Because Texas spans so many ecological regions, it has more butterfly species than any other state–at least 400. (The total lepidopteran species–butterflies, moths, and skippers–in Texas number more than 1,600.) This is more than half the butterfly species known from North America north of Mexico. The western limb of the state, bordering New Mexico at Guadalupe Mountains National Park, holds the southernmost island peaks of the Rocky Mountains, whose pine, juniper, and oak woodlands harbor butterflies typical of the Rockies. In stark contrast, the dense eastern pine forests host butterflies typical of the Gulf Coast through Louisiana, Mississippi, and eastward. At the southern tip of the state, near Brownsville, subtropical species find a home, while the arid west across the Rio Grande from Chihuahua provides dry mountain habitats for endemic desert butterflies. The resulting variety is staggering.
The state's geologic past has further enhanced the diversity of its butterfly fauna. Cold and warm cycles caused by the advance and retreat of continental glaciers have pushed distant species into the region. Many of these have survived, the northern forms occupying niches in cool, shady valleys, and the southern forms colonizing arid hills. In addition to resident butterflies, numerous species stray into the state from the subtropics of Mexico. Most of these cannot survive the cold winters of Central Texas and northward, but many establish temporary colonies for one to several years until freezes annihilate them. Autumn is the best time to look for these visitors.
Although the number of families of North American butterflies has not been settled, all are liberally represented in Texas. The larvae of two families (Hesperiidae and Satyridae) feed upon grasses, and most others eat a wide variety of herbs, shrubs, and trees. One carnivorous species (Feniseca tarquinius) feeds on wooly aphids, and two other species (Calycopis isobeon and C. cecrops) seem able to survive on ground litter and detritus. Most adult butterflies feed on nectar, though some avoid flowers and prefer tree sap. Others visit fermenting or decaying organic matter, including carrion and feces.
The Monarch, Danaus plexippus, with which most Texans are familiar, makes its annual migratory flight through Texas en route to its wintering grounds in Mexico. The northward flight in April and May is not as easily recognized as the southward flight in late summer and fall, when the large, soaring butterflies at times fill the air and gather on trees by hundreds or thousands to rest.
Other butterflies are less well known, but at least a few of the nearly twenty species of swallowtails that inhabit the state are familiar to gardeners. A patient nature enthusiast can learn to recognize dozens of other species, and with practice suburbanites in smaller cities such as Austin or San Antonio can find at least sixty species in the backyard.
Although a few of the state's butterflies are threatened by land developments that destroy fragile habitats, human activities have not generally harmed butterfly populations. In fact, because many species prefer open areas, clearing of space for highways, railroads, and recreation spaces and parklands has encouraged the spread of butterflies throughout the state. Only when suburbs become lawn-to-lawn and eliminate vacant lots do butterflies avoid them.
A few of the butterflies of Texas are commercial pests of minor import. The cabbage butterflies and mustard whites (the genera Artogeia and Pontia) feed on some domestic cabbage-family crops. The alfalfa butterflies (Colias eurytheme and relatives) feed on various clovers. And the orange dog, or giant swallowtail (Heraclides cresphontes), attacks citrus orchards. None of these is a major threat to crops unless unusual population explosions occur.
In the northern part of the state, where freezes are regular and hard, butterflies are predictably seasonal; they emerge from their pupae at about the same time each year. Farther south, however, butterflies are subject to wet and dry cycles as well as the photoperiod, and seasonal cyclicity is less pronounced. A species that flies in July of one year might not be on the wing at all in July of the following year. In addition, population density varies greatly from year to year, depending upon the same variables. The lepidopterist is hard-pressed to predict the flight of a given species and often is surprised by unexpected periods of scarcity or abundance.