1 Agriculture Victoria – Rutherglen, RMB 1145, Rutherglen Vic 3183 Australia (
2 Evolutionary Biology and IPM Unit, Department of Genetics and Evolution, La Trobe University, Bundoora, Vic 3058 Australia ()
3CSIRO Division of Entomology, Private Bag, Wembley, WA 6014 Australia ()
Redlegged earth mite (Halotydeus destructor) and the blue oat mite (Penthaleus spp.) complex are soil dwelling mites that comprise the most regular and economically important group of pests confronting Australian canola growers. Current control is largely dependent on insecticides, used prophylactically or in response to pest occurrence; practices which are unlikely to be sustainable without other measures. Research is providing a knowledge base that will allow the development of enduring and robust management approaches primarily aimed at preventative strategies. For example, we now know that the blue oat mite group contains several cryptic species, which vary in their response to different cropping and insecticide environments. Strategically managed crop and pasture rotations should provide the key to reducing densities prior to sowing canola. Spring spraying in pastures at optimum timing can be used to control mites in following crops. The development of host plant resistance in canola is progressing slowly, although success could be accelerated by direct genetic modification. Similarly, work on classical biological control has had limited impact, but current screening of control agents may reveal new opportunities.
KEYWORD: Halotydeus destructor, Penthaleus, integrated pest management, plant resistance
Most Australian canola crops are sown in autumn following the first substantial rains. Seedling establishment is often slow because of declining temperatures and day length. This exposes the initially small and weak plant to prolonged attack by a range of pests, but most importantly a complex of earth mite species including Halotydeus destructor (Tucker) (Acarina: Penthalediae) (redlegged earth mite (RLEM)) and Penthaleus spp. (blue oat mite). These mites cause similar plant damage and losses to an establishing canola crop as the crucifer flea beetle (CFB), Phyllotreta cruciferae (Groeze), found across the northern great plains of America and the prairie provinces of Canada (Burgess 1977). Work on the biology, ecology and control of earth mites was summarised in two research workshop proceedings (Ridsdill-Smith 1991, McDonald and Hoffmann 1995) and in a more recent RLEM review (Ridsdill-Smith 1997).
Earth mites are small and taxonomically complex organisms. They attack canola during early establishment, and are capable of destroying crops within days of emergence. These characteristics, particularly the rapidity of attack, make the pest group difficult to manage with strategies that are most commonly used in classical pest management programs. The current approach is largely dependent on chemical control as a single tactic. Although most growers have few options beyond the use of insecticides, the approach is unlikely to be sustainable because of the risk of insecticide resistance and secondary pest incursions, rendering that tactic ineffective. A more robust strategy is required, using the principles of integrated pest management (IPM).
“Responsive IPM programs” require sampling regimes and economic thresholds for each pest. The double dilemma facing canola growers is that these tools are not available, and even if they were available, the economic margins associated with the crop and the uncertainty of profitable returns makes their use cost-prohibitive or risky. This suggests that preventative strategies should form the research focus. This paper highlights recent progress in developing the strategic elements of an IPM program for earth mites in Australian canola, providing a model for other Australian canola pests.
Earth mite control is currently achieved through one of three relatively inexpensive methods of chemical application: “bare earth” treatment prior to germination, aimed at control of mites as they hatch (James and O’Malley 1992), seed dressings and foliar applications after emergence. Bare earth treatments provide residual activity for several weeks but are non-selective and may be hazardous to beneficial invertebrates. Natural enemies in crops and pastures provide some regulatory influence on earth mites (Michael 1995), and are also likely to play a significant role in the regulation of the important post-establishment pests of canola, particularly the caterpillar and aphid pests (different natural enemies). Secondary pest outbreaks in the vegetative and flowering crop may result from an elimination of natural enemies.
Insecticides applied as seed dressings are more selective but often fail because the small canola seed is unable to carry sufficient active ingredient to provide adequate or sustained protection under heavy mite pressure. Many foliar sprays are systemic and more selective, and are used in response to an infestation that is perceived to be a threat to the crop. Timing of foliar sprays is crucial if only one application is to be used, as the mites must be controlled before the appearance of the first reproductive females. Once these have laid eggs, repeated applications of insecticide may be required because eggs are not killed by these chemicals.
There are at least five elements that are required to establish a durable IPM program based on preventative principles. Each of these has been recently or is currently being addressed by research, and include: (a) a knowledge of the taxonomy, distribution and biology of the species complex, (b) strategic use of insecticides (c) cultural control options (d) host plant resistance and (e) the use of natural enemies. These elements are largely based on the principles of preventative pest management, where management tactics act to reduce future pest populations (Pedigo 1995). These tactics need to be applied before the pest causes economic damage and are usually conducted as part of the production system (e.g., tillage, crop rotation). They are usually based on historical perceptions by the grower to a pest that is unpredictable.
During autumn and winter, RLEM females deposit their winter (non-diapause) eggs on the soil or foliage of host plants. During spring, environmental cues induce the development of diapause eggs that are resistant to desiccation and high temperatures of summer. Diapause is terminated by an exposure to 52ºC for ca one month (Wallace 1970a) and then emergence occurs following exposure to lower temperatures and a rainfall event (Wallace 1970b). The proportion of eggs surviving summer can be highly variable. The extent of carry-over of eggs from spring to autumn is a major factor in the size of crop infestations in autumn.
H. destructor has been confirmed as a single species and biotype distributed across southern regions of Western Australia, South Australia, New South Wales, and throughout Victoria and Tasmania (Qin and Halliday 1995, Weeks et al. 1995). The Penthaleus complex has now been identified as consisting of at least 3 species (P. major, P. falcatus and P. sp. x) and many biotypes (Weeks and Hoffmann 1999). DNA electrophoretic analysis and the complete absence of males indicate that all three Penthaleus species are obligate thelytokous parthenogens, unlike H. destructor which is sexual. The blue oat mite species are pests of pastures and several crops, although the preferred plant hosts differ. Both P. falcatus and P. sp. x appear to be more important pests of canola than P. major. Two of the Penthaleus spp. (P. major, P. falcatus) are distributed more widely than H. destructor. The other species (P. sp. x) only occurs in two disjunct areas, the mallee region in north-western Victoria and a region in north-eastern New South Wales.
Partial resistance in RLEM to some insecticides (e.g., omethoate, alphamethrin) has been reported, probably locally induced as a direct effect of repeated spraying of the same chemical over a series of years (Hoffmann et al. 1997). This may help to explain occasional reports of unsatisfactory control levels. The repeated use of insecticides from the same chemical group carries the risk of further induced resistance, so the recent introduction of new insecticides (eg. bifenthrin, imidacloprid) has widened the control options. More recently, a significant difference was shown in the response of the four earth mite species to a range of common insecticides. P. falcatus is significantly more tolerant of the four most commonly used insecticides than the other species, possibly explaining apparent failures in areas of NSW where P. falcatus is common (Umina and Hoffmann 1999). These outcomes highlight the importance of determining species composition when considering control options.
Strategic decisions on spray treatments require a knowledge of the size of the mite population once the autumn break has commenced, but are complicated by the large and variable egg mortality during the summer (Annells and Ridsdill-Smith 1991). Prediction of the timing of egg hatch is possible based on weather conditions (Wallace 1970b, Ridsdill-Smith and Annells 1997).
Recent work now focuses on methods to achieve control of mites in the previous spring, before diapause eggs are produced, thus preventing carry-over of eggs in to autumn (Ridsdill-Smith and Annells 1997). A model has been developed to predict the optimum time for spraying RLEM in spring, just before the production of diapause eggs (Ridsdill-Smith and Pavri 1998).
The potential for conventional crop management activities to reduce mite densities prior to crop sowing has been recognised for over 50 years (e.g. Newman 1925, Swan 1934). These include the use of spring fallows, burning of pasture and stubbles in summer, tillage, delayed plantings, and maintenance of weed-free paddocks. Nonetheless, these practices are incompatible with the use of sustainable cropping systems. In fact, there is now strong anecdotal evidence that more sustainable systems (minimum tillage, stubble retention and crop rotations) result in increased mite problems.
More sophisticated cultural management strategies are now being developed which are compatible with current farming practices and rely on the prevention of eggs being carried over from spring to autumn. For example, heavy and regulated stocking rates in pastures during early to mid-spring are very effective in significantly reducing mite densities and the carry-over of aestivating eggs into a subsequent crop (Grimm et al. 1995). Similarly, McDonald et al. (1995) have identified particular crop plants that have an antibiosis affect on RLEM. Laboratory and field experiments showed that mites have a significantly shorter longevity and lay few or no eggs when maintained on any of several common crop plants including two cereals (wheat and oats) and three pulses (chick peas, lentils and narrow leafed lupins). Consequently, when these “non-host” crops precede canola in rotation, few eggs enter summer diapause and thus few are expected to emerge the following autumn. The use of non-host crops relies on these being kept weed-free so that the mites do not have alternative hosts for multiplication. The success of rotations will also depend on low rates of movement of mites between paddocks with alternative hosts, particularly during the diapause stage in summer when eggs may be carried passively by wind and stock movement.
In the process of domesticating rapeseed to produce today’s canola (Busch et al. 1994), breeders are likely to have eliminated naturally occurring pest resistance, especially by removing glucosinolates. In current work, which aims to select RLEM resistance from a bank of 480 wild lines of B. napus, about 10% were found to have enhanced tolerance to RLEM damage under intensive greenhouse screening (G. McDonald, unpublished data). A small number of these lines were derived from seed with low levels of glucosinolate. Under intensive mite pressures in the field (ca 10-15,000 mites per m2), a few lines survived significantly longer than the controls, but all eventually succumbed. Subsequent crossing to F2 does not appear to have elevated resistance levels, although the work is continuing. The mechanisms of this resistance are not currently understood. However, a study of resistance to RLEM in subterranean clover has revealed that a number of mechanisms are involved, and in trifoliate leaves a suite of isoflavones act as deterrents to the mites (Ridsdill-Smith and Nichols 1998). Resistance to RLEM also was effective against Penthaleus spp. Thus cultivars can contain one of a number of compounds in order to exhibit resistance.
Despite its vulnerability, seedlings and older plants of commercial lines of B. napus are, in fact, poor hosts of RLEM. Adults that have prolonged exposure to B. napus plants have a shorter longevity and lay infertile or non-viable eggs. In contrast, mite populations actually grow when they feed on B. juncea (G. McDonald, unpublished data); a surprising result considering the high levels of aliphatic glucosinolates in this species. Canadian work on screening B. napus for flea beetle resistance over the last decade has also been slow, with only minor levels of resistance identified (J. Soroka, Pers. Comm.). In contrast, significant resistance to flea beetle has been achieved through introgressing resistance from wild relatives of Brassica into B. napus (A. Good, Pers. Comm).
Biological control has been attempted for RLEM with the importation to Western Australia by CSIRO a general predator, Anystis wallacei Otto, from the Mediterranean coast of France in 1965. It causes RLEM mortality but has a slow rate of dispersal (Michael 1995). There are clearly “background” levels of predation (e.g., Weiss and McDonald 1998) and pathenogenesis (Ridsdill-Smith and Annells 1997) by native species which exist in the Australian environment, but whose activity is insufficient to control pest populations. A current study is systematically comparing predatory fauna from Australia and South Africa with the aim of identifying new agents for screening (B. Halliday and M. Keller, Pers. Comm.).
The new approaches to an IPM package for earth mites are based on a better understanding of the biology and ecology of the mites in farming systems. The challenge for researchers is to cumulatively reduce mite populations by pyramiding options. These will include the strategic use of insecticides to reduce the spring to autumn carry-over of mites, to maintain and enhance natural enemies, the use of disruptive farming systems with crops/plants that are hostile to mites and not beneficials, and management practices that limit mite access to the emerging plants. Longer term components will be the use of resistant plants, and the use of biological control.
We thank Melina Miles, Alexei Rowles (previously and currently Agriculture Victoria, respectively), Michelle Robinson, Ursi Kolliker (La Trobe University), Celia Pavri (CSIRO) and Royce Holtkamp (NSW Agriculture) for their contributions to aspects of this work.
Burgess, L. (1977). Flea beetle (Coleoptera: Chrysomelidae) attacking rape crops in the Canadian prairie provinces. Canadian Entomologist 109, 21-32.
Busch, L., Gunter, V., Mentele, T., Tachikawa, M., and Tanaka, K. (1994). Socializing nature: Technoscience and the transformation of rapeseed into canola. Crop Science 34, 607-614.
Grimm, M., Michael, P., Hyder, M. and Doyle, P. (1995). Effects of pasture pest damage and grazing management on animal production efficiency. Plant Protection Quarterly 10, 62-64.
Hoffmann, A.A., Porter, S. and Kovacs, I. (1997). The response of the major crop and pasture pest, the redlegged earth mite (Halotydeus destructor) to pesticides: dose-response curves and evidence for tolerance. Experimental & Applied Acarology 21, 151-162.
James, D.J and O’Malley, K.J. (1992). Control of redlegged earth mite Halotydeus destructor on bare earth. Plant Protection Quarterly 7, 10-11. McDonald, G. and Hoffmann, A.A. (eds) (1995). Proceedings of the Second National Workshop on Redlegged Earth Mite, Lucerne Flea and Blue Oat Mite, 99 pp. Victoria Printing, Victoria, Australia.
McDonald, G. and Hoffmann, A.A. (eds) (1995). Proceedings of the Second National Workshop on Redlegged Earth Mite, Lucerne Flea and Blue Oat Mite, 99 pp. Victorian Printing, Blackburn, Australia.
McDonald, G., Moritz, K., Merton, E. and Hoffmann, A.A. (1995). The biology and behaviour of redlegged earth mite and blue oat mite on crop plants. Plant Protection Quarterly 10, 52-54.
Michael, P. (1995). Biological control of redlegged earth mite and lucerne flea by the predators Anystis wallacei and Neomolgus capillatus. Plant Protection Quarterly 10, 55-57.
Newman, L.J. (1925) Red-legged earth mite. Journal of Agriculture, W.A. 2,49-54
Pedigo, L.P. (1995). Closing the gap between IPM theory and practice. Journal of Agricultural Entomology 12, 171-181.
Qin, T.K. and Halliday, R.B. (1995). Systematic studies of redlegged earth mite Halotydeus destructor (Tucker) and related species (Acari: Penthaleidae). Plant Protection Quarterly 10, 50-52.
Ridsdill-Smith, T.J. (ed.) (1991). Proceedings of the National Workshop on Redlegged Earth Mite, Lucerne Flea and Blue Oat Mite, 169 pp. Department of Agriculture, Perth, Australia.
Ridsdill-Smith, T.J. (1997). Biology and control of Halotydeus destructor (Tucker) (Acarina: Penthaleidae): a review. Experimental & Applied Acarology 21, 195-224.
Ridsdill-Smith, T.J. and Annells (1997). Seasonal occurrence and abundance of redlegged earth mite Halotydeus destructor (Acari: Penthaleidae) in annual pastures of southwestern Australia. Bulletin of Entomological Research 87, 413-423.
Ridsdill-Smith, T.J. and Nichols, P.G.H. (1998). Development of pasture legumes resistant to redlegged earth mite. In Pest management future challenges (eds MP Zalucki, RAI Drew and GG White) Vol 2 Proc. 6th Australasian Applied Entomology Conference. 382-389.
Ridsdill-Smith, J. and Pavri, C. (1998). Spring spraying for redlegged earth mites. Australian Grain (Oct-Nov) i-iv.
Swan, D.C. (1934). The red-legged earth mite Halotydeus destructor (Tucker) in South Australia: with remarks upon Penthaleus major (Duges). Journal of Agriculture, S.A. 10, 353-67.
Umina, P.A. and Hoffmann, A.A.(1999). Tolerance of cryptic species of blue oat mites (Penthaleus spp.) and the redlegged earth mite (Halotydeus destructor) to pesticides. Australian Journal of Experimental Agriculture (in press).
Wallace, M. M. H. (1970a). Diapause in the aestivating eggs of Halotydeus destructor (Tucker) (Acari: Eupodidae). Australian Journal of Zoology. 18: 295 - 313.
Wallace, M.M.H. (1970b). The influence of temperature on post-diapause development and survival in the aestivating eggs of Halotydeus destructor (Acari: Eupodidae). Australian Journal of Zoology 18, 315-329.
Weeks, A.R., Fripp, Y.J., and Hoffmann, A.A. (1995). Genetic structure of Halotydeus destructor and Penthaleus major populations in Victoria (Acari: Penthaleidae). Experimental & Applied Acarology 19, 633-646.
Weeks, A.R. and Hoffmann, A.A. (1999). The biology of Penthaleus species in south-eastern Australia. Entomologia experimentalis et applicata (in press).
Weiss, M. J. and McDonald, G. (1998). European earwig, Forficula auricularia L., (Dermaptera: Forficuladae), a predator of the redlegged earth mite, Halotydeus destructor, (Tucker) (Acarina: Penthaleidae). Australian Journal of Entomology 37, 183-185.