Variability in host plant chemistry effects flight muscle development, gene expression, and flight capacity of adult lepidopteran herbivores

This dissertation examines how variation in host plant chemistry can affect flight muscle development and dispersal ability of adult insects. Three mechanisms known to cause variation in host plant chemistry were tested: inbreeding, gene mutation, and the presence of fungal symbionts. Many studies that measure the effect of plant differences on herbivores only focus on the immature stages of the insect. Because reproduction and dispersal generally occurs during the adult stages of insects, adults make a great contribution to the overall dynamics and demographics of the population. Understanding how host plant heterogeneity influences adult mobility may provide us with better predictive power when confronting problems dealing with insect pests, invasive species, or the preservation of biodiversity. Chapter 2 documents the effects of host plant (Solanum carolinense) inbreeding on the growth and flight capacity of a specialist insect herbivore (Manduca sexta). Manduca larvae grew 9.7% faster and molted into 38.4 % larger pupae when reared on inbred plants, compared to larvae reared on outbred plants. These differences in pupae mass also carried over to the adult stage of the insect. On average, mass-adjusted flight metabolic output was 12.7% higher for moths reared on inbred plants than those reared on outbreds. Changes in adult flight metabolism were also associated with changes to the relative abundance of an isoform of Troponin t (Tnt), a flight muscle protein that regulates muscle contraction. These results show that variation in host plant chemistry caused by plant inbreeding creates developmental effects that cascade through the larvae and pupae to affect dispersal-related traits of the adult stage. This suggests that plant inbreeding may increase herbivore dispersal ability. It also suggests that changes to the population biology of host plants can affect the flight capacity and population dynamics of an animal at a higher trophic level. iv Chapter 3 documents how two generalist Noctuid herbivores (Fall Armyworm and Southern Armyworm) responded differently to variation in the host plant’s (Solanum lycopersicum) defense response. This study compares the changes to growth and flight capacity of two lepidopteran herbivores caused by a gene mutation, in the host plant, that alters the production of jasmonic acid (JA). JA acts as a key molecule in the biochemical pathway that leads to the up-regulation of genes involved in a plant’s defense response to herbivory. Manipulating the defense response of tomato host plants through the JA pathway produced differences in survival, growth and flight metabolic output in the 2 species. The survival rate for Southern Armyworm (SAW) was 71.4% compared to just 14.1 % survival rate for Fall Armyworm (FAW). Compared to defenseless (def1) plants, plants that exhibited normal levels of defense response (Castlemart and methyl jasmonate induced) had no effect on SAW adult body size, but methyl-jasmonate induced plants (MeJA) caused a significant reduction in adult body mass (31.4%) of FAW. FAW adults reared on MeJA plants also showed a 16.9% reduction in flight metabolic rate. The reductions in FAW flight metabolism were also associated with a 42.5% a reduction in the ratio of Tnt A and F isoforms. These results show that SAW appears to be better adapted to feeding on tomato than FAW because SAW was relatively unaffected by variation in tomato’s induced defense response. Compared to FAW, SAW larval survival was higher, and adult fitness traits such as body size and dispersal capability were less affected by host plant defense response. Our results also suggest that changes to host plant defenses could have a greater impact on the spatial ecology and population dynamics of poorly adapted herbivores because it may decrease both their survival and ability to disperse longer distances. Chapter 4 examines how adult flight is affected by variation in host plant (Poaceae) chemistry caused by the presence of fungal symbionts (endophytes). Endophytes have the ability to alter the levels of toxic secondary metabolites, thus decreasing host plant quality for herbivores. This chapter compared the survival, body size and flight capacity of a lepidopteran v herbivore (FAW) when reared on 6 different species of grass, both with and without endophtyes. The effect of endophytes on insect survival and flight capacity varied depending on the species of host plant-endophyte symbiotum. Endophytes we associated with a 77.4% and 68.2% reduction in relative survival for insects that fed on grass species Agrostis perennuns and Poa autumnalis. Endophytes were also associated with reduced growth performance in individuals that fed on Cilamagrostis arundinacea (CIAR), but increased flight metabolism. This was a surprising result because it indicates that the presence of endophytes in CIAR caused these insects to invest more metabolic resources into the development of high performing flight muscles rather than larger bodies. Overall these results shows that fungal symbionts can be associated with changes to adult insect growth and flight capacity, but the effects are dependent upon the species of symbiota that the larval stages feed on.