Swarms of butterflies or moths are quite a sight. Different species form swarms in various parts of North America.
Small, brown American Snouts (Libytheana carinenta) form migratory swarms. These huge clouds of butterflies often stop people in their tracks. Snout migrations are thought to be related to the intensity and duration of dry periods.
The small Cabbage White Butterfly (Pieris rapae) can appear in large numbers during long periods of hot weather when three to four generations are born in one summer. Parts of Montana and Saskatchewan (see news report) have reported large numbers of these butterflies during the hot summer months.
The charismatic Red Admiral (Vanessa atalanta) undergoes seasonal migrations, and these insects can form huge swarms. For more information on these migrations, go to The Red Admiral and Painted Lady Research Site.
Lyside Sulphurs (Kricogonia lyside) make frequent mass flights across south Texas. "These mass movements appear to be induced by an extensive drought followed by heavy, widespread rains which induce the production of fresh leaves on the butterfly's host plant. The drought evidently serves to knock back the insect predators and parasites so the butterfly caterpillars enjoy an unusually high survivorship." (BugGuide) For more details see:
Gilbert, L.E. 1985. Ecological factors which influence migratory behavior in two butterflies of the semi-arid shrublands of South Texas. Pp. 724-747 in: M.A. Rankin. (editor). Migration: Mechanisms and Adaptive Significance. Contributions in Marine Science Supplement Vol. 27. Marine Science Institute, Port Aransas.
The Pandora Pinemoth (Coloradia pandora) can be seen in large numbers in the western United States in some years. They can be a pest and have outbreak cycles (around 7-10 years apart) though they are a native species. They can plaster the sides of buildings, particularly under lights at night. They are grey to tan with pinkish tint on the hindwing.