An analysis of data in Illinois has found a link between higher county-level use of an herbicide called glyphosate and reduced abundance of adult monarch butterflies, especially in areas with concentrated agriculture. This association was only evident during the initial years of the adoption of herbicide-resistant crops (1994-2003), however, when glyphosate use was increasing most quickly.

Investigators also found that wetter and, to a lesser degree, cooler springs in Texas were associated with higher summer abundances of monarchs in Illinois, as were relatively cool local summer temperatures in Illinois.

The findings suggest that both seasonal climate and land use conditions are associated with trends in adult monarch abundance.

"Current studies on migratory species rarely integrate seasonal processes occurring outside the core breeding period or range. Our research was motivated by this knowledge gap," said Dr. Sarah Saunders, lead author of the Ecography study. "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."