Michigan fights a long-feared aquatic invader

In October 2023, hydrilla – one of the world’s most invasive aquatic plants – was found in two West Michigan ponds, its first detection in Michigan. 

Invasive hydrilla can spread easily and quickly in lakes, ponds, and streams. Courtesy of Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. 

 

The Michigan Department of Environment, Great Lakes, and Energy (EGLE) confirmed two small populations of the plant, Hydrilla verticillata, in two of three interconnected private ponds on residential properties in Berrien Springs. They were discovered during routine monitoring of the ponds after treatment for another invasive plant, parrot feather milfoil (Myriophyllum aquaticum). Both species are prohibited in Michigan, meaning it is unlawful to sell, possess, or import them into the state, and both are on Michigan’s invasive species watch list. 

Hydrilla can quickly fill a lake or pond and choke off recreational access. The low hydrilla abundance found in the ponds suggested that the population was either not flourishing or was recently introduced. 

Hydrilla’s root tubers, buds, and even small plant fragments can develop into new plants, allowing easy dispersal through water or by hitchhiking on boats, gear, or attached to ornamental plants sold for water gardens. 

While it’s difficult to be certain, hydrilla and parrot feather likely hitchhiked into the ponds as fragments with ornamental pond plants previously planted there. Both hydrilla and parrot feather are found near patches of ornamental water lily, suggesting a connection between the species. Genetic analysis confirmed hydrilla to be the monecious biotype, but its specific lineage could not be determined. Sediment coring showed a hydrilla tuber bank to be present but with low overall abundance that will aid response actions.

The chain of ponds drains into a stream that flows five miles to the St. Joseph River. EGLE surveyed the connected ponds, stream, and river for hydrilla, and the ponds were treated with herbicide to prevent further tuber production in the 2023 season.

Currently, the Southwest by Southwest Corner Cooperative Invasive Species Management Area (CISMA) has funding from the Michigan Invasive Species Grant Program to continue surveillance efforts through 2025. This effort will focus on looking for hydrilla in surrounding water bodies, including the St. Joseph River, Lemon Creek, and nearby ponds and creeks.

EGLE’s long-term plan is to eradicate hydrilla from the ponds to protect Michigan’s waterways from this invader. Herbicide is being used to suppress hydrilla growth while plans are underway to completely dredge the two ponds later this summer. Dredging provides the highest likelihood of eradication in the shortest time period. Though hydrilla can be treated with herbicide, it is difficult to eradicate. Herbicide treatments alone take six to eight years to deplete hydrilla tuber banks.

Native to parts of Africa and Asia, hydrilla was introduced into Florida in the 1950s and spread across the Southeast. A separate strain detected in Delaware in 1976 has since made its way to several Great Lakes states.  A third genetically distinct strain was recently discovered in the Connecticut River.

In 2006, hydrilla was discovered in an Indiana lake during a routine aquatic plant survey. It was the first time hydrilla had been confirmed in the Midwest, about 40 miles from the Great Lakes basin. The plant was scattered through a large part of the 735-acre Lake Manitou, indicating it had probably been present for a few years. It took a decade and $2.6 million to eradicate the infestation. 

EGLE is nonetheless optimistic about responding effectively to the pond finding, noting successes in eradicating other invasive plants from Michigan waterways. While discovering hydrilla is disappointing, EGLE’s experience with other aquatic invasive plants has prepared responders to manage this situation and protect water resources across the state. 

EGLE is communicating with regional hydrilla experts including SePro Corp., the U.S. Army Corp of Engineers, Cleveland Metroparks, and the Indiana Department of Natural Resources who have extensive experience controlling and eradicating hydrilla. 

Early detection provides a better opportunity for successful control and eradication. Michigan Sea Grant and Michigan State University Extension first launched the Michigan Hydrilla Hunt in 2004 with EGLE’s Office of the Great Lakes as a statewide early detection and education effort. 

The Michigan Invasive Species Program, cooperatively implemented by EGLE and the Michigan departments of Natural Resources and Agriculture and Rural Development, relies on reports from the public to help in early detection and response efforts. 

The hydrilla discovery is also an important reminder for boaters and anglers to clean, drain, and dry all watercraft, trailers, and gear to help prevent the spread of aquatic invasive species. 

What to watch for 

Here’s what to look for in identifying suspected hydrilla: 

• A submerged aquatic plant with slender, green, saw-toothed leaves whorled in groups of four to eight, with five the most common.  
• A leaf mid-vein that’s reddish and may contain a row of spines, giving it a rough texture. 
• Slender stems up to 30 feet long that branch out considerably near the water surface. 
• White to reddish or brown three-petaled flowers. 

Report hydrilla or any suspicious aquatic plants you encounter. Note that hydrilla has some look-alikes in Michigan including native elodea, mare’s tail, and invasive Brazilian elodea. Take a close-up picture of the suspect plant and email the photo and sighting details to EGLE-WRD-AIP@Michigan.gov. Find more information at Michigan.gov/Invasives. 

Adapted from an article in the 2023 Michigan State of the Great Lakes Report by Joanne Foreman, Michigan Department of Natural Resources.