The western corn rootworm (Diabrotica virgifera virgifera) is a chrysomelid beetle originating from North America, where it is regarded as the main pest of maize. Nowadays it is present in many European countries, including Italy, where it is widespread in the maize-growing areas in the north. Maize is the only crop in which high population densities can develop, and consequently it is susceptible to serious damage. Adults may also be found on Poaceae, Asteraceae, Fabaceae and Cucurbitaceae.
Life cycle and appearance
In Europe, the western corn rootworm produces a single generation each year. The pest overwinters in the soil in the egg stage. Egg hatching start in the middle of May and continues until the begin of July, with the presence of larvae peaking around the middle of June. The whitish-coloured larvae move around in the soil, feeding on the maize roots. Their development lasts approximately one month, during which they pass through three larval instars. The pupa is hard to find in the soil, as this stage lasts only a few days.
The adults, 5-6 mm long, are dark yellow in colour and there are three black stripes on the wing covers. They emerge throughout the summer period with a peak between the end of June and the middle of July. The adults may mate several times during their lifetime, and females lay an average of 400 eggs each. They feed on both the leaves and the silk of the female inflorescences of maize.
Damage is caused by both larval and adult feeding, although it is the former that causes the most extensive damage.
The larvae feed on the rootlets and dig tunnels in the larger roots. This leads to a reduction of the root system, making the plant more susceptible to lodging, reducing its water and nutrient uptake capacity and leading to yield loss. When water supply is sufficient, the plants continue to grow after lodging. This causes a typical gooseneck-like shape also called goose necking.
The adults, feeding on the silk of the female flowers, can cause flower abortion and, in the case of heavy infestations, significantly reduce the number of seeds. During the milk and dough stages of maturity, the adults may also feed on the kernels. This does not usually result in heavy yield losses, but it can facilitate the development of toxic fungi in the cob, with the resulting production of mycotoxins.