by Kevin Schofield

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Growing up in a rural area, as I did, one learns that a regular rite of summer is bugs getting smashed on your car windshield. There’s the odd one here and there, and then occasionally when driving by a field a swarm will cross the road and … well, it’s pretty disgusting. But over the past couple of decades, many people have started to notice that there don’t seem to be nearly as many flying insects as there used to be. At the same time, scientists have been gathering data that seems to confirm the phenomenon: Insect populations are in decline in many parts of the world.

Measuring insect populations is difficult: There is no “census taking,” and they don’t like to stay still long enough to be counted. Scientists’ primary approach to counting insects is through sampling: They collect them from specific locations at regular intervals and track how the numbers (and species) change over time. They use a variety of methods to gather the samples: “sweep nets” to catch what is in the air, as well as “sticky plates” and catch basins that lure them in and capture them. Scientists also sometimes will look for proxies instead of direct measurements, such as the feeding behaviors of birds and lizards that prey on insects.

This weekend’s read is a 2019 research paper from a dedicated European scientist who spent 21 years collecting his own samples of the insect population: the number of bugs splatted on his windshield. From 1997 to 2017, he drove 1,375 times along two paths — one 1.2 kilometers, and one 25 kilometers — in a rural area of Denmark, and counted the bugs smashed on his windshield. The trends in his data matched those collected through other sources, which validated that his was a reliable sampling method for insect populations in the region. But alarmingly, he confirmed that over the two decades of his study, the insect population declined by 80%.

Scientists haven’t identified a single cause for the rapid decline in insect populations, but they have several possible contributors. Climate change is certainly one: Ambient temperature increases may have made the environment less hospitable for insects and their eggs. Agricultural practices may be another cause, with both pest-resistant crops and pesticides in wide use. Public health measures, such as removing sources of standing water where insects lay their eggs, may also play a part. And humans’ use of bug sprays and lotions could play a role. Likely it’s a combination of all these factors.

The researcher also looked at the behaviors of three local species of birds to try to identify accurate proxies for the insect population. Indeed, he found that the number of times that the parent birds brought food to their hatchlings declined in proportion to the overall insect population. That’s a good reminder for us that even though we might enjoy the short-term benefits of fewer bugs on our windshield and in our homes, such a dramatic decline also has effects on the species that feed on insects. Biologists call this a “trophic cascade”: when an increase or decline in the population of one species in a food web ripples out to the other species. Trophic cascades can look very different depending upon where the change happens. If an apex predator is removed (for example, grey wolves) then the population it preys upon (such as elk) will expand — and likely overfeed on species that it depends on for food. Those species will then decline in numbers, and in turn their prey will expand their population — a repeating pattern down the chain. But contrast that with what happens when species near the bottom of the food web, such as insects, decline: Their predators have less food and see a population drop as well (as we are now beginning to see with bird populations around the globe), and then over time the entire ecosystem collapses, species by species, as their food sources disappear.

The ability to detect and measure these population changes, before dramatic trophic cascades begin, is critically important to our stewardship of the environment. So as silly as it is, we will need to both develop new, accurate measurements of species populations and pay attention to the daily “samples” that we see every day but may not give much consideration to. Or, as the author puts it, “Insects are commonly killed on the windscreen of cars, although this has generally been considered a nuisance rather than a biologically relevant measure of insect abundance.” Perhaps to stimulate some creative thinking as to where we should be looking, he also suggests that sales of car windshield cleaner might also be an accurate proxy of insect populations. And that, if indeed accurate, is probably an easier approach than counting dead insects on your windshield 1,375 times over 21 years (though all respect to the researcher for his dedication to seeing it through).

Read the full research paper “Parallel declines in abundance of insects and insectivorous birds in Denmark over 22 years” online.

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📸 Featured Image: Photo by Colnago 95310/Shutterstock.com

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