Purple loosestrife and a beetle’s journey

By Daniel Barringer, Preserve Manager.

Here’s a story I have been thinking about, but knew I needed to write about after I received an inquiry from Preserve Manager (retired) and former co-blogger Tim Burris. He asked, in essence, “Is it me, or are you seeing a lot more purple loosestrife than in recent years?”

Yes, I am.

Photo: Daniel Barringer

This beautiful but highly invasive wetland plant (Lythrum salicaria) has been held in check in recent decades in the United States by a biocontrol beetle in the genus Galerucella. Various agencies and university researchers evaluated the beetle, native to loosestrife’s native habitat, and decided it posed little risk to other species of plants. It was released decades ago and through its feeding, eventually had the effect of greatly reducing the size and density of purple loosestrife populations. From an invasive management standpoint, if a species’ behavior is no longer invasive, then the impact has been mitigated.

Photo: Daniel Barringer

We don’t normally have populations of purple loosestrife at Crow’s Nest Preserve, so it isn’t a major concern of mine (whew!). But it occurs upstream around Hopewell Lake and so could re-enter Crow’s Nest at any time, so we stay vigilant. One year when Sean Quinn (manager of Cheslen Preserve) was an intern, I told him we didn’t have any purple loosestrife here. He didn’t say, “Here, hold my beer,” but might as well have: within a day he’d found a population in a wetland along French Creek south of Harmonyville Road. After another day I had dispatched those plants and haven’t seen it here in the almost 20 years since.

Then while I was participating in the Invasive Species Scavenger Hunt this August, I stumbled upon a single plant in a power line right-of-way at the boundary of the preserve. The location isn’t even a wetland as the plant seems to prefer, but seed or vegetative propagule must have gotten here on equipment the power company uses to maintain the right-of-way. So I shot these photos before removing the plant before it could spread, and will make a note to monitor the location next year.

But in our region, purple loosestrife had become ubiquitous in the 1990’s and early 2000’s. Every roadside drainage ditch and stormwater basin was full of it, and it was showing up in some naturally-occurring wetlands, to the exclusion of the plants that would otherwise grow there. And animal species that need sunny spots to bask in wetlands, such as cold-blooded turtles, could be crowded out by the dense growth of purple loosestrife.

(By the way, there are other species that use the name “loosestrife” that are unrelated to this one, and we are talking here only about the invasive one.) Purple loosestrife was a widely sold ornamental plant before it became apparent how invasive it is and was eventually named a noxious weed in several states (including Pennsylvania) and banned from sale.

Long-touted as a successful biocontrol, the Galerucella beetles did their job. But now all of a sudden loosestrife is back as though the beetle had never been here. What’s going on?

I don’t think that loosestrife populations have evolved to resist the beetle in just 20 – 30 years. What I think has happened is just the normal seesawing of predator and prey, the cyclical swing of population dynamics. With greatly reduced numbers of purple loosestrife available, the Galerucella beetle populations also shrank, allowing the invasive plant populations to recover. With some years’ lag, the beetle may again catch up and manage the plant.

The lesson learned is that no invasive management technique is so encompassing that we can dust off our hands and say, “mission accomplished.” There are no silver bullet solutions and the natural community in which we live is complex, nuanced, and dynamic. Biocontrol was once considered the longest-lasting and most self-sustaining method of invasive plant control. Now we know better.