Development of red-eyed treefrog eggs affects efficiency and choices of egg-foraging wasps

The arboreal eggs of red-eyed treefrogs, Agalychnis callidryas, are preyed on by social wasps, Polybia rejecta. Embryos hatch rapidly up to 30% before their typical undisturbed hatching age when attacked by wasps, so many embryos escape. We hypothesized that the escape ability of older embryos would reduce wasp foraging success or efficiency, and that wasps would therefore prefer to prey on younger eggs. To characterize wasp foraging abilities and preferences, we videotaped the behaviour of individual foragers at an outdoor feeding station. We presented wasps with pairs of clutches differing in age by 1 day, from newly laid versus 1 day through 4 versus 5 days. Eggs become competent to hatch at 4 days. Wasps foraged more efficiently and successfully on older eggs than on younger ones. Among not-yet-hatchable clutches, wasps preferred older eggs, but there was no evidence for discrimination between late unhatchable eggs and hatchable eggs (3-5 days), and wasps readily attacked embryos likely to escape. The thick egg jelly of younger clutches makes it difficult for wasps to break into eggs. The poorly developed embryos also disintegrate easily, making them harder to carry. Thus, at least when wasps have experience with a range of egg stages, they kill few young eggs. Older eggs frequently escaped by hatching, so the heaviest mortality fell on intermediate developmental stages, both at the feeding station and at a natural breeding site. Structural variation in egg clutches and developmental changes in embryos may affect interactions between other small, mobile predators and amphibian eggs.