DEFENSE-MECHANISMS OF ARTHROPODS .118. 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 preda tors, eggs of the arctiid moth Utetheisa ornatrix were offered to labo ratory colonies of the ant Leptothorax longispinosus. The ants rapidly devoured Utetheisa eggs produced by parents reared on an alkaloid-fre e diet, but left eggs that had been endowed with parental alkaloid lar gely unmolested. That defense can be attributed directly to the presen ce of a pyrrolizidine alkaloid since topical application of monocrotal ine in either its free base or N-oxide state conferred protection on o therwise palatable eggs. The N-oxide and free base were not equally ef fective 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 se ldom absolute. That difference was resolved only when the otherwise fo od-deprived ants exceeded a certain threshold of hunger. The efficacy of applied monocrotaline as a predator deterrent increased with increa sing concentration of N-oxide, but was lowest for eggs treated with an intermediate concentration of free base. The latter concentration eff ect likely represents an artefact of the behaviour of the ant colonies offered eggs treated with an intermediate concentration, as those col onies were the most voracious predators in reference tests with palata ble eggs. In addition to the immediate deterrent value of pyrrolizidin e 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 avoi dance of chemically protected lepidopteran prey by an invertebrate. Th e discovery of a mechanism promoting learned avoidance on the part of ant predators has important ramifications