Summertime in NCW: Why Do Flies Swarm Above Our Trees?

A few weeks ago, I met up with some friends at a house, just outside of Dryden, along the Wenatchee River. Summertime here is perfect for late afternoon dinners on the back deck with drinks and great conversation. While we were chatting, I noticed a swarm of flies or gnats swirling above every tree along the river.

Why is it that flies like to gather and swarm above tree tops at sundown?

Flies gather and swarm above treetops at sundown primarily due to a behavior called hilltopping, where certain insects, including flies, congregate at elevated points like treetops to find mates. A combination of factors drives this behavior:

Mating Behavior: Many fly species, such as dance flies or midges, form mating swarms at dusk when light levels are low, reducing predation risk. But why do they seem to swarm along the treetops? It's because treetops provide a prominent, open location where the males and females can meet. Males often swarm to attract females, using visual or pheromonal cues. Think of the treetops of a kind of bar during happy hour.

Get our free mobile app

Optimal Conditions: Sundown offers excellent conditions for the flies. It's usually cooler, with higher humidity and reduced wind, which is ideal for small, delicate insects like flies to fly and cluster without getting pushed away from the happy hour environment.

Landmark Navigation: Treetops serve as perfect natural landmarks, acting as a rendezvous point for flies. The silhouette of trees against the sky at dusk provides a clear visual cue. This prominent location helps the flies to meet up in a specific area.

Thermoregulation and Resting: Some flies may gather in these areas to regulate body temperature or prepare for nighttime rest, as treetops can retain warmth and offer shelter from ground-level predators.

This behavior is most common in species like midges, gnats, or sure fruit flies, and the swarming is often more noticeable at dusk due to the contrast against the fading Dryden light.