Introduction

Clover mites, Bryobia spp. (Acari: Tetranychidae), have recently been identified as emerging pests of winter crops and pastures in southern Australia. Reports of clover mites in Australia date back to the early 1900’s, however it has only been in the last decade that these mites have been reported as causing significant damage to a variety of agriculturally important plants. Management of these mites is complicated by the large number of species present, as well as their small size and lack of suitable morphological characters which makes identification difficult. In Australia there are at least seven cryptic Bryobia species found in broad-acre crops and pastures. Of these seven species, there are three species that appear to be most common.

Identification

Adult clover mite

Clover mites (also referred to as ‘Bryobia mites’) are often misidentified with other pest mites, such as the redlegged earth mite and blue oat mites. However, clover mites are smaller than these mites, only reaching about 0.75mm in length when adults. They have an oval shaped flattened dorsal body that is dark grey, pale orange or olive in color. Adults have 8 pale red-orange legs with a pair of elongated front legs. These elongated front legs can help distinguish clover mites from other pest mites.

Eggs are spherical and red in color. They measure approximately 0.15-0.25 mm in diameter, and are laid either separately or in small groups and generally covered with dust. Newly hatched larvae are very small in size, bright orange-red in colour and have 6 legs. The larval stage is followed by two nymphal stages in which the mites have eight legs and resemble adults, but are smaller.

Males (although rare) are also found within a few species and these are quite easily recognised from females by their much smaller body size and extremely long front legs, which are up to two times longer than their body size.

Distribution

Distribution of clover mite in Australia

Clover mites are broadly distributed 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 have also been recorded in Tasmania and Queensland. However, there has been no systematic sampling conducted in these states so their full distribution remains unclear. Furthermore, the distribution of each Bryobia species in Australia is currently unknown and will require surveys with molecular markers that can distinguish the species.

Similar to other pest mites, long dispersal range is thought to occur via the movement of eggs in soil adhering to farm machinery and livestock as well as through the transportation of plant material. Movement is also thought to occur during summer when diapause eggs are transported by summer winds.

Biology and lifecycle

In Australia, there are at least seven cryptic species of clover mites found in broad-acre agriculture, which complicates management. Distinguishing between these species is extremely difficult as these mites are extremely small and lack suitable morphological characters. Of the seven species identified, there are three species that appear most important as pests of grain crops and pastures.

Clover mites are typically most abundant during the warmer months of summer, autumn and spring, however they can be found throughout the year. Their active periods are dependent on species, with the lifecycle and occurrence of diapause varying considerably between species. Some mite species have no diapause stage with the succession of generations proceeding continuously throughout the year, while others have one or more generations a year, separated by summer and/or winter diapause.

The lifecycle on average takes about one month to complete but is dependent on temperature and humidity. The developmental stages of clover mites consist of four active stages (larvae, protonymph, deutonymph and adult) and three resting stages. Although further research is needed, is it assumed that clover mites within Australia are largely asexual, given that the majority of populations consist of only females, and only a very limited number of males have been found.

Behaviour and damage

Typical clover mite damage to canola

Unlike redlegged earth mites and blue oat mites, clover mites are mostly found on the leaf surfaces of plants. They can also be found on twigs or debris on the ground. Clover mites are active in the warmer parts of the day and tend to feed mainly on the upper leaf surfaces. When disturbed they drop from the plants, curl their legs and remain still. All species of clover mites are phytophagous and they feed on plant material by puncturing plant cells with their mouth-parts and sucking out the contents. Damage caused by these mites consists of winding, etiolated trails composed of whitish-grey spots. They tend to cause most damage in autumn where they attack newly establishing pastures and emerging crops, greatly reducing seedling survival and retarding development.

Clover mites attack a variety of agriculturally important plants and are reported to cause considerable damage to pastures and oilseed, pulse and cereal crops. They appear to have a preference for broadleaf plants such as canola, lupins, vetch, lucerne and clover. In pastures, clover mites tend to have a preference for medics and clovers over grasses. They occasionally attack wheat and barley, as well as some weeds.

Monitoring

Susceptible pastures and crops should be carefully monitored from autumn to spring for the presence of mites and evidence of damage. It is especially important to regularly inspect crops in the first three to five weeks after sowing. Crops most at risk are those sown into paddocks that contain summer and autumn weeds and/or were pasture the previous year. A warm mild autumn will favour mite survival. Clover mite numbers generally decline over the cool wet winter months and they tend not to be much of a problem during this period.

Weeds present in and around paddocks should be checked for the presence and abundance of clover mites. Mites are best detected on warm days from midday until late afternoon. Unlike many other species of mites, which spend a lot of time on the soil surface, clover mites are mostly found on the lower and upper leaf surfaces of plants. Clover mites are difficult to detect during early mornings or in wet conditions.

An effective way to sample mites is to use a standard petrol powered garden blower/vacuum machine. A fine sieve or stocking is placed over the end of the suction pipe to trap mites vacuumed from plants and the soil surface.

Control

Chemical control

Chemicals are the most commonly used method to manage clover mites, and there are several pesticides registered for clover mite control. These chemicals are only effective against the active stages of mites; they do not kill mite eggs.

Differences in tolerance levels between species complicates the management of clover mites. At least one species of clover mite has a high natural tolerance to a range of pesticides registered against earth mites in Australia. Furthermore, there have been many field reports of pesticide control failures involving clover mites.

Pesticides with persistent residual effects can be used as bare-earth treatments. These treatments can be applied prior to, or at sowing to kill emerging mites and protect the plants during the early seedling stage. If damage warrants control, apply pesticides three weeks after the first appearance of clover mites in autumn. Pesticide sprays are rarely warranted to control clover mites once crops are established, although some pastures can be impacted significantly in spring.

Information on the registration status, rates of application and warnings related to withholding periods, OH&S, residues and off-target effects should be obtained before making decisions on which pesticide to use. This information is available from cropping consultants, chemical resellers, Australian Pesticides and Veterinary Medicine Authority (APVMA) and the pesticide manufacturer. Always consult the label and MSDS before using any chemical product.

Biological and cultural control

There have been no biological control agents (predators or parasites) identified in broad-acre crops and pastures in Australia that are effective in controlling clover mites. However, there are a number of predator species known to attack other earth mites such as redlegged earth mites, which may also potentially prey upon clover mites. These include predatory mites, small beetles and spiders.

Cultural controls can be effective in controlling mite numbers. Crops that follow pastures with a high clover content are most at risk. Avoid planting susceptible crops such as canola, lupins, vetch and lucerne into these paddocks. Early control of summer and autumn weeds within and around paddocks, especially broadleaf weeds such as capeweed and clovers, can help prevent mite outbreaks.

Acknowledgements

This AgNote was written by Aston Arthur and Paul Umina from cesar and The University of Melbourne. Support from the Grains Research and Development Corporation is acknowledged.