Weather and Pests


Temperature and rainfall are very important for insects and can cause their populations to grow or decline. Why are pests worse some years than others?

Unlike humans, insects are cold-blooded. Some can produce their own heat (like honey bees vibrating their wing muscles) and retain heat (like bumble bees with their fuzzy, insulating hairs), but they cannot generate their own heat while inactive. Because of this, they are greatly influenced by changing temperatures and the seasons.

Insect development is highly dependent upon temperature. The warmer it is, the faster insects grow. Consistent high temperatures result in faster development through the life cycle and higher insect populations. Forensic entomologists can determine a victim’s approximate time of death based on the species of flies colonizing the body, age of the maggots, and the ambient temperature. Indoor pests that can live and breed indoors (like those that infest dried foods or the soil of potted plants) can survive year-round because they don’t experience extreme fluctuations of hot and cold as they might in nature.

The colder it is, the slower insects grow, until it gets so cold that their development stops, or kills them. That’s why you don’t see insects flying around in the winter months. Insects either enter hibernation (called overwintering) or endure the winter in a dormant life stage (like an egg or cocoon). Overwintering insects have a variety of methods to survive freezing temperatures, like antifreeze chemicals in their blood, or expelling as much water from their bodies as possible to prevent water crystals from forming in their bodies and damaging their cells. Harsh winters can kill overwintering insects, resulting in lowered populations in the spring. Warm temperatures followed by a frost can wake up some of the hibernating insects early and kill them. In this case, late wakers are more likely to survive into spring. Extreme winter temperatures can also kill off prey or host plants that would feed many of these insects after they emerge from hibernation.

Moisture is important for insects, with low humidity causing dehydration and death. Subterranean termites prefer to swarm on sunny afternoons after a rain, as this increases their survival rate from drying out. On the opposite end of the spectrum, too much moisture can be damaging. Heavy rainfall can drown insects, disrupt feeding, and encourage the growth of fungi which might infect them. During dry spells, fire ants tunnel deeper into the ground to find water. After a rain, they resume their mound-building activities, with mounds seemingly popping up overnight. Although the ants were always there, they become much more noticeable to people after a rain.

The University of Arizona, College of Agriculture and Life Sciences. Website: Accessed March 2018.