Diabrotica virgifera

Scientific Name Diabrotica virgifera subsp. virgifera LeConte
Common Names English: Western corn rootworm; German: Westlicher Maiswurzelbohrer; French: Chrysomele des racines du mais; Spanish: Gusano de la raiz del maiz; Italian: Diabrotica del mais; Croatian, Serbian: Kukuruzna zlatica
Description The adult is 5-7 mm (0.19-0.27 inch) long. It has a yellow body with long black stripes on the elytrae. The antennae are as long as the rest of the body.
The larva is up to about 15 mm (0.58 inch) long and white in color.

Biology

Damage

Although the adults can sometimes be harmful, most injury is done by the larvae of D. virgifera. The first instar feeds on root hairs and the outer layers of root tissue. Older larvae bore deep into the roots, tunneling and destroying them. Each plant may be attacked by several of the „worms“, which can move up to 1 m (3.28 ft.) through the soil in their search for a suitable host. At first, the loss of root tissue causes reduced water and nutrient uptake in the host, which alone may be enough to kill small plants. With progressing damage, the plants lose their stability and become extremely prone to lodging. If sufficient soil humidity allows them to compensate for rootworm feeding by increased root growth, they may straighten up again, but the resulting „goose-necking“ (bent stems) will still make harvesting very difficult.
The adult beetles feed on aerial plant parts, mainly pollen, young kernels, leaves and silks. The latter type of damage („silk clipping“) can become economically important as it interferes with pollination, and may reduce kernel formation considerably.

Lifecycle

The Western corn rootworm has one generation per year. In June or July, the first larvae will hatch. Depending upon temperature, larval development will last from three to four weeks. After three instars, pupation occurs in the soil and lasts another week. The adult beetles emerge from late July until September. They can actively fly more than 20 km (12.4 miles) in a single flight. After mating, the females will feed for about two weeks on pollen, which contains the amino acids they need for producing their eggs (approximately 500 each). These are then laid 10-30 cm (3.93-11.81 inch) deep into the soil in batches of ca. 80. They are the only overwintering stage, and can resist temperatures of -15° C (59°F) in diapause.
A small fraction (~0.2%) of eggs will overwinter twice.
The adults of D. virgifera may feed on pollen of some other host plants, and young larvae are occasionally found on the roots of other grasses, but the insect can only complete its development on corn. So most eggs are deposited in corn fields. However, a small proportion of them is laid into other crops. In some areas of the United States, where corn has routinely been rotated only with soybean over many years, the selection pressure has led to the development of a local strain in which a higher percentage of females lay their eggs into soybean fields so that the larvae emerge to find corn as a food source.
The genus Diabrotica is of North American origin and is found from Mexico to Canada. Since the beginning of the nineteen-nineties, however, D. virgifera has been spreading in Europe too. In recent years, it has passed the Alps northwards.

Occurrence

Agricultural Importance

Probably the most important insect pest of corn in the world. In North America, it is called the „billion dollar pest“. Crop losses of 90% are possible.

Control

Integrated Crop Management

Despite recent, still controversial findings that corn may not be the only food source for D. virgifera, proper (longer) crop rotation is still the most effective means of fighting this pest. However, the adaptability of the pest's oviposition behaviour means that this should not only comprise alternating between corn and soybean, but should also include at least one more - wherever possible dicotyledonous - crop.

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