Which aquatic plant was released as a biological agent for control in 1965?

The aquatic plant that was released as a biological agent for control in 1965 is alligatorweed. In the 1960s, alligatorweed was recognized for its ability to spread rapidly in aquatic ecosystems, leading to significant ecological and economic impacts. As a means of managing this invasive species, the South American flea beetle (Agasicles hygrophila) was introduced to help control its growth. This biological control method leverages natural predators to reduce the population of the invasive plant without the use of chemical herbicides, promoting a more sustainable approach to managing aquatic vegetation.

While water hyacinth is another invasive species that has been targeted for control, it was not released as a biological agent in 1965. Brazilian pepper is a non-native shrub that has also been the subject of control efforts but is not an aquatic plant, and Chara, a type of green algae, does not fit the criterion of being released as a biological agent in that time frame. Therefore, alligatorweed stands out as the correct answer in relation to the history of biological control efforts for invasive aquatic plants in Florida.