Mesquite bugs and other insects in the diet of pallid bats in southeastern Arizona

The pallid bat (Antrozous pallidus) is a species of arid and semiarid western North America, inhabiting ecoregions ranging from desert to oak and pine forest. Considered primarily insectivorous predators on large arthropods but taking occasional small vertebrate prey, pallid bats were recently shown to be at least seasonally omnivorous; they demonstrate unusual dietary flexibility and opportunism in certain parts of their geographic range and at different times of year. In a few areas they take nectar from cactus flowers and eat cactus fruit pulp and seeds. Until recently mesquite bugs were primarily tropical-subtropical inhabitants of Mexico and Central America but have since occupied the southwestern United States where mesquite trees occur. Pallid bats regularly use night roosts as temporary shelters in which to process and consume large arthropods caught near their foraging areas. Using a noninvasive method, we investigated the bats’ diet by collecting food parts discarded by the bats beneath three night roosts in soil-piping cavities at the Cienega Creek Natural Preserve, Arizona. We also made phenological and behavioral observations of the mesquite bugs, Thasus neocalifornicus, and their interactions with the mesquite trees. The bats discarded inedible parts of at least 36 species in 8 orders of mainly large-bodied and nocturnal insects below the night-roosts. In addition, one partial bat wing represents predation upon a phyllostomid bat, Choeronycteris mexicana. About 17 of the insect taxa are newly reported as prey for pallid bats, as is the bat C. mexicana. The large majority of culled insect parts (88.8%) were from adult mesquite bugs. As nymphs, mesquite bugs are aposematically colored and secrete noxious pheromones; nymphs did not appear in the bat-culled insect parts. Adult mesquite bugs are darkly colored and secrete different noxious pheromones than the nymphs. During daytime hours in the summer adult bugs are abundant, flying around the canopy and alighting on the edges of the trees. In late summer and early fall they breed and lay eggs that overwinter on the mesquite branches to hatch in January. Soon after breeding, the adult bugs die. When summer heat diminishes and nighttime low temperatures drop below 21°C, the adult bugs become immobile on the periphery of the trees where they probably make easy prey for foliage-gleaning pallid bats. The historically subtropical-tropical mesquite bugs may have moved into the southwestern United States with the spread of cattle and mesquites. In this area of Arizona, pallid bats provide an important natural control on the local mesquite bug population. The high diversity of other insect remains and the remains of another species of bat provide additional supportive evidence of a diet for pallid bats that reflects their plasticity across a variety of habitats. This behavioral plasticity probably enhances the bats’ survival across their range in the face of climate change.