Bug o’the Week – Milbert’sTortoiseshell Butterfly

Greetings, BugFans,

When the BugLady started this little enterprise back in the summer of 2007, her main criteria for an episode were that she had taken a respectable picture of the bug, and that it had a good story to tell.  She doesn’t have any digital shots of a Milbert’s Tortoise at all, because she hasn’t seen one in at least 20 years (they were last recorded on the Riveredge Nature Center Butterfly count in 1998).  She may have an ancient color slide of one, but when she tries scanning slides, the results are always murky.  Thank goodness (once again) for bugguide.net.

Milbert’s Tortoiseshells (Aglais milberti.) are in the family Nymphalidae, the largest butterfly family.  A distinctive family trait is that their front pair of legs is hairy and much-reduced in size, which has earned them the nicknames “brush-footed” and “four-footed” butterflies (they get around just fine on their second and third pairs of legs).  Although some are very colorful, the upper wing surfaces of many family members are variations in orange and brown, and their underwing surfaces look like dried leaves (allowing them to disappear when they fold their wings https://bugguide.net/node/view/1730918/bgimage).

There are about 6,000 species of Nymphalids worldwide (only 209 in North America); for an introduction to the family, see http://www.biokids.umich.edu/critters/Nymphalidae/ (one of the BugLady’s favorite sources of information).

The spectacular Milbert’s Tortoiseshell https://bugguide.net/node/view/46852/bgimage https://bugguide.net/node/view/476935/bgimage, is also known as the Fire-rim Tortoiseshell (males and females look similar).  It’s pretty unmistakable (but at a very quick glance, might be mistaken in flight for a Red Admiral https://bugguide.net/node/view/1682166/bgimage).

MTs are found across North America, but their range skews north and they’re absent from the Southeast https://www.butterfliesandmoths.org/species/Aglais-milberti.  MT numbers are unpredictable from year to year, their distribution is described as “locally common,” and they like moist grasslands, roadsides, woods openings, the edges of wetlands, and higher elevations – anywhere nettle grows.

They are quick and alert.  Male MTs scan for flying females from perches in their territories, often sitting with their wings open.  Females deposit eggs in clusters on the undersides of leaves of nettle plants in the genus Urtica (she may lay as many as 900 but 150 is more likely) https://bugguide.net/node/view/1675002/bgimage.  The young caterpillars stay together, creating communal, silken nests and feeding within them, and older caterpillars https://bugguide.net/node/view/1367818/bgimage fold leaves around themselves.  They may form chrysalises gregariously, and, say Douglas and Douglas in Butterflies of the Great Lakes Region, and “when the butterflies emerge, their communal meconium stains surrounding areas red.”

There are at least two generations a year, maybe three.  Some of the final brood will overwinter as chrysalises https://bugguide.net/node/view/1459379/bgimage (“chrysalis” describes both the life stage and the exterior case), to emerge in early spring, but some fall butterflies will overwinter as adults in sheltered spots, often with a group of their confreres, flying on warm winter days and emerging, battered, even earlier in spring to heed the reproductive imperative.

The excellent Butterflies of Massachusetts website tells us that MTs are probably a species whose numbers increased as the European settlers cut the forests and established agricultural fields and pastures https://www.butterfliesofmassachusetts.net/milberts-tortoiseshell.htm They were given a boost, too, because the Europeans arrived with the common or stinging nettle (Urtica dioica), a food/medicine/fiber plant that loves disturbed soils and that quickly made itself at home in the New World, outcompeting the native nettle.  In 1899 Scudder described roadside nettles in Massachusetts as being black with MT caterpillars https://www.buglifecycle.com/?p=278.  Climate change could cause MTs to retreat to the more northern parts of their range.

Depending on the time of year, adults feed on sap drips, animal droppings, fermenting fruit or flower nectar.

MTs are “obligate dorsal baskers.”  Some insects can heat up their thorax by quivering their flight muscles in preparation for flight, but on cooler days, MTs have to warm their thorax by basking with open wings in order activate their wing muscles.

The MT is the only North American species in its much-fought-over genus (depending on the sources you look at, you can find them in three genera), but two non-native genus members have also been reported.  There are sporadic records along the East Coast of the very similar European Small Tortoiseshell https://www.ukbutterflies.co.uk/species.php?species=urticae, which ranges across Eurasia (there’s a great anecdote of a Small Tortoiseshell in Nova Scotia flying out of a box that had recently arrived from England).  The Peacock butterfly, a western European butterfly, arrived in Montreal in 1997 and is establishing a small population there https://www.ukbutterflies.co.uk/species.php?species=io.

Kate Redmond, The BugLady

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