European corn borer: when to spray and how to spray tall maize

By Nikola Mojsa, MSc in Agricultural Engineering

How to recognise European corn borer

The damage is done by the larva, not the moth. The moths are yellowish to light brown, 13 to 15 mm long, and fly at dusk and at night, hiding in the crop during the day (AgrotopVSG, in Serbian). That is why you rarely see them. Look for the eggs instead, on the underside of the leaves, around the leaf bases.

The eggs are small and whitish, laid overlapping like fish scales, in masses of 20 to 40 (Savacoop). Just before hatching they turn translucent and black dots appear on them: the caterpillars' heads. The embryo develops in 5 to 7 days (Savacoop). The young larva feeds first in the leaf sheath and then bores into the stalk (AgrotopVSG).

The damage shows as plants breaking above the entry hole, and a cob with a larva in it is worth less on the market. The holes the larvae make are an open door for fungi (Agrosava), and for the second generation Savacoop also mentions mycotoxins, aflatoxin among them. Agrosava puts the yield loss at 2 to 25%, and AgrotopVSG at up to 20%.

When to spray for corn borer: moth flight and forecasting

First-generation moths fly from late May to early July, with the main flight in June. Emergence is driven by temperature, not by the date (Agrosava, Savacoop). The flight is monitored with pheromone and light traps (Agro Info Net, in Serbian), and treatment recommendations are published by the advisory services, the Serbian forecasting and warning service (PIS) and the Vojvodina PIS council (Agro Info Net). For Banat and Vojvodina, that is the Vojvodina Plant Protection Forecasting and Warning Service (in Serbian).

Our agronomist Nikola follows the corn borer flight and the advisory service warnings, and before spraying he also inspects your field: eggs on the underside of the leaves, eggs turning black, and the share of plants with egg masses.

• Late May to early July: first-generation flight, peaking in June. Treat when hatching starts (according to Agrosava, early to mid-June).
• July: some of the caterpillars pupate.
• Late July and August: second-generation flight. Agrosava gives early August for the treatment.

Treatment threshold

The treatment threshold is the share of plants carrying egg masses. You treat when a field inspection shows the threshold has been reached:

Sources (in Serbian): Agrosava, Savacoop, Agro Info Net. When hatching is spread out, Agrosava says the treatment is repeated after 10 to 14 days. Your agronomist chooses the insecticide and the rate according to the label.

Second-generation corn borer in tall maize

Sources disagree on how much it matters. Savacoop says the second generation hits commercial maize harder and bores straight into the cob (Savacoop). AgrotopVSG says it has traditionally been considered insignificant here, but that recent data show heavy attacks in some years (AgrotopVSG). So here, too, the decision comes from inspecting the field and the advisory service warning, not from a rule.

The practical problem is crop height: chemical control is carried out until the plants outgrow the tractor's ground clearance (AgrotopVSG). In the experience of our agronomist Nikola, the tractor has to stop once maize reaches 1 to 1.5 m, and in its wheel tracks it tramples 5 to 6% of the crop.

Drone spraying maize against corn borer

The drone flies about 3 m above the crop and follows its height, so it makes no difference whether the maize is 1 m or 2.5 m tall. The eggs sit on the underside of the leaves and around the leaf bases, and the downdraught from the rotors drives the fine droplets down to the lower leaves, so the product reaches the eggs. The drone sprays 15 to 20 litres of water per hectare, which gives a more concentrated solution than a tractor using 150 to 400 litres. The product and the rate are set by the label and your agronomist, who checks whether the product may be applied in a low water volume.

After rain, only the access road to the field needs to dry, so the treatment does not wait for the whole field to dry out. We work in wind up to 3 to 5 m/s. If the wind picks up, we pause and carry on as soon as it drops.

Bees: before an insecticide flight, Nikola notifies nearby beekeepers so they can close their hives. If you keep bees, or know someone who keeps them next to the field, tell us when you call. The pre-harvest interval is set by the product label or the advisory service recommendation. More on this in Pre-harvest and re-entry intervals.

For a side-by-side comparison with a tractor in numbers, see drone or tractor. For the whole maize season, see drone spraying maize.

What to do after harvest to reduce corn borer

The larva overwinters in the maize stalks. So at harvest the stalks are cut as low as possible and the crop residue is ploughed in (AgrotopVSG, Agrosava). Agrosava also recommends deep ploughing and weed control, and AgrotopVSG lists cocklebur, mugwort and foxtail as hosts of the second generation. These measures do not replace spraying, but they reduce the number of moths the following spring.

Price of drone spraying maize against corn borer

Drone spraying maize against corn borer costs €25 per hectare for areas up to 20 ha, €22 from 20 to 100 ha and €20 over 100 ha. You supply the product and the water. Travel within 50 km of Opovo is free, and for new clients the first hectare is free on a first order of at least 10 ha.

If a field gets three treatments in a season, for example one against corn borer, a repeat because hatching is spread out, and a fungicide, the package price applies: 5% off for three treatments and 10% for five or more. For 40 ha near Kovačica (22 km from Opovo), three treatments cost €2,487.10 in total, with the first hectare free on the first order. Package clients get priority for dates at the height of the season. Check the price for your field in the calculator.