Residues of pesticides in dairy cow rations and fly treatments reduce the number of Coleoptera in dung

There is a global pattern of a decline in insect abundance and diversity. The presence of residues of pesticides in animal feed and fly treatments results in polluted animal dung, which may hamper the survival of insects feeding and breeding in and on dung. To study the route from pesticide contamination in the feed and the dung to insect survival in dung pats, two experiments were performed. In Experiment 1, the feed and dung of dairy cows were collected on eight Dutch dairy farms (six conventional, two organic). All the feeds in the ration as well as the dung were analysed for the presence of pesticides. In Experiment 2, dung was collected from a herd of dairy cows and was spiked with four concentrations of deltamethrin. In both experiments, a field experiment was implemented with artificial dung pats of 2 kg of fresh dung. The dung pats were retrieved after 7 and 14 days (Experiment 1), or 7, 14, 21, and 35 days (Experiment 2), and invertebrates in the dung were counted. A total of 70 pesticides were detected in the different types of feed of the eight farms. The concentration of pesticides in the dung was on average 423 µg kg⁻¹ DM (range of 112–1980 µg kg⁻¹ DM). In Experiment 1, the number of Coleoptera present in dung was negatively correlated with the pesticide concentration, and more Coleoptera were present in the dung of organic farms (23.7 on organic farms vs. 11.4 Coleoptera on conventional farms, per 500 g freshly deposited dung). In Experiment 2, the dung which was not spiked with deltamethrin contained more than twice as many Coleoptera larvae and adults (5.6 Coleoptera per 500 g freshly deposited dung on treatment without deltamethrin vs. 2.2 with highest concentration of deltamethrin). We conclude that pesticides are widely present in feed for dairy cows and that a number of these pesticides are transferred to dung. The pesticides have a negative effect on the number of Coleoptera and as a consequence may also affect ecosystem services such as the disappearance of dung pats and the presence of insects in dung pats as feed for farm birds or bats.