Mosquitoes transmit a range of pathogens, such as malaria protozoans, dengue, Ross River and Barmah Forest viruses. Human infection with any of these pathogens may lead to the onset of debilitating disease, and due to limited vaccine availability, mosquito control is essential to interrupt the transmission of disease. During control operations, the larvae are targeted and there are two classes of biologically-based insecticides available: microbials (Bacillus thuringiensis var. israelensis de Barjac [Bti] and Bacillus sphaericus Neide [Bs]) and insect growth regulators (s-methoprene and pyriproxyfen). The aims of this thesis were to: 1) undertake specific research to enhance mosquito control operations; and 2) evaluate the ecological impacts of insecticide use in saltmarsh habitats.
Initially, the efficacy of new formulations of biologically-based insecticides were investigated in the laboratory. Bioassays were conducted using third-instars of six common Australian mosquito species, Aedes aegypti (Linnaeus), Aedes vigilax (Skuse), Aedes notoscriptus (Skuse), Culex sitiens Wiedemann, Culex annulirostris Skuse and Culex quinquefasciatus Say. The normal model for log-linear mortality was used to determine LC50 and LC95 values. The newly developed VectoBac WG (3,000 Bti International Toxic Units [ITU]/mg) was highly effective against the range of larvae, contrasting with the unregistered insect growth regulator, pyriproxyfen, which was not effective against some species.
Sequentially, the efficacy of Bti formulations was assessed in the field. Larvae were exposed to Bti as free-swimming larvae and in mesh cages, and mortality was calculated after 48 h to examine appropriate sampling techniques. The accuracy of sampling free-swimming larvae with 250-ml ‘dips’ was highly variable, where monitoring mortality of caged larvae was highly accurate; this information was used to design the sequential field trials. In freshwater pools, replicated cohorts of caged Cx. annulirostris were exposed to the water dispersible (VectoBac WG) and liquid (VectoBac 12AS: 1,200 Bti ITU/mg) formulations. Treatment concentrations of 0.008 ppm VectoBac WG and 0.04 ppm VectoBac 12AS and above produced significant larval control (>96% mortality) at 48 h, with no residual control after 1 week. In saltmarsh pools, cohorts of caged Ae. vigilax were exposed to the granular (VectoBac G: 200 Bti ITU/mg) formulation; which was effective (>99% mortality) at application rates of 4 kg/ha and above at 48 h.
Next, the distribution of the granular (VectoBac G) formulation was assessed after an aerial treatment using catch-trays. The accuracy of the catch-trays was defined using mathematical models. Specifically, the analysis revealed that the size of catch-trays can affect the interpretation of results, especially if smaller than 2 m2. The mass of product captured in 1 m2 catch-trays, due to random sampling processes alone, would be expected to range between the equivalent of 2.9 to 7.8 kg/ha for 95% of replicates when targeting 5 kg/ha. During the field trial, 1 m2 catch-trays were used, this catch-tray size was selected as it was the nominated size used by contractors for quality assurance of aerial granular applications. The average flight lane separation of the rotary-wing aircraft was 14.70 m (SD: ± 4.52 m) and the average treatment rate was 5.76 kg/ha (SD: ± 3.46 kg/ha; CV = 60%). This was close to the targeted lane separation of 14 m and treatment rate of 5 kg/ha. However, the product was not distributed evenly. Nonetheless, there was 100% mortality of third-instar Ae. vigilax that were exposed to the treatment in mesh cages. The most important factors that affected the observed spatial distribution of product were the uneven flight path of the helicopter and the low sensitivity of the small catch-trays.
Despite the fact that Bti and s-methoprene are considered to be among the most target specific of insecticides, there are indications that non-target organisms may be impacted in different ecosystems. In response, changes in the density and diversity of non-target communities, after application of either Bti or s-methoprene, were examined. The main taxa collected from ephemeral pools were Copepoda; and from terrestrial plots were Collembola, Coleoptera, Heteroptera, Hymenoptera and Diptera. Applications of both products altered the community composition; however, differences were not consistent over the two localities. After applications of Bti to ephemeral pools, lower numbers of Copepoda were recorded at only one location. No differences were recorded after treatments of s-methoprene to ephemeral pools. After applications of s-methoprene to terrestrial plots, higher numbers of Acariformes were recorded at both localities, and this was also recorded after application of Bti to one of the locations. However, these differences were not spatially and temporally consistent or in agreement with predictions. The results of these trials suggest that applications of Bti and smethoprene will not impact on the long-term structure and composition of arthropod assemblages in saltmarshes. Authors of previous studies from the northern hemisphere had indicated that s-methoprene was more broadly toxic to non-target organisms than Bti; however, this is not true in Australia.
The results of this thesis found that Bti can be used to effectively control mosquito immatures under different field conditions. Applications of Bti and s-methoprene did not decrease the diversity or abundance of non-target arthropods in south-east Queensland. As such, future applications of Bti and s-methoprene are supported in preference to organophosphate alternatives; this is based on a comparison with previously published literature that has demonstrated organophosphate insecticides to be directly toxic to non-target arthropods. Considering that the appropriate use of Bti and s-methoprene can reduce the incidence of arbovirus transmission among the local human population, the future application of these products is supported. The use of insecticides should be integrated with public education, biological control, physical habitat modification and early detection systems.