Bio21 - Theses
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ItemFitness trade-offs involved in adaptation to host plants, diapause-egg phenotypes and pesticides pressure in the redlegged earth mite, Halotydeus destructor( 2018)The redlegged earth mite, Halotydeus destructor, is an invasive species introduced into Australia from South Africa. In Australia, this species enters summer diapause at the egg stage which can survive desiccation, heat exposure and applications of pesticides over summer, while annually it produces approximately three generations of active mites in the cool and wet period. Hatching of diapause eggs approximately synchronize the germination of annual plants and the post-diapause mites (1st generation), thus causing serious damage on seedlings. Understanding the mechanism and fitness cost of adaptation to host plants, environmental changes and pesticides pressure in this species is crucial for development of an integrated pest management (IPM) strategy. My experiment reveals that host performance of H. destructor is influenced by the interaction between mite populations and host plants. Mite survival, net reproductive output, development and feeding damage on plants are reduced if the previous host plant of a population is different from the host plant of the introduced microcosm, revealing that a fitness cost is involved in host adaptation. A typical type of diapause eggs with a thick chorion is known to survive sprays of pesticides and stressful summer. A program, Timerite®, has been designed in Western Australia to predict the onset of the typical diapause (TD) egg production based largely on daylength, and farmers spray pesticides according to Timerite® predicted date to suppress populations of pre-diapause mites. However, effectiveness of Timerite® is less in eastern areas of Australia than Western Australia. My study reveals a cryptic type of diapause (CD) egg which lacks a thick chorion and thus is morphologically similar to the non-diapause (ND) egg. Production of diapause eggs (including both TD and CD types) was triggered by longer-daylength, as well as hotter and drier conditions in comparison with conditions leading to ND eggs. However, CD egg can be produced under shorter daylength and cooler temperatures in comparison with TD eggs. Coexistence of CD and ND eggs reflects a bet-hedging strategy in that some individuals can enter diapause for unpredictable conditions. Nevertheless, a fitness penalty of this strategy is revealed by lower diapause intensity of CD eggs when parental mites were reared in cooler and moister conditions. Heavy reliance of synthetic pyrethroid chemicals targeting active mite has led to target-site resistance (kdr) evolving. The L1024F substitution in the voltage-gated para sodium channel is incompletely recessive. Heterozygote (RS) individuals which are detectable in genetic screening are therefore undetectable by bioassays. Only the resistant homozygote (RR) survive 100 mgL-1 bifenthrin which is the discriminating dose and also the field rate, while RS heterozygotes do not survive. Refuges untreated by pyrethroids could reduce resistance evolution because susceptible homozygous (SS) mites in refuges mate with RR survivals in sprayed areas to produce RS. Furthermore, the resistant population has decreased net reproductive output in comparison with susceptible populations, revealing a fitness cost of the L1024F substitution. The frequency of resistant alleles in a population is therefore decreased in the absence of pyrethroid chemicals.