Quantifying Insectivorous-Bat-Mediated Pest Control in Cotton Production Landscapes

Abstract

<p>Insectivorous bats and birds consume invertebrates in agricultural systems, regulating and suppressing agricultural pests of economic importance. However, aspects of this natural pest control service remain unclear. In particular, the diversity of natural crop predators is underexplored, with a lack of information evaluating the economic and agricultural role of vertebrate predators of crop pests in Australian cropping systems. Quantifying the pest control service provided by insectivorous birds and bats in high-value crops (such as cotton) is crucial to evaluating the effectiveness of integrated pest management (IPM) and incorporation of wildlife-friendly habitat in agrienvironmental schemes.</p> <p>I addressed these knowledge gaps by investigating the pest control service provided by insectivorous bats and other crop pest predators in a major cotton production area in New South Wales. Specifically, this research: (1) examined the composition, abundance and ratio of pest to beneficial arthropods in the diet of insectivorous bats in cotton crops, using DNA metabarcoding; (2) partitioned the impact of bats and other large vertebrate predators on crop pests using prey removal trials and exclosure experiments in cotton crops; (3) explored biophysical factors at farm and landscape scale that appear to maximise bat-mediated pest control at the crop edge and in the crop interior, and (4) estimated the economic value of bats as natural pest control agents.</p> <p>Dietary analysis revealed that bats consume major arthropod pests and exploit a narrow selection of preferred pest taxa (mainly lepidopterans). This indicated that insectivorous bats can play an important role in controlling lepidopteran irruptions on cotton farms. This was confirmed by the spatial modelling of farm biophysical factors in crop interiors, where increased prey abundance (Lepidoptera and Hemiptera) was positively related to bat feeding attempts. Furthermore, bat feeding attempts increased with Hemiptera abundance on high moonlit nights of >75% illumination. Therefore, the timing of crop insecticide spray regimes might be adjusted to target pests, such as hemipteran sucking bugs, on nights of low moon illumination to maximise batmediated bug control.</p> <p>The pest control service provided by birds and bats was difficult to partition and remains uncertain. Nevertheless, prey removal trials and exclosure experiments (used as a proxy for insect pest control) showed that bird and bat activity was variable at the crop edge and dependent on the phenological stage of the cotton crop. Non-crop woody vegetation cover and increasing crop foliage cover facilitated movement by edge-sensitive birds and bats inside the crop later in the growing season. Prey removal trials revealed that birds likely have a greater impact on experimental prey (and, by inference, a greater role in pest control) than bats, but camera traps showed the opposite, suggesting that many more bats were present and foraging in the cotton crop, just not selecting the experimental prey. The effect of exclosure treatment on insect abundance was generally not significant, except for an elevated abundance of aphids, fleas and mites in the treatment excluding vertebrate predators late in the season, and evidence of insect predation by birds or bats more generally.</p> <p>My research highlights the importance of managing bat roosting habitat at different landscape scales to enhance bat diversity in agroecosystems and likely in-crop pest suppression. Modifying crop configurations to increase exposure to non-crop habitat and crop edges would facilitate increased diversity and foraging activity of insectivorous bats in cotton crops. However, an increase in bat diversity (although benefiting bat conservation) may not translate into a reduction in important crop pests below an economic threshold, as different species of bat differentially impact insect species. Thus, land managers may wish to conserve habitat to promote certain species of bat that consume economically important pest irruptions or promote natural predator biodiversity which complements overall insect reductions.</p> <p>Finally, I estimated the direct value (savings) to the Australian cotton industry of an average non-reproductive bat in dryland Bt-cotton to be $99–126 ha⁻¹ and $286–361 ha⁻¹ in irrigated Bt-cotton (or $63.6 million annually), by averting damage caused by the major cotton pest Helicoverpa armigera (cotton bollworm). Valuing the pest control service provided by bats is crucial information needed to engage land managers to conserve habitat for natural pest enemies, a key strategy in existing IPM programs that should receive greater prominence as result of this study. </p>