Does rain kill fleas?

Summary

Rain can negatively impact and sometimes kill developing fleas. The water dissolves and washes away larval food, heavy rains can physically damage immature stages, and larvae can drown if the soil becomes saturated. However, immature fleas prefer living in sheltered micro-habitats where they’re protected from the negative effects of rain.

Surveys done on seasonal flea populations show conflicting results related to rainfall. Some conclude rain has no effect on flea populations. Some conclude rain has a negative effect. And some conclude rain has a positive effect. The data is confusing and more research needs to be done.

Details

How Rain Affects Flea Survival

Dissolving Flea Dirt

Flea larvae live in the environment and require the feces of adult fleas (flea dirt) for food. They’ll starve to death without it. Water from rain and irrigation can dissolve and wash away the feces, leaving flea larvae without food.

Mechanical Action of Rain

The mechanical action of heavy rainfall can destroy flea eggs and larvae. Fleas are known for developing well in sandy regions. This may be partially explained by the sand protecting the ground, rendering heavy rains less destructive to fleas in the soil.

Relative Humidity

The larval stage is most vulnerable to changes in humidity. Larvae quickly desiccate in low relative humidity (RH). They’ll die in lower than 50-60% RH. Between 72-92% RH is optimal. Larvae actively uptake water from the air in greater than 75% RH, and become larger, healthier adults. While humidity from rain is a good thing, too much humidity can kill the larvae. At 95-100% RH, fungi begins growing and the larvae can’t survive.

Soil Moisture

Outdoors, most larval activity occurs in the upper few millimeters of soil. Flea larvae burrow to an average depth of 2.36 mm into sand, but can reach a maximum depth of 7.5 mm. Similarly, another study found larvae live 5-20 mm below the soil surface at 2% moisture.

12% RH is lethal to larvae. However, larvae can survive the desiccating air if there’s moisture in the soil. Survival increased from 0% to 100% by merely increasing soil moisture from 0% to 1% (1% isn’t sufficient if there’s is too much clay, as the clay particles absorb the moisture). In all soils, larval mortality decreases significantly when moisture is raised to 5%. Another study found similar results, with larvae thriving at 2-7% soil moisture.

At 7-10% moisture, the soil becomes saturated and gas exchange is blocked. Thus, the larvae are forced to the soil’s surface and onto objects like blades of grass. At moisture exceeding 20%, a film of water develops on the soil’s surface which drowns some larvae. The soil also becomes compacted, impeding any attempts to take shelter beneath the surface from predators, such as nematodes. Most larvae still survive in up to 22% soil moisture. However, once moisture exceeds 22%, there’s over 20% mortality. Thus, heavy rain and irrigation may drown larvae.

This is why flea larvae thrive in micro-habitats that are protected from rainfall, irrigation, and flooding. They prefer dark areas with a stable, high RH. It’s common knowledge that fleas occur in greatest abundance in sandy regions. Sand maintains moisture more uniformly, and thus permits the immature stages of the flea to develop with greater success.

Surveys

Published papers have come to vastly different conclusions about how rain and moisture levels affect flea population growth. It’s difficult to make heads or tails about how rain correlates to the severity of flea seasons.

One paper claims that years with rainy summers will result in a flea population boom. While dry, hot summers will limit the amount of fleas that year. And still, excessive moisture in breeding areas could be just as detrimental as excessive dryness. Another paper shares the idea that summer rains affect the variation in flea populations. It’s believed that the greatest abundance occurs in above normal rainfall, and the lowest abundance in extremely heavy rainfall.

A survey in Spain concluded that rainfall was negatively correlated with the abundance of dog fleas (C. canis) and human fleas (P. irritans), and that there was no correlation with rainfall and cat flea (C. felis) populations.

Two separate surveys in Mexico couldn’t find any statistical relationship between flea prevalence and total rainfall in spring, summer, autumn, or winter.

A survey in Spain found that mean annual rainfall was positively correlated with overall cat flea (C. felis) and dog flea (C. canis) abundances. Similarly, surveys done in Iran found that infestation rates were largest in areas with higher rainfall.

A survey on rodent fleas found that populations decreased following an increase in rainfall.

A seasonal study in Florida found that the highest infestation rates occurred during in the dry autumn months. While wet summer months saw higher levels of mortality. Pest control operators have also described flea infestations decreasing during the summer months. This may be explained by daily thunderstorms and rain showers during the summer.

References