Purple Loosestrife Biocontrol Update

About a month ago, I blogged about using Galerucella spp. beetles to control Purple Loosestrife (Lythrum salicaria). At that time, Galerucella beetles had just recently emerged, and we saw eggs, larvae, and a few adult beetles, but not much damage to the newly emerged leaves. I visited the same site a few days ago, and the damage to Purple Loosestrife was a much different story.

In the photograph above, you can see what looks somewhat like an herbicided plant. That's Purple Loosestrife after the beetles have done their job. This plant surely won't be producing any seed this year.

The photograph above is a close-up of the damage to Purple Loosestrife leaves.

In the photos above and below, you can see numerous Galerucella beetles covering Purple Loosestrife plants. I counted nine beetles in the photograph below!

There were several comments to my previous post on this topic. We don't know for sure what will happen long-term as a result of the introduction of these beetles. They may feed on our native Winged Loosestrife (Lythrum alatum) and cause similar results to that species. I had suggested that Hybrid Cattail (Typha x glauca) often becomes dominant in areas where Galerucella have been released; I have to correct myself. At these sites, Hybrid Cattail is in fact becoming the dominant in some areas, but native species including Blue Joint Grass (Calamagrostis canadensis), Water Sedge (Carex aquatilis), Jewelweed (Impatiens capensis), and Dotted Smartweed (Polygonum punctatum) are becoming dominant in other areas.

Battling Purple Loosestrife

Purple Loosestrife (Lythrum salicaria) is a perennial herbaceous plant native to Europe and Asia. Many people think this is a "pretty" plant, though to me, it is quite gaudy.
Because of its supposed attractive physical appearance, Purple Loosestrife has been introduced into North America and New Zealand since the 1800s as an ornamental plant. It has since spread to nearly every state in the United States, creating monocultures in wetlands that lead to the exclusion of native plant and wildlife species. It is common in ditches, as seen below, as well as in emergent marshes and sedge meadows, and on pond edges. If you're wondering how a few introduced plants can spread and create such a problem, you will be astounded to hear that a single purple loosestrife plant can produce up to 3 million seeds per year!

In recent years, however, researchers at Cornell University have been studying the natural enemies of this invasive plant in its native range. Where Purple Loosestrife is native, it occurs in small patches, at least in part because it has natural biological enemies; in the absence of these enemies, populations of Purple Loosestrife can run rampant. The researchers have discovered that several beetle species in the genus Galerucella (as well as a few weevil species in the genera Hylobius and Nanophyes) have the ability to keep Purple Loosestrife in check while not impacting other plant species. As these insects reduce populations of Purple Loosestrife, populations of these biological control insects will also decrease. The goal for this method of control is not to completely eliminate Purple Loosestrife, but rather to reduce it to a point that plant and resulting animal diversity can increase.

The photograph below shows eggs of Galerucella sp. on a Purple Loosestrife plant. You may need to click on the photo to increase its size to see the clusters of small cream-colored eggs with hairlike black projections. You'll also notice some damage to the leaves, a result of the introduced insects.
Below is a photograph showing a Galerucella sp. larva, also on a Purple Loosestrife plant. Larvae of these insects are only a few millimeters long.

Finally, the adult stage of Galerucella sp., shown below, on the tip of a Purple Loosestrife leaf.

While botanizing in northern Indiana, I've noticed several places where Galerucella spp. were not introduced but where they are now present, indicating that introduced populations of this biological control agent are spreading to other populations of Purple Loosestrife. Last week, while in Lake Station, I saw the following...

This is definite evidence that the beetles are reproducing, damaging Purple Loosestrife, and spreading to other populations of the invasive plant. Galerucella spp. were introduced at this site, but not in the area of the site where this photograph was taken. Hopefully in a few years, you won't be blinded by the purple haze as you drive through northwest Indiana.