Dung beetles, Ivermectin, and soil fertility: a rangeland study

Abstract

Dung beetles provide essential ecosystem services in rangelands by recycling Nitrogen, Phosphorus, Potassium, Sulfur, and Organic Matter from cattle dung pats back into the soil. However, the use of macrocyclic lactones, like ivermectin, may hinder dung beetles from performing this ecosystem service. This study evaluated the specific impact of ivermectin-treated dung on local populations of dung beetles and resulting soil fertility. To test this, three dung treatments were established from cattle dung that was either left untreated and contained no ivermectin residues or from dung that was manually treated with ivermectin to achieve one of two residue levels, 0.4 (low) and 4.0 (high) ppm. A randomized complete block design with four treatments was established at two sites in northcentral Montana in June 2024 or 2025, where eight-inch dung pats were manually formed from untreated, low-residue, and high-residue dung and placed in the corresponding research cells. A fourth 'empty' treatment was established by leaving certain cells without a dung pat to measure the soil changes in the absence of dung. I found that dung beetles transferred more (p<0.05) nutrients from untreated and low-residue pats than from high-residue pats, which did not differ (p>0.05) from empty cells. With the onset of a drought in the second year of the study, I hypothesized that the lack of dung beetle effects in 2025 are due to drought conditions. This oxidation relationship resulted in no statistical differences (p>0.05) among treatments in the second year of the study for phosphorus and sulfur. Yet, for nitrogen and potassium, analyses indicated that greater (p<0.05) transfer occurred in the control and low compared to the high and empty treatments. Levels of organic matter did not differ (p>0.05) among treatments in either year of the study. These results show that while dung beetle ecosystem services remain a primary driver of rangeland ecosystems, the presence of ivermectin needs to be monitored within a proposed management plan to help benefit cattle ranchers and the environment.