A Traprock Ridge Specialty

Imagine walking in what is now central Connecticut 200 million years ago, when magma rose through cracks in the land as the great supercontinent Pangaea split apart. In the New Haven area, the magma cooled and formed intrusive, or underground, diabase rock. For millions of years the softer sandstones that later were deposited above this rock eroded, leaving the harder, igneous East Rock and West Rock ridges standing as sentinels surrounding Connecticut’s Central Valley today.

---

These kinds of hills are commonly called traprock ridges, and Connecticut is known for them. Trap is from the Swedish trappa, which means “stairway.” These igneous rocks often cleave off at 90 degree angles, forming walkable steps in many places. The drier microclimates and shallow soils of these ridge tops are unique habitats for flora and fauna.

West Rock Ridge State Park, with its dark basalt rocks, is a heat island for some butterfly species normally found much farther south. One such species that is relatively rare here in Connecticut is the Falcate Orangetip (Anthocharis midea). In the southern part of its range it is found in open, wet woods along streams and rivers and in open swamps. But here, at the northern part of its range, it is confined to traprock ridges. Why the difference? It may need warmer temperatures and these ridges can be warmer than the surrounding valleys. Also, it is ecologically tied to the plants found growing in this habitat.

After mating, Falcate Orangetip females search for plants that are members of the mustard family. They usually deposit only one egg on each plant. At West Rock, females often lay eggs on Lyre-leaved Thale-cress (Arabidopsis lyrata).The larvae (caterpillars) often feed at night on flower buds, flowers, and fruit. They must feed and then develop as pupae before the spring-blooming plants complete their cycle.

Unfortunately, Falcate Orangetip females sometimes lay their eggs on invasive, non-native Garlic Mustard (Alliaria petiolata). The larvae that eclose (hatch) and eat this plant, poisonous to them, die. To protect this butterfly and the other plants it feeds on, it is important to both control Garlic Mustard and preserve traprock ridge habitats.

Falcate Orangetips are sexually dimorphic. Both males and females are white above with a back spot on the upper forewing. Males also have a bright orange patch on the outer forewing. Falcate means “hooked” and refers to the hooked upper forewings. Along with the orange patch on the males, this hook is a way to differentiate this species from the more common, introduced Cabbage White butterfly (Pieris brassicae).

Now is the time to find this springtime butterfly on traprock ridges. Falcate Orangetips are out hilltopping, or patrolling for females. They fly just above the ground and often repeat the same route. If you would like to get out this weekend to see this butterfly, join Larry Gall, Entomology Collections Manager at the Yale Peabody Museum, for a free Connecticut Butterfly Association spring walk at West Rock Ridge State Park in New Haven on Saturday, May 8, 2021. For details and directions visit the CBA website.