Predation on Larval and Embryonic Amphibians by Acid-Tolerant Caddisfly Larvae (Ptilostomis postica)

We examined effects of predation by larvae of the caddisfly Ptilostomis postica on survival of embryonic and larval wood frogs (Rana sylvatica) and spotted and Jefferson salamanders (Ambystoma maculatum and A. jeffersonianum). Studies of predation were conducted both in the laboratory and natural ponds. We also estimated the tolerance to low pH of P. postica in laboratory trials. There were significant effects of predation by P. postica on overall hatching success of A. maculatum in one laboratory experiment; there was no difference between embryos from opaque versus clear egg mass morphs. Ptilostomis postica preyed on larvae of R. sylvatica and A. jeffersonianum in the laboratory and showed no preference in choice of prey. There were no differences in survival of prey due to density of amphibians or presence or absence of leaf and twig cover. Ptilostomis postica was considerably more tolerant to low pH than the amphibians. We observed no mortality of P. postica in any pH treatment (as low as pH 3.2 for 96 h). Field surveys showed that densities of P. postica were quite variable among ponds and years. Embryos of A. maculatum suffered severe (>90%) mortality due to predation by caddisflies in three ponds, whereas caddisfly predation was not observed in two ponds in which it had been observed in previous years. These results suggest that predatory outbreaks of these acid-tolerant insect larvae can dramatically affect amphibian recruitment, especially in systems already stressed by acidic conditions.