Hydrilla, Hydrilla verticillata, is an invasive aquatic weed native to Asia. Hydrilla was introduced in the 1950's as an aquarium plant. It was released in waterways of Miami, Florida and spread to Tampa, FL soon after. By the 1970's it had already spread to the Atchafalaya River Basin in south Louisiana.

Hydrilla can be found in a variety of freshwater aquatic habitats such as lakes, ponds, rivers, and canals. The plant can become established by fragments of another plant being introduced in an area. The plant takes root in the substrate and begins to grow. The best way to prevent these fragments from spreading to different locations would be to clean your boats and other water vessels along with your aquatic equipment (boat trailers, motors, fishing poles, etc.) after every use. Keep in mind: "Clean, Drain, and Dry".

Image courtesy of www.nyis.info/?action=invasive_detail&id=16.

Distribution

Hydrilla has been reported in almost 30 parishes across the Louisiana, and in 29 states nationwide. The most dense and concentrated infestations are found to be at marinas and boat launches nationwide. Map courtesy of EddMaps.

Impact

This image shows how Hydrilla can take over a habitat. Hydrilla causes many problems in aquatic systems because it can form dense mats on the water's surface, which can block out sunlight to native aquatic plants. Large densities in ponds like the image above can decrease the property value of a home due to its unappealing aesthetic.

Hydrilla can cause dissolved oxygen levels to decrease which could result in fish kills as well as displacement of other aquatic plants. Native fishes and wildlife are negatively affected because many depend on submerged aquatic vegetation for food, shelter, and habitat.

Thick mats of Hydrilla as shown in the image above can cause reduced navigability of waterways, damage to boats or personal property, and be hazardous for people swimming in the infested lakes. Hydrilla is dangerous to swim in because people can easily become tangled in the plant due to the root system. The recreational fishing industry has also been negatively impacted by Hydrilla. Fishermen often get lines, fishing poles, and bait caught in the plant, which can lead to additional expenses to replace the equipment. Fishermen are also affected by the lack of fish availability due to the decrease in dissolved oxygen levels.

Control

There are many methods of control for Hydrilla. One method is physical or mechanical removal which can be very effective in small ponds but not in larger water bodies. Other methods include chemical control with the use of herbicides. Fluctuating water levels can also be effective, but it is not an option for certain habitats. Combining several methods of control is one of the best ways to reduce large densities in widespread areas. The best way to control Hydrilla is prevention. This can be done by cleaning your boats and other equipment to prevent spread to other water bodies.

Scientists are currently attempting to control hydrilla by using biological control agents. Biological control agents are organisms with highly selective appetites that feed only on a target plant. Ducks feed on Hydrilla, but they are not considered a primary control agent, because if consumed in large amounts it can become toxic to them.

Grass Carp (Ctenopharyngodon Idella) are used in managing high densities of Hydrilla in some areas. The Grass Carp is originally from Asia and was introduced into the United States in 1963 for experimental purposes to see if they would be an effective control agent for aquatic weeds. Introducing sterile, triploid grass carp are excellent biocontrol agents of Hydrilla if stocked in large systems with enough resources for the carp to survive.

Biological control agents released in United States include the hydrilla stem weevil, Asian hydrilla moth, hydrilla leaf-mining flies, hydrilla stem borers, and hydrilla tuber weevil. The hydrilla stem weevil (Bagous hydrillae) is a highly specific, and feeds on the stems and submerged leaves of hydrilla plants. Adult weevils can be identified by their dark brown appearance with a lighter abdomen. They have also been known to have large white spots on their wing covers.

Larvae of the hydrilla tip mining midge eat the growing shoot tips eventually killing the tips. Larvae of the midge are approximately 3/64 inches (1-2mm) in length. Fresh or live specimens can be identified by their light green body and dark blue band located near the head. Older specimen samples that have lost their color appear to have a greyish white body with a dark purple or black band near the head.

Additional Resources

Cuda, J., J. Shearer, E. Weeks, E. Kariuki, J. Baniszewski, and M. Giurcanu. 2016. Compatibility of an insect, a fungus, and a herbicide for integrated pest management of dioecious hydrilla. Journal of Aquatic Plant Management. 54:20-25.

“Hydrilla (Hydrilla Verticillata) - EDDMapS Distribution.” EDDMapS.org, 2018, www.eddmaps.org/distribution/uscounty.cfm?sub=3028. (Accessed on 25 April 2018).

“Hydrilla (Hydrilla verticillata (L.f.) Royle).” NYIS, 3 Oct. 2012, www.nyis.info/?action=invasive_detail&id=16. (Accessed on 25 April 2018).

“Invasive Species.” Louisiana Sea Grant, www.laseagrant.org/education/topics/invasive-species/species/hydrilla/. (Accessed on 25 April 2018).

Kelso, W., J. Colon-Gaud, and A. Rutherford. 2004. Spatial distribution of macroinvertebrates inhabiting hydrilla and coontail beds in the Atchafalaya Basin, Louisiana. Journal of Aquatic Plant Management: 42:85-91.

Manning, J. and R. Johnson. 1975. Water level fluctuation and herbicide application: an integrated control method for Hydrilla in a Louisiana reservoir. Aquatic Biologists. 1:11-17.

Sutton, D., V. Vandiver Jr., and J. Hill. 1986. Grass Carp: a fish for biological management of hydrilla and other aquatic weeds in Florida. University of Florida. 1-6.

Authors: Emily Cressoine, Darby Simmons, Caleb Tate-Goff

Instructor: Dr. Rodrigo Diaz, rdiaz@agcenter.lsu.edu