During which stage does the Birch Leafminer overwinter?

The Birch Leafminer overwinters as a fully grown larva inside a cocoon in the soil. This stage provides the insect with protection from harsh winter conditions, allowing it to survive until the temperatures warm up again in spring. The larval stage is well-suited for this overwintering strategy, as they can remain inactive and in a relatively safe environment, shielded from predators and extreme weather.

Other stages, such as adults in the canopy of trees, eggs on the underside of leaves, or pupae in leaf tissue, do not offer the same level of protection necessary to survive winter conditions. For instance, adult insects are exposed to the elements and potential ecological threats, while eggs and pupae would also be subject to environmental hazards. Thus, the larval stage in the soil cocoon is evidently the most advantageous for overwintering.