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Biology and population dynamics

The Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) is a leaf beetle belonging to the family Chrysomelidae. It is a major pest of maize in North America and, increasingly, in Europe. There are hundreds of species of the genus Diabrotica, of which Diabrotica virgifera virgifera is the only one which has established itself in Europe.

The beetles are about 5-7 mm long. Adults of the Western Corn Rootworm have a dark head, a yellow pronotum and a yellow abdomen. Their legs can be yellow or dark. The elytra are yellowish, often with three distinct darker stripes. Males and females differ to some extent in their markings. On males, nearly the entire posterior half of each wing cover is black (Fig. 1b), whereas on females, the dark stripes are more pronounced     (Fig. 1a). Generally, males are smaller than females; they have longer antennae and are darker.



  female beetle   male beetle

Fig. 1a: Female beetle

Fig. 1b: Male beetle

(Source: Dr. M. Zellner, LfL)



Western Corn Rootworm beetles usually begin to emerge from the soil in late June, and continue to emerge until late October. They are active in fields until frost. Female beetles lay 300-400 eggs in the upper 5-20 cm of soil among the roots of maize during later summer and early fall; with 80 % of the eggs in the upper 10 cm. The eggs are oval, beige and approx. 0.6 mm in size. Western Corn Rootworm eggs pass winter in the soil; a diapause of three to four months is necessary for the further development. Some of the Western Corn Rootworm eggs (0.21 %) may not hatch until the second spring due to "extended diapause", a resting period during which the insect does not develop. The first larvae hatch in April shortly after seeding and the peak appearance is in Mai and June. The larval development passes through three stages and is finished after three to four weeks (Fig. 2). The size varies depending on the larval stage between 3-15 mm. When the larvae hatch they burrow into the developing roots and base of maize stems.




Fig. 2: Larvae of the Western Corn Rootworm

(Source: Dr. M. Zellner, LfL)



The first instar can move up to one meter maximum to find a suitable food source (maize roots). After the larvae finish feeding, they change to the pupal stage in pupal cells in which they change into the adult or beetle stage. The whitish pupae are 3-4 mm in size. The pupal stage requires approx. seven days before transformation to the adult stage. Adults emerge from pupae in the soil and work themselves to the soil surface. Normally, males hatch before females.

Adults consume pollen of flowering maize, silks, leaves and developing seed heads. If these are not yet developed, the beetles also feed on maize leaves. In the beginning, they stay in the field where they developed, after that they move actively by flying or travel by wind. They often move to later ripening maize fields where the plants still have fresh silks and tassels. Thereby, the beetles increase their area of distribution every year. After emergence, the females feed for about two weeks before they start laying eggs. Nevertheless, the Western Corn Rootworm has only one generation per year     (Fig. 3).



  Life cycle of Diabrotica


Fig. 3: Life cycle of the Western Corn Rootworm

(Source: S. Wagner, Dr. M. Zellner, LfL)






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