BIOLOGY AND CONTROL OF VERRALLINA FUNEREA (DIPTERA: CULICIDAE)

Jeffery, Jason Andrew Loh (2006). BIOLOGY AND CONTROL OF VERRALLINA FUNEREA (DIPTERA: CULICIDAE) PhD Thesis, School of Population Health, University of Queensland.

       
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Author Jeffery, Jason Andrew Loh
Thesis Title BIOLOGY AND CONTROL OF VERRALLINA FUNEREA (DIPTERA: CULICIDAE)
School, Centre or Institute School of Population Health
Institution University of Queensland
Publication date 2006
Thesis type PhD Thesis
Supervisor Dr Peter Ryan
Abstract/Summary Verrallina funerea (Theobald) is a brackish water mosquito species common in the north eastern coastal regions of Australia. It is a recognised pest species and has been implicated in the transmission cycles of Australia’s two most common arboviruses, Ross River virus (RRV) and Barmah Forest virus (BFV). The aims of this study were to investigate aspects of the biology and ecology of Ve. funerea as a prerequisite to designing more appropriate control strategies against this species. To prioritise the control of Ve. funerea, the vector competence of this species for BFV and RRV was investigated. Experiments were performed on laboratory-reared Ve. funerea to evaluate the susceptibility of this species to the BF1611 strain of BFV. Verrallina funerea were found to be moderately susceptible to infection, with an ID50 of 103.6 CCID50 (Vero) per mosquito. Virus was detected in 5% of salivary glands only 2 days (d) after infection: however, maximum salivary gland infection (65%) was not observed until 8 d after infection. Transmission to mice was demonstrated, with 52% of mosquitoes transmitting 9􀀐12 d after infection. Vector competence experiments were also carried out with laboratory-reared Ve. funerea and the B94/20 strain of RRV. Virus was detected in 10% of salivary glands only 2 d after infection: maximum salivary gland infection (50%) was not observed until 10 d after infection. To determine whether the susceptibility of Ve. funerea to BFV and RRV was consistent between populations of this species across southeast Queensland (Qld) and northern New South Wales, adult mosquitoes were trapped from across that region on the same night and fed the same blood/virus (either BFV or RRV) mixture. No statistically significant variations in whole body, disseminated and salivary gland infection rates were detected for either virus. Using wing length as an indicator of adult body size, there was no apparent association between smaller mosquitoes and their ability to become infected. To assess whether the results of experiments using Ve. funerea from southeast Qld would be applicable to other populations of Ve. funerea throughout Australia, the genetic population structure of this species was investigated. Sequence analyses were performed on sections of the cytochrome oxidase I (COI) and internal transcribed spacer 2 (ITS2) genes from individuals collected from localities representative of the distribution of Ve. funerea in Australia. Verrallina funerea populations were relatively genetically consistent. No ITS2 variation was found. For COI, only 12 haplotypes were identified from 94 individuals. Haplotype 1 was found at all locations, and represented the most common haplotype, being present in more than 73% of individuals at all localities. There was some indication of isolation by distance based on significant Fst values and Mantel regressions. These significant values were between the Darwin/Cairns and the Ballina/Russell Island groups of populations, which represent the extremes of the distribution of Ve. funerea in Australia. Therefore, although Ve. funerea has a relatively genetically consistent population structure in Australia, there is some evidence of the beginnings of a cline in variation between populations at the extremes of its distribution. Since insecticides need to be applied when mosquito immatures are at their most susceptible stage, immature development time and survival of Ve. funerea was defined in the laboratory. This was done in response to a range of temperatures (17􀀐34oC) and salinities (0􀀐35 parts per thousand [ppt]). The expression of autogeny in this species was also assessed. Salinity only had a slight effect on mean development time from hatching to adult emergence (7.0􀀐7.4 d at salinities of 0, 17.5 and 31.5 ppt) and survival was uniformly high (97.5􀀐99.0%). Mean development times were shorter at 26, 29, and 32oC (7.0, 6.8, and 6.8 d, respectively) and longest at 17oC (12.2 d). The threshold temperature (t) was 5.8oC and the thermal constant (K) was 142.9 degree-days above t. Survival to adulthood decreased from greater than 95% (at 17􀀐29oC) to 78% (at 32oC) and 0% (at 34oC). No expression of autogeny was observed. Immature development times of Ve. funerea, Aedes vigilax (Skuse) and Aedes procax (Skuse) were then determined under field conditions at Maroochy Shire. Following tidal and rain inundation, cohorts of newly hatched larvae were monitored daily by dipping, and time until pupation was noted. Tidal inundation triggered hatching of Ve. funerea and Ae. vigilax larvae whereas Ae. procax larvae were found only after rain inundation. Estimates of Ve. funerea and Ae. vigilax field development times were similar (8􀀐9 d) while Ae. procax development time was slightly longer (9􀀐10 d). Mean salinities of pools containing Ve. funerea and Ae. vigilax ranged from 17 to 28 ppt and 10 to 28 ppt, respectively. Mean salinities for Ae. procax positive pools ranged from 9 to 21 ppt. The efficacy of various registered and novel insecticides against Ve. funerea was then assessed using laboratory dose-response assays. Verrallina funerea was highly susceptible to Abate 100E (temephos), VectoBac 12AS (Bacillus thuringiensis var. israelensis), Altosid Liquid Larvicide (s-methoprene) and technical grade pyriproxyfen, but was highly refractory to VectoLex WG (Bacillus sphaericus). The survival of adults exposed as larvae to sub-lethal doses of ALL was reduced for those doses greater than the EI50, although sex ratios were unaffected. The efficacy of VectoBac 12AS was observed to increase in higher salinities whereas the efficacy of ALL decreased. Following these laboratory assays, small-plot field trials using caged larvae in saline water pools demonstrated that Ve. funerea was susceptible to VectoBac G at 7 kg/ha and Altosand at 4 kg/ha, with these products giving greater than 99 and 98% control, respectively. A significant increase in immature mortality in Altosand treated pools was observed, suggesting that s-methoprene field trials based solely on emergence inhibition of pupae may underestimate the efficacy of this insecticide. The results presented in this thesis provide evidence that Ve. funerea is a vector of BFV, and confirms and further elucidates its role as a vector of RRV. Based on moderate susceptibility to experimental infection with these viruses, this species is likely to be an amplification vector of both viruses during epidemics. Because of its genetic structure, this is most likely applicable to all populations of Ve. funerea throughout Australia. This study provides justification for prioritising the control of Ve. funerea in areas where it is abundant. Although Altosand and VectoBac G were shown to be effective in small-plot field trials, the potential effectiveness of sustained-release formulations and the ability of these products to penetrate the Casuarina canopy that covers Ve. funerea habitats should be evaluated.

 
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