Wireworm-true and false

True wireworm (adult and larva)

False wireworm (adult and larva)

Wireworm damage to sweet potato

True and false wireworms are the larval stages of 2 types of beetles. The adults of true wireworms are commonly called click beetles while the adults of false wireworms are known as darkling beetles.

There are 667 different species of true wireworms found in Australia. BSES entomologists identified 26 species of true wireworm in established cane fields in north, central and southern Queensland and in New South Wales.

Scientific name

True wireworms of recorded agronomic importance in Australia include:

• potato wireworm (Hapatesus hirtus)
• sugarcane wireworm (Agrypnus variabilis).

False wireworms are major soil insect pests of establishing summer and winter crops in Queensland and New South Wales. False wireworms of agronomic importance in Australia include:

• northern false wireworm (Gonocephalum carpentariae)
• southern false wireworm (Gonocephalum macleayi)
• small false wireworm (Gonocephalum misellum)
• eastern false wireworm (Pterohelaeus darlingensis)
• striate false wireworm (Pterohelaeus alternatus).

Life history

The lifecycle of true wireworms varies greatly between species. For example, sugarcane wireworm takes 1 year with 4 larval instars to complete its cycle whilst the potato wireworm takes approximately 4 years with 10 larval instars.

Summary of the lifecycle of the sugarcane wireworm (a true wireworm): Beetles emerge in October–November and migrate if habitat is not suitable. Female beetles require food (green leaf material) before they begin egg laying. Females lay eggs in November–February in batches of approximately 10 to 15 eggs. Eggs are laid on soil surface or in small crevices.

Once larvae have emerged they generally remain in the top 50mm of the soil profile during the first instar stage. Later larval instars will move deeper in the soil profile in response to moisture and temperature. Sugarcane wireworm take 10 months and 4 instars for larvae to fully develop in SE Queensland. Once larvae mature (in September–October) they hollow out small soil cells to pupate within. This stage generally takes about 2 weeks.

The lifecycles of the major false wireworm pest species are all similar: In central Queensland beetles emerge in October–January and migrate if the habitat is not suitable. Female beetles require food (green leaf material) before they begin egg laying. One month after emergence the females begin laying eggs on the soil surface or just below. Egg laying may continue for up to 20 weeks but is terminated by cool weather. Females are capable of laying 1000 eggs or more.

Once larvae have emerged they generally remain in the top 50mm of the soil profile and are relatively immobile. Larvae develop through autumn, winter and spring. It is not possible to distinguish between larvae instars. They are believed to have up to 11 instars.

Pupation occurs in September–October and is triggered by rainfall. This stage generally takes about 2 weeks.

Distribution

Common in horticultural districts throughout Queensland and New South Wales.

Host range

Sweet potato, potato, capsicum, chilli, sweetcorn and tomato as well as cover crops used in horticultural systems such as sorghum, oats and sugarcane.

Damage

Larvae chew holes in storage tubers and storage roots (e.g. potato and sweetpotato) and chew on roots and stems of several other vegetables. Adults chew on stems just below the ground.

'Shotgun' is the name Australian sweetpotato growers use for the random scattering of small holes caused by wireworm larvae feeding on sweetpotato storage roots. Even though holes may be shallow, a few holes can make a sweetpotato unmarketable.

Wireworms also chew on roots causing poor establishment of transplanted seedlings.

Control

Larvae

The recommended strategy to control wireworms is to incorporate insecticides into the soil before planting. Controlling wireworm larvae can be difficult as they move up and down the soil profile in response to temperature and soil moisture. They move down the soil profile as the topsoil dries out and heats up.

Soil insecticides are generally incorporated by a rotary hoe to a depth of approximately 20cm. It is important that the wireworms are in the treated zone so that the larvae come in contact with the insecticide. If wireworms are below this zone, insecticides will only repel larvae from the developing storage roots for as long as the insecticide remains active. Aim to incorporate insecticides into an even soil moisture profile. If the top 20cm of soil is dry, control may be improved by irrigating the block to moisten the topsoil, thus encouraging wireworm larvae to move up into the treatment zone.

Beetles

You may also need to control wireworm beetles during the cropping period, because if suitable egg laying sites are not present when adults emerge they will migrate to seek actively growing crops to lay their eggs. An irrigated sweetpotato block provides all the necessary environmental conditions for the successful development of eggs and larvae.

Lifecycle research indicates peak flights can occur anywhere from October through to January.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.