Corn rootworm

Diabrotica spp.

Emergence and egg deposition: 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.
Larvae: Given German weather conditions, the first larvae hatch in the second half of May and the peak appearance is in June and July. In warmer countries (Italy), the hatching takes places much earlier. The larval development passes through three stages and is finished after three to four weeks. 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.
Larval instars: Larvae at the first stage can move up to one meter in the soil 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.
Pupae: 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.
After emergence, the females feed for about two weeks before they start laying eggs. A female can have up to three periods of egg-laying. Each egg-laying period follows a period of feeding of two weeks and copulation. Nevertheless, the Western Corn Rootworm has only one generation per year.

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