Performance of aerial Bacillus thuringiensis var. israelensis applications in mixed saltmarsh-mangrove systems and use of affordable unmanned aerial systems to identify problematic levels of canopy cover

Abstract

BACKGROUNDIn the Australian southeast, the saltmarsh mosquito Aedes vigilax (Skuse) is the focus of area-wide larviciding campaigns employing the biological agent Bacillus thuringiensis var. israelensis (Bti). Although generally effective, frequent inundating tides and considerable mangrove cover can make control challenging. Here, we describe the efficacy and persistence of an aqueous Bti suspension (potency: 1200 International Toxic Units; strain AM65-52) within a mixed saltmarsh-mangrove system and the use of affordable unmanned aerial systems (UAS) to identify and map problematic levels of mangrove canopy cover. RESULTSHigh mangrove canopy density (>40% cover) reduced product deposition by 74.5% (0.013{+/-} 0.002 l/cm2 vs. 0.051{+/-} 0.006 l/cm2), larval mortality by 27.7% (60.7{+/-} 4.1% vs. 84.0{+/-} 2.4%), and ground level Bti concentrations by 32.03% (1144 {+/-} 462.6 vs. 1683 {+/-} 447.8 spores ml-1) relative to open saltmarsh. Persistence of product post-application was found to be low (80.6% loss at 6 h) resulting in negligible additional losses to tidal inundation 24 h post-application. UAS surveys accurately identified areas of high mangrove cover using both standard and multispectral imagery, although derived index values for this vegetation class were only moderately correlated with ground measurements (R2=0.17-0.38) at their most informative scales. CONCLUSIONThese findings highlight the complex operational challenges that affect coastal mosquito control in heterogeneous environments. The problem is exacerbated by continued mangrove transgression into saltmarsh habitat in the region. Emerging UAS technology can help operators optimize treatments by accurately identifying and mapping challenging canopy cover using both standard and multispectral imaging.