Although insects play crucial roles in most ecosystems, surprisingly little is known about how anthropogenic pollutants or their mixtures affect insect populations. While the antagonistic toxicological relationship between selenium and mercury has been documented for decades in fish, birds, and mammals, no investigation has been initiated to determine if a similar relationship occurs in the insects. In this study we investigated the individual and combined effects of selenate and methylmercury on a ubiquitous insect detritivore Megaselia scalaris. Females displayed no ovipositional preference between uncontaminated and food sources contaminated with either selenate or methylmercury. As a result larval survival was significantly decreased and larval development was significantly prolonged by selenate or methylmercury at low or intermediate treatment levels that are ecologically relevant. In adults, female fecundity was significantly reduced by methylmercury treatments. In addition, potentiation, as opposed to antagonism, was strongly evident as mixtures containing concentrations as low as only 1% of the respective mean toxic doses caused significantly more mortality and delayed larval development than would be expected from the responses selenate and methylmercury elicit individually. This is supported by the relative toxicity to M. scalaris of each of the individual and joint treatments; selenate (LC₅₀=260 µg/g) < methylmercury (LC₅₀=22 µg/g) < mixture (LC₅₀=13 µg/g of selenate plus 1.1 µg/g of methylmercury). The increased mortality and delayed larval development within sites contaminated by selenate, methylmercury, or their combination, have substantial implications for the ecology, population dynamics, and sustainability of M. scalaris populations.