Bug of the Week

Swamp Metalmark (Family Riodinidae)


Greetings, BugFans,

Recently the BugLady was privileged to photograph this lovely butterfly at Riveredge Nature Center, one of its few known breeding areas in Wisconsin, (thanks, Mary). Riveredge tallied a record number of Swamp Metalmarks (30) in their July, 2010 Butterfly count (and the BugLady hopes the butterflies and their offspring were all accounted for after her size 12’s left the area).

Everyone agrees that Swamp Metalmark populations are in trouble, and the species is listed as rare/endangered in the areas where it is known to live (it is among the first insects whose status earned it a spot on the Wisconsin Endangered and Threatened Species List in 1989). Why? Its habitat needs are very specific (location, location, location); its caterpillar eats a single, uncommon species of thistle; the adults nectar on only a half-dozen or so flowers; and they don’t stray far from their never-common-and-increasingly-rare habitat. Their habitat is under attack by the normal succession of plant communities and by invasive alien woody plants. Since the caterpillars are vulnerable to fire, the kinds of burns normally used to maintain habitat can’t be used.

Metalmark
This little beauty is little – its less-than-one-inch-wingspan is described by Wagner as "dime-size."

There are about 1000 Metalmark family members worldwide, and North America boasts almost all of them. The family is primarily tropical, and as Wagner says in Caterpillars of Eastern North America, “Not far into the Neotropics this subfamily[family] explodes into a boundless array of wondrous butterflies ... they are a study in variation and complexity, both as larvae and adults.” The BugLady thinks there’s nothing shabby about the Swamp Metalmark either. The top surface of its wings is dark, the underside is orangey, and its name comes from the rows of silvery markings on its wings. The front wings of males are more pointed than are the females’ wings, and females have six full-length legs, while males have reduced front legs. Despite its name, the Swamp Metalmark prefers open spaces that are wet underfoot, not wooded wetlands (“Fen Metalmark” would be a better name). So, if you spot a dark brown/orange, tiny, moth-like butterfly that flies slowly, below knee-level, through alkaline fens with hummocky sedges and low vegetation, and then hides under leaves, it’s worth a second look.

Eggs are laid only on the undersides of the downy, first year leaves of the swamp thistle (it’s a biennial, and its first year leaves lie close on the ground). The resulting caterpillars hatch, feed briefly, and hibernate under those same basal leaves, lying along the leaves’ mid-rib. The caterpillars are described as “slug-like” and covered with long hairs (Wagner offers the tantalizing tidbit that caterpillars are often attended by ants). In spring they resume eating, are ready to pupate in leaf litter by late June/early July and to fly (briefly) shortly afterwards. As adults, they nectar on black-eyed Susan as well as swamp thistle, and they have been seen on swamp milkweed, mountain mint, meadowsweet, yarrow and shrubby cinquefoil.

What is interesting about the Swamp Metalmark is that we can’t accurately chart how abundant it has ever been. There are three species of metalmarks – the Little, the Swamp, and the Northern - that divide the eastern half of the country by habitat. The Little Metalmark and the Northern Metalmark are long-standing species (described by the mid-1800’s), but the Swamp Metalmark was not recognized as a species until 1937, when it was split off from the Northern Metalmark. So, Swamp Metalmarks were being recorded, but as something else (sometimes even as a western species with the awesome name of the Fatal Metalmark). Now that we know who they are, we also know that small colonies of the Swamp metalmark are presently known to exist in only two counties in Wisconsin. Its total range is limited to just a few areas in the Midwest (Missouri is considered the historic heart of its range), and it is assumed that many of its former locations are now empty of Swamp Metalmarks.

What happens now? Riveredge Nature Center is taking a leadership role in strengthening its metalmark population by increasing suitable habitat (and known metalmark “hang-outs” were salted with imported caterpillars in order to increase genetic diversity). Habitat parameters are being studied. Riveredge Habitat Healers are also managing habitat to encourage a small population in a neighboring county. And, Wisconsin metalmarks are being exported to help grow a population at a natural area in northern Illinois. A big story for a very small butterfly.

The BugLady