Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 1996
Publication Date: N/A
Interpretive Summary: Hydrilla is an aquatic weed that constitutes the major biological impediment to water flow in the southeastern US. This manuscript describes research results that investigated methods of improving control of this weed with biological methods, namely the use of insect flies that feed on the hydrilla foliage and thereby reduce its importance. These flies were originally introduced from India after intensive testing and are now widely distributed throughout the southeastern US. However, their impact on the weed populations may be compromised by their variable ability to feed and utilize the weed when grown under different environmental conditions. This report shows that when the weed bears leaves of relatively low nutritional quality the flies have lower survival, slower growth and reduced size. This information is useful to improve methods to culture plants that are most nutritious for the production of large numbers of flies for massive releases and to predict the best sites for further redistribution of flies. Furthermore, additional insect species are needed that either utilize hydrilla of lower nutritional quality or species that are not as sensitive to foliar variability.
Technical Abstract: Hydrilla leaf quality was studied as a factor that may influence larval survival, growth and development of the biological control agent Hydrellia pakistanae. Hydrilla plant nitrogen and leaf toughness varied among different sites, within a site, and within each hydrilla apical stem. Percent nitrogen of the plants averaged from 1.2 - 3.6% (dry mass = dm) at different sites and the hydrilla stem tips (2.7 - 3.7% dm) had the highest concentration of nitrogen compared with tissues 15 cm from the tip (1.5 - 2.9% dm). Leaf toughness also varied at different sites, averaging from 190.4 to 235.3 g/mm2. The stem 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. pakistanae individuals 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 tipghness). Additionally, when fed leaves of poorer quality, most often in the fifth whorl from the apical tip, possibly to avoid predation by tip-foraging natural enemies. The biological control of