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Dung-dwelling larvae of ectoparasites of livestock such as the horn fly, Hematobia irritans (L.), may be exposed to =1 different alkaloid species in dung from animals ingesting herbage of the tall fescue (Festuca arundinacea Schreb.)—endophyte association (Neotyphodium coenophialum (Morgan-Jones & W. Gams) Glenn, Bacon & Hanlin comb. nov.). First-instar horn flies were exposed to bovine dung supplemented with up to 50 µM each of N-formyl loline and ergotamine tartrate in factorial combination. In the absence of ergotamine tartrate, N-formyl loline caused a linear decline in the number of pupae recovered, and probit analysis indicated an LC50 of 36 µM. In the absence of N-formyl loline, significant quadratic responses of larvae to ergotamine tartrate were established, and probit analysis indicated a LC50 of 34 µM. An interaction (P < 0.001) was found between the 2 alkaloids for larval survival. This interaction showed that ergotamine tartrate moderated the toxicity of N-formyl loline and indicates that a membrane-bound receptor may be involved. There was no evidence of carryover of effects of alkaloids on subsequent stages of development or expressed as abnormalities of pupae or adults. Interactions between alkaloids probably are involved in other plant-herbivore relationships of endophyte-infected grasses.