Which biological control agent is known to significantly reduce existing hydrilla biomass?

The biological control agent known to significantly reduce existing hydrilla biomass is the hydrilla stem borer. This specific insect feeds on the hydrilla plant, effectively reducing its growth and overall biomass. The hydrilla stem borer targets the stalks of the plant, leading to reduced photosynthetic capability and overall vigor of the hydrilla. Over time, this can lead to a significant decline in hydrilla populations in infested areas.

Other options, such as manatees, while herbivorous and able to consume hydrilla, do not specialize in controlling its growth in a targeted manner as the hydrilla stem borer does. The biological control process is most effective when the agent used has a specific relationship with the target weed, which in this case is best exemplified by the hydrilla stem borer.