Weevils Make Positive Impacts in Giant Salvinia Management

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AUSTIN – Giant salvinia is a destructive, invasive, free-floating aquatic fern that poses a major threat to Texas lakes. The Texas Parks and Wildlife Department (TPWD) implements an integrated pest management strategy to control giant salvinia and other aquatic invasive vegetation. Biological controls, like giant salvinia weevils, are one of the tools in the management effort, and biologists are encouraged by some recent discoveries.

Giant salvinia was first identified in Texas during the spring of 1998 in the Houston area. Later that year, the invasive plant was discovered in Toledo Bend Reservoir, Texas’ largest water body. Giant salvinia produces thick surface mats that make fishing, boating, swimming and other water recreation nearly impossible. It can double its coverage area in a week — one football field can become two in seven days.

The giant salvinia weevil feeds only on giant salvinia and can be an effective management tool for this highly invasive plant. The adult weevil feeds on the giant salvinia plant, creating holes in the leaves and feasting on any fresh, new growth. Additionally, the larvae will burrow through the plant “stem” disrupting the plants’ ability to move water and nutrients, ultimately killing the plant.

Although the weevils and larvae are susceptible to cold weather, some recent observations by TPWD biologists may indicate the weevils are more tolerant of low temperatures than originally thought. Early qualitative observations suggested that weevils released in water bodies north of highway 190 between Jasper and Huntsville died off every winter due to below freezing temperatures. Meanwhile, the more southern water bodies in the state that experienced milder winters have been able to establish self-sustaining populations once the weevils are introduced.

Giant salvinia is currently present on 36 Texas lakes and numerous rivers, creeks and marshes. With the spread of giant salvinia to more lakes in recent years, TPWD has worked to attempt to establish weevil populations in colder portions of the state while continuing stockings to augment or rebuild existing populations in areas more hospitable to weevils.

TPWD biologists have found that a few lakes north of highway 190 have maintained weevil populations year-to-year, most notably Lake Naconiche and, to a lesser extent, Lake Murvaul. TPWD biologists indicate the Lake Naconiche weevil populations are correlated with decreased salvinia coverage over the last five years.

“After the winter freeze event in February 2021, our office lost most or all of the weevils we had in our uncovered rearing facilities,” said John Findeisen, TPWD aquatic habitat enhancement director. “In March, we went to a few lakes to sample giant salvinia, looking for surviving weevils. Surprisingly, we found surviving weevils at Lake Naconiche, Lake Nacogdoches, Toledo Bend Reservoir, Lake Raven, Sheldon Reservoir, various marshes and creeks around the Lower Sabine River, and even a few in Caddo Lake.”

TPWD biologists were surprised to see adult weevil overwintering populations in the mid 20’s per kilogram shortly after the 2021 severe winter weather event. High single digit densities are considered viable for overwintering numbers, with 35 adults per kilo regarded as the minimum density to begin controlling a salvinia population.

“It is possible that these population densities were that high due to surviving weevils finding and rafting on the remaining floating salvinia, but it showed that the weevils had better survival rates than previously thought,” said Findeisen. “We collected as many of those Toledo Bend weevils as we could, and the weevils we are raising now for release are primarily descended from that population.”

Sampling in various lakes after the December 2022 freeze event also found weevil survival, indicating that the 2021 samples weren’t a chance occurrence or only made possible by insulation from snowfall.

While TPWD’s biocontrol and herbicide treatment efforts have kept giant salvinia from limiting angling or boating access in Texas public waters, the plants could still hitchhike from one lake to another on a boat, trailer or other equipment. TPWD urges boaters to follow these three simple but crucial steps to clean, drain and dry boats and gear before traveling from lake to lake. Remove plants, mud and debris, and drain all water from the boat and gear. Once back home, open compartments and allow everything to dry completely for at least a week, if possible, before visiting another lake.

For more information on how to properly clean, drain and dry boats and equipment to prevent the spread of giant salvinia and other aquatic invasive species, visit the TPWD YouTube channel for a short instructional video. To learn more about zebra mussels and other invasive species in Texas, visit tpwd.texas.gov/StopInvasives.

TPWD and partners monitor for invasive species in Texas lakes, but anyone who spots them on boats, trailers or equipment that is being moved can help prevent new introductions by reporting the sighting to TPWD at (512) 389-4848. Anyone who finds invasive species such as giant salvinia in lakes where they haven’t been found before can help identify new introductions by emailing photos and location information to aquaticinvasives@tpwd.texas.gov.