Friday, August 10, 2012

Giant Swallowtails Return to Breed and a Spicebush Swallowtail is Seen at the Preserve

True to its name, The Giant Swallowtail is a sizable butterfly
Over the last five years, Giant Swallowtail Butterflies have become increasingly common in Upstate New York. Mainly a species if the deep south, most of the main foodplants for this swallowtail are in the citrus family --so you'd think that our regions lack of orange trees would necessarily mean a dearth of Giant Swallowtails.

This species, like the other swallowtails, usually feed while hovering next to the flower
As it turns out there are 2 alternate food plants that these butterflies can lay their eggs on --both of them (Prickly Ash and Hoptree) are uncommon if not altogether absent from our particular area. Still in mid summer, the butterflies press through our region, nectaring on an infinite supply of meadow flowers and searching (I suspect) largely in vain, for food plants to lay their eggs on.

The egg of the Giant Swallowtail is relatively large and easy to find on the host plant
Giants that fly by my house are in luck since we have a potted Grapefruit Tree that spends the summer on our back porch. For over 25 years the tree has never produced a flower or a single Grapefruit, but last year it produced a crop of 6 Giant Swallowtails. Butterflies in general have an uncanny ability to locate their foodplants, and these Swallowtails are certainly no exception.
 An early instar of the Giant Swallowtail larva (left) and an unhatched egg (right)
A recent close inspection of my Grapefruit Tree revealed at least a dozen Giant Swallowtail eggs on its leaves --likely laid by several different passing Giants. The eggs are orange and relatively large for butterfly eggs, and they tend to be deposited on the top face of the leaf.
While hovering, the female Giant lays single eggs on the Grapefruit Tree leaves
The first thing that young caterpillars do upon hatching is to eat their egg shell; after that they start gnawing on the tree's leaves. By their final instar, they are quite large –as big as an adult person’s index finger, and by this point they can really munch down a lot of leaves. In the south, in the citrus groves, they are considered to be pests and are referred to as Orange Dogs.
The Giant Swallowtail larva is a convincing bird-dropping mimic
Giant Swallowtail caterpillars are bird dropping mimics. That is to say, they are camouflaged to resemble bird droppings. This understandably dissuades predators from picking them off. Another way this caterpillar keeps from being a menu item is to erect 2 reddish, foul smelling horns from its head. Other swallowtail species also have this ability.
When it senses danger, the swallowtail caterpillar erects 2 reddish horns
Held to the tree branch by a silk harness, the larva sheds its skin and becomes a chrysalis
After shedding its final larval skin, the Giant Swallowtail becomes a chrysalis, and the chrysalis also employs camouflage. At this stage the insect is trying to deceive predators into believing it’s just a broken off twig.
A freshly emerged Giant Swallowtail Butterfly hangs next to its empty chrysalis
The Giant Swallowtail is not the only swallowtail species visiting the nature preserve these days. We’ve also seen a Spicebush Swallowtail, which for us in the northeast, is a rare treat. Our last  sighting of this species took place around 10 years ago.
The Spicebush Swallowtail is seen at the nature preserve after a decade-long  absence
As its name implies, the main foodplant for this species is Spicebush, and though that small tree is not particularly common this far north, we do have some occurring in our woods. With some luck the Spicebush Swallowtail will discover them.

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