Causal agent(s) and transmission
Wireworms are insects of the order Coleoptera (family Elateridae). The adult is blackish brown, measures 10 to 15 mm and is not harmful. It is also named click beetle because of the characteristic sound made when getting upright on to its legs after falling on its back. The female lays eggs from May to July below the soil surface in fields covered with plant foliage (grassland, fallow land, cereals, maize, potatoes, etc.).
Damage is caused by the larvae (photo 1) which are bright orange-yellow in colour. They are hard-skinned worms ranging in size from 5 to 25 mm depending on their age and the larval stage.
Many species of wireworms may attack potatoes but the four most harmful species belong to the genus Agriotes. These are either long-cycle species such as A. lineatus, A. sputator and A. obscurus or short-cycle species such as A.sordidus (photo 2). The long-cycle species have an average 5-year cycle, including 4 years of underground larval stages, whereas the short-cycle species has a typical 1-to-2 year cycle in the South of France, where this species is now predominant, but it may have a longer cycle in colder areas where it is also currently spreading.
Damage occurs during two major periods of the wireworms’ activity, in spring and at the end of the summer, when the larvae are in the topsoil and feed on underground plant organs before moulting.
When climatic conditions are less favourable in winter and summer, larvae move deeper into the soil to about 40-50 cm and stop feeding.
Wireworms do not usually affect the growing cycle of the potato crop but they can downgrade the aspect of the tubers by the presence of holes or galleries, dug by the larvae when feeding on the tuber flesh. In the case of severe soil infestation, tubers can be heavily damaged thus downgrading their visual quality, even making them unsuitable for marketing.
Symptoms on tubers
Wireworms have no effect on the vegetative development of the potato.
But they feed on the flesh of tubers by digging galleries affecting the presentation of potato (Photos 3 to 5) and making it unfit for marketing.
Tuber damage (holes or galleries) caused by wireworms is intensified when the previous crop has favoured the development of larva populations; for example, permanent pastures or grassland, fallow land, irrigated corn, which are suitable for egg laying.
The damage is exacerbated in wet soils (rain, irrigation), which are favourable to the presence of larvae near the soil surface.
Tests have shown that damage thresholds of 20 to 40 larvae/m2 (evaluated before planting by boring with an auger) represent a significant risk to the potato crop, either to the root system or to the yield.
It is easier to evaluate the size of the population in a field by means of either larva traps containing a bait (pots filled with vermiculite containing a mixture of wheat and corn), or by means of a pheromone trap that captures adults. Pheromone traps are commercially available with species specific pheromones for A. lineatus, A. sputator, A. obscurus and A. sordidus.
The potato crop can be protected either by a full soil treatment or a localised planting furrow treatment, using an authorized insecticide for this purpose.
If ploughing heavily infested grass or fallow lands, the soil must be thoroughly cleansed for 3 years by appropriate cultivation methods and by using efficient insecticide on the corresponding crops (maize, sunflower, beetroot, etc.).
Avoid planting in short rotations or in wet fields that are favourable to both the survival of the larvae and to the damage.
To be effective, the soil cultivation must be done during dry conditions or during the laying period, in late spring or late summer; this partly destroys the eggs and the young larvae, which are very susceptible to dry conditions, but does not affect the older larvae.
This method is not as efficient with A. sordidus, which has a longer flying period and a heterogeneous larval development.