THE INFLUENCE OF HYDRILLA LEAF QUALITY ON LARVAL GROWTH AND DEVELOPMENT OF THE BIOLOGICAL-CONTROL AGENT HYDRELLIA-PAKISTANAE (DIPTERA, EPHYDRIDAE)

Hydrilla leaf quality was studied as a factor that may influence larva l survival, growth, and development of the biological control agent Hy drellia pakistanae. Nitrogen content of plants and leaf toughness vari ed among different sites, within a site, and within each hydrilla apic al 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 1 5 cm from the tip (1.5-2.9% dw), Leaf toughness also varied at differe nt sites, averaging from 190.4 to 235.3 g/mm(2). The apical tip (136.9 -210.3 g/mm(2)) contained the softest leaves, whereas leaf toughness w as greater in leaves 15 cm from the tip (159.0-252.9 g/mm(2)). H. paki stanae individuals reared from neonate larvae to the adult stage on po orer quality hydrilla (low nitrogen, higher toughness) had higher mort ality, longer developmental periods, and at one site, decreased female biomass compared with larvae fed the more nutritious hydrilla (high n itrogen, lower toughness). Additionally, when fed leaves of poorer qua lity, 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 p redation by tip-foraging natural enemies. The biological control of hy drilla by H. pakistanae may be reduced by the latter's apparent sensit ivity to poor plant quality; however, the ability of the larvae to exp loit the more nutritious leaves may mitigate these negative effects. ( C) 1996 Academic Press, Inc.