There are two types of Wireworm larvae found in cotton. The True Wireworm (Agrypnus variabilis Candeze), and the False Wireworms, which are a mixture of species, principally Eastern False Wireworm (Pterohelaeus darlingensis Carter), Large False Wireworm (Pterohelaeus alternatus Pascoe), Southern False Wireworm (Gonocephalum macleayi Blackburn) and Saragus spp. The True Wireworm is the larval stage of a Click beetle whereas False Wireworms are the larvae of beetles of the family Tenebrionidae which includes the Flattened Pie Dish beetles.
Identification: The True Wireworm is a soft-bodied, cream-coloured larva growing to about 20mm with a flattened, dark brown head. False Wireworms are all very similar, hard-bodied, shiny, tan-coloured larvae growing to about 30mm long. True Wireworms have fine hairs on their bodies while False Wireworms are completely smooth.
Lifecycle: Eggs are laid in summer and autumn, and larvae feed on organic matter in the soil until they reach full size in spring. Adult females lay eggs either singly on the soil surface or in batches of 10-15 eggs in crevices to 5cm deep in the soil. There are eight larval instars with a total average larval duration of 315 days; the last instar, the most damaging, occupies 48% of this time. Larvae pupate in cells in the soil during October to January. Adults emerge after 14 days.
True Wireworms prefer wet soil for egg laying (eg. irrigated summer crops), whereas False Wireworms prefer drier conditions protected by stubble or weeds. Spring infestations can be expected to be worse in newly developed fields, in fallow fields (particularly with heavy trash cover) and in fields following summer crops (particularly soybeans). There should be no problems with fields following conventional cotton, as conventional insecticide use usually prevents beetle survival, however the Bollgard II cotton varieties which are less dependant on pesticides may allow more soil insects to survive.
Host range: All field crops.
Damage: Both True and False Wireworms damage cotton by either boring into germinating seeds or chewing through young seedlings just below ground level. Damage can occur for up to 4 weeks after sowing; resulting in patchy plant stands which can necessitate replanting.
Monitoring: Wireworms are most difficult to sample. They descend in the soil as it dries out and venture near the surface only after rain or an irrigation event. Hence, they are often not noticed until damage appears. However, by this time it is not possible to control them.
Control: Any control measures must be applied before, or at sowing. The most effective control is an in-furrow band application of either a granular or liquid insecticide, though seed treatments may provide some control.
The True Wireworm is a soft-bodied, cream-coloured larva with a flattened, dark brown head. 20mm Photo: L. Wilson
Adult Ttrue Wireworms are grey to brown elongated beetles that jump and click when disturbed. They are known as click beetles. 25mm Photo: J. Wessels
Several species of False Wireworms may occur in any particular crop, depending on locality, soil type, organic matter and tillage practices. Larvae feed on germinating seed and chew on seedling roots and shoots, resulting in patchy stands. 35mm Photo: L. Wilson
Adults of the Large False Wireworm (20mm - left) and the Southern False Wireworm (9mm - Right). Photo: D. Ironside.
