Variation in the abundance of foraging insectivorous bats across an agricultural landscape

Brazilian free-tailed bats (Tadarida brasiliensis) form extremely large colonies in caves within and around an agricultural area called the Winter Garden region in south-central Texas. These bats forage nightly and consume massive amounts of insects, including Helicoverpa zea, a noctuid moth responsible for over a billion dollars of annual losses and costs for control each year in the US. This study analyzed bat activity, in correspondence with its prey, across a regional spatial scale within the Winter Garden to understand the factors determining bat activity. Using Anabat II bat detectors, nightly bat activity was surveyed across the landscape from March to December of 2007. In addition, landscape cover data, nightly insect abundances and climatic data were collected. Though analysis shows no overall correlation between increases in bat activity and peaks in moth abundance, there appears to be a temporal relationship between bat activity and moth numbers during the time of highest moth abundance. In weeks when moth counts are higher, there is a trend towards an increase in bat activity. Spatial analysis shows a significant positive relationship between relative bat activity and sorghum during the early summer months, as well as a significant positive relationship between bat activity and natural habitat type during late summer months, corresponding to periods of peak bat activity. During the late summer period, sites with a higher proportion of natural habitat had higher bat activity than did sites with more agricultural land. Numbers of Brazilian free-tailed bats have been consistently and continuously declining, and these data indicate the importance of maintenance and restoration of natural areas within and around the Winter Garden agricultural region.