Cabbage moths, corn borers and other plant-eating insects crucial to ecosystems have declined dramatically in East Asia over the past two decades — along with dragonflies and other predator insects that eat them, according to a study published Friday in the journal Science Advances.
They found that overall levels of insects flying through a key migration corridor between China, the Korean Peninsula and Japan declined nearly 8 percent, and summer levels of predator insects went down almost 20 percent. The drop in plant-eating bugs contributed to the decrease in predator insects, reducing their ability to act as a control at the top of the food chain.
“Everything is connected,” said Kris Wyckhuys, a visiting professor at the Chinese Academy of Agricultural Sciences and one of the co-authors of the study published in the U.S.-based journal. “One species will start to disappear or experience dramatic declines in abundance, and those linkages in the food web will start to weaken as well — ultimately the whole web will unravel.”
Dragonflies, beetles and other predator bugs keep plant-eating insects in check. Without predators, plant eaters such as aphids and caterpillars are free to gorge. The consequences can spill over for humans when the plant eaters target crops such as corn, soybeans and cotton that people depend on for food and economic growth.
Each year from 2003 through 2020, the scientists tracked insects flying over Beihuang Island in Bohai Bay between China and the Korean Peninsula. Using radar and light traps that attract insects flying at high altitudes, they found that as the plant-eating insects declined, so did the insects that eat them. Eventually, the absence of the predator species let others expand unchecked, creating ripple effects down the food chain.
The study echoes other research that has found similarly alarming rates of insects vanishing, including butterflies on the American prairie, beetles in the forests of Puerto Rico and flies in Germany’s swamps.
A decline in insects might not seem like it would pose broad problems. But the bugs play a crucial role in ecosystems by pollinating plants and controlling pests that menace essential crops. Some scientists are concerned that a massive bug die-off could throw ecosystems around the world out of balance, cause food chains to unravel, and lead to an overabundance of some species and the extinction of others. Other scientists have debated whether a die-off is underway worldwide or confined to specific locations.
The findings by the team in Beijing underscore a core principle in ecology: that an imbalance at one level of the food chain can cascade to throw all the others off.
“In the future as the climate continues to change, food webs are going to change dramatically, depending on who wins and who loses,” said Matthew Moran, emeritus professor of biology at Hendrix College in Arkansas. Losing a keystone species could have a larger impact than the decline of a comparatively rare species, said Moran.
The iconic wolves in Yellowstone National Park have come to symbolize how important predators are for biodiversity. In the 20th century, wolves were absent from the park for 70 years. Without its top predator, the ecosystem fell out of balance; elk and deer overgrazed trees that were essential shelter for birds. In 1995, a group of Canadian wolves was released into the park. Just a few years after the wolves’ reintroduction, the elk and deer had declined, while trees and birds had rebounded.
“The same thing happens in the insect world,” said Wyckhuys. “Predators such as dragonflies and lady beetles, those are the wolves of the insect world.”
As the top levels of the food chain disappear, the lower levels — such as aphids and caterpillars — overindulge on their food sources, throwing plants out of balance, too.
“These predators who sit at the apex of a food web are very vulnerable to extinction,” said David Wagner, an ecologist at the University of Connecticut. “They need all the pieces down below to remain intact. They’re among the most vulnerable species on the planet.”
There are a number of reasons the insects are dying off, according to ecologists. Roads, farmland and invasive species encroach on their habitat, and pesticides pollute what remains. Climate change has led to irregular weather and seasonal patterns.
“The new climate that humans are creating is going to benefit some insects and be a detriment to others,” said Moran.
The die-off might not be irreversible. Studies such as the one conducted by the scientists in Beijing showing the food web relationships between predators and prey can help ecologists develop restoration projects that include actions like reintroducing predator species.
“When we implement restoration measures, we’re usually successful,” said Wagner. “We have the knowledge and the tools to turn things around.”
The research could also help scientists develop new methods of pest control for food crops, reducing the need for pesticide use by deploying predator insects instead. “Insects — they do something in the world,” said Wyckhuys. “They provide important services to humanity.”