 20 mm in length, and has similar body colours and marking to the large false wireworm. Adults are generally dark brown-grey, oval beetles, which sometimes have a coating of soil on the body. Adults have the edges of the body flanged, hence the common name "pie-dish" beetles.
Usually only one generation occurs each year. However, some species may take up to 10 years to complete the life cycle. Adults emerge from the soil in December and January and lay eggs in or just below the soil surface, mostly in stubbles and crop litter. Hence, larvae are most commonly found in stubble-retained paddocks.
Larvae of most false wireworm species prefer to feed on decaying stubble and soil organic matter. When the soil is reasonably moist, the larvae are likely to aggregate in the top 10-20 mm where the plant litter is amassed. However, when the soil dries, the larvae move down through the soil profile, remaining in or close to the subsoil moisture, and occasionally venturing back to the soil surface to feed. Feeding is often at night when the soil surface becomes dampened by dew.
Nothing is known of the conditions which triggers the switch in the false wireworm feeding from organic matter/ litter to plants. Significant damage is, however, likely to be associated with soils that remain dry for extensive periods of time. Larvae are likely to stop feeding on organic matter when it dries out, and when the crop plants provide the most accessible source of moisture.
Affected crops may develop bare patches, that could be large enough to require resowing. Damage is usually greatest when crop growth is slow due to cold, wet conditions.
Infestations of the small false wireworm can be as high as hundreds of larvae per square meter, although densities as low as 5 larger false wireworm larvae per square meter can cause damage under dry conditions.
The larvae of the small false wireworm are mostly found damaging canola and other fine seedling crops shortly after germination. They feed on the hypocotyl (seedling stem) at or just below the soil surface. This causes the stem to be "ring-barked", and eventually the seedling may be lopped off or it wilts under warm conditions. Larger seedlings (eg. grain legumes) may also be attacked, but the larvae appear to be too small to cause significant seedling damage.
The larger false wireworms can cause damage to most field crops. The larvae can hollow out germinating seed, sever the underground parts of young plants, or attack the above surface hypocotyl or cotyledons. In summer, adult beetles may also chew off young sunflower seedlings at ground level. Damage is most severe in crops sown into dry seedbeds and when germination is slowed by continued dry weather.
Sampling and detection
The principles for detection and control of false and true wireworms are generally similar, although different species may respond slightly differently according to soil conditions.
Crops should be sampled immediately before sowing. There are two methods available, although neither provide a 100% reliable method of detection. This is because larvae change their behaviours according to soil conditions, particularly soil moisture and temperature.
- Soil sampling. Take a minimum of five random samples from the paddock. Each sample should consist of the top 20 mm of a 0.50 m 0.50 m area of soil. Carefully inspect the soil for larvae. Calculate the average density per meter squared by multiplying the average number of larvae found in the samples by 4. Control should be considered if the average exceeds 10 small false wireworm, or 10 of the larger false wireworms.
- Seed baits. Seed baits have been used successfully to sample true and false wireworms in Queensland and overseas. In Victoria, they have not been rigorously tested. Preliminary work has shown that they can be used to show the species of larvae present, and give an approximate indication of density. Take about 200-300 gm of a large seed bait, such as that of any grain legume, and pre-soak over 24 hours. Select 5 - 10 sites in the paddock and place a handful of the soaked seed into a shallow hole (50 mm) and then cover with about 10 mm of soil. Mark each hole with a stake, and re-excavate each hole after about 7 days. Inspect the seed and surrounding soil for false wireworm larvae. This technique is most likely to be successful when there is some moisture within the top 100 mm of soil.
Larvae of the small false wireworms are relatively difficult, although not impossible, to see in grey and black soils because of their small size and dark colour. However, they can be found in the top 20 mm of dry soil by carefully examining the soil in full sun light. Larvae of the other false wireworm species are more prominent because of their relatively pale colour and large size.
Crop residues and weedy summer fallows favour survival of larvae and over- summering adult beetles. Clean cultivation over summer will starve adults and larvae by exposing them to hot dry conditions, thus preventing population increases. Suitable crop rotations may also limit increases in population numbers. Seed beds must be sampled prior to sowing if control is to be successful. Insecticides may be applied to soil or seed at sowing. Most chemicals registered for false wireworm control are seed treatments, although these may not be consistently reliable. They probably work best when the seedling grows rapidly in relatively moist soils. Adults may also be controlled with insecticide incorporated into baits.If damage occurs after sowing, no treatment is available, other than resowing bare patches with an insecticide treatment.
Figure 1. Two common false wireworm larvae and a 'generalised' true wireworm larva.
(family Elateridae; numerous species)
These slow moving larvae tend to be less common, although always present, in broadacre cropping regions and are generally associated with wetter soils than that of false wireworms.
Larvae grow to 15-40 mm, are soft-bodied, flattened and slow moving; they can be distinguished from false wireworms which are hard bodied, cylindrical and fast moving. Their colour ranges from creamy yellow in the most common species to red brown; their head is dark brown and wedge-shaped. The tail piece is characteristically flattened and has serrated edges (Figure 1). Adults are known as "click" beetles, due to their habit of springing into the air with a loud click when placed on their backs. They are dark brown, elongated and 9-13 mm long (Figure 2).
There may be one or several generations per year, depending on species. Most damage occurs from April to August and adults emerge in spring. Wireworms prefer low-lying, poorly drained paddocks and are less common in dry soils. Larvae are reasonably mobile through the soil and will attack successive seedlings as they emerge. Adults are typically found in summer and autumn in bark, under wood stacks or flying around lights.
There is little known of the biology of most species, but one species (Hapatesus hirtus) is better understood. This species is known as the potato wireworm although it is found in many other crop and pasture situations. It is very long-lived and probably takes 5 years or more to pass through all the wireworm stages before pupating and finally emerging as an adult beetle.
Adult click-beetles emerge in spring and summer, mate and lay eggs, and then may spend a winter sheltering under the bark of trees. The connection between trees and adult beetles is probably why damage is often, but not always, most pronounced on tree -lines. The wireworms have a long life in the soil and are active all year, even in winter.
The damage caused by wireworms is similar to that of false wireworms, except that most damage is restricted to below the soil surface. Larvae eat the contents of germinating seed, and underground stems of establishing plants, causing wilting and death.
Sampling, detection and control
See "false wireworm" for full details. Wireworms and false wireworms can only be controlled if they are detected in the seedbed before sowing. Insecticide can be applied to the soil with fertiliser, or seed can be treated.
Figure 2. The adult (beetles) of the false wireworm (left) and true wireworm (right)