The primary resource of temperate forest treeholes is leaf litter, and different insects specialize on particular stages of decay. Helodes pulchella (scirtid beetle) takes part in a processing chain by shredding leaf litter and creating material for other consumers. We hypothesized that variation in scirtid density and resources influences the insect community. To test this, we manipulated scirtid beetle and resource densities in field mesocosms. We used a two-factor design (3 litter levels by 3 scirtid densities), and monitored insect communities from April 2004 to June 2005. We detected no statistically significant effects of scirtids on leaf decay. However, during the first season, species richness was higher in mid-summer in the presence of low scirtid density compared to treatments with high scirtid densities or those with no scirtids. The dominant species was Aedes triseriatus (eastern treehole mosquito) and its abundance was unaffected by either scirtid or leaf litter. However, mean pupal mass of mosquitoes was greater at low scirtid densities. Facilitation is suggested by the combination of high mosquito densities and large pupal mass. Densities of Culicoides guttipennis (ceratopogonid midge) were higher at intermediate and high resource levels without scirtids, compared to treatments with any scirtids, suggesting a negative interaction between midges and scirtids. We demonstrated strong, and not always positive, effects of scirtid beetles on insect communities in water-filled treeholes.