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Western corn rootworm (Diabrotica virgifera virgifera) – pest management
Prevalence. The western corn rootworm is a species native to America and was first reported in Europe in 1992 and has permanently expanded its range. In the United States, this worm is considered one of the most damaging to the corn crops, causing very high annual losses.
Description. The adults have a yellow to green body and display sexual dimorphism (the female may differ from the male in appearance). The female has a body length of 4-6.8 mm and has on the elytra 3 longitudinal bands that can merge. The antennae represent about ¾ of the body length and the abdomen is pointed at the extremity. The male has a body that is 4.4-6.6 mm long and the antennae are longer than the body. The elytra are darker on the anterior side and the abdomen is rounded terminally. The egg is yellow and oval. The larva is thin, white, with a black head, and can reach 13 mm in length at maturity.
Biology and ecology
The western corn rootworm produces one generation per year and overwinters in the egg stage in the ground. Eggs can be found in the ground from July to June of the following year. Egg hatching and the appearance of larvae begin in mid-May and end in early August.
The adults appear in the corn crops from June to October with a maximum number registered in the middle of July and continuing in August. They are more active in the first hours after sunrise and in the last hours before sunset. The adults mate a few days after hatching and the laying of eggs begins in July, reaching maximum values in August. A female lays between 400-1000 eggs, almost exclusively in corn crops, in the soil, to a depth of up to 35 cm.
After hatching, the larvae start migrating to find corn roots. If they can’t find them, they die. The compact soil is not favorable to larvae, causing their death. In contrast, soil moisture allows them to survive. Near maturity, the larvae move to the surface layer of the soil and pupate.
The maximum appearance of adults occurs during the flowering of corn crops.
Attacked plants and damages
The western corn rootworm attacks both in the adult and larval stages, but the biggest damage is caused by the larvae. Before the flowering of the corn crops, the adults feed on the leaves, on which appear stripes in the form of elongated, whitish bites. When the corn blooms, the adults move to the inflorescences and feed on pollen and silk.
The larvae are oligophagous and feed on the roots of several grass species. The young larvae attack at first the root trichomes and the outer layer of the root and later the thicker roots and their internal tissue. The most dangerous is the last stage larvae because, by destroying the root system, the plants can be easily loosened by wind and rain, causing them to fall. Wounds caused by the larvae also provide the opportunity for other pathogens to penetrate the plant (Fusarium). Plants strongly attacked by the larvae have a destroyed root system, wither and dry out prematurely. Plants that have been lying on the ground can recover by producing adventitious roots, but the plant’s stem remains curved and causes the “swan neck” symptom. Insufficient hydration during summer days associated with larval feeding leads to higher production declines.
After finding no more pollen, young silk, and cornstarch in the grains, the adults fly to crops that were sown later or to other host plants such as red-root amaranth (Amaranthus retroflexus) or various species of Cucurbitaceae. Adults also cause damage to pumpkin, melon, cucumber, and bean seedlings interspersed with corn.
Control
Monitoring adults and larvae are very important in predicting the occurrence of the pest. It has to take into account both the density of the pest and the climatic conditions and the cultivation system in the area. Adhesive yellow traps, attractive plants, and pheromones are used for this.
Limiting the attack and preventing damages
Avoiding monocultures is one of the most important measures, especially in areas where the pest is present. Crop rotation, with corn returning every 4 years on the same soil is the most indicated and it significantly reduces the number of pests, depending on the crop sown in the following year after corn. It is recommended to perform deep plowing in autumn and to properly prepare the seedbed in spring to destroy some of the eggs.
Fertilization contributes to the development and regeneration of the root system of the plants. Tillage ensures the contact of the eggs with airy and dry areas that lead to increased mortality.
A number of parasites and natural predators are known to help reduce the population: species of viruses, bacteria, and fungi infest the insect in their various stages of development. The bacterium Bacillus thuringiensis and fungi of the genus Beauveria infest the adult and larval stages. Among the parasitoid, organisms are nematodes species such as Steinernema feltiae.
Adult control involves carrying out chemical treatments with approved insecticides.
To control larvae, it is recommended to apply insecticides to the soil at the time of sowing or with the first soil hoeing or during the vegetative growth period.
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