A Butterfly Invasion

Forrest M. Mims III

Most people in the United States are aware of the spectacular spring and fall migrations of monarch butterflies. Did you know that other butterflies also stage migrations?

For the past few weeks large numbers of small butterflies have been migrating across portions of South Central Texas. These are American snout butterflies (Libytheana carinenta (Cramer, 1777)).

You can quickly determine how the snout butterfly got its name by looking at one up close. The mouth parts of the snout are extended into a slender cone.

When snouts roost, they often hang upside down from a thin branch or twig and place their snout and antennae upward against the twig. This makes them resemble a dead leaf attached to the twig.

This is pretty much what they resemble while roosting on the dead, giant ragweeds along Geronimo Creek (Fig. 1). Their drab appearance causes them to look much like the few dead leaves hanging from the ragweeds.

Snout butterflies are found from South America through the United States. They especially like the brush country of Mexico and Texas.

In most years snout butterflies are relatively uncommon. But during drought conditions interrupted by heavy rain, their population sometimes explodes.

Scientists Raymond Neck and Larry Gilbert have found that this occurs when the spiny hackberry shrubs that snout larvae feed on sprout new growth. The fresh leaves provide much more food than normal, and the snout butterfly population skyrockets.

Huge flocks of snout butterflies migrate away from where they stripped the hackberries of all their leaves. Perhaps they are looking for a new place to lay their eggs, but the various scientific papers I checked didn't answer this question.

Whatever the reason, migrating clouds of snout butterflies can include many millions of insects over hundreds of square miles.

On 9 August 1966, snout butterflies were so dense over Tucson, Arizona, that they blocked sunlight. Butterfly expert W. H. Howe reported that street lights had to be turned on!

The last big migration in our area occurred in September 1996. Drivers complained of reduced visibility caused by hundreds of snout butterflies on their windshields. The butterflies even clogged car and truck radiators.

I recall this well, for I was making trips to San Antonio to visit my hospitalized father. Each evening before driving home, I had to remove a layer of snout butterflies from the radiator of my pickup and clean the windshield.

So if the present snout nose invasion seems bad, it could be much worse!

Forrest M. Mims III and his science are featured online at www.forrestmims.org and www.sunandsky.org.