12 June 2009

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.

6 comments:

Justin said...

I recall reading a paper that showed that the Loosestrife Beetles also feed on our native members of Lythraceae. Perhaps all that glitters is not gold.

Scott said...

I agree. The verdict for me is still out on introducing a non-native to fight a non-native. We've all seen what's happened with species like the Asian Lady Beetle, which was also introduced as a biocontrol agent. Also, at many of the sites I've seen where Galerucella spp. have been released, Hybrid Cattail (Typha x glauca) has become the dominant. Only time will tell, I guess.

MartyL said...

Isn't it kind of typical for cattails to dominate for a while when growing room opens up in a marsh? So, wouldn't you expect that as a phase on the road to recovery? Maybe I'm just being optimistic.

Scott said...

Hi Marty. As far as I know, cattails are early colonizers, but I'm not sure that they will be outcompeted by anything native. Especially this Hybrid Cattail (Typha x glauca), which seems to be much more aggressive than either parent species. Have you seen cattails give way to natives after a few years?

MartyL said...

Hi Scott, that's my impression, FWIW. I can think of one marsh in particular, supposedly formed by a muck fire, that was predominantly cattails in my youth (say late 60s) and now is buttonbush, common bladderwort, river birch, etc. But that likely was not hybrid cattails. I have old photos of Lena Park Marsh from the late 70s early 80s, I suppose, with many cattails, now none or very few (but some of that area is now reed canary grass). At our place, cattails don't seem to persist without disturbance. Muskrats eat them, for one thing. Marsh iris is more persistant in similar situations. Just anecdotes, I know, but I do have photos going back several decades.

Scott said...

Interesting, Marty. I hope you're right. It will be interesting to see what happens long-term with these areas.