Redlegged earth mite

The redlegged earth mite (RLEM) (Halotydeus destructor) is a major pest of broadleaf crops and pastures in regions of Australia with cool wet winters and hot dry summers. RLEM was accidentally introduced into Australia from the Cape region of South Africa in the early 1900s.

These mites are commonly controlled using pesticides. However, non-chemical options are becoming increasingly important. This is due to evidence of chemical resistance and concern about long-term sustainability.

Description

Adult RLEMs are 1 mm in length and 0.6 mm wide (the size of a pin head). They have 8 red–orange legs and a completely black velvety body (Figure 1).

Newly hatched mites are pinkish–orange with 6 legs. They are only 0.2 mm long and are not generally visible to the untrained eye.

The larval stage is followed by 3 nymphal stages. The nymphs have 8 legs and resemble the adult mite, but are smaller and sexually undeveloped.

Other mite pests are sometimes confused with RLEMs in the field:

• Blue oat mites can be distinguished from RLEMs by their blue–black body and an oval orange/reddish mark on their back.
• Balaustium mites have a rounded, dark red–brown coloured body covered in short stout hairs. They can grow to twice the adult size of RLEM.
• Bryobia mites are smaller than RLEMs. They have an oval-shaped body with a flattened back. They are dark grey, pale orange or olive in colour.

Unlike other species that tend to feed singularly, RLEMs generally feed in large groups of up to 30 individuals.

Feeding damage

RLEM is called an earth mite because it spends 90% of its time on the soil surface, rather than on the foliage of plants.

The mites feed on the foliage for short periods. Then they move around before settling at another feeding site. Other mites are attracted to volatile compounds released from the damaged leaves. This results in groups of mites feeding together.

Typical mite damage is ‘silvering’ or ‘whitening’ of the attacked foliage (Figure 2). Mites use adapted mouthparts to tear up the leaf tissue of plants and suck up the discharged sap. The resulting cell and cuticle damage dries out the leaf and slows photosynthesis. It also produces the characteristic silvering that is often mistaken for frost damage.

RLEMs are most damaging to newly establishing pastures and emerging crops. They greatly reduce seedling survival and development. In years with a late ‘break of season’ or with late-sown crops and pastures, seedlings may emerge among established populations of RLEM. This can cause significant damage. In severe cases, entire crops may need re-sowing after RLEM attack.

RLEM feeding reduces the established plants’ productivity. It can also reduce the palatability of pasture to livestock.

Hosts

RLEMs have a very broad host range:

• Canola
• Wheat
• Barley
• Oats
• Lupins
• Sunflower
• Faba beans
• Field peas
• Poppies
• Lucerne
• Vetch
• Pasture legumes and grasses.

While RLEMs are less of a concern in cereal crops and in some pulses, they can cause economic damage in some years. They also feed on a range of weed species, including:

• Paterson’s curse
• skeleton weed
• variegated thistle
• capeweed.

Life cycle

RLEMs are active in the cool, wet part of the year, usually between April and November. During this period, they may pass through up to 4 generations. Each generation survives 6 to 8 weeks.

RLEM eggs hatch in autumn following cooler temperatures and adequate rainfall (Figure 3). It takes about 2 weeks of exposure to favourable conditions for over-summering eggs to hatch. This releases swarms of mites. These then attack delicate crop seedlings and emerging pasture plants.

RLEM eggs laid during the winter–spring period are orange in colour and about 0.1 mm in length. They are laid singly on the underside of leaves, the bases of host plants (particularly stems) and on nearby debris. They are often found in large numbers clustered together. Female RLEMs can produce up to 100 winter eggs. The eggs usually hatch in 8 to 10 days, depending on conditions.

Towards the end of spring, physiological changes in the plant, hot dry weather and changes in light conditions induce the production of over-summering or ‘diapause’ eggs. These are stress-resistant eggs that are retained in the dead female bodies. Diapause eggs can successfully withstand the heat and desiccation of summer. They produce the autumn generation the following year.

RLEMs reproduce sexually, with an adult sex ratio that is female biased. The male RLEM (which is smaller than the female) produces webbing, usually on the surface of the soil. It then deposits spermatophores on the threads of this webbing. The female mite picks up the spermatophores to fertilise her eggs.

Distribution and spread

RLEM is widespread throughout most agricultural regions of southern Australia (Figure 4). They are found:

• in southern New South Wales
• on the east coast of Tasmania
• in the south-east of South Australia
• in the south-west of Western Australia
• throughout Victoria.

Genetic studies have found high levels of gene flow and migration within Australia. Individual adult RLEMs only move short distances between plants in winter. Long-range dispersal occurs via the movement of eggs in soil stuck to livestock and farm machinery, or when fodder and plant material are transported. In summer, over-summering eggs can be dispersed by wind.

Monitoring

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. This is when crops are most susceptible.

In spring, RLEMs are best detected feeding on the leaves in the morning or on overcast days. In the warmer part of the day, they tend to gather at the base of plants, sheltering in leaf sheaths and under debris. They will crawl into cracks in the ground to avoid heat and cold. When disturbed during feeding they will drop to the ground and seek shelter.

An effective way to sample mites is to use a standard petrol-powered garden blower/vacuum machine:

1. Place a fine sieve or stocking over the end of the suction pipe
2. Vacuum mites from plants and the soil surface.
3. Place the contents onto a black tray to make mites easy to identify.

Economic thresholds

Crops can compensate for RLEM damage, so it is important to applying thresholds before using insecticides.

For example, canola and wheat are susceptible to feeding damage by RLEMs at early growth development stages. In contrast, both crops can tolerate damage at the later growth stages,

Wheat tolerates and compensates for mite feeding damage to a larger extent than canola. For more information about economic thresholds for, visit the Cesar Australia RLEM PestNote.

Control

Chemical control

Chemicals are the most commonly used control option against earth mites. A number of chemicals are registered for control of active RLEMs in pastures and crops. However, no currently registered pesticides are effective against RLEM eggs. An increased reliance on chemical control has led to increased levels of insecticide resistance in the pest.

Resistance to synthetic pyrethroid and organophosphate chemicals has been detected in parts of Western Australia and South Australia. Organophosphate resistance has also been detected in Victoria.

We encourage growers and advisers to download the Redlegged earth mite best management practice guide.

The guide aims to help growers effectively control this pest, while minimising the selection pressure for further resistance.

For low to moderate mite populations, insecticide seed dressings are an effective method. Avoid prophylactic sprays; apply insecticides only if control is warranted and you are sure of the mite identity.

Research shows that spraying an appropriate insecticide once at the right time in spring can significantly reduce populations of RLEMs the following autumn. This kills mites before they start producing diapause eggs in mid–late spring. The optimum date can be predicted using climatic variables. Tools such as TIMERITE can help farmers identify the optimum date for spraying.

Spring RLEM sprays will generally not be effective against other pest mites and may exacerbate other mite problems, so caution is advised.

Selectively rotating insecticides with different modes of action will help minimise the risk of further resistance evolution in RLEM.

Chemical control is often used on plant pests as part of an integrated pest management program. Consult your local nursery, cropping consultant, chemical reseller or pesticide manufacturer for advice.

For information on currently registered and or permitted chemicals, check the Australian Pesticides and Veterinary Medicines Authority (APVMA) website. Always consult the label and safety data sheet before using any chemical product.

Biological control

At least 19 predators and one pathogen are known to attack RLEM in eastern Australia. The introduced French Anystis mite can effectively suppress populations in pastures. Other effective natural enemies include:

• snout mites
• ther predatory mites
• small beetles
• earwigs spiders.

Leaving shelterbelts or refuges between paddocks will help maintain natural enemy populations.

Natural enemies  in windbreaks and roadside vegetation can suppress RLEM in adjacent pasture paddocks. Insecticides with residual activity are applied as border sprays to prevent mites moving into a crop or pasture. However, they may inadvertently reduce beneficial insect numbers, which will protect RLEM populations.

Cultural control

Using cultural control methods can decrease the need for chemical control. Rotating crops or pastures with non-host crops can reduce mite colonisation, reproduction and survival. For example, before planting a susceptible crop like canola, a paddock could be sown to cereals or lentils. This will help reduce the risk of RLEM population build-up.

Clean fallowing and controlling weeds (particularly broadleaf weeds like capeweed) before and after sowing can also reduce mite numbers. This is because the weeds can be important breeding sites for RLEM. Avoid sowing paddocks with high-clover pastures if they they have a history of damaging, high-density RLEM populations.

Appropriate grazing management can reduce RLEM populations to below damaging thresholds. A shorter pasture with exposed soil will limit growth of mite populations. This can be an effective strategy in late spring. It reduces the number of RLEMs producing over-summering diapause eggs. This results in fewer RLEMs hatching the following autumn.

Acknowledgements

This information was originally developed by Paul Umina from Cesar Australia and reviewed by Leo McGrane and Paul Umina in 2021. For more information on the work Cesar Australia is doing on RLEM, visit the Cesar Australia website

Image credits

Figure 1: A. Weeks (Cesar Australia).
Figures 2, 3 and 4: P. Umina (Cesar Australia).