(CN) – A group of Michigan State University researchers say there is a nexus between large-scale deaths of Monarch butterflies and the application of the widely applied herbicide glyphosate.

Their study looked at the application of glyphosate in counties in Illinois and Texas during different seasons in an effort to better understand its potential effect on the population of Monarch butterflies – which has been in sharp decline in recent years.

“We provide the first empirical evidence of a negative association between county-level glyphosate application and local abundance of adult monarchs, particularly in areas of concentrated agriculture,” wrote Sarah Sanders, an MSU biologist and lead author of the study.

Glyphosate is a weed killer originally created by agrochemical giant Monsanto, which sells the chemical under the brand name Roundup.

The nonprofit Center for Biological Diversity reported in February 2017 that the overwintering population was at 109 million, down 27 percent from 2016 and down more than 80 percent from counts in the mid-1990s.

In addition to the decline of milkweed – which the distinctive butterflies use for procreation – agricultural development and the increasing use of pesticides and herbicides are seen as contributing to the insect’s decline.

Researchers also attempted to integrate seasonal differences in the nexus between glyphosate and butterfly population decline, finding longer and cooler springs led to a correlative growth in the population in Illinois.

“Current studies on migratory species rarely integrate seasonal processes occurring outside the core breeding period or range,” Sanders said in a statement. “Our research was motivated by this knowledge gap Monarchs are an ideal species to study because the hypotheses proposed to explain their decline include climate and land-use factors occurring during every season of their annual cycle and incorporating broad geographic extents.”

Those “broad geographic extents” include much of North America as the Monarch butterfly, an iconic pollinator species, is the most common butterfly in the region.

Also, Monarchs undertake an annual migration, traveling from Canada and the northern United States down to the southern United States and Mexico in the autumn.

The species is not currently listed as endangered under the Endangered Species Act, but the Center for Biological Diversity petitioned the U.S. Department of Fish and Wildlife for listing in 2014.

Fish and Wildlife did a review, but the environmental organization sued last year claiming the agency was dragging its feet. A federal court ruled Fish and Wildlife must decide on the listing by 2019.

Glyphosate is one of the most widely used herbicides in the United States. The chemical compound is used in both industrial agricultural production and home use.

The chemical has come under considerable scrutiny in recent years, as various groups and individuals have accused Monsanto of hiding its adverse effects on human health and wildlife.

In California, a lawsuit centers on whether the company has covered up a connection between human exposure to glyphosate and the development of a certain virulent strain of non-Hodgkin’s lymphoma.

Recently disclosed documents appear to show Monsanto paid scientists to write studies that cast doubt on the causal link between glyphosate and cancer. In some cases, Monsanto employees wrote the studies themselves, which the U.S. Environmental Protection Agency used to base its findings about whether or not to restrict or ban the herbicide.

A parallel case in the state court has ruled that California’s Office of Environmental Health Hazard Assessment can add the chemical compound to its Proposition 65 list of carcinogenic chemicals.

Environmental organizations have often decried the use of the compound in agricultural areas, saying it kills pollinators like bees and butterflies.

The Michigan State University study adds to the mounting evidence indicating a correlation between the herbicide and pollinator-population declines.