By John Upton

Ever noticed that your car’s windshield is smattered with fewer bugs after long country drives than used to be the case? That’s good news for gas-station squeegee duties — but it’s foul news for the planet.

The world’s bug populations are crashing faster than a swarm of mosquitoes into a backwater bug zapper. And that has reverberating yet little-understood consequences for the species and ecosystems that rely on insects for food, pollination, pest control, nutrient cycling, and decomposition.

A Science paper dealing with the planet’s sixth great extinction, underway since 1500, warns that invertebrate species are faring even worse than vertebrates. Two-thirds of monitored bug populations have declined by an average of 45 percent. Their habitat is being destroyed, and they are being drenched with agricultural insecticides.

The July 25 paper was penned by an international team of researchers led by Stanford University’s Rodolfo Dirzo following an exhaustive literature review. The following chart from the paper shows the percentage of species of insects in the orders Coleoptera, Hymenoptera, Lepidoptera, and Odonata that have declined by as much as 40 percent (shown in dark red) during just the past four decades.

The next chart also includes data on Orthoptera, which includes grasshoppers and crickets.

Vertebrate populations, meanwhile, have fallen by an average of a quarter, the researchers found. The largest of these species are faring the worst.

“In the past 500 years, humans have triggered a wave of extinction, threat, and local population declines that may be comparable in both rate and magnitude with the five previous mass extinctions of Earth’s history,” the researchers wrote in the paper. “This recent pulse of animal loss, hereafter referred to as the Anthropocene defaunation, is not only a conspicuous consequence of human impacts on the planet but also a primary driver of global environmental change in its own right.”

The problem of creepy-crawly declines could be worse than anybody realizes. Unlike charismatic mammals, very few invertebrate species, including lowly centipedes, slugs, spiders, and worms, come under the trained eyes of scientists or conservationists. Fewer than 1 percent of the 1.4 million described species of invertebrates have been assessed for threats by the International Union for Conservation of Nature, which maintains the Red List of Threatened Species. Of those few species that have been assessed, 40 percent were found to be threatened.

Lepidoptera, which includes butterflies, moths, and of course their caterpillar larval stages, are the best studied and monitored order of insects, and evidence suggests that their abundance has fallen by 35 percent worldwide. Lepidopteran species richness is nearly 8 times greater in undisturbed sites than in developed areas, and abundance appears to be 60 percent higher on average in near-pristine environments than elsewhere.

As bad as that might be, it appears that these plant-munchers, which metamorphose into nectar-sucking plant pollinators, are faring far better than lesser-studied orders of insects.

Snails and their shell-less terrestrial gastropod mollusk cousins, the slugs, by sticky contrast, are among the least-well studied. These species may seem like mere pests to many gardeners, but they help break down organic material and they provide food for larger animals. Grasping how slug and snail populations are coping with the defaunation of the Anthropocene relies right now on little more than educated guesswork.

“We mentioned slugs as a point of reference regarding the fact that many groups of invertebrates have been very poorly studied in rigorous, quantitative ways and over long, consistent periods,” Dirzo told us.

“Given the fact that at least some species of terrestrial mollusks seem to do well in disturbed areas, I suspect several of them might not be so severely impacted and might be thriving,” Dirzo said. Then again, he added, “given their strong dependence on moist, relatively mild-to-cool habitats, climate disruption might have a strong, negative impact on them.”