Biological Control Of Invasive Plants: Hydrilla (Hydrilla verticillata)

Cyperus articulatus -- Center for Aquatic and Invasive Plants

Hydrilla is a submersed plant that was introduced into Florida as an aquarium plant. Since the 1960s, hydrilla has become the "number 1" aquatic weed in the state, filling lakes and rivers. It has spread rapidly throughout the southern US, and now occurs as far north as Delaware, as far west as California.

The first biocontrol insect released against hydrilla in the US was the Pakistani hydrilla tuber weevil (Bagous affinis). First releases were made in 1987. Unfortunately, this weevil feeds on hydrilla tubers only when water recedes from the plants, a situation that occurs in Pakistan but is rare in Florida, except when lakes are drained. Therefore it has been difficult to establish field populations. The efficacy of this hydrilla tuber weevil, as well as another released Bagous species is still being evaluated.

Another hydrilla biocontrol agent, Hydrellia pakistanae, was first released in Florida in 1987. The hydrilla leaf-mining fly is from Pakistan. Only the larvae of this insect damage hydrilla by mining the leaves. Its life cycle is about 20 days. Initial establishment problems were overcome by 1990, and now field populations are established in several places in Florida.

Other Hydrellia species have been identified, investigated and released against hydrilla, including Hydrellia balciunasi, an ephydrid fly from Australia.

The hydrilla miner Cricotopus lebetis is a midge that has been associated with hydrilla declines in several Florida locations since 1992. Developing larvae mine the growing shoot tips of hydrilla, which severely injures or kills them. The feeding damage alters the plant’s architecture by preventing new hydrilla stems from reaching the water surface. The life cycle of the hydrilla miner is completed in 1-2 weeks. (See Plant Management in Florida Waters for more information)