Performance of an aerially applied liquidBacillus thuringiensisvar.israelensisformulation (strainAM65-52) against mosquitoes in mixed saltmarsh-mangrove systems and fine-scale mapping of mangrove canopy cover using affordable drone-based imagery

BACKGROUND In the Australian southeast, the saltmarsh mosquitoAedes vigilax(Skuse) is the focus of area-wide larviciding campaigns employing the biological agentBacillus thuringiensisvar.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 aqueousBtisuspension (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. RESULTS High mangrove canopy density (>40% cover) reduced product deposition by 75.2% (0.01 +/- 0.002 mu L cm(-2)versus0.05 +/- 0.006 mu L cm(-2)), larval mortality by 27.7% (60.7 +/- 4.1%versus84.0 +/- 2.4%), and ground levelBticoncentrations by 32.03% (1144 +/- 462.6versus1683 +/- 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 (R-2=0.17-0.38) at their most informative scales. CONCLUSION These 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.