To test the link between wasp attack and frass accumulation, Weiss introduced the leaf shelters of the silver-spotted skipper caterpillar (Epargyreus clarus) into captive paper wasp (Polistes fuscatus) colonies. The roomy leaf pockets contained either hidden frass pellets or similar-looking black glass beads.
She found that the wasps that visited leaf shelters spent more than 70 percent of their time on those containing frass.
When caterpillar larvae were added to the leaf shelters, the evidence was equally as dramatic. During 5-minute trials, 14 of the larvae housed with frass were devoured by wasps. In contrast, only three were eaten in the shelters containing the black beads.
Related experiments showed that accumulation of frass in their leaf shelters did not affect the caterpillars in terms of crowding or disease. Skipper caterpillars in close contact with 30-day accumulations of frass were no less likely to make it to a healthy adulthood than those in frass-free homes.
The findings are reported in the April issue of the journal Ecology Letters.
Predatory wasps appear to be attracted to the odor of caterpillar frass, said Weiss. "Evolutionarily, it seems that frass ejection helps to protect larvae from predation by natural enemies," she said. "If they retained frass in their shelters they would be more likely to be killed by wasps or other enemies."
The findings are "novel, interesting, and significant," said Caveney. "[The study] shows convincingly that frass may be used as a homing signal to locate prey."
Predator avoidance is a significant factor driving the evolutionary development of silver-spotted skipper caterpillars. Earlier experiments conducted by Weiss have shown that up to 30 percent of caterpillars in some wild populations can be killed by predators over just a few days.
In fact the risk of being found by predators is so great that many distantly related caterpillar lineages have independently developed the ability to fire frass, said Weiss.
That leaves just one question: Why are pellets ejected so far, so fast?
"A shot distance of a few centimeters or so would generally be sufficient to propel the pellet off the leaflet surface," said Weiss. "Perhaps it's just a by-product of caterpillar physiology."
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