Competitive interactions among four pest species of earth mites (Acari: Penthaleidae)

Earth mites are major winter pests of a variety of crops and pastures in southern Australia. Competition between four earth mite species was investigated using field and shadehouse experiments. The influence of different plant hosts on the frequency and intensity of competitive interactions also were examined. This information is important, because control attempts that eradicate one species of mite could be directly followed by an increase in abundance of another earth mite species. There were strong effects of intraspecific competition on the reproductive rate of species, while interspecific interactions between Halotydeus destructor (Tucker) and Penthaleus species and between the three Penthaleus species also were detected. Competitive abilities were altered on the different plant types. On pasture, the competitive advantage swayed between Penthaleus major (Dugés), H. destructor, and Penthaleus falcatus (Qin & Halliday). Penthaleus sp. x was the strongest competitor in a mixture of wheat, Triticum aestivum (L.), and oats, Avena sativa (L.), whereas on canola, Brassica napus (L.), and bristly ox-tongue, Picris echioides (L.), P. falcatus, and H. destructor were superior competitors. These results suggest that competition is a strong force influencing the abundance of earth mites in the field and that host plant factors are important in shaping the type of interactions. This highlights the importance of identifying mite species when considering control options and suggests that effective control recommendations need to be developed for each individual species.     

Plant host associations of Penthaleus species and Halotydeus destructor (Acari: Penthaleidae) and implications for integrated pest management

Integrated pest management programs seek to minimise reliance on pesticides and provide effective long-term control of pests. Cultural control strategies, such as crop rotations, trap and border crops, and weed management, require a thorough understanding of pest host associations. This paper examines the effects of different plant hosts on the persistence and reproduction of blue oat mites, Penthaleus spp., and the redlegged earth mite, Halotydeus destructor (Tucker), which are major agricultural pests in southern Australia. Field and shade-house experiments were conducted testing several crop and plant types. All species survived and reproduced from one mite season to the next when confined to pasture. Canola and a common weed, 'bristly ox-tongue', were suitable hosts for H. destructor and Penthaleus falcatus (Qin and Halliday), whereas Penthaleus sp. x and Penthaleus major (Dugés) failed to persist on these plants. A mixture of wheat and oats sustained P. sp. x and H. destructor, but not P. falcatus or P. major. Lentils were generally a poor host plant for all mite species. These findings show that earth mite species differ in their ability to persist on different plant types, highlighting the importance of distinguishing mite species before implementing control strategies. Results are discussed with respect to cultural control options for the management of these winter pests.     

Genetic structure of Halotydeus destructor and Penthaleus major populations in Victoria (Acari: Penthaleidae)

The redlegged earth mite (Halotydeus destructor) and the blue oat mite (Penthaleus major) are major pests of pastures and crops in southern Australia. Reproductive modes, migration rates and levels of differentiation between populations were investigated using allozyme electrophoresis. Collections were made throughout Victoria and a sample was also obtained from Western Australia. Three enzyme loci were polymorphic in H. destructor (Mdh-1, Mdh-2 and Idh). Genotype frequencies of these loci did not differ between phenotypic males and females, providing no evidence for haplodiploidy. Allele frequencies were in Hardy-Weinberg equilibrium, indicating that H. destructor is diploid and sexual. This was confirmed via crosses between males and females. Allele frequencies differed between Victorian sites, although F statistics indicated little differentiation over all loci. A sample from Western Australia did not differ in allele frequencies from the Victorian sites. Four polymorphic loci were found in P. major (Mdh-1, Mdh-2, Idh and Gpi). Only a few multilocus genotypes occurred in a sample, indicating that P. major is parthenogenic. No male P. major were found in this study. A number of colour morphs were also identified and a genetic association between genital plate colour and clonal type was found in one population of P. major. Two different body colour morphs were associated with different clonal types.     

Diapause and implications for control of Penthaleus species and Halotydeus destructor (Acari: Penthaleidae) in southeastern Australia

Blue oat mites, Penthaleus spp., and redlegged earth mites, Halotydeus destructor (Tucker) are major winter pests of a variety of crops and pastures. In southern Australia earth mites exhibit a facultative egg diapause to survive unfavorable summer conditions. The initiation of diapause egg production in earth mites was investigated using field and shade-house experiments. Species differed in their timing of diapause. H. destructor mainly produced diapausing eggs towards the end of the active mite season in spring, although small numbers were also produced in winter. In contrast, Penthaleus major (Dugés) produced diapause eggs almost immediately after emergence in autumn and continued producing these eggs throughout the season. Penthaleus falcatus (Qin and Halliday) also produced diapause eggs in early winter, although the first appearance of these eggs was slightly later in the season than for P. major. The diapause response of an undescribed species was also somewhat later than in P. major and P. falcatus, but earlier than in H. destructor. Electrophoresis of P. major samples indicated that clones of this parthenogenic species may differ in their timing of diapause egg production, providing another potential selective factor contributing to the maintenance of clonal diversity within this group. The results highlight the importance of determining species composition when devising control strategies for earth mite outbreaks.     

Host Plant Acceptance by Redlegged Earth Mite, Halotydeus destructor (Tucker) (Acarina: Penthaleidae)

Host plant acceptance behavior of redlegged earth mites (Halotydeus destructor) on cotyledons of subterranean clover (subclover) lines was investigated in the laboratory. Sustained feeding consisted of a series of short bouts of feeding; rather than one long period of feeding. H. destructor preferred to feed at mite-damaged rather than undamaged sites on cotyledons. With time the numbers of mites feeding in aggregations increased, as mites were attracted to damaged patches. Most feeding occurred in aggregations, and mites in such groups benefited by greater weight gain. The sequence of foraging leading to feeding was similar between subclover lines, however, feeding and aggregation activities were markedly reduced on a resistant line. Host plant acceptance occurred during probing but only after some feeding had occurred.     

The pest status and distribution of three cryptic blue oat mite species (Penthaleus spp.) and redlegged earth mite (Halotydeus destructor) in southeastern Australia

Earth mites are pests of crops and pastures in southeastern Australia.Recent studies show differences between earth mite species in their mode ofreproduction, preferred hosts and pesticide tolerance. This paper examines thedistribution and pest status of each species. The southeastern Australiandistribution for each species is mapped, incorporating new data from easternNewSouth Wales, South Australia and Tasmania. A new population of an undescribedspecies previously identified from northwestern Victoria was found in northernNew South Wales. CLIMEX was used to identify climatic factors limiting thedistribution of P. major and P.falcatus, the most broadly distributed species. This analysissuggests tolerance to heat and desiccation limits the inland distribution ofthese two species. A three-year survey of agricultural outbreaks indicates thatall Penthaleus species are major agricultural pestsalthough their pest status on crop types appears to differ. All speciescontributed to chemical control failures. However P.falcatus, previously identified in laboratory tests as havingincreased tolerance to pesticides, was the most common species associated withcontrol failures. A laboratory experiment indicated that mites are sometimespests on crops on which they cannot persist for a generation. Results arediscussed with respect to management of these agricultural pests.     

Dispersal patterns of pest earth mites (Acari: Penthaleidae) in pastures and crops

Control methods in pest earth mites and other mites often depend on low dispersal rates, yet there are no experimental estimates of these rates. To rectify this, adult movement rates were estimated in the earth mite Halotydeus destructor Tucker and the winter grain mite, Penthaleus major (Duges), using mark-release-recapture techniques. Mean square dispersal distances were used to estimate diffusion coefficients. In pasture, coefficients were in the range 0.3-1.3 for these species. This suggests that 90% of the population moves < 5-11 m in a 10-d period, or 7-16 m within their adult lifetime. Releases of mites in adjacent pea/wheat crops indicated directional movement toward the more favored pea host. However, there was no directional movement when adjacent plots of peas and lupins were compared, even though lupins are poor hosts. These results indicate that broad border sprays or border culturing will be needed to prevent mite movement from adjacent paddocks.     

Cold storage of Halotydeus destructor (Acari: Penthaleidae) for use in experiments

Survival of medium sized nymphal stages of redlegged earth mite Halotydeus destructor (Tucker) (mainly tritonymphs and deutonymphs) stored under low temperature (1.5°C) in sealed plastic boxes remained more than 50% after 12 days of storage, with some mites surviving for up to eight weeks. Adding fresh subclover leaves into the storage box increased the survival rate of mites from 12% to 28%, 19 days after the storage started. Mites stored for two weeks at low temperatures showed feeding activity in a screening experiment similar to mites collected directly from the field. This indicated that cold storage of redlegged earth mite can be used to build up mite numbers for large screening experiments, or to extend the period of availability of mites collected from the field. However, their reproductive ability was greatly reduced after three weeks at low temperature. Thus, care should be taken when using mites for experiments measuring reproduction. The implications of low temperatures for reducing field populations of mites in midwinter are also discussed.     

Predictions of summer diapause in the redlegged earth mite, Halotydeus destructor (Acari: Penthaleidae), in Australia

A prediction for the onset of a summer diapause in the eggs of the redlegged earth mite, Halotydeus destructor, was developed for Australia. In this species diapause eggs pass the summer in the cadavers of adult female mites. Adult female mites were collected for several weeks from pastures in spring at 18 sites in south-western Australia and dissected to determine the timing of the production of diapause eggs. Some sites were sampled for several years between 1990 and 1997. A model was developed to predict the time for onset of diapause. The week at which 90% of eggs were in diapause was predicted best by daylength (80.1% of the variability), then by duration of the long-term plant growing season (10.4%, of variability), leaving 9.5% due to other factors. A single chemical spray in spring 2 weeks before the production of 90% diapause eggs resulted in 99% fewer mites present in autumn 7-8 months later at three sites. The timing of the spring spray was the factor leading to successful control. This model was tested at 17 sites across the whole geographical distribution of the redlegged earth mite in Australia between 1998 and 2001. The observed week of 90% diapause was within 1 week of the predicted week on 81% of occasions, and 2 weeks earlier on 15% of occasions. A database was created for the predicted date of onset of 90% diapause for the whole distribution of the redlegged earth mite in southern Australia on a 10 km(2) grid. Australian farmers are using this for timing a spring spray to control mites in the following autumn.     

An improved method for rearing Halotydeus destructor (Acari: Penthaleidae) in the laboratory

We developed an improved method for rearing Halotydeus destructor in the laboratory. Mite numbers increased rapidly over the summer of 1992–1993 when fed on vetch, Vicia sativa cv Blanchefleur; in contrast mite numbers were low and fell over the second half of the summer of 1990–91 when fed on subterranean clover, Trifolium subterraneum cv Junee. On V. sativa at a fluctuating temperature from 11 to 18°C 200 mites produced a mean of 1308 and 1455 progeny per transfer in 2 years, a 6–7-fold rate of increase per transfer. The mean transfer time was 35 days and using the interval between transfers as a measure of generation time, the mites completed six to seven generations with no evidence for development of diapause eggs from August until April. There was considerable variability in the numbers of mites produced per pot, associated in part with the presence of Verticillium sp. fungus in 35% of the pots during transfers. Improvement in H. destructor rearing resulted from the use of V. sativa cv Blanchefleur as a food source, maintenance of high humidity and adequate ventilation within cages, and the transfer of mites to fresh food sources at early nymphal stages, which reduced the spread of fungal infections.     

Biology, ecology and control of the Penthaleus species complex (Acari: Penthaleidae)

Blue oat mites, Penthaleus spp. (Acari: Penthaleidae), are major agricultural pests in southern Australia and other parts of the world, attacking various pasture, vegetable and crop plants. Management of these mites has been complicated by the recent discovery of three cryptic pest species of Penthaleus, whereas prior research had assumed a single species. The taxonomy, population genetics, ecology, biology and control of the Penthaleus spp. complex are reviewed. Adult Penthaleus have a dark blue-black body approximately 1 mm in length, and eight red-orange legs. Within Australia, they are winter pests completing two or three generations a season, depending on conditions. The summer is passed as diapausing eggs, when long-distance dispersal is thought to occur. The Penthaleus spp. reproduce by thelytokous parthenogenesis, with populations comprising clones that differ ecologically. The three pest Penthaleus spp. differ markedly in their distributions, plant hosts, timing of diapause egg production and response to pesticides, highlighting the need to develop control strategies that consider each species separately. Chemicals are the main weapons used in current control programs, however research continues into alternative more sustainable management options. Host plant resistance, crop rotations, conservation of natural enemies, and improved timing of pesticide application would improve the management of these pests. The most cost-effective and environmentally acceptable means of control will result from the integration of these practices combined with the development of a simple field-based kit to distinguish the different mite species.     

The response of the major crop and pasture pest, the red-legged earth mite (Halotydeus destructor) to pesticides: dose-response curves and evidence for tolerance

We describe an assay for testing the response of the adult red- legged earth mite, Halotydeus destructor, to pesticides. This assay is used to generate dose response curves for an organophosphate (omethoate) and an organochlorine (endosulphan) commonly used to control these mites, as well as a synthetic pyrethroid (alfamethrin). Comparisons of the populations from five sites around Victoria indicated tolerance to omethoate. A heritable basis for the tolerance was demonstrated by comparing the stocks after they had been reared under laboratory conditions for a generation. No tolerance of endosulphan was detected, although this chemical was not used heavily at any of the sites. Some tolerance of alfamethrin was also suggested. These results suggest that alternative methods for controlling this pest will eventually need to be considered.     

Effect of alphacypermethrin and bifenthrin on the survival of five acarine predators of Halotydeus destructor (Acari: Penthaleidae)

The synthetic pyrethroids, alphacypermethrin and bifenthrin, were evaluated in the laboratory and field for their impact on the survival of five acarine predators of the pasture pest, Halotydeus destructor (Tucker). In laboratory bioassays, alphacypermethrin at the proposed field rate (5 g a.i. ha^-1) for H. destructor and the scarab beetle pest, Aphodius tasmaniae Hope caused medium-high mortality to Walzia australica Womersley (Anystidae), Bdellodes affinis Atyeo (Bdellidae), Balaustium murorum (Hermann) (Erythraeidae) and Parasitus fimetorum (Berlese) (Parasitidae) but was not toxic to Cyta latirostris (Hermann) (Bdellidae). In the field, populations of W. australica and B. affinis treated with this rate were significantly reduced but recovered after 8 weeks. Bifenthrin at 5 g a.i. ha^-1 killed 96–100% of W. australica, B. affinis, B. murorum and P. fimetorum in laboratory bioassays and significantly reduced field populations of W. australica and B. affinis for 17 weeks. In bioassays both products at 2.5 g a.i. ha^-1 permitted 10–30% survival of B. affinis and B. murorum and alphacypermethrin permitted 59% survival of W. australica. A 2.5 g a.i. ha^-1 rate of alphacypermethrin is recommended for control of H. destructor in pasture when A. tasmaniae is not present, thus minimizing the impact of chemical control on the survival of some predatory mite species.     

Population growth and predation interference between two species of predatory phytoseiid mites (Acarina: Phytoseiidae) in interactive systems

Summary Populations of two species of phytoseiid mite predators, Phytoseiulus persimilis Athias-Henriot and Amblyseius degenerans (Berlese), feeding on a tetranychid prey, Tetranychus pacificus McGregor, were allowed to grow separately as well as together on bush lima bean (Phaseolus lunatus Var.) arenas in the laboratory. The population plateau attained by P. persimilis was nearly 5 times higher than that for A. degenerans when each species was on separate leaf arenas. When they were on the same arena, P. persimilis was outcompeted by A. degenerans after about 70 days of population growth. When dispersal to other arenas was necessary for the predators to find prey in another experiment, P. persimilis survived well but not A. degenerans. The mechanisms underlying species displacement were explored further. The differential mortality of immature predators at different developmental stages due to interspecific predation was concluded to be responsible for the population decline of P. persimilis, and the decline of A. degenerans in another experiment was attributed to its sedentary tendency regardless of prey distribution and to the lack of alternative food sources in the system. The implications to biological control of mutual predation between predator species is discussed briefly.