Rainfall and temperature – these are two factors that regulate hatching of over-summering eggs of the redlegged earth mite (Halytodeus destructor) in autumn.
Research suggests that in south-eastern Australia, 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.
With much of south-eastern Australia receiving rainfall during April, these requirements for redlegged earth mite egg-hatch were fulfilled relatively early this season, particularly in cooler regions.
For example, this year, peak egg-hatch was expected in the Victorian regions of Ballarat, Hamilton, and Bendigo from early to mid-April. This is earlier than in 2019, when peak egg-hatch wasn’t expected until May due to a relatively dry, warm April.
As icy temperatures swept over eastern states in early May, areas that weren’t quite cool enough for egg-hatch during April (e.g. spots in the northern Victorian Mallee and NSW Riverina), will have well and truly satisfied the requirements for egg-hatch by now.
And agronomists and growers have been spotting the redlegged earth mite in grain cropping regions in south-eastern Australia.
But not all populations will be at economically damaging levels.
Severity of redlegged earth mite depends on several factors, many of which can be considered to help determine what the risk of damage is in 2020, in a given paddock.
Predicting risk of the redlegged earth mite
What was last year’s crop?
What is this year’s crop?
What is the extent of broadleaf cover in the paddock?
What is the spray history of the paddock?
These are a few of several factors to consider when predicting whether the redlegged earth mite will be a problem in a paddock.
To assist growers and advisers in weighing up these various factors, researchers have developed a method to calculate your ‘redlegged earth mite risk-rating’.
This method asks users to assess various factors and assign their responses a designated ‘weighting’ score between -3 and 3.
For example, if last year’s crop was undersown with clover a score of 2 is assigned. But, if cereals were in the paddock last year, a lower score of 0 is assigned as the environment would not have been as conducive to build up of redlegged earth mite as with clover.
Once all the factors are considered, the user sums up their scores and has a low (<0), medium (0-5) or high-risk (>5) result.
This risk calculating method is featured in the ‘Redlegged earth mite: best management practice guide’, created by cesar, Birchip Cropping Group and SARDI, and is available for download through the GRDC website.
Redlegged earth mite identification
The redlegged earth mite can be mistaken for other common mites in broadacre crops and pastures.
For identification assistance, watch the YouTube video below or contact the PestFacts south-eastern (firstname.lastname@example.org).
Thanks to the following for providing field reports and observations: Jesh Smith (Growmore Agronomy Services), Mitch Dwyer (Elders), Claire Pickles (BCG), James Maino (cesar), Cameron Morris (Nutrien Ag Solutions).