Effect of rearing conditions on the allocation of larval and adult acquired essential and nonessential fatty acids to flight in two adult Lepidoptera: Danaus plexippus and Mythimna unipuncta

Libesha Anparasan

Abstract

Insects which are nectivorous as adults acquire essential fatty acids, which are important for many biological processes, almost exclusively from the larval diet. Thus, adult allocation of this limited resource may result in trade-offs in migrant insects that delay reproduction. I used the true armyworm, Mythimna unipuncta, and monarch butterfly, Danaus plexippus, to test the hypothesis that environmental cues (fall migratory or summer reproductive) would influence the use of fatty acids during flight (0-6h). I used larval and adult diets manipulated isotopically (δ¹³C) and chromatographic analyses to determine fatty acid composition and source in the fat body. C4 carbohydrate feeding increased the δ¹³C value of lipid in moths (-29.1 vs -16.5‰) and monarchs (-31.2 vs -22.1‰) and increased total fatty acid concentrations reflecting the important role of adult feeding. Fuel use during flight differed with essential fatty acids being more conserved under fall than summer conditions for moths (0% vs 33% loss) and monarchs (21% vs 33% loss) indicating that the environmental cues responsible for the onset of migration result in physiological changes that modify lipid use. The isotopic composition of fall-reared monarch fat body remained constant during flight but declined in fall-reared moths (-18.7‰) indicating an increased use of adult-derived resources in moths. Extended multi-day flight experiments focused on fall-reared moths with differing diet availability. When diet was available, moths conserved essential fatty acids reflecting the important role nectar availability during migration has on fatty acid allocation. I evaluated the effect of flight under differing diet provisions on egg count and fatty acid composition in virgin female moths. Fasting during flight period reduced egg count compared to fed females, but eggs had similar fatty acid compositions. I explored the role of male-derived essential fatty acids in reproduction. There was incorporation of spermatophore essential fatty acids into eggs produced by mated females and thus male donated fatty acids may play a role in reproductive success. The importance of migratory strategy, nectar availability and life history on the conservation of essential fatty acids during migration and potential mechanisms behind the differential allocation of fatty acids in migratory insects are discussed.