Aston L. Arthur

Investigating latitudinal clines for life history and stress resistance traits in Drosophila simulans from eastern Australia.

Latitudinal clines have been demonstrated for many quantitative traits in Drosophila and are assumed to be due to climatic selection. However, clinal studies are often performed in species of Drosophila that contain common cosmopolitan inversion polymorphisms that also show clinal patterns. These inversion polymorphisms may be responsible for much of the observed clinal variation. Here, we consider latitudinal clines for quantitative traits in Drosophila simulans from eastern Australia. Drosophila simulans does not contain cosmopolitan inversion polymorphisms, so allows the study of clinal selection on quantitative traits that are not confounded by associations with inversions. Body size showed a strong linear cline for both females and males. Starvation resistance exhibited a weak linear cline in females, whereas chill‐coma recovery exhibited a significant nonlinear cline in females only. No clinal pattern was evident for development time, male chill‐coma recovery, desiccation or heat resistance. We discuss these results with reference to the role inversion polymorphisms play in generating clines in quantitative traits of Drosophila.

Emerging pest mites of grains (Balaustium medicagoense and Bryobia sp.) show high levels of tolerance to currently registered pesticides

Balaustium medicagoense and Bryobia sp. (clover or pasture mite) have recently been identified as potential emerging pests of crops and pastures within southern Australia. Recorded damage by these mites has markedly increased in the past decade. There is limited information on the pesticide tolerance of these mites relative to other earth mite pests. This study examined the response of Ba. medicagoense and Bryobia sp., using the redlegged earth mite [Halotydeus destructor (Tucker)] as a comparison, to several currently registered pesticides against earth mites (omethoate, bifenthrin, chlorpyrifos, methidathion and α-cypermethrin). Ba. medicagoense had a much greater level of tolerance to all pesticides tested than H. destructor. Similarly, Bryobia sp. had a higher level of tolerance to bifenthrin, methidathion and α-cypermethrin than H. destructor. However, in the case of omethoate and chlorpyrifos, the tolerance levels were similar for Bryobia sp. and H. destructor. Ba. medicagoense had a higher level of tolerance than Bryobia sp. to the organophosphates tested (omethoate, chlorpyrifos and methidathion), but there was no difference for bifenthrin. We were unable to compare tolerance levels between Ba. medicagoense and Bryobia sp. to α-cypermethrin because of inconsistencies between replicate tests. These emerging pest mite species, therefore, have a high natural tolerance to currently registered pesticides and may prove difficult to control in the field. These findings suggest that other strategies that are not reliant on chemicals should be considered for the control of Ba. medicagoense and Bryobia sp.

Genetic Markers Indicate a New Species Complex of Emerging Pest Mites in Australian Grains

ABSTRACT Balaustium medicagoense Meyer & Ryke (Acari: Erythraeidae) and Bryobia spp. Koch (Acari: Tetranychidae) are significant emerging pests in Australian grains and pastures. Despite this, there is limited information known on their basic biology, such as species status and reproductive modes, making it difficult to develop effective and sustainable control strategies. The species/strain status of Balaustium and Bryobia mites from southern Australia was examined using a combination of mitochondrial and nuclear DNA sequence data. In addition, the amplified fragment length polymorphism (AFLP) method was used to examine the reproductive mode and genetic diversity of Ba. medicagoense from 16 populations within southern Australia. Results indicate that there is only one species of Balaustium (Ba. medicagoense) and as many as seven species of Bryobia mites currently present in grain crops, pastures and roadsides within southern Australia. The Bryobia species complex consists of four recently described lineages (Bryobia sp. I, Bryobia sp. IV, Bryobia sp. VII, and Bryobia praetiosa Koch) and three additional genetic lineages (Bryobia sp. VIII, Bryobia sp. IX, and Bryobia sp. X) that have not been described previously. Bryobia sp. VIII, B. sp. IX, and B. sp. I seem to be the most abundant species present in Australian broadacre agriculture (i.e., land suitable for farms practicing large-scale crop [agriculture] operations). The AFLP data revealed that Ba. medicagoense reproduces asexually and that genetic diversity was low with only 10 genotypes found. These findings indicate a new complex of pest mites are present within Australian grain crops and pastures and this has implications for their control.

Impact of Halotydeus destructor on crop seedlings at different plant developmental stages and levels of moisture stress.

ABSTRACT Tolerance and compensatory ability after invertebrate damage are important components of plant defense and are affected by various endogenous and exogenous factors. Here, we examine the impact of feeding damage caused by different densities of the redlegged earth mite (Halotydeus destructor Tucker) on the performance and recovery of canola and wheat seedlings at different plant developmental stages, and when grown under three levels of moisture stress. Both canola and wheat were susceptible to feeding damage caused by H. destructor at early growth development stages (canola—cotyledon, first and second true leaf stage; wheat—GS10 and GS12), but tolerated damage at the later growth stages (canola—third true leaf stage; wheat—GS14). Wheat plants compensated completely for mite feeding damage, while canola plants showed partial compensation when feeding occurred at the cotyledon and first true leaf stages. Canola was susceptible to immediate feeding damage from H. destructor under all levels of moisture stress and did not compensate for this damage over time when grown in the unstressed and moderately stressed treatments. Under severe moisture stress conditions, plant performance traits and mite density often showed a nonlinear relationship, with growth initially increasing as mite density increased, but then decreasing markedly as densities exceeded 30 mites/100 cm2. Wheat seedlings were susceptible to immediate feeding damage when grown in the unstressed and severe stress conditions, but were generally able to compensate for this damage regardless of moisture conditions. The implications of these results for the management of H. destructor are discussed.

Survival and reproduction of the pest mites Balaustium medicagoense and Bryobia spp. on winter grain crops

Balaustium medicagoense and Bryobia spp. have recently been identified as emerging pests of winter crops and pastures in Australia. These mites have a high natural tolerance to currently registered pesticides, highlighting the need to develop alternative control strategies such as cultural controls which require an understanding of plant associations. In shade-house experiments, Bryobia spp. survived and reproduced successfully on pasture, lupins and oats, but progeny failed to reach the adult stage on canola and wheat. Balaustium medicagoense progeny failed to produce a generation on any crop but parental adults survived a few months on all crops, particularly wheat. Bryobia spp. damaged canola, pasture and lupins, but caused minimal damage to oats and wheat, whereas Ba. medicagoense caused considerable damage to wheat and lupins, but only moderate damage to canola, oats and pasture. Field survey data, taken from approximately 450 sites across southern Australia, combined with analysis of historical pest reports, suggest broadleaf crops such as canola, lucerne, lupins and weeds appear particularly susceptible to attack by Bryobia species. Balaustium medicagoense was more commonly found on cereals and grasses, although they also attacked broadleaf crops, particularly canola, lucerne and lupins. These findings show that the mites have the potential to be an important pest on several winter grain crops and pasture, but there are important differences that can assist in management strategies such as targeted crop rotations.

A Framework for Identifying Selective Chemical Applications for IPM in Dryland Agriculture

Shifts to Integrated Pest Management (IPM) in agriculture are assisted by the identification of chemical applications that provide effective control of pests relative to broad-spectrum pesticides but have fewer negative effects on natural enemy (beneficial) groups that assist in pest control. Here, we outline a framework for identifying such applications and apply this framework to field trials involving the crop establishment phase of Australian dryland cropping systems. Several chemicals, which are not presently available to farmers in Australia, were identified as providing moderate levels of pest control and seedling protection, with the potential to be less harmful to beneficial groups including predatory mites, predatory beetles and ants. This framework highlights the challenges involved in chemically controlling pests while maintaining non-target populations when pest species are present at damaging levels.

Challenges in devising economic spray thresholds for a major pest of Australian canola, the redlegged earth mite (Halotydeus destructor).

BACKGROUND A key component for spray decision-making in IPM programmes is the establishment of economic injury levels (EILs) and economic thresholds (ETs). We aimed to establish an EIL for the redlegged earth mite (Halotydeus destructor Tucker) on canola. RESULTS Complex interactions between mite numbers, feeding damage and plant recovery were found, highlighting the challenges in linking H. destructor numbers to yield. A guide of 10 mites plant(-1) was established at the first-true-leaf stage; however, simple relationships were not evident at other crop development stages, making it difficult to establish reliable EILs based on mite number. Yield was, however, strongly associated with plant damage and plant densities, reflecting the impact of mite feeding damage and indicating a plant-based alternative for establishing thresholds for H. destructor. Drawing on data from multiple field trials, we show that plant densities below 30-40 plants m(-2) could be used as a proxy for mite damage when reliable estimates of mite densities are not possible. CONCLUSION This plant-based threshold provides a practical tool that avoids the difficulties of accurately estimating mite densities. The approach may be applicable to other situations where production conditions are unpredictable and interactions between pests and plant hosts are complex.

Estimating Densities of the Pest Halotydeus destructor (Acari: Penthaleidae) in Canola

ABSTRACT Development of sampling techniques to effectively estimate invertebrate densities in the field is essential for effective implementation of pest control programs, particularly when making informed spray decisions around economic thresholds. In this article, we investigated the influence of several factors to devise a sampling strategy to estimate Halotydeus destructor Tucker densities in a canola paddock. Direct visual counts were found to be the most suitable approach for estimating mite numbers, with higher densities detected than the vacuum sampling method. Visual assessments were impacted by the operator, sampling date, and time of day. However, with the exception of operator (more experienced operator detected higher numbers of mites), no obvious trends were detected. No patterns were found between H. destructor numbers and ambient temperature, relative humidity, wind speed, cloud cover, or soil surface conditions, indicating that these factors may not be of high importance when sampling mites during autumn and winter months. We show further support for an aggregated distribution of H. destructor within paddocks, indicating that a stratified random sampling program is likely to be most appropriate. Together, these findings provide important guidelines for Australian growers around the ability to effectively and accurately estimate H. destructor densities.