The sudden appearance of a large cloud of dragonflies, known as a swarm, is a natural phenomenon. A dragonfly swarm is an aggregation of adult dragonflies driven by two distinct biological imperatives. These gatherings represent a temporary concentration of individuals either pursuing a localized food source or undertaking a seasonal, long-distance journey. Understanding the type of swarm explains the behavior and purpose of these insects.
Aggregation for Hunting (Feeding Swarms)
The most frequent type of swarm observed is a feeding aggregation, where dragonflies gather to capitalize on a concentrated abundance of prey. These swarms are not coordinated movements but result from many individuals independently flying to the same location rich with small, soft-bodied insects. This behavior is most common during late summer and early fall, often occurring near water sources where insects like gnats, midges, and mosquitoes hatch in large numbers.
The dragonflies in these feeding swarms exhibit an erratic, non-directional flight pattern, darting and weaving within a localized area to intercept prey. They are highly efficient aerial predators, capable of catching small insects mid-flight. The dragonfly uses its spiny legs to form a “basket” to scoop up the prey, which it often consumes while still flying. These localized hunting swarms are temporary, dissipating quickly once the concentrated food source is depleted or environmental conditions, such as temperature, shift.
Seasonal Long-Distance Movement (Migratory Swarms)
The second type of swarm involves a true migration, which is a coordinated, long-distance movement undertaken by certain species. Only a small fraction of the world’s dragonfly species migrate, such as the Common Green Darner (Anax junius), the most recognized North American example. These migratory flights occur in the late summer and fall as a response to cooling temperatures in their northern breeding grounds.
Migratory swarms are directional, with individuals moving along a southward path, often utilizing geographic features like coastlines and major lake shores as navigational guides. Unlike feeding swarms, their movement is sustained, and they can travel impressive distances. Some individuals of the Green Darner species cover more than 1,500 miles. The Wandering Glider (Pantala flavescens) is the global record holder for insect migration, capable of crossing oceans. This movement is a survival strategy, ensuring the species can lay eggs in warmer regions where their offspring complete development.
The Dragonfly’s Role in the Ecosystem
A large gathering of dragonflies indicates a healthy local ecosystem, and their presence provides significant benefits. Dragonflies are apex insect predators throughout their life cycle, controlling pest populations in both aquatic and terrestrial environments. The aquatic larval stage, known as a nymph, spends up to several years in the water, where it consumes the larvae of mosquitoes and other aquatic insects.
As adults, dragonflies continue this beneficial predatory role, hunting flying pests, including flies, moths, and mosquitoes. A single adult dragonfly can consume up to 100 mosquitoes in a single day, making them an effective form of natural pest control. By preying on large numbers of smaller insects, dragonflies help maintain a natural balance and reduce the populations of insects that can be nuisances or disease vectors.