Earth mites

With the onset of wet and cooler autumn conditions, just when will earth mites appear in damaging numbers?

 

Predictions of egg hatch

Redlegged earth mites (Halotydeus destructor) and blue oat mites (Penthaleus spp.) survive hot summers in the ‘diapause’ egg stage. With the onset of adequately wet and cooler conditions in autumn, these eggs will hatch and thereafter will take about 3 weeks to reach their most damaging potential.

We are currently testing a model developed to predict the median redlegged earth mite hatch dates, which proved moderately accurate in 2013. The model was run for Birchip, Bendigo and Wagga Wagga. The rainfall that occurred widely on April 9th & 10th was adequate to trigger egg development (>10 mm), but the temperatures that followed in most areas was too warm to allow the majority of eggs to complete development. Accordingly, we do not expect the majority of eggs to hatch until early May or later, although there are small areas in Victoria’s Central District where egg hatch may occur in late April. We anticipate that the growth of crops germinating over the next few weeks, while the soil is still relatively warm, will be able to outgrow the damage caused by young mites in many cases.

To further validate this model, we would appreciate hearing when mite damage first becomes evident in crops.

Our advice

Growers are encouraged to check seedling crops as they emerge, being aware that damage can be patchy. Blue oat mites and redlegged earth mites attack a wide variety of plant types including cereals, oilseeds and pastures. They feed by piercing plant cells and sucking out the contents, which results in silver or white patches. High mite populations can kill plants at emergence, when crops are most vulnerable to attack, even when insecticide seed treatments have been applied.

Control options

Before deciding on the most appropriate control measure, ensure the correct mite species has been identified. Autumn insecticide sprays for redlegged earth mites and blue oat mites should ideally target mites 2-3 weeks after their emergence. This provides time for the majority of diapause eggs to hatch, but should be before the second-generation eggs have been laid. One species of blue oat mite, which is frequently found on canola, has increased levels of natural tolerance to organophosphates and synthetic pyrethroids.

Distinguishing redlegged earth mites and blue oat mites

These mites grow up to 1 mm in length with a globular shaped dark body and eight red legs. Blue oat mites can be distinguished from redlegged earth mites by the presence of a characteristic orange-red mark on their back. Redlegged earth mites have a completely black coloured body.

Bryobia and Balaustium mites

Bryobia and Balaustium mites tend to occur earlier in the season than other mites and can cause severe crop damage, particularly when emerging seedlings are under moisture stress.

It is important to distinguish Bryobia mites (Bryobia spp.) and Balaustium mites (Balaustium medicagoense) from other mite species before deciding on control options. Bryobia mites are less than 1 mm long with a fawn-orange coloured body and orange legs; they have characteristically long forelegs, which are about 1.5 times their body length. Balaustium mites are larger and can grow up to 2 mm in length, and are covered in short dense hairs. For assistance with identification download the GRDC Back Pocket Guide - Crop Mites.

If chemicals are required, application rates for Bryobia mites need to be higher than for redlegged earth mites and blue oat mites. Balaustium mites are highly tolerant to a range of insecticides and there are no chemicals currently registered for their control. Moreover, Balaustium mites will often persist in paddocks following chemical treatments targeting other mites.