A reader recently asked us how high an apartment needs to be above ground level before insects stop getting into your home through the window. The reader notes that they live pretty high up in an apartment building, and there are some insects they see a lot, some they see only rarely, and some native insects they never see. What's going on?

This is a fun question, and we turned to entomologist Elsa Youngsteadt for answers. Youngsteadt is an urban ecologist, an associate professor of applied ecology at NC State, and an accomplished science writer. She wrote the remainder of this post. Take it away, Elsa!

Altitude Matters

The overall density of insects in the air does decline with altitude. From the 1920s through the 1950s, entomologists conducted a series of clever studies using kites, balloons and airplanes to sample insects at different heights, from about 3 meters above the ground to more than 4,000 meters above ground. (Those studies are compiled in this paper.) This ranges from the height of a 1-story building, to much higher than any manmade structure. The tallest building in N.C. is 60 floors and 265 meters high, and the tallest in the world is 163 floors and 828 meters high. In all these sampling schemes, insect density in the air dropped off pretty exponentially with height.

No single study covered the entire range of heights. But to give a sense of the density of insects in the air we can look at one study (pdf download here) that used insect traps attached to airplanes flying over a mix of agricultural, forested and swampy land in Louisiana in the 1920s and '30s. The plane-mounted traps had a catching surface of 5 square feet (a bit less than half a square meter). At an altitude of 6 m (a 2-story building), their traps caught 26 arthropods per 10 minutes of flying time. At 180 to 300 m (around the highest buildings in N.C.) they were down to 3 - 5 arthropods every 10 minutes. By 4,000 m, they were catching less than 1 insect every 10 minutes.

But not all insects dropped off at the same rate. Flies were the most abundant insects in the airplane samples at all heights. But as flights got higher, the proportions changed. Beetles were also abundant in lower samples-second only to flies-but their abundance plummeted within the first 100 meters above ground. In contrast, aphids and plant hoppers were more evenly distributed across altitudes; by 300 m they, not beetles, were in second place after flies.

How Big Is That Bug?

How high an insect flies depends on the insect's body size and its immediate objectives.

In terms of objectives, if an insect is foraging, it's usually going to stay close to the ground. Most insects feed on plants or detritus (dead plants and animals) that are on the ground, and short flights to find a new resource patch don't need to be high in the air. For example, bumble bees flying among flower patches seem to stay 1-3 meters above the ground.

But at least three activities could bring insects high in the air: mating, migrating and dispersing.

Ants and honey bees are known for their aerial mating swarms, where they congregate dozens of meters above the ground to mate (even though these same species would stay close to the ground when foraging). And long-distance migrants or dispersers take advantage of thermals to rise high in the air and cover long distances in the thin, low-friction air at high altitudes. Monarch butterflies are a great example of this. This species forages near ground level, but has been spotted 600-1,250 m above ground during their annual migration.

In terms of body size, the smaller an insect, the higher it is likely to be.

One clever study in 2016 studied this pattern using birds - specifically, purple martins that were catching insects for their offspring. The birds wore altitude loggers, then the researchers intercepted each bird's prey back at the nest. The researchers then connected the prey type back to the altitude of the flight in which it was caught. (The researchers did give the baby martins substitute food to replace the prey insects they took!) At low altitudes, the birds caught insects of all sizes - but as they went higher, larger species dropped out and the birds retrieved ever smaller prey the higher they flew, up to 922 meters! Smaller insects tend to have more wing area per unit of body weight, so it is easier for them to "float" higher into the air.

Buildings (May) Change Flight Behavior

All these studies of insects in the air have been done in open areas. If an insect encounters a building, does it turn around and go back? Does it fly up the building? Around it? We are still trying to answer these questions!

In my own work on urban bees, I often wonder about effects of built structures on insect flight. My lab recently published a study showing that buildings were barriers to bee movement. Bees foraging on potted cucumber plants were 7-9 times more likely to move directly between plants that were separated by a lawn or forest than plants that were separated by a building. So buildings are redirecting bees in some way, although we can't yet see exactly how.

Studies of pollinator communities on green roofs also corroborate the lower flight heights of bees. For example, one study in Toronto put identical flowering plant communities in ground-level gardens and on rooftops from 6 to 20 meters high. Bee and wasp abundance declined steeply with garden height - but flies had no clear trend (although even flies were missing on the very highest roof). (That said, not all green roof studies find an effect of roof height on insect community!)

Which Insects Get Into a Skyscraper Apartment?

For flying insects that get into a building by accident, the higher you are, the smaller these insects are going to be, the fewer beetles there will be, and they're probably going to be insects who were trying to mate, migrate, or disperse. But for pesty insects that are actually making a living in an apartment - such as cockroaches and certain ants that disperse within a building - I'm not sure building height matters much. They're probably moving through the building interior from one apartment to the next. (But those aren't the kinds of insects I study.)