Written by: Simon Leake | Agronomist | 0439 999 173
In Western Australia, the red-legged earth mite (RLEM) has become a major pest of concern due to its increased levels of insecticide resistance. The reliance on chemical control methods has contributed to the development of resistance, particularly to synthetic pyrethroid (SP) and organophosphate (OP) chemicals. Cumulative exposure to the same chemical group has been identified as the primary factor behind the emergence of resistance, even when (or especially because) the chemicals are not directly targeting RLEM. Resistance to SPs has been detected in various locations, while mites in some areas have also developed resistance to OPs. Once resistance is established, it tends to persist for many years. As the dispersal ability of mites is limited, resistance tends to remain relatively localised and spread slowly, however spread over larger distances does occasionally occur. Therefore, effective management strategies are crucial to mitigate the impact of RLEM resistance.
So how do we manage these bad boys? In a nutshell the answer isn’t all that helpful and this is to basically just reduce insecticide use as much as possible. Now that sounds simple enough, but I thought I would compile a comprehensive list as there are several out of the box ways of doing this.
- Identify mites correctly – Balaustium mite, Blue oat mite and Bryobia mite are all easy enough to confuse with RLEM and are all controlled differently. If misidentified and sprayed with the wrong thing, that’s one extra application that can contribute to more resistance.
- Keep weeds out of crops and firebreaks – simple, less habitat = less insects. Keeping weeds controlled up to seeding will also provide a benefit amongst others.
- Intensive grazing in spring appears promising as a tactic for suppressing RLEM for the following season. Studies have shown that heavy grazing during spring at the TIMERITE®, period can achieve a significant reduction in mites the following season. More can be read here: https://www.agric.wa.gov.au/livestock-research-development/controlling-red-legged-earth-mites-using-intensive-spring-grazing
- Rotate chemistry – don’t use the same insecticide or same group of insecticides over and over. This is a good guide on how to approach many different scenarios:
- Use soft insecticides – avoid using insecticides that will affect beneficial insects and avoid using SP and OPs to control spring pests that aren’t RLEM as although they aren’t the target, they will still get a dose of the chemical and select for resistance. For example, use Primicarb for aphids or Versys for caterpillars which don’t affect RLEM.
- Avoid preventative sprays like bare earth / PSPSE or the “throw it in anyway” attitude where possible. Perhaps its time to stop growing canola on pasture? If we put canola on cereal stubbles (that have been kept weed free – remember above!) moving forward might be the most sensible option.
- Crop rotations – use a crop rotation that decreases risk as per above.
- Know hatch dates to assess risk – https://cesaraustralia.shinyapps.io/RLEM-hatch/ – if you know when you’re most likely to be exposed to risk this will help on decisions of your management options like using a seed treatment in higher risk setting for instance.
- Resistance testing – if you suspect you have resistance take samples and get them tested, instead of guessing and potentially throwing fuel onto the fire. In areas where resistance is present, steps should be taken to limit the movement of resistant mites and their eggs by carefully managing the transport of hay, silage, farm machinery, and livestock into resistant-free paddocks/ farms.
- Seed Treatments – Use seed treatments in place of bare-earth but also try to avoid using seed treatments where possible without sounding contradictory!
- Grow mite tolerant pasture species and crops – prima gland clover or sub clovers like Narrikup Bindoon or Rosebrook are supposedly more tolerant to mite damages and obviously mite tolerant crops like cereals.
- Use economic thresholds to only use insecticide when absolutely required. These are the thresholds recommended by GRDC: Canola: Cotyledon: mite damage (silvering) affects 20%+ of plants and mites are present. 1 true leaf: >10 mites/plant , 2 true leaves: when plant numbers are low <30/m2 and mites are present, More than 3 true leaves: only when plants are under severe stress and >2000 mites/m2, Wheat/ barley: 50 mites/m2, Pulses: 50 mites/m2.
- Use higher seeding rates – more plants will minimise yield impact on crop, to a degree.
- Barrier sprays – use these if mites are attacking from edges as opposed to spraying the whole paddock.
- When spraying – I already said no spraying!!! I’m kidding but not really. If you absolutely must spray, try to follow these guidelines:
• A well-timed spring spray of an appropriate insecticide can effectively reduce red-legged earth mite (RLEM) populations in the following autumn. This approach targets mites before they lay diapause eggs in mid-late spring. By predicting the optimum spraying date using climatic variables and tools like TIMERITE®, farmers can maximize the effectiveness of control measures. Rememberonly spray if absolutely necessary. Find your TIMERITE®, here: https://www.wool.com/land/timerite/calculatedate/ keep in mind the website is set up poorly – you can’t enter your full latitude and longitude (who knows why) so just enter the first number of your coordinates as per below:
- When spraying autumn pastures, it is recommended to target the first generation of mites before adults lay eggs, ideally within three weeks of mite appearance. This method works best in years with
a concentrated hatching period, characterized by favourable autumn rains and temperatures dropping to 20°C or lower. It’s important to note that chemical sprays do not kill mite eggs, so applications should be timed when most mites have emerged. Remember, only spray if absolutely necessary.
- To avoid ineffective treatments, it is advised not to re-spray a paddock in the same season if a known spray failure has occurred with the same product or another product from the same insecticide
group. Additionally, if the cause of a spray failure is unknown, it is best to refrain from re-spraying.