Pyrrolizidine alkaloid deters ant predators of Utetheisa ornatrix eggs: effects of alkaloid concentration, oxidation state, and prior exposure of ants to alkaloid-laden prey.

To examine the chemical defense of lepidopteran eggs towards ant predators, eggs of the arctiid moth Utetheisa ornatrix were offered to laboratory colonies of the ant Leptothorax longispinosus. The ants rapidly devoured Utetheisa eggs produced by parents reared on an alkaloid-free diet, but left eggs that had been endowed with parental alkaloid largely unmolested. That defense can be attributed directly to the presence of a pyrrolizidine alkaloid since topical application of monocrotaline in either its free base or N-oxide state conferred protection on otherwise palatable eggs. The N-oxide and free base were not equally effective in conferring protection however; free-base treated eggs were subject to greater predation than N-oxide treated eggs after 24 h. The emergence of differential alkaloid effectiveness only after prolonged exposure is consistent with the notion that chemical protection is seldom absolute. That difference was resolved only when the otherwise food-deprived ants exceeded a certain threshold of hunger. The efficacy of applied monocrotaline as a predator deterrent increased with increasing concentration of N-oxide, but was lowest for eggs treated with an intermediate concentration of free base. The latter concentration effect likely represents an artefact of the behaviour of the ant colonies offered eggs treated with an intermediate concentration, as those colonies were the most voracious predators in reference tests with palatable eggs. In addition to the immediate deterrent value of pyrrolizidine alkaloid, ant colonies that had been exposed to alkaloid-laden eggs subsequently avoided even palatable, unprotected eggs offered 33 days later. Our data provide the first demonstration of such long-term avoidance of chemically protected lepidopteran prey by an invertebrate. The discovery of a mechanism promoting learned avoidance on the part of ant predators has important ramifications to egg-laying strategies of female Utetheisa, both in terms of the dispersion of eggs, and the extent to which eggs are provisioned with alkaloid.