A tiny insect with a big appetite for hydrilla could help reduce the presence of this troublesome invasive water weed in lakes, springs and rivers, says an entomologist with the University of Florida Institute of 
Food and Agricultural Sciences. During National Invasive Species Awareness Week (Feb. 26 to March 2), researchers with UF/IFAS want to make residents aware of this potential solution to the state’s longstanding hydrilla problem.
Known as the hydrilla tip miner, the insect helped native eelgrass plants gain a competitive advantage over hydrilla in a recent study, said James Cuda, a professor and Fulbright Scholar with the UF/IFAS entomology and nematology department. Hydrilla is Florida’s most intensively managed aquatic plant, costing the state about $12 million annually.
In its larval stage, the insect is an aquatic worm-like creature and feeds exclusively on the growing tips of hydrilla shoots, Cuda said. Hydrilla plants respond to the damage by branching out horizontally, changing the architecture of underwater hydrilla beds, he said.
“This outcome is good news, absolutely,” Cuda said. “The results clearly demonstrated that the insect’s feeding activity could shift the competitive balance between hydrilla and the native eelgrass, in favor of the latter.”
American eelgrass, known scientifically as Vallisneria americana, is an important component of many lake and river ecosystems throughout the Southeast, he said. Its long, grasslike leaves provide habitat for small aquatic animals and food for waterfowl. When hydrilla invades these ecosystems and forms thick mats at the surface, it shades out eelgrass beds rooted in the bottom, slowing their growth.
The UF/IFAS study was part of a dissertation that led to a doctoral degree for Eutychus Kariuki, one of Cuda’s graduate students. The study involved multiple experiments where hydrilla was grown in large outdoor tanks. In some tanks, American eelgrass grew alongside hydrilla. The insect was released in selected tanks with mixed hydrilla/eelgrass cultures, so that the researchers could determine whether eelgrass grew more vigorously with the insect present.
Overall, the results showed that in mixed hydrilla/eelgrass tanks, eelgrass produced more than twice as much biomass when the insect was present, Cuda said.
Scientists are uncertain whether the hydrilla tip miner is native to Florida or if it’s a non-native that arrived in the Sunshine State with hydrilla plants and became established, he said. At present, the insect is found in small numbers at many southeastern locations, but populations may have been overlooked because the insect is so small – an adult is about the size of a sesame seed.
Cuda has been investigating the hydrilla tip miner as a potential biological control agent for two decades, and has developed protocols for breeding the insect in captivity. Eventually, Cuda says, he hopes to develop a program for mass-rearing the insect and distributing its egg masses or young larvae at hydrilla-infested water bodies to augment naturally occurring populations.
“If funding is available, we would like to make releases of the insect in the Silver River, in Marion County near Ocala, to change the underwater architecture of the hydrilla,” he said. “This should increase the water flow and reduce the algae problem in that water body.”
Cuda also is part of a team developing an integrated pest management, or IPM, system for hydrilla, using the insect, a native plant fungus and an herbicide. The goal is not to eliminate hydrilla but to reduce existing stands to smaller, fully submerged patches that do not interfere with sunlight penetration, drainage or boat travel.
Learn more about the hydrilla tip miner in this “Featured Creatures” document from the UF/IFAS entomology and nematology department — http://entnemdept.ufl.edu/creatures/aquatic/hydrilla_tip_mining_midge.htm.
By Tom Nordlie, University of Florida Institute of Food and Agricultural Sciences