The Balaustium mite, Balaustium medicagoense (Acari: Erythreidae), is emerging within the southern Australian grains industry as a significant 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. They attack a variety of agriculturally important plants.
Identifying Balaustium mite
Balaustium mites are quite often confused with other pest mites, such as the redlegged earth mite and blue oat mites (Penthaleus spp.).
Similar to other pest mites, Balaustium mites have a rounded dark red-brown coloured body and red legs. However, they have distinct short stout hairs that cover their entire body, which gives them a velvety appearance.
Adults reach about 2mm 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.
In the Balaustium mite's earlier stages:
- Newly laid eggs are light maroon in colour, becoming darker before egg hatch.
- Larvae are bright orange in colour and have 6 legs.
- The larval stage is followed by a number of nymphal stages in which mites have 8 legs and resemble adults, but are much smaller.
Balaustium mite in Australia
Balaustium mites are widespread in most agricultural regions in southern Australia with a Mediterranean-type climate. They are found in:
- Western Australia
- New South Wales
- 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, but there has been no systematic sampling conducted and the distribution across the state remains unknown.
How Balaustium mite spreads
Similar to other pest mites, long range dispersal is thought to occur via the movement of eggs in soil stuck 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.
Life cycle and biology
Balaustium mites are active in the cool, wet part of the year. Mites generally emerge in March to April and can be found through until December. Numbers peak in autumn and spring when they are most active. They typically go through 2 generations a season:
- one in autumn or winter
- one in spring or early summer.
The second generation hatching of larvae occurs in August to September. Balaustium mites typically have a longer active season than other earth mites and are still found in late spring or early summer. It is thought that they enter summer diapause at the egg stage in December and are not active again until late summer or early autumn.
There has been little research examining the life cycle of Balaustium mites. It takes approximately 5 to 6 weeks for larvae to reach adulthood, while adults have been found to live for at least 8 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.
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:
However, they have a preference for grasses, cereals and weeds, particularly:
- barley grass
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.
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, but when numbers are high and plants are already stressed due to other environmental conditions, they can cause significant damage to crops.
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. Though, Balaustium mites 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 Balaustium mites.
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 3 to 5 weeks after sowing.
Crops sown into paddocks that were pasture the previous year should be regularly inspected for Balaustium mites. Weeds present in paddocks before cropping should also be checked for 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 or wet.
One of the most effective methods to sample mites is using a D-vac on plants and soil. Often you can use a standard petrol-powered garden blower/vacuum machine. Place a sieve over the end of the suction pipe to trap any mites vacuumed from plants and the soil surface.
Chemical control methods
Chemical control is often one of the methods available for plant pests as part of an integrated pest management program. More information is available from:
- your local nursery
- cropping consultants
- chemical resellers
- the pesticide manufacturer.
For information on currently registered and or permitted chemicals, check the Australian Pesticide and Veterinary Medicine Authority (APVMA) website.
Always consult the label and Safety Data Sheet before using any chemical product.
This species has a high natural tolerance to many insecticides and will generally survive applications aimed at other mite pests.
Biological and cultural control methods
There have been no biological control agents (predators or parasites) identified in Australia that are effective in controlling Balaustium mites. But alternative methods such as cultural control can prove to be effective at controlling this mite.
These cultural control methods include:
- 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, before planting a susceptible crop like cereals or canola, a paddock could be sown to a broadleaf plant that Balaustium mites have not been reported to attack, such as vetch.
This information has been updated from what was originally written by Aston Arthur and Paul Umina from the Centre for Environmental Stress and Adaptation Research (CESAR).
Assistance was provided by Andrew Weeks, Dale Grey and the Grains Research and Development Corporation through the National Invertebrate Pest Initiative.
Figures 4 and 5 courtesy of A.Weeks (CESAR)