Why are there more flies after it rains?

Why do flies come out after it rains?

After a refreshing rainfall, have you ever noticed an influx of flies seemingly appearing out of nowhere? This curious phenomenon has puzzled many of us. While rain itself might not directly summon flies, there is a strong correlation between increased fly activity and post-rain conditions. 


Understanding the relationship between rain, humidity, and fly behavior provides us with a clearer perspective on this intriguing occurrence. Humidity, a measure of the amount of moisture in the air, plays a pivotal role in the lifecycle, movement, and behavior of flies.

Humidity’s Impact on Breeding and Development

Flies are remarkably attuned to changes in humidity levels. Following a rain shower, the air is often laden with moisture, raising the overall humidity in the environment. This increased humidity provides an ideal setting for flies to lay their eggs and for larvae to thrive. Flies, particularly those of the house fly and fruit fly variety, require moist environments for their eggs to hatch and develop. The moisture from the rain thus becomes the trigger for a surge in fly population, as breeding cycles are accelerated under these conditions.

Hydration and Movement

Just as humidity affects the fly’s reproductive cycle, it also influences their daily activities. Flies, like many insects, are prone to desiccation, or drying out, due to their small size and relatively large surface area. High humidity levels help prevent excessive moisture loss from their bodies, allowing them to stay hydrated and active for longer periods. After a rain, flies are more likely to venture out in search of food, mates, and suitable breeding sites due to the increased humidity, as it reduces the risk of dehydration.

Humidity and food scarcity

The relationship between humidity and food availability cannot be overlooked when discussing fly behavior. Rain not only introduces moisture to the environment but also affects food sources. The combination of rain and humidity can break down organic matter more quickly, leading to an increase in available nutrients for flies. This abundance of food acts as a beacon, attracting flies to areas where nourishment is plentiful, leading to an apparent surge in their numbers.

Behavioral Responses to Humidity

Flies have developed behaviors that are finely tuned to humidity changes. Rainfall is often accompanied by a decrease in temperature and flies instinctively seek shelter during these conditions to avoid being washed away or exposed to cold temperatures. Once the rain ceases and humidity levels rise, flies interpret this as a suitable time to venture out and capitalize on the improved environmental conditions.

In Conclusion, flies thrive in humidity which plays a multifaceted role in influencing fly breeding, development, hydration, and movement. But how do we stop flies attacking your horses?

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