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. 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. That difference was resolved only when the otherwise food-deprived ants exceeded a certain threshold of hunger.
The emergence of differential alkaloid effectiveness only after prolonged exposure is consistent with the notion that chemical protection is seldom absolute. 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. 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 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.
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.
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