The appearance of the crane fly (family Tipulidae) often signals a distinct shift in the seasons, especially when they suddenly seem to be everywhere. These long-legged, mosquito-like insects are commonly, though incorrectly, referred to as “mosquito hawks” or “daddy long-legs.” The sudden influx prompts the question of when this seasonal event will end. The quick disappearance of the adults is a natural part of their brief reproductive phase, marking the transition to their hidden, subterranean life stage.
The Timing of the Adult Flight Season
Crane flies are most visible during their adult flight season, which is surprisingly short, lasting only a week or two for any single individual. Their main purpose as flying adults is mating and egg-laying, directly tying their visibility to their reproductive cycle. Many regions experience two main periods when large numbers of adults emerge, causing a noticeable seasonal influx.
The first major emergence typically occurs in the spring, often around late April or May, depending on the local climate. This is generally followed by a second, larger wave of adults emerging in the late summer and fall, usually from August through October. For instance, the European crane fly (Tipula oleracea) often has two generations per year in North America.
When people ask when crane flies “go away,” they are observing the end of these short flight periods. The adults complete their reproductive cycle, die off quickly, and the population of flying insects rapidly drops. The disappearance means the population has returned to its hidden existence in the soil.
The Full Life Cycle and Subterranean Stages
The flying adult is the final, brief stage of the crane fly’s four-part life cycle: egg, larva, pupa, and adult. After mating, the female deposits hundreds of tiny, black eggs into moist soil or turf within 24 hours. These eggs hatch into larvae in one to two weeks, initiating the longest part of the insect’s life.
The larvae are commonly known as “leatherjackets” due to their tough, grayish-brown skin. This larval stage is the primary growth phase and can last from several months to nearly a year, depending on the species and conditions. Leatherjackets spend this time burrowed in the top layers of the soil, often feeding on decaying organic matter, but sometimes consuming the roots of grasses and plants.
The larvae spend the winter underground, surviving freezing temperatures by burrowing deeper. They feed actively in the fall and again in the spring as the soil warms, which is often when they cause the most noticeable damage to turf. Finally, around mid-to-late spring or summer, the larvae stop feeding and enter the pupal stage just below the soil surface. This non-feeding transformation period lasts a few weeks before the adult fly emerges to start the cycle anew.
How Climate and Geography Affect Timing
The precise timing of crane fly emergence is not fixed but is sensitive to local environmental conditions, mainly temperature and moisture. Since crane flies are cold-blooded, warmer temperatures cause larvae to develop faster, potentially leading to an earlier adult emergence. Conversely, a prolonged cold spring can delay pupation and the subsequent adult flight season.
Soil moisture is a key factor, as eggs and young larvae require a damp environment to survive. Regions experiencing summer drought or dry, sandy soil may see lower populations or delayed emergence, as dry conditions cause significant mortality in early larval stages. In contrast, areas with mild winters and cool, moist summers, such as the Pacific Northwest, provide ideal conditions for large populations and predictable flight seasons.
Geographical location also plays a role. Some species, like the European crane fly (Tipula paludosa), have a single generation per year, typically emerging in the fall. However, the common crane fly (Tipula oleracea) and native species in warmer climates may complete two generations annually, resulting in both a spring and a fall flight period. This variation means the exact month crane flies appear or disappear can shift by several weeks based on the specific species, latitude, and yearly weather patterns.