Doctor of Philosophy
Hobson, Keith A.
McNeil, Jeremy N.
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.
Summary for Lay Audience
Many insects migrate or overwinter in response to potential habitat deterioration. Migration is typically performed by sexually immature adults who reproduce when they arrive at suitable habitats. However, long-distance flights are costly, and if resources cannot be replaced (i.e. limited resource) this could result in reduced reproductive success. This trade-off could be managed by differentially allocating limited resources, thereby reducing overlapping requirements between migration and reproduction. A limited resource for moths and butterflies is essential fatty acids. These insects feed on nectar during their adult stages so essential fatty acids can only come from the larval diet. My goal was to evaluate how reproductive (summer) and migratory (fall) rearing conditions affect the source and use of fatty acids during flight. I tested this by comparing fatty acid use in true armyworm moths (Mythimna unipuncta) and monarch butterflies (Danaus plexippus) using isotopic analysis and gas chromatography. Both insects were used as both migrate similar distances but differ in terms of fuel loading, flight behaviour and migratory strategy. I found that although environmental conditions did not affect how insects accumulated fatty acids, they did affect how insects used them when exercised. Fall-reared insects conserved non-replenishable essential fatty acids vs. other fatty acids for fuel during exercise, especially in moths. Due to this, multi-day flight experiments were conducted on fall-reared moths. The pattern of differential fatty acid use persisted in moths over multiple days of flight but only when provided with food. The impact of diet restriction during flight on subsequent reproduction was assessed in virgin female moths through egg count and fatty acid analysis. I found that the fatty acid composition did not change when insects were flown with no food, but the number of eggs produced declined. The role of male donations of these limited fatty acids was also evaluated. There was a clear incorporation of male-derived essential fatty acids into eggs which could compensate for resources lost during flight by females. This study indicates that environmental cues responsible for migration result in modified lipid use and the important role nectar availability during migration can have on reproductive success.
Anparasan, Libesha, "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" (2023). Electronic Thesis and Dissertation Repository. 9641.