The Monarch Butterfly (Danaus menippe) is classed as the "only true migratory insect in North America." The adult is quite a hardy individual; but in no stage of its life history is the Monarch able to withstand freezing conditions. Consequently, it cannot winter in the North. All individuals found in the North in summer have themselves migrated from the South where winters are mild, or they are the late summer offspring of such migrants.
Early spring broods of adults migrate northward in search of milkweed upon which to lay eggs. Successive broods appear farther and farther north as summer advances, following the milkweed season. This migration is not noted because no large numbers of butterflies are seen together. Thus, Monarchs reach Mount Rainier National Park usually in mid July.
The migration to the South is made in the early Fall, and is not noted this far north except by the absence of the species. Farther south large swarms of Monarchs are observed in autumn, migrating their way to a warmer clime.
Other insects make extensive flights seemingly for no reason at all. The flight of the Tortoise-shell Butterfly common to mountains (Aglais californica) is an example. This flight takes place after oviposition and does not constitute a true migration. An extensive flight of the Tortoise-shell was observed on Mazama Ridge in July of 1934. These butterflies were feeding busily on the blossoms of Mountain Ash and seemed to be moving up the ridge. Up to Labor Day of 1935, the Tortoise-shell had not made its appearance in numbers at Paradise.
During the summer of 1934 a large number of Lady-bird beetles were observed at McClure Rock. These insects undergo extensive flights after oviposition, and always move uphill.
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