Asia
Hydrilla (Hydrilla verticillata)
This species and Hydrellia balciunasi are virtually indistinguishable; experts must use genitalia features to differentiate adults. Eggs of both species are yellow-white and elongate with longitudinal ridges. Larvae are yellow-green to white, have two anal spines, and are up to 3½ mm long at maturity. Pupae are enclosed in cocoons that turn dark brown with age. Adults are dark gray with grayish-orange legs and typically bronze faces and are 1½–2 mm long.
At warm locations, both Hydrellia spp. and hydrilla develop throughout the year; larvae and pupae are the stages most often observed over winter. Adults emerge in spring and lay eggs singly or in small groups (average of 35 eggs per female) on hydrilla leaves and stems at or just above the water surface. Larvae mine within leaves (4–12 per lifetime) and develop through three instars. Leaves are mined completely before larvae exit to find another. Mature larvae attach themselves to leaf axils and pierce the stems with their anal spines while their outer layer hardens into cocoons. Pupae obtain air from the stems via the anal spines. Adults emerge and float to the surface in a bubble of air to repeat the cycle. There are up to seven generations per year depending on site conditions.
Leaves mined by Hydrellia larvae decay and sink. The subsequent reduction in photosynthesis reduces plant biomass as well tuber biomass and density.
Harms, N.E., M. Purcell, and G.S. Wheeler. 2022. Hydrilla Biocontrol Agents: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-32-HYDRILLA-A. https://bugwoodcloud.org/resource/files/23223.pdf