(a) Tracking resources

Our results demonstrate that the consumption of CEW moths by Brazilian free-tailed bats tracks the abundance of these moths. While the seasonal patterns of moth abundance differ among the pheromone trap sites, they are significantly correlated at three of the four sites, and the numbers of bats consuming CEW moths are significantly associated with moth abundances at each of these same sites. At the fourth trap site (Uvalde) moth abundance was neither correlated with those at the other sites, nor associated with the numbers of bats consuming CEW moths. Our estimates of moth abundances combining data from the four sites are indicators of the distribution of CEW moth availability over the landscape. With regard to the use of this landscape by bats, the estimates based on the maximum numbers of moths captured consistently outperform the estimates based on the average numbers of moths captured in providing the closest associations with the numbers of bats consuming CEW moths (Table 3; Figure 3). This result is expected if bats forage at the locations where moths are most abundant, and is consistent with the hypothesis that the bats track moth abundance in space as well as time.

In contrast to the significant associations observed between the numbers of bats eating CEW and estimates of CEW moth abundance, the numbers of CEW gene sequences in the feces of bats show no associations with our estimates of moth abundance (Figure 4). There are many complications in relating gene copy numbers to estimates of biomass or numbers of prey consumed [14]. These complications include differences in the DNA content of different prey species, differences in digestive efficiencies and DNA content of different body parts of prey, possible variation in DNA content related to the age or life history stage of prey items, the age of the meal that produced the feces, and inherent variation in the PCR process [14]. Our captive feeding experiments show a positive relationship between the percent mass of CEW moths in a bat's meal and the numbers of CEW gene copies in resulting feces (Information S1; Table S1; Figure S1). However, feces from meals with a similar proportional CEW content often differed by orders of magnitude in numbers of gene copies (Table S1). In addition, the relationship between gene copy content of feces and the numbers of moths eaten was affected by what else a bat ate in the same meal. This is illustrated by the separate feedings of bat # 3 (Table S1) where the consumption of five moths comprising 100% of a meal yielded over 4 million gene copies per mg feces; whereas consumption of five moths comprising 26% of a meal yielded less than 200,000 gene copies per milligram. The occurrence of such variation in qPCR estimates, even in a comparatively simple captive feeding situation, demonstrates that our attempt to calibrate qPCR gene copy numbers to CEW consumption did not provide measures of the biomass or number of prey consumed. Given that a single Brazilian free-tailed bat typically consumes many different insect taxa in a single night [8], we suspect even greater variability in gene copy numbers from field samples. Our qPCR results are consistent with the assessment by King et al. [14] that estimates of gene copies obtained in field studies are likely to provide, at best, some semiquantitative measure of predation.

It is nonetheless of interest to investigate the gene copy data in the contexts of the seasonal patterns of CEW moth abundance and the frequency of moth consumption by bats. Our results illustrate two periods, one in spring (April–early May), and the other in mid-late summer (late July–September), when moths are rare and few bats are feeding on them. However, during spring gene copy numbers are low, whereas in mid-late summer, gene copy numbers are higher. Earlier research on the insect resource base and dietary breadth of Brazilian free-tailed bats has shown greater insect diversity and broader dietary breadth in spring than in late summer [7]. Earlier work on the seasonal life history and behavior of the bats also shows that in late summer they feed for longer periods, lose body mass, and appear to be under much greater food stress than in spring [33], [34] (GFM, pers. observation). Taken together, these observations and our qPCR data suggest that, to the bats that eat them, CEW moths are a more important part of their diet in mid-late summer than in spring.

With the spike in CEW moth abundance in mid-season (late May–mid July) and again with their abrupt increase during the fall migration of moths, the numbers of bats eating CEW increases, and the high frequency of consumption is often coupled with high gene copy numbers. This suggests that in mid-season during pregnancy and lactation and in fall prior to migration CEW moths are an important part of the diet of many bats.

All earlier studies investigating the diets of Brazilian free-tailed bats and links to agronomic impacts were confined to mid-season months, focusing on the arrival of migrating moths in late May–early June and subsequent infestations of crops into summer [5], [6], [8]. This is the same period that the bats must cope with high energetic demands associated with pregnancy and lactation [35]. The returning southward migrations of insects on advancing cold fronts has been well established by entomological researchers, and in fact, some of the highest densities of moths ever recorded aloft were observed in September [36]. However, because these late-season migrants do not have the same immediate regional agronomic impact, the migrations and population dynamics of insects in autumn have been less studied than those during spring and summer. Our data are the first to implicate the late season migrations of insects on advancing cold fronts as a resource during another period that is critical for the survival of the bats; the time when they must accumulate fat reserves for their migratory flight in advance of the approaching winter.