Flies often bite more frequently when rain approaches, a phenomenon rooted in their biological responses to changing environmental conditions. Many insects can detect shifts in weather patterns before humans. This article explores the scientific explanations behind this increased biting activity, including the specific types of flies involved, atmospheric triggers, and their survival strategies during rainfall.
Identifying the Biting Flies
Not all flies bite; only certain species actively seek out blood meals from humans and animals. These include horse flies, deer flies, stable flies, black flies, and biting midges. Horse and deer flies are known for their painful bites, often using scissor-like mouthparts to cut the skin. Stable flies also bite humans and animals, while biting midges, tiny in size, can inflict irritating bites. Female flies of these species require protein from blood meals for egg production.
Atmospheric Triggers for Biting
The surge in biting activity before rain is primarily linked to changes in atmospheric conditions that signal impending adverse weather. A significant factor is the drop in barometric pressure, which often precedes a storm. Many insects, including flies, can detect these pressure changes, prompting them to become more active and agitated. This response is a survival mechanism, as they instinctively seek to feed before conditions worsen and make foraging difficult or impossible.
Increased humidity levels, another common precursor to rain, can also affect fly behavior. Flies generally thrive in warm, moist environments, and higher humidity can make them more active. Some studies suggest that increased humidity can make it easier for flying insects to remain active for longer periods. The combination of falling barometric pressure and rising humidity creates an urgency for female flies to secure a blood meal for egg development, driving them to feed quickly before heavy rains and strong winds arrive.
Survival Strategies During Rain
Flies and other small insects are significantly impacted by rainfall, which can impede their flight and even threaten their survival. Insects do not fly in wet weather, as raindrops, though small to humans, are massive in comparison to a fly. A single raindrop can weigh many times more than a fly, making flight hazardous. If hit directly, a fly can be knocked out of the air, or the water can adhere to its body, causing it to drown by covering its spiracles, which are breathing openings.
When rain begins, flies typically seek shelter to protect themselves from the downpour. They often find refuge under leaves, in crevices in tree bark, or beneath the eaves of buildings. This need for shelter reinforces the urgency of their pre-rain feeding frenzy, as female flies ensure they have the necessary nutrients for egg production before finding a safe place to wait out the storm.
Reducing Bites
Minimizing exposure to biting flies, especially before rain, involves several practical steps. Wearing protective clothing, such as long-sleeved shirts, long pants, and hats, can create a physical barrier against bites. Light-colored clothing is often recommended, as some biting flies, like black flies, are attracted to dark colors. For added protection, clothing and gear can be treated with permethrin, an insecticide that repels and kills flies.
Applying insect repellents containing active ingredients like DEET or picaridin to exposed skin is another effective measure. Natural alternatives with essential oils such as citronella, lemongrass, peppermint, or rosemary may also offer some repellency. Avoiding outdoor activities during peak fly activity times, which often coincide with the periods just before rain, can further reduce bites. Using fans in outdoor spaces can also deter flies, as many species are not strong fliers and struggle against air currents.