Is the insect apocalypse happening in our area?

Missing insects in the local mountain ecosystem

[  by David W. Inouye, RMBL  ]

If you spend much time outside, you’re surely familiar with some of the local insects. Pollinating bumblebees, pesky mosquitoes (although they do pollinate one of our local orchids), beautiful butterflies, and determined biting flies are among them. Although you may have noticed that their populations vary from year to year, have you thought about how stable their populations are?

A few years ago, articles began appearing in scientific journals showing that the number of insects is declining at an alarming rate, a phenomenon coined the “insect apocalypse.” Most of these studies took place in heavily developed areas, including fields adjacent to agricultural areas or houses, raising the possibility that those declines are localized results of disturbances to habitat, use of pesticides, increases in streetlights, or other human-caused changes that could be detrimental to insects.

Unfortunately, there aren’t many places in the world where long-term data are available about the status of insect populations. But in 1984, the late Dr. Michael Soulé, internationally famous for his work in conservation biology, began sampling insects every week at the Rocky Mountain Biological Laboratory. I and collaborators have kept this project going since Dr. Soulé stopped his RMBL work. 

Every week during the summer we set up a tent-like structure (a Malaise trap) to collect flies, wasps, moths and other insects for 48 hours. We then bring the insects into the lab to sort them into broad taxonomic groups (i.e., different types of flies, wasps, grasshoppers, etc.) and count and weigh them.

By 2020 we had over 35 years of data and wanted to see if the “insect apocalypse” is happening at Gothic. In a recent scientific publication, we report that the number of insects has declined by 61% and the biomass by 48% since 1986. Declines this large would certainly have been conspicuous and caused alarm if they had occurred in local populations of people, big game animals or cows. 

RMBL is a relatively undisturbed habitat, surrounded on three sides by national forest land, so causes suggested for the alarming insect declines at most other long-term sampling sites are unlikely to affect the results here. For example, there have been no local changes in land use, no application of pesticides or fertilizers and no new streetlights. 

Using long-term climate data, we found that summers have become warmer and winters drier over this time period. The number of insects in a given year tended to be lower when there was less snowfall the previous winter, less rainfall in the summer, and to a lesser degree, warmer temperatures. A conclusion is that as the climate continues to change to warmer and drier in this area, insect populations may continue to decline.  

The local decline in insects may also be a result of other factors that we couldn’t test for, such as increases in the atmospheric dispersal via wind, rain, and snow of pollutants such as pesticides or other man-made chemicals such as endocrine disrupters. Studies elsewhere have documented the global dispersal and presence of such toxins, but their effects on insect populations are largely unknown.

Malaise traps aren’t good at trapping some kinds of insects, including butterflies and bees. RMBL researchers are conducting other studies that focus on these two groups. Dr. Carol Boggs and her students are following up on some butterfly surveys in the Copper Creek valley above Gothi, to see how populations have changed since surveys conducted by Dr. Soulé in the 1980s. She has already shown that the earlier springs and late spring frosts that have become more common are detrimental to the Mormon fritillary butterfly because of the loss of nectar from frost-killed flower buds. 

Dr. Rebecca Irwin now has 16 years of data on the phenology and abundance of about 200 of Colorado’s 1,000 species of native bees. This is one of the longest and most comprehensive surveys of native bees in the world, and is documenting both how the bee and wildflower populations are interacting and how they are being affected by the changing climate. Dr. Irwin and I are participating in a USDA-funded project to establish a national monitoring program for native bees (about 4,000 species) in the U.S., and the RMBL data will contribute to this project.

Dr. Nora Underwood has decades of data on grasshopper populations near RMBL. Her annual census this year shows a record high number of them, and this is also evidenced by their effects on some of the local wildflowers. Aspen sunflowers this year lost a lot of their yellow petals to the grasshoppers. The local dusky grouse population is probably benefiting from the grasshopper abundance as they eat a lot of them, and the garter snakes may too.  

A decline in flies might not seem so bad—well, at least for hikers avoiding biting flies or motorists cleaning dead insects from their windshields. However, insects are extremely important in mountain ecosystems. They provide many ecosystem services, including pollination of wildflowers, breaking down organic matter and serving as a food source for other animals. The steep nationwide decline in insectivorous birds may be a consequence in part of the significant decline in their insect food source. 

These insights into our local ecosystem, which have global implications, are only possible because of the decades-long research that dedicated researchers, funded in part by the National Science Foundation, have carried out at the Rocky Mountain Biological Laboratory. We’re fortunate to have this world-class research facility in our community.