Temperature and body mass drive earthworm (Eisenia fetida) sensitivity to a popular glyphosate-based herbicide

Researchers have analyzed the effects of glyphosate and glyphosate-based formulations on earthworm health. A full literature review shows negative, positive, and non-significant impacts on a suite of outcomes. Pilot studies from our lab have produced similar inconsistencies. This study aimed to determine which factors drive earthworm sensitivity to Roundup-Ready-To-Use III®, a popular herbicide. Taking cues from literature and our pilot work, we examined the effects of soil temperature, initial earthworm body mass, and the interaction between the two on a) final earthworm mass after a month of exposure to contamination and b) survival time during a stress test. We found that both earthworm responses varied with intrinsic worm characteristics (e.g. initial body mass) and environmental characteristics (e.g. soil temperature). Specifically, only initially heavy worms grown in heated soil responded to contamination, and they responded by growing significantly heavier than their uncontaminated counterparts. Worms of lighter mass and worms grown in a cooler temperature did not respond to contamination by altering final body mass. Additionally, earthworms living in unheated soil survived significantly fewer minutes during the stress test, with herbicide-exposed worms surviving the shortest number of minutes overall. Initial earthworm body mass did not affect the outcome of this test. This study shows that by varying initial body condition and soil temperature, earthworms can respond positively, negatively or not at all to exposure to a glyphosate-based formulation. Earthworm sensitivity varies with specific environmental and somatic conditions, and this may explain variation in published literature, where neither initial earthworm body mass nor soil temperature is generally considered.