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Tuesday, October 1, 2013


American Snout, Libytheana carinenta

The change from late summer into early fall can trigger some unusual natural events, and at this time of year in Central Texas, we can often see periodic population explosions of the American Snout butterfly.

Often mistaken for migrating Monarch butterflies, the American Snout (Libytheana carinenta) is a medium-sized butterfly with a brown upperside, wings orange at the base, and white spots near the tips of the forewings.  Their underside is a mottled grayish-brown pattern, much like a dried, dead leaf.  Snouts are named for their elongated mouthparts, and when they hang from the underside of a branch, which they most often do, their ‘snout’ resembles the stem or ‘petiole’ of a leaf and their folded wings appear to be the dead leaf itself.  It’s the perfect camouflage for defense against avian predators. 

In the caterpillar stage, snouts are dark green with yellow stripes along the top and sides of their body, with two small, black tubercles on the top of their thorax.  These caterpillars feed on all hackberry species, but they prefer spiny hackberry.  While Austin is at the northernmost boundary of this plant’s range, it is close enough that we get to experience a ‘sn-outbreak.’  After a good rain, spiny hackberry plants (also known as granjeno or desert hackberry) grow numerous new leaves.  In response, the snout butterfly lays its eggs on these new leaves, which provides the fuel for a significant number of its caterpillars.  In Texas, it only takes 12 days to go from egg to caterpillar to adult butterfly.

Spiny Hackberry, Celtis ehrenbergiana

The ecology behind this event is related to several factors.  First, the population of snouts is positively correlated to the intensity and duration of dry periods that immediately precede significant rains.  These droughts seem to greatly diminish parasites that can harm and kill snout larvae.  Second, adult snouts wait out these long, dry periods by remaining in ‘reproductive diapause’, a state of arrested development/reproduction and decreased metabolism in response to the adverse environmental conditions.  This condition reverses when the rains arrive and trigger the third factor, new growth on the spiny hackberry host plants.  Female snouts will only lay eggs on this prolific new growth, and coupled with the lack of parasites, this creates a population boom.

Most often, sn-outbreaks occur in South Texas and Mexico, where the spiny hackberry is plentiful.  However, one of the largest recorded snout emergences occurred in late September of 1921, when over the course of 18 days more than 6 billion southeasterly-bound snout butterflies flew over San Marcos to the Rio Grande River.  That’s an estimated 25 million per minute!  While not every year is quite that spectacular, keep your eyes open about two weeks after a rain, and you should see region-wide migrations of snout butterflies as they waft by in pretty impressive numbers.