Effects of water cations on treehole insect communities

We monitored water sodium concentration ([Na]), water volume, and larval densities of insects in natural treeholes and larval insects in simulated treeholes. In natural treeholes, water [Na] was inversely correlated with mosquito and helodid beetle densities; water [Na] was positively correlated with water volume in 1996. The culicid Aedes triseriatus (Say), the ceratopogonid Culicoides guttipennis (Coquillett), and the syrphid Mallota posticata (F.) were the 1st to colonize simulated treeholes. Densities of the helodid beetle, Helodes pulchella (Guerin), and Prionocyphon discoideus (Say), were significantly higher and mosquito populations had a significantly higher population of early instars in microcosms that were lower in [Na] and [Mg]. On the other hand, more early instars of midges were found in microcosms higher in [Na] and [Mg]. The consistency of the relationship between low [Na] and high insect density in natural treeholes, and the effects of [Na] on microcosm communities, suggest that [Na] and perhaps other cations may be important resources and cues to treehole organisms and may partially determine treehole community structure.