The Influence of Hydrilla Leaf Quality on Larval Growth and Development of the Biological Control AgentHydrellia pakistanae(Diptera: Ephydridae)

Hydrilla leaf quality was studied as a factor that may influence larval survival, growth, and development of the biological control agentHydrellia pakistanae.Nitrogen content of plants and leaf toughness varied among different sites, within a site, and within each hydrilla apical tip. Percent nitrogen of the plants averaged from 1.2 to 3.6% (dry weight = dw) at different sites and the hydrilla apical tips (2.7–3.7% dw) had the highest concentration of nitrogen compared with tissues 15 cm from the tip (1.5–2.9% dw). Leaf toughness also varied at different sites, averaging from 190.4 to 235.3 g/mm2. The apical tip (136.9–210.3 g/mm2) contained the softest leaves, whereas leaf toughness was greater in leaves 15 cm from the tip (159.0–252.9 g/mm2).H. pakistanaeindividuals reared from neonate larvae to the adult stage on poorer quality hydrilla (low nitrogen, higher toughness) had higher mortality, longer developmental periods, and at one site, decreased female biomass compared with larvae fed the more nutritious hydrilla (high nitrogen, lower toughness). Additionally, when fed leaves of poorer quality, all instars moved among the leaves of the hydrilla stem and most frequently fed and pupated on the more nutritious leaves in the tip. The larvae fed hydrilla of higher nutritional quality fed and pupated most often in the fifth whorl from the apical tip, possibly to avoid predation by tip-foraging natural enemies. The biological control of hydrilla byH. pakistanaemay be reduced by the latterʹs apparent sensitivity to poor plant quality; however, the ability of the larvae to exploit the more nutritious leaves may mitigate these negative effects.