Rarely common and commonly rare, or how a super weed loses its might

Post by Jenny Seifert, UWEX Aquatic Invasive Species Outreach Specialist

It turns out an aquatic invasive plant feared by many as a lake-choking menace may not be so universally problematic after all.

The “super weed” in question is Eurasian watermilfoil, or EWM for short, and a decade’s worth of research is dismantling some commonly held beliefs about the non-native plant’s impact on lakes and how to effectively control it.

This mat of Eurasian watermilfoil may look menacing, but it’s actually a rare sight in Wisconsin.

To Michelle Nault, a water resources management specialist with the Wisconsin DNR who is among those studying the plant, one of the most surprising things they’ve found is just how much the amount of EWM can vary within one lake and across lakes in the same region.

In other words, it’s not really behaving in a way that would make an “invasion” inevitable.

“We could be on a lake with relatively dense milfoil one year, and then return to that same exact lake the following year and barely be able to find any it. We observed this even in lakes with no active management,” says Nault, explaining statewide research is important for helping them understand the extent of the problem and making wise management decisions.

But the weed hasn’t lost its might completely, cautions Nault. It can certainly become problematic in some lakes, making it still critical for lake users to always take the basic steps to prevent invasive species – that is, inspect, remove, drain and dry whenever you leave the water.

“There are still many lakes within Wisconsin that do not have Eurasian watermilfoil, and preventing its introduction is much more ecologically and economically cost-effective than dealing with the invader once it’s introduced,” says Nault.

Think you know what’s up with EWM? Read on to test your knowledge against what researchers at the WDNR and elsewhere are finding out.

True or false: EWM has invaded most of Wisconsin’s lakes.

False. The research indicates EWM is “rarely common and commonly rare.” According to Wisconsin’s statewide aquatic invasive species database, fewer than five percent of Wisconsin’s 15,000 lakes contain EWM, and the majority of lakes with public access, particulary those up north, still don’t have a shred of it.

Not only that, most lakes that do contain EWM have just a little bit of it – not even enough for most lake users to consider it a nuisance. For some of these lakes, strategic aquatic plant management helps to keep the non-native plant at bay, while natural conditions seem to be enough to keep it in check in other lakes.

The rare cases of dense EWM populations are most commonly found in reservoirs, not natural lakes, and in more southerly parts of the state, where the species began its invasion decades ago.

True or False: Herbicide treatments are the only way to control EWM effectively.

False. There are many ways to control EWM effectively, and choosing the right method depends on your specific lake and management goals. The toolbox of methods includes pulling it by hand, harvesting it with machines, diver-assisted suction harvesting (a method that entails using hoses to transport pulled plants to the water’s surface for easier removal), drawing down lake levels, and protecting shoreline habitat for weevils that eat the plants (a method called biocontrol). Since several of these methods require a permit, consult the WDNR before you make your management plan.

Can you spot the baby bug? This young weevil will grow up to be biocontrol for EWM.

As for chemical herbicides, they seem to be more effective when EWM is caught early or with purebred Eurasian watermilfoil, but they can also have unintended consequences.

Like most invasive species, EWM is clever about finding ways to claim its new territory. One of its tactics is to hybridize with a common native milfoil species, which has resulted in a variety of milfoil mutts, some of which seem to have a tolerance for herbicides or a knack for growing back quickly after treatments.

Moreover, research is showing the cure may be as bad as or worse than the disease sometimes. Large-scale or long-term herbicide control can potentially harm native plants, fish and other beneficial organisms, necessitating a balancing act between treating the problem and protecting the things we’re trying to protect in the first place.

Ultimately, there is no one size fits all solution to controlling EWM. An adaptive approach, continued research and an acceptance that total eradication may be the unobtainable pot o’ gold can help us make careful management decisions.

True or False: Once EWM invades, it quickly dominates the whole lake.

Sometimes true, but usually false. It turns out it is hard to predict what exactly EWM will do once it gets into a lake, but contrary to long-held beliefs, its population doesn’t always explode upon introduction.

A ten-year study of unmanaged lakes with EWM showed that sometimes the plant remains relatively sparse in a lake, while other lakes experienced an immediate spike in EWM that was followed by a natural drop over time. Still other lakes found equilibrium with their EWM population.

Moreover, year-to-year events like drought or lots of rain – the latter of which can lead to floods or bursts of nutrient runoff – can make annual EWM populations boom or bust.

True or False: Prevention and management are still important for keeping our lakes healthy.

True! (Just checking to make sure you’ve been reading carefully.) The best – and cheapest – way to protect your lake from becoming one of the “commonly rare” is to not allow EWM and other invasive species to get in it. Enter the Inspect, Remove, Drain and Dry mantra.

If EWM does find its way into a new lake, the second best thing is early detection, which can reduce the amount of time and effort spent on trying to control it in the long run. A lake-specific management plan with a realistic goal and an approach that integrates multiple control techniques may bring the most success to suppressing EWM’s sub-super powers.

For a deeper look into the latest WDNR research on EWM, read the article ‘The science behind the “so-called” super weed’ in the August 2016 issue of Wisconsin Natural Resources.

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