Nutrient limitation and its consequences for performance and the homeostatic regulation of macronutrient composition in two phytophagous insects with divergent life-history strategies
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Insect herbivores have a decidedly higher nitrogen and phosphorus than their host plants, an elemental mismatch that places severe constraints on their ability to meet nutritional demands. This study examined the consequences of macronutrient (nitrogen and phosphorus) limitation for two wing-dimorphic, phloem-feeding planthoppers with very different life-history strategies: the sedentary <i>Prokelisia dolus</i> and the migratory <i>P. marginata</i>. As a consequence of the constraints dispersal imposes on ingestion capacity, I argue that species using dispersal as a strategy for acquiring limiting nutrients are unable to adequately obtain nutrients when dispersal is not an option. The effect of nitrogen and phosphorus limitation for planthopper performance (Chapter 1) and homeostatic regulation of macronutrient composition (Chapter 2) were determined, as were the constraints dispersal places on other traits (ingestion capability) used to cope with nutrient limitation (Chapter 3).
The planthopper species responded differently to nutrient limitation. The survival, body size, and development rate of both species was adversely affected on nutrient-deficient host plants, but <i>P. marginata</i> was more negatively affected than <i>P. dolus</i>. Nitrogen was more limiting than phosphorus for both species. <i>Prokelisia marginata</i> was also less able to regulate its macronutrient composition (C:N:P) and incurred greater growth penalties than <i>P. dolus</i>. Overall, the migratory species was far more sensitive to nutrient limitation.
Divergent life-history strategies (migratory versus sedentary) and the differential muscle allocation patterns associated with such strategies provide the mechanism underlying the consistently different performance responses of the two planthopper species on nutrient-deficient host plants. Morphometric and gravimetric measures of investment in flight versus feeding indicate that the sedentary <i>P. dolus</i> allocates more muscle mass to feeding whereas <i>P. marginata</i> invests more heavily in flight. Due to its greater investment in feeding musculature and associated enhanced ingestion rate, the immobile <i>P. dolus</i> is better equipped to meet macronutrient demands when faced with nutrient-poor food than the migratory <i>P. marginata</i>. Results of this research demonstrate the importance of considering life-history strategies, and associated constraints imposed on ingestion, when assessing how the macronutrient stoichiometry of plants (C:N:P content) interfaces with the nutritional requirements of phytophagous insects to affect their growth and performance.