By Doug Taron, Ph.D, Chief Curator of the Chicago Academy of Sciences
Of all the species that we work with in the butterfly conservation lab, by far the most challenging has been the species that is also the most seriously endangered, the Swamp Metalmark. This species has proven difficult at virtually every stage of the captive breeding process. The populations where we can obtain founder stock are small. The few females that we are able to collect don’t lay many eggs.
We feel very lucky to get more than 90 or so out of a single female. Contrasts that to Regal Fritillary females that can each produce upwards of 800 eggs. Hatching, larval growth and survival to pupation are all modest at best.
In northern Illinois, the species has but a single generation per year, which means that we are confronted with the challenge of successfully carrying caterpillars over the winter, a process that has proven difficult for many species. Despite these odds, we continue attempting to breed the species in the lab so that we can return the species to the fens of northeastern Illinois where it formerly flew.
This past August we were able to obtain 4 females from southern Indiana. True to form one of the females died after laying only a single egg. All told, we were were able to harvest about 80 metalmark eggs. Only 63 hatched. We began feeding them leaves of swamp thistle, their preferred host plant The goal is to have adult butterflies next spring that we can release onto a fen in northwest Cook County.
Throughout September and early October we experienced the kind of gradual attrition that is typical of our experience with the species. We were faced with a dilemma: should we try moving the larvae to cages where they would spend the winter outdoors? We have never succeeded with this approach. Or should we raise them through to adulthood and try to get an additional generation with perhaps greater numbers. We have only once before succeeded in rearing the species to adulthood, but did not get any offspring. Despite the uncertainty, the latter course of action seemed less perilous, so we retained the caterpillars in the lab and continued to offer them food.
By mid October we were down to 21 caterpillars. The numbers stabilized as the caterpillars continued to eat and grow. With few additional losses, we obtained 19 pupae. At the time of this writing we have about 10 adults, four of which are females. We have paired them in small cages where we hope that mating will occur. After a few days we will move the females into egg laying cages and hope for the best.
Although this species is proving difficult to work with, I believe that it is well worth the effort. Swamp metalmarks were once part of the great species diversity that was found in the fens of Illinois. It my firm hope that they will one day fly there again.