A new study looks at data from a 29-year moth-trapping effort and suggests a minimum of 15 years of data for detecting true long-term trends in insect populations—and researchers say 20 to 25 years would be even better. In a light trap operated spring through fall from 1967 through 1995, the moth Phlogophora meticulosa, shown here, was found in persistently low levels of abundance interrupted by peaks of increased abundance. (Photo by Jan Kulfan)

By Paige Embry

Paige Embry

For 29 years, from early spring through late fall, a 250-watt mercury vapor lamp glowed from sunset to sunrise by the south side of a building in Prague in the Czech Republic. The light was placed 8 meters (about 26 feet) above the ground, higher than most of the foliage in the surrounding ornamental garden, and therefore visible to nocturnal flying insects in the nearby agricultural fields and sparsely populated residential areas. The light was backed by a white panel and fronted by a grid of wires charged with an electric current. Anything trying to fly to the light or the white panel got zapped and fell into a glass bottle filled with chloroform where it was killed by the fumes. The light ran from 1967 to 1995.

Recently, a group of scientists analyzed this old data using a mathematical framework called state-space modeling to assess local changes in abundance of 110 species of moths. Their results are reported this month in Environmental Entomology.

How insects are doing can be a difficult question to answer. Their abundance tends to bounce around from year to year, so to see reliable long-term trends requires many years of data collection. But how many? That’s one of the key questions the scientists studying the Prague moths were asking. Additionally, they wanted to be able to recognize both short-term changes (a few years) and more long-lasting shifts in abundance. State-space modelling can accommodate that, doing more than putting a best-fit line through a cloud of data.

Marek Brabek, Ph.D., is a senior scientist in the Department of Statistical Modeling at the Czech Academy of Sciences and a co-author of the study. He says one of the main reasons he and his collaborators chose this approach is that it “allows for flexible estimation of the long-term trend, without the need to dwell on unrealistic assumptions, such as exact linearity. … Very often, people mistakenly reduce the study of systematic trends to study of linear trends, neglecting or distorting other trend shapes—e.g. those going first up, then down, those increasing but at different speed, first fast and then substantially slowing down, etc. etc.” He adds that while linear trends may be easy to calculate they are “rarely plausible” for biological systems.

Using a state-space model on the Prague data, the scientists found that out of the 110 species analyzed, 65 showed no significant change in abundance, 29 showed decreases, and 16 showed increases. Some of those changes were clearly temporary, with the increase or decrease returning to the norm before the end of the study period. For others the change was ongoing when the study ended—and potentially represent a long-lasting change.

A new study looks at data from a 29-year moth-trapping effort and suggests a minimum of 15 years of data for detecting true long-term trends in insect populations—and researchers say 20 to 25 years would be even better. In a light trap operated spring through fall from 1967 through 1995 in Prague in the Czech Republic, moths were collected and quantified. Shown here are examples of long-term changes in abundance in four species of the family Noctuidae collected in the study. Columns indicate the sizes of the catches in individual years. Agrochola lychnidis (A) showed a significant decrease in abundance. Lacanobia aliena (B) showed a significant increase in abundance. Mythimna ferrago (C) showed a consistently high abundance with the absence of a long-term trend in change of abundance. Phlogophora meticulosa (D) showed a persistently low level of abundance interrupted by peaks of increased abundance. (Figure originally published in Honěk et al. 2026, Environmental Entomology)

The researchers also looked for reasons why the abundance was stable for some species while it increased or decreased for others. They looked at specific characteristics of the moths: diet, forewing length, body mass, species abundance, number of generations per year, and in what life stage (egg, larva, or pupa) they overwintered. They found no link. They also found no link to local habitat or climate change, both of which can impact moth abundance. Alois Honěk, Ph.D., with the Czech Agrifood Research Center and lead author on the study, says, “These changes [in abundance] were already occurring in the 1970s and 1980s, when local climatic conditions were stable. These periods also did not witness significant depletion of plant community species or significant intensification of agriculture associated with excessive pesticide use.”

One of the study’s key findings was to calculate how many years of data needed to be collected before reliable trends appeared using this model: 15 years for increasing abundance, 17 for decreasing. “The results of our study clearly indicate that a many-year data series is necessary to determine a long-term trend in insect abundance,” Honěk says. “To demonstrate reliably the changes in abundance of a number of species, the study should be certainly longer than 20−25 years. … The 15-year value was calculated for our species set. It could therefore be different for a different set of species caught in a different geographical location.”

Honěk lays out the basic requirements for a decades-long project: finding an appropriate site, ensuring institutional and financial support that won’t peter out, maintaining adequate staff. The work-load can be prodigious. On a single summer night, he reports, the Prague light trap sometimes caught several thousand moths. Despite the obstacles, Honěk says, “I think our study is a strong reason for long-term monitoring of abundance of insect species. It can encourage entomologists to start and maintain such studies and serve as an argument for raising financial resources for starting and for continuing long-term studies.”

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.