The biology, ecology and population genetics of Balaustium medicagoense and Bryobia spp.; emerging mite pests of grains in southern Australia

© 2010 Dr. Aston Lindsay ArthurBalaustium medicagoense (Acari: Erythraeidae) and Bryobia spp. (Acari: Tetranychidae) have recently been identified as emerging pests of winter crops and pastures by the Australian grains industry. The relative importance of these mites and damage caused by them in Australia has markedly increased in the last decade, and pest reports indicate extensive damage to a variety of winter grain crops and pastures. Furthermore, pest reports suggest that these mites have been difficult to control in the field, with some pesticides described as being ineffective. Despite this, there is very little information about the biology and control of these species, making it difficult to devise effective and sustainable management strategies. This thesis addresses some of these concerns by investigating various aspects of the biology, ecology and genetics of these mites in broad acre agriculture in southern Australia. Laboratory bioassays were undertaken to examine the response of Ba. medicagoense and B. sp. I (along with the redlegged earth mite (Halotydeus destructor) as a point of reference) to several currently registered pesticides against earth mites. Balaustium medicagoense and B. sp. I generally had a much greater level of tolerance to the pesticides tested than H. destructor. These mites therefore have a high natural tolerance to currently registered pesticides and may prove difficult to control in the field. These findings suggest other strategies that are not reliant on chemicals should be considered for control of Ba. medicagoense and B. sp. I. The effects of different crop plants on the survival and reproduction of Ba. medicagoense and Bryobia spp. (B. sp. VIII and B. sp. IX) and the plant damage caused by these mites were investigated under shade-house conditions. In addition information from field surveys and pest report bulletins was collated to test for outbreak patterns. The findings showed that these mites were polyphagous, attacking a variety of agriculturally important plants as well as numerous weeds within southern Australia. Therefore, these mites have the potential to be important pests on several winter grain crops and pasture, but there are important differences in host responses that might assist in management strategies such as targeted crop rotations and weed management. The distribution, life cycle, seasonal abundance patterns and the pest status of Ba. medicagoense and Bryobia spp. were examined in southern Australia. Both mites are widely distributed throughout Mediterranean-type climate areas within southern Australia. Balaustium medicagoense has two generations per year and was active from March until December, with a likely diapause period in summer. There were differences in the seasonal abundance and life cycle of two species of Bryobia (B. sp. IX and B. sp. I). Bryobia sp. IX had two generations per year, was active from March until December, and was likely to be in diapause over the summer months. Bryobia sp. I did not have a diapause stage, was active year round and had approximately four generations per year. Seasonal abundance patterns of Ba. medicagoense and B. sp. IX overlapped with those of the major pest mite species H. destructor and Penthaleus major. A survey of pest outbreaks and chemical control failures showed that while H. destructor and the Penthaleus species remained important pests, outbreaks and control failures involving Ba. medicagoense and Bryobia spp. had increased over the last decade. The species/strain status of Balaustium and Bryobia mites from broad acre environments was examined in southern Australia using a combination of mitochondrial and nuclear DNA sequence data. In addition, the reproductive mode and genetic diversity of Ba. medicagoense was examined using the amplified fragment length polymorphism (AFLP) method. Findings showed that Ba. medicagoense is the only species present in grain crops, pastures and roadsides within southern Australia, and at least seven species of Bryobia mites are present. The AFLP data revealed that Ba. medicagoense reproduces asexually and that genetic diversity was low with only 10 genotypes found from 16 populations. Overall, the data presented in this thesis helps to explain why Ba. medicagoense and Bryobia spp. are emerging pests and points to ways in which these mites may be controlled in the future. The results show that these mites are difficult to control with currently registered pesticides, have a widespread distribution, can potentially increase in abundance rapidly under favourable conditions and persist across a wider range of seasonal conditions than other major pest earth mites. However, there are differences in pesticide susceptibilities and plant host responses that have the potential to be exploited when developing targeted control options for these pest mites