Yes, house flies can 'wake up' in the spring after staying dormant during cold weather

House flies can be a common nuisance inside a home. Warmer seasons, like the summer, can be “prime time” for house fly infestations. 

Many people describe an annual phenomenon where flies that seemed to be dead in their home throughout the winter wake up when it gets warmer. As the weather starts to cool down, people are wondering where flies in their homes actually go. VERIFY viewers Alvin and Sara reached out to ask about the life cycle of these flies and if they hibernate all winter before waking up when it's warmer. 

THE QUESTION

Can house flies “wake up” in the spring after staying dormant during cold weather? 

THE SOURCES

• Floyd W. Shockley, Ph.D.,  collections manager for the Department of Entomology at the Smithsonian Institution

• Jonathan Larson, assistant professor of Extension Entomology at the University of Kentucky

• University of Florida Department of Entomology and Nematology

• University of Florida Institute of Food and Agricultural Sciences

• Colorado State University Extension 

• University of Missouri Extension

• Terminix

• Plunkett’s

• ABC Home and Commercial

THE ANSWER 

Yes, house flies can “wake up” in the spring after staying dormant during cold weather.

WHAT WE FOUND

House flies can enter a hibernation-like state and wait out winter. This state, referred to as diapause, is a different form of dormancy from hibernation because flies slow down their development process and reduce their metabolic activity instead of going into a deep sleep, according to an article by ABC Home and Commercial. 

Most vertebrates that hibernate won’t break their sleep cycle, but with flies in diapause, if the temperature increases, “they will snap out of it and begin to become active again,” said Floyd Shockley, Ph.D. 

This state of diapause means that if they “get cold enough, they can kind of like stop moving and they won’t necessarily die,” according to Jonathan Larson, Assistant Professor of Extension Entomology at the University of Kentucky. This process slows their development, which can lead to a slightly longer lifespan.   

Diapause is pretty common among house flies. The flies end up in places like window sills, where they’re attracted to sunlight coming in, but then get impacted by the cold temperatures by the window compared to the rest of the home. This, Shockley says, can “trigger them to go into a dormancy, a diapause, until the temperature raises.” The diapause can last throughout the winter until the springtime when temperatures increase. 

The lifespan of a fly is typically pretty short, usually lasting up to around a month of active life. “They start life as an egg, the egg hatches into a maggot, then the maggot will go through multiple stages of development, and then it will pupate, and then out will come the adult fly,” Larson says. 

The metamorphosis process can happen completely inside a home, and a fly can survive its entire lifespan indoors if it has a proper food source, according to a University of Missouri Extension article. 

House flies can also enter diapause at any stage of their lifespan. An article published by University of Florida’s Institute of Food and Agricultural Sciences says the flies can also become dormant in the larval or pupal stages, and “under suboptimal conditions the life cycle may require up to two months.” 

In general, house flies typically can live in a home for as long as a month, but this is often dependent on the temperature of the home and a fly’s access to food. 

A house fly’s longevity is “enhanced by availability of suitable foods,” according to an article from the University of Florida’s Entomology and Nematology department. Without food or water, active flies will typically starve to death after 2 or 3 days. “If a fly is trapped in your home, and it can't get access to sugar water, whether that be a cup of sugar water that you've made to pour into a mixed beverage, or pop or something like that, then they're they're not going to be able to survive very long, they only have a reserve of fat bodies in them that’s limited,” says Larson.

A house fly’s life cycle can also “be impacted by just how hot it is outside and then what temperature you keep your house at,” adds Larson. Since flies develop faster in warmer temperatures, cooler weather can extend the life cycle. But this depends on both biological and environmental factors, according to Terminix, which adds that the minimal to no activity of diapause “can potentially prolong the lifespans significantly.” In general, house flies inside homes “can essentially continue carrying on their lifecycle for longer than they could outside,” Shockley says. 

Female flies also tend to live longer than male flies, and “an adult female house fly typically lives for about 25 days (males live for about 15 days),” according to Terminix.