By Pete Bauman
South Dakota researchers have taken a closer look at the function of dung beetles in Eastern South Dakota over the last few years. This article summarizes findings related to management of livestock grazing and chemical pesticides in relation to dung beetle and insect community health (see the references at end of this article for more information).Source : sdstate.edu
Researches evaluated impacts of various grazing management factors on dung beetle and arthropod communities across 16 grazing operations in eastern South Dakota. Selected grazing operations had a minimum of a 5-year grazing history and ranged in size from 20 to 120 head. Grazing operations were categorized as regenerative, intermediate, or conventional based on incorporation of three primary livestock/range management practices (stock density, frequency of rotation, and applications of pesticides [avermectin]). High stock density, frequent rotation, and no or very limited use of chemical pesticides was considered ‘regenerative’ and low stock density, infrequent rotation, and frequent use of chemical pesticides was considered ‘conventional’. The grazing operations were scored based on frequency of use of the three practices, with higher scores reflecting practices correlated with the promotion of biodiversity and soil quality (i.e. regenerative). Of the 16 grazing operations studied, five were categorized as regenerative, five as intermediate, and six were categorized as conventional.
Over 116,000 arthropod specimens were collected and evaluated over the course of the study, with beetles being the most observed type of arthropod. Specimens were categorized as those who foraged on dung, pests, predators, herbivores, or parasites. Early in the season (May) yielded the lowest overall arthropod use, with the subsequent months showing significantly higher occurrence. The study also showed that the arthropod community changes over the course of the growing season.
Generally, time of year and the type of management system had varying effects on different categories of arthropods, with conventionally managed pastures having both reduced dung beetles and pest predators. Both are desirable in grazing systems. Regenerative pastures with higher stock density, frequent rotations, and no ivermectin use had significantly more individual dung beetles and a greater overall diversity of dung beetle species. Higher stock density coupled with frequent rotation tended to create a system where forage was consumed at a more palatable state resulting in more dung deposited in a smaller area and dung that was less dense, had higher initial moisture, and was ultimately attractive to dung beetles and other arthropods.
The effects of ivermectin use and the residual effects in the dung over time are outlined in the paper, with conventionally managed pastures having roughly twice as much residual ivermectin remaining in dung pats than did the intermediately managed pastures, and nearly 14 times as much as those categorized as regenerative. Ivermectin has been shown to be toxic to dung beetle larvae, and the conventionally managed systems had a 66% decrease in dung beetle occurrence when compared to the regenerative systems. Interestingly, fly maggot abundance was highest in conventional pasture systems utilizing the most frequent ivermectin applications, calling into question the efficacy of the use of the product for the desired control of flies and other pests while also negatively impacting pest predators and dung beetles. This dual negative effect may ultimately reduce the potential for the dung pats to degrade while potentially creating increased livestock pest issues. Livestock managers experiencing poor breakdown of manure pats coupled with continued pest problems should consider reducing or eliminating ivermectin while also exploring options for improve herd management and forage quality options.