Weevils might conquer milfoil

Kathryn Herzog

Ask any Twin Cities area water enthusiast what the biggest nuisance this summer is and they won’t say mosquitoes — they’ll say milfoil.
University professors and graduate students are researching environmentally sound ways to control the growth of Eurasian water milfoil, an exotic aquatic weed damaging lakes and streams across the country. The researchers are focusing on weevils, native microscopic insects that eat milfoil.
Milfoil is believed to have been introduced to North America in the late 1940s. The first Minnesota sighting of the aquatic weed was in 1987 at Lake Minnetonka. The weed now infests more than 70 Minnesota lakes — most of them in the Twin Cities area.
Milfoil plant fragments can reproduce and grow rapidly in water. Sometimes the weed forms dense mats on the water’s surface, blocking other native aquatic species from sunlight and becoming entangled in boat propellers.
If it isn’t removed from a propeller, the weed fragment can be carried to another body of water where it can reproduce and spread.
The plant’s population grows out of control because it is not a native plant, and most native aquatic species do not eat it.
Associate Professor of the Department of Fisheries and Wildlife and aquatic ecologist Raymond Newman said milfoil’s growth has caused a decrease in the number of native plants. Newman has studied the Eurasian milfoil since 1992, focusing on four metro area lakes: Lake Minnetonka, Otter Lake, Lake Auburn and Cedar Lake.
Historically, if you look at a lot of exotic species such as milfoil, “they will end up reaching an abundant state on a given lake or area and then decline,” Newman said. “Sometimes they will decline to a level that is no longer a nuisance. Other times they end up remaining a nuisance, and that’s usually in a 15- or 20-year time frame.”
Newman and other University researchers are looking at insects as a means to control the weed without using chemicals.
“There are several insects that have been found elsewhere in the country that have some potential for biological control,” Newman said. “The most promising of these insects is the weevil.”
Newman, who first found the weevil in Minnesota lakes, reports that weevils raised in tanks with the Eurasian milfoil devour the weed and reproduce successfully.
Results show that once the weevil is exposed to the exotic milfoil, it will eventually prefer it over native milfoil species.
Researchers are optimistic about lab experiment results, but further study is required.
“Right now we don’t have a high enough density of weevils at our sights that cause any rapid control,” said Newman. Research is aimed at finding out what is limiting the growth of the weevil population.
There is definite concern about environmental effects of milfoil, Newman said, but the bigger concern is how to control the plant’s population using environmentally and ecologically sound methods.
The Minnesota Department of Natural Resources uses mechanical aquatic plant harvesters to remove milfoil and on some lakes chemical herbicides will be used to rid waters of the weed and prevent further infestations.
“Chemical treatments either kill everything or they don’t kill anything,” Newman said. “Many people that want control are going to tend to err on the kill everything approach, and that clearly has implications on environmental health.”