Beet fly
How to combat the beet fly
From the end of April and in July/August, eggs can be found on the underside of (usually) fully developed leaves. After the larvae hatch, their feeding tunnels become visible on the leaves. The burrows can burst open over time and cause additional damage to the leaves. After 2-3 weeks the larvae pupate in the soil.
Treatment
Neem oil helps against the larvae
Victor M. Vicente Selvas, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
