The yellowish-white, legless larvae of the frit fly (up to 5 mm) create rows of holes across the leaf veins, from which feeding grooves extend into the leaf tissue. Tissue near the feeding grooves turns yellow, and the edges appear silvery. The leaves twist, curl, or become stuck in the leaf sheath. Wind causes the leaves to tear or snap off in the damaged areas. This damage typically has little effect on plant development and yield. However, the damage becomes more severe when the larvae reach the growing point and damage the main shoot, leading to side shoot formation or plant death. Additionally, feeding sites can serve as entry points for secondary infections, such as maize blight.
The adult frit fly is 3-4 mm long and shiny black. It can produce up to three generations per year, with the first generation causing damage to maize. The frit fly lays eggs on emerging maize plants in spring. A few days later, larvae hatch, feed on the leaves, and move down to the growing point. Late seedlings are particularly affected, as they germinate quickly but grow slowly due to unfavorable weather conditions. The frit fly stops laying eggs on maize after the 4-leaf stage.
There is no reliable damage threshold for frit flies, as the damage is often not noticed until it has already occurred.
The most effective way to control frit flies is through preventive measures, such as seed coating with suitable seed treatments that also protect against bird predation. Ensuring rapid juvenile development of the crop also helps. In Germany, chemical control is only permitted between BBCH stages 11-13 using contact agents, but it is considered unreliable. Late treatments are ineffective.