Redlegged earth mites – new research insights

Recent applied research enhances our toolkit to manage redlegged earth mite

Where have they been reported?

The first reports of redlegged earth mites (Halotydeus destructor) this season have been from irrigated pastures in Mitiamo (Victoria’s Northern Country), and near Gunbar and Deniliquin in the NSW Riverina. 

Redlegged earth mites (Source: cesar)

 

Predictions of egg hatch

Peak hatching of redlegged earth mite eggs may be delayed in parts of Victoria and NSW due to an unusually warm autumn. Research by Dr Garry McDonald suggests that the process of egg development in autumn requires at least 5 mm of rain accumulated over five consecutive days or less, followed by 10 days of average daily temperatures remaining below 16°C.

We ran a prediction model to estimate the date of peak egg hatch in five areas in Victoria and southern NSW. Whilst the rainfall requirements were met in each location, temperatures have been too warm around Albury, Wagga Wagga and Birchip for egg hatch. However, with colder conditions having arrived at Hamilton and Ballarat, we predict peak egg hatching to occur around the 10^th-11^th of May in these regions.  

The egg-hatch model is a useful predictive tool for dryland broadacre cropping; the presence of redlegged earth mite activity in irrigated pastures is presumably stimulated by irrigation and extra soil moisture.

Knowing when to spray

Recent research conducted by The University of Melbourne has led to the development of preliminary economic thresholds for redlegged earth mites in canola. These thresholds are based on shade-house and field trials conducted over 3 years as part of a research project funded by GRDC. These new thresholds should reduce many uneconomic sprays. Avoiding unnecessary spraying will not only save growers money but also reduce the risk of mites developing insecticide resistance.

It is important to note that the data underpinning these thresholds is still preliminary and needs further validation in the field. It is hoped that PestFacts subscribers will provide feedback over the coming months so the thresholds can be refined. Please send your comments and feedback to Paul Umina at pumina@cesaraustralia.com or phone 03 9349 4723.

Preliminary economic thresholds in canola

At the cotyledon stage:

If visual mite feeding damage (silvering or whitening) to 20% of plants or more and the presence of mites, treatment is warranted. If not, recheck at the 1^st true-leaf stage.

At the 1^st true-leaf stage:

If there are 10 mites per plant, treatment is warranted.

If there are fewer numbers of mites don’t spray. Recheck paddock in 5 days if crop growth is slow, or in 10 days if crop growth is rapid.

At the 2^nd true-leaf stage:

If there are fewer than 30 plants/m² and the presence of mites, treatment is warranted.

If there are greater than 30 plants/m² and the majority of plants show no visual mite feeding damage, don’t spray. Recheck paddock in 5 days if crop growth is slow, or in 10 days if crop growth is rapid.

Later plant development stages:

Once plants reach the 3^rd true-leaf stage there is no benefit in spraying, except when plants are under severe stress (moisture stress or waterlogging) coupled with mite numbers greater than 2000/m².

Our advice

It is important that crops are monitored for mite activity, especially during plant emergence when crops are most vulnerable. Damage manifests itself in silver or white patches, which can be sporadic. If mite activity is suspected, it is important that the correct species is identified. For assistance with identification download the GRDC Back Pocket Guide - Crop Mites.

Resistance to synthetic pyrethroid and organophosphate chemicals has been detected in redlegged earth mite populations in Western Australia. Only apply insecticides if control is warranted and avoid preventative sprays. For further information on redlegged earth mite management visit redlegged earth mites within our PestNote series. 

 

Sources of field reports of redlegged earth mites

Early reports of mites, all in irrigated paddocks, have been reported through Twitter.