Corn borer (Ostrinia nubilalis Hbn.)
Economically important pests (phytophages) in corn:
Corn borer (Ostrinia nubilalis Hbn.) is the most dangerous pest of corn. It inflicts biological and technical damage:
– Seed losses reach 15–40% at biological damage.
Apart from direct damage, the feeding of the corn borer results in conditions for the penetration of disease agents such as smut, Fusarium, Fusarium spp., etc.
The technical damage is from the breakage of the stems and cobs resulting from the damaging activity (feeding) of the corn borer. They hamper mechanized harvesting — losses reach 50–80%.
The Ostrinia genus includes about 20 species of pests. There are 6 species in Europe, of which only the corn borer damages corn.
In Bulgaria, the corn borer develops 1–2 generations per year.
The corn borer overwinters in crop residue (in tunnels at the base of stems) as a fully developed caterpillar. In spring, before pupation, the caterpillar gnaws a round hole from which the imago (adult) will emerge. Butterflies actively fly at a distance of up to 2–3 km. The average fecundity is 270–780 eggs.
The harmfulness of corn borer increases significantly with increasing rates of mineral fertilizers, especially nitrogen.
Damage to corn by the corn borer leads to a sharp increase in smut and Fusarium infestations.
Displacement of trichogramma to control corn borer
Egg-laying in the corn borer begins 2 to 7 days after the start of flight. The eggs are laid on the underside of the leaves in groups of 10–25–90 eggs.
We apply two methods of colonization (displacement) of the trichogramma — free and by capsules. Both methods are used and are successful when the technology is properly applied:
– In free displacement, trichogramma colonizes in portions in the form of eggs from a height of 7–12 m, thus dispersing the eggs over a large area from the tops of the plant and to the soil surface. The trichogramma becomes imago within 2–3 hours after colonization and immediately begins searching for eggs in which to lay its own eggs.
Displacement should be carried out in the evening or early morning at temperatures of +14 to +25 °C.
The efficiency of such displacement is huge and instantaneous, but it should be repeated — two or three displacements are needed every 4–5 days. This ensures that the eggs laid by the pest are parasitised throughout the egg-laying period. The method is applied in Canada, China and the former Soviet republics.
– The capsule is a cellulose container, resistant to moisture and temperature fluctuations, self -degrading in nature. It colonizes from 7–30 m height (depending on the aircraft), 10 capsules per decare.
In capsule displacement, we colonize the trichogramma in 3 different stages of development (in Germany even six), 1/3 — becoming imago within 2–3 hours; 1/3 — on the fourth day; and 1/3 on the eighth day. With this method, we are guaranteed to cover a period of up to 12 days with just one displacement.
First trichogramma displacement begins when the threshold number of males captured by light and pheromone traps, or by visual analysis, is reached at a density of 2–2.5 egg clutches per 100 plants.