Lifestyle Entomology
Windshield dzika_mrowka/iStock

A windshield.

May 04, 2016

Infrequently Asked Questions: Why do so many bugs hit my windshield?

The world is full of questions we all want answers to but are either too embarrassed, time-crunched or intimidated to actually ask. In the spirit of that shared experience, we've embarked on a journey to answer all of the questions that burn in the minds of Philadelphians — everything from universal curiosities (Why do disposable coffee cups still leak?) to Philly-specific musings (How does one clean the Liberty Bell?). 


If you've driven through a rural area, you're painfully familiar with the experience of having your squeaky clean car pelted by a smattering of insects, whose innards suddenly find a new home on your windshield. Curious as to why and under what conditions this tends to happen, we reached out to the Academy of Natural Sciences at Drexel University Entomology Collection Manager Jason Weintraub for an explanation.

In general, why are there so many insects hitting windshields?

The concise answer I would give to someone is it depends on where they are, what time of year it is, exactly where they’re driving relative to habitats of insects that are abundant in that season, insect behavior as it relates to vehicles moving along roads, the shape and aerodynamics of the vehicle they’re driving and the speed they’re driving. Without knowing those factors, it’s hard to answer questions as to why there are bugs on windows.

So let's say we're talking about a wooded part of Bucks County. Is the insect population there any bigger or more diverse than other parts of the country?

It doesn’t necessarily have greater diversity or abundance than compared to a comparable rural area in, say, Michigan where I’m from, or Washington state or Northern California, but what’s more significant, in terms of the numbers and diversity of insects that end up squashed on people’s windshields, is what type of vehicle they're driving, how fast they drive, where they drive and when they drive it there. That bears much more on the question.

Why would the vehicle matter?

What happens with small insects, is if the vehicle isn’t going very fast, they get caught in the laminar airflow over the surface of the vehicle — especially if it’s an aerodynamic vehicle and they don’t end up impacting the windscreen. They fly over the top of the vehicle. Whereas if it’s a Greyhound bus with a flat front, everything that hits it gets squashed. So the physics of airflow around vehicles, the flight patterns and behavior of the flying insects likely to get hit, are likely to influence how many insects hit the windshield. 

The other thing that's important is whether the vehicle is driving through an area where there’s insect aggregation for some reason. So you have aquatic species like mayflies that are very short-lived — they may only be flying for two or three days in one given mayfly population, near a stream or river, and if a car is driving at a high speed during that one-, two- or three -day emergence, over a place where they are having their annual mass emergence to reproduce when the adults are all around looking for mates, then one vehicle crossing one bridge at one point in time may have thousands upon thousands of mayfly bodies impacting on its front end as it speeds across a bridge.

What other factors influence when or how many bugs you're hitting?

The smallest flying insects are not going to hit a windshield if they are impacted by the laminar airflow on a car only going 30 miles per hour, as opposed to 60 miles per hour. They might, however, hit the grill of the car below the windshield. So, it depends on how high above the ground they tend to fly. Most flying insects tend to fly anywhere from two to five feet above the ground. That tends to be in the path of automobiles as they cross roads. 

There are other factors, such as whether the car is driving with the headlights on and whether it’s a moonless night or a night with heavy overcast and cloud cover. Nocturnal insects use moonlight to navigate, to keep flight paths going in one direction. They maintain flight path in a specific angle to the moon. When there is no moon, on the night of a new moon, or if the moon is invisible because of dense cloud coverage, they get disoriented because of artificial light sources — and that includes headlights of oncoming vehicles. And many nocturnal insects get attracted to porch lights at your home, but they also get attracted to lights of oncoming cars, and when that happens they’re much more likely to fly toward that light source on a moonless night than a night with the moon shining. That’s another factor in terms of when the person is driving their vehicle: Is it near the new moon? Full moon? If it’s a new-moon night, or a night when the moon is not visible, they might end up with a lot more nocturnal insects splattered on the front grill of the car and on the headlights of the car.

And it’s not something influenced significantly from year to year. It’s obviously going to be different between driving around in winter when a majority of insects are in a dormant stage, hibernating as adults or in a cocoon in the case of moths. There are going to be fewer insects in most parts of North America impacting the front windshields of cars during winter, late fall and early spring than in the mid-to-late spring and summer.

What does it say about an area if I'm hitting a bunch of them?

If you hit them all at once it means you’ve flown through an aggregation. And flying insects aggregate for different reasons. Some species of insects have what I guess could be described as ‘mating swarms.’ This is particularly common in some species of flies like midges  and they will often aggregate to find a mate. And often most individuals in big assemblages in one place will be males, and females looking for a mate will come to find a mate in a big swarm of males and usually the swarm tends to orient itself around a landmark. And another common way insects get together when looking for a mate is to do what’s called ‘hill-topping.’ In that case, they’re essentially doing the same thing that swarming flies are that hang out near a specific landmark, except that they're going to the highest point in a given region. At times when there was a lot more forests in the eastern United States, that highest point might have been the highest tree in a forest canopy, but after we cut down most of the forests it ends up often being the highest point on a hill. And biologists who study insects use that behavior to look for aggregations of males of particular species by going to the highest point in an area, and they’ll often find a lot more individual insects hanging out there, sometimes in large numbers.

But that means if you’re driving over a landscape, where the road happens to go across the crest of a number of hills — passes and the Rocky Mountains and places like that — you’re likely to hit a lot more insects when you end up driving through those areas where they’re aggregating. Other places they’ll often aggregate is along edges of streams and rivers, in the case of aquatic insects like mayflies.

Any way to tell what kind of bugs are on your car by their guts?

A field guide was written that actually enables people to do that. It’s in print. ‘That Gunk on Your Car.’ People use it to identify splattered bugs on their windshields. It was written by an entomologist at the University of Florida about 20 years ago. Mark Hostetler. It's a field guide for figuring out what you just killed. It’s a tongue-in-cheek field guide, but it’s based on actual research he did.


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