Pesticides and deposited fine sediment have been associated independently with changes in relative abundance and number of sensitive species in aquatic ecosystems, but the interplay between these stressors in agricultural streams is poorly understood. We used a 28-d experiment in outdoor streamside mesocosms to examine the effects of varying levels of fine sediment and a glyphosate-based herbicide on 2 important aspects of macroinvertebrate community dynamics, invertebrate drift and adult insect emergence. We applied 4 levels of each stressor in a full factorial, repeated-measures design (4 glyphosate × 4 sediment levels × 2 replicates of each treatment combination). We focused on the propensities of the invertebrates to drift or emerge (number drifting per mesocosm/benthic individuals per m²). Sediment addition affected drift propensities of 4 of the 10 most common invertebrate taxa, community-level measures of drift (total taxon and Ephemeroptera, Plecoptera, Trichoptera [EPT] richness) and adult emergence (total propensity and taxon richness), whereas glyphosate addition affected drift propensity of only 1 taxon. These results suggest that glyphosate entering streams during terrestrial herbicide operations may be less problematic for aquatic invertebrate community dynamics than fine sediment from catchment erosion. No significant sediment × glyphosate interaction was detected for any individual taxon drift propensity or community-level drift or emergence measure, indicating that the 2 stressors acted additively, rather than synergistically or antagonistically. The dynamic measures (invertebrate drift and adult insect emergence propensities) often showed the opposite pattern compared to benthic invertebrate densities in the same experiment, a result highlighting the value of using dynamic aspects of stream invertebrate communities to identify mechanisms underlying stressor effects on benthic standing stocks.

Interactions of pesticides with biotic or anthropogenic stressors affecting stream invertebrates are still poorly understood. In a three-factor laboratory experiment, we investigated effects of the neonicotinoid imidacloprid, food availability, and population density on the New Zealand mayfly Deleatidium spp. (Leptophlebiidae). Larval mayflies (10 or 20 individuals) were exposed to environmentally realistic concentrations of imidacloprid (controls, 0.97 and 2.67 μg L⁻¹) for nine days following five days during which individuals were either starved or fed with stream algae. Imidacloprid exposure had severe lethal and sublethal effects on Deleatidium, with effects of the lower concentration occurring later in the experiment. The starvation period had delayed interactive effects, with prior starvation amplifying imidacloprid-induced increases in mayfly impairment (inability to swim or right themselves) and immobility (no signs of movement besides twitching appendages). Few studies have investigated interactions with other stressors that may worsen neonicotinoid impacts on non-target freshwater organisms, and experiments manipulating food availability or density-dependent processes are especially rare. Therefore, we encourage longer-term multiple-stressor experiments that build on our study, including mesocosm experiments involving realistic stream food webs.