Note Number: AG1413
This Information Note provides information regarding the behaviour and management of the Balaustium mite in field crops and pastures.
The Balaustium mite, Balaustium medicagoense (Acari: Erythreidae), has recently been identified within the Australian grains industry as an emerging pest of winter crops and pasture. This mite is the only species of the genus Balaustium recorded in Australia and was probably introduced from South Africa, along with the redlegged earth mite (Halotydeus destructor), in the early 1900s. Balaustium mites are found throughout areas of southern Australia that have a Mediterranean-type climate, attacking a variety of agriculturally important plants.
Balaustium mites are quite often confused with other pest mites, such as the redlegged earth mite and blue oat mites (Penthaleus spp.). They have a rounded dark red/brown coloured body and red legs similar to other pest mites, however they have distinct short stout hairs covering their entire body giving them a velvety appearance. Adults reach about 2 mm in size, which is twice the size of other earth mite species. Balaustium mites also have distinct 'pad-like' structures on their front legs and move slower than redlegged earth mites and blue oat mites.
Newly laid eggs of Balaustium mites are light maroon in colour, becoming darker prior to egg hatch. Larvae are bright orange in colour and have six pairs of legs. The larval stage is followed by a number of nymphal stages in which mites have eight legs and resemble adults, but are much smaller.
Balaustium mites are widespread throughout most agricultural regions in southern Australia with a Mediterranean-type climate. They are found in Western Australia, Victoria, New South Wales and South Australia. They are generally restricted to coastal areas and do not occur too far inland or in the drier Mallee areas of Victoria and South Australia. Balaustium mites have been found in Tasmania, however there has been no systematic sampling conducted and the distribution across the state remains unknown.
Similar to other pest mites, long range dispersal is thought to occur via the movement of eggs in soil adhering to livestock and farm machinery or through the transportation of plant material. Movement may also occur when over-summering eggs are moved by summer winds.
Lifecycle and biology
Balaustium mites are active in the cool, wet part of the year. Mites generally emerge in March-April and can be found through until December. Numbers peak in autumn and spring when they are most active. They typically go through two generations a season, one in autumn/winter and one in spring/early summer. The second-generation hatching of larvae occurs in August-September. Balaustium mites typically have a longer active season than other earth mites and are still found in late spring/early summer. It is thought that they enter summer diapause at the egg stage during December and are not active again until late summer/early autumn.
There has been little research examining the lifecycle of Balaustium mites. It takes approximately five-six weeks for larvae to reach adulthood, while adults have been found to live for at least eight weeks in field enclosures. Eggs are likely to be laid in batches of up to 30. Recent molecular studies have shown that Balaustium mites reproduce asexually, with populations dominated by a limited number of genetic or 'clonal' variants. Males have been found but are rare and probably non-functional. This means populations are made up of clones that can respond differently to environmental and chemical conditions. This may influence the likelihood of populations developing pesticide resistance and means Balaustium mite populations could respond differently to control strategies.
Behaviour and damage
Unlike redlegged earth mites and blue oat mites, Balaustium mites are most active in the warmer parts of the day, where they can be found on the foliage, particularly near the tips of plants. They attack a variety of agriculturally important plants and are reported to cause considerable damage to canola, lupins and lucerne. However, they have a preference for grasses, cereals and weeds, particularly barley, wheat, oats, barley grass and capeweed. Even in pastures, Balaustium mites tend to have a preference for grasses and weeds over clovers and other medics.
Balaustium mites are unusual in that they not only feed on plants, but also prey upon other small invertebrates. They have been reported to feed on a number of different groups, including various collembolan species and other mites. Balaustium mites were originally thought to be a beneficial predator with some reports suggesting they provided localised control of redlegged earth mites. It is only recently that Balaustium mites have been confirmed to feed on plant material.
Fig 4. Typical Balaustium damage on canola. Photo: A. Weeks (CESAR).
Fig 5. Typical Balaustium damage to lupin. Photo: A. Weeks (CESAR).
Balaustium mites feed on plants using their adapted mouthparts to probe leaf tissue of plants and suck up sap. In most situations Balaustium mites cause little damage, however when numbers are high and plants are already stressed due to other environmental conditions, significant damage to crops can occur.
Balaustium mite feeding damage in canola is characterised by distorted and cupped cotyledons, which may have a leathery appearance. Typical damage to cereals, grasses and pulses is 'silvering' or 'whitening' of the attacked foliage, similar in appearance to damage caused by redlegged earth mites and blue oat mites. However, Balaustium mite damage differs in that they tend to attack the edges and tips of plants. Adult mites are likely to be responsible for the majority of feeding damage to plants.
The impact of mite damage is increased when plants are under stress from adverse conditions such as prolonged dry weather or waterlogged soils. Ideal conditions for seedling growth enable plants to tolerate higher numbers of Balaustiummites. Carefully inspect susceptible pastures and crops from autumn to spring for the presence of mites and evidence of damage. It is especially important to inspect crops regularly in the first three to five weeks after sowing.
Crops sown into paddocks that were pasture the previous year should be regularly inspected for Balaustium mites. Weeds present in paddocks prior to cropping should also be checked for the presence and abundance of Balaustium mites. Mites are best detected feeding on the leaves, especially on or near the tips, during the warmest part of the day. Balaustium mites are difficult to find when conditions are cold and/or wet.
One of the most effective methods to sample mites is using a D-vac which is based on the vacuum principle, much like a vacuum cleaner used in the home. Typically, a standard petrol powered garden blower/vacuum machine is used, such as those manufactured by Stihl® or Ryobi®. A sieve is placed over the end of the suction pipe to trap mites vacuumed from plants and the soil surface.
Currently no product has been registered to control Balaustium mite in any state or territory of Australia. The Australian Pesticides and Veterinary Medicine Authority (APVMA) maintain a database of all chemicals registered for the control of agricultural pests in Australia. Reference to the APVMA website (www.apvma.gov.au) will confirm the registration status of products for Balaustium mite, or consult chemical resellers or a local chemical standards officer.
Ensure the relevant Maximum Residue Limits (MRLs) for the chemical in the end market is met, be it domestic or export.
Chemical users must read and understand all sections of chemical labels prior to use.
There have been no biological control agents (predators or parasites) identified in Australia that are effective in controllingBalaustium mites. Alternative methods such as cultural control can prove to be effective at controlling this mite. Early control of summer weeds, within and around paddocks, especially capeweed and grasses can help prevent mite outbreaks. Rotating crops or pastures with non-host crops can also reduce pest colonisation, reproduction and survival. For example, prior to planting a susceptible crop like cereals or canola, a paddock could be sown to a broadleaf plant that Balaustiummites have not been reported to attack, such as vetch.
This AgNote was written by Aston Arthur and Paul Umina from CESAR, The University of Melbourne. Assistance was provided by Andrew Weeks, Dale Grey and the Grains Research and Development Corporation through the National Invertebrate Pest Initiative.
This Note was developed by Dale Grey, Farm Services Victoria. May 2010.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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