Controlling red legged earth mites using intensive spring grazing

Key messages

• Maintaining feed on offer at about 2t DM/ha for four weeks in spring can effectively control RLEM in the following growing season.

Aims

To demonstrate that short periods of intensive grazing in spring can control red legged earth mites (RLEM) effectively without the use of insecticides.

Introduction

Research conducted in the 1990s found that grazing pastures intensively for the whole of spring could control RLEM as effectively as insecticides. However, the tactic has not been adopted by producers due to the impractical number of sheep required for months in one paddock. We investigated whether some, if not all, of the benefits could be realised using shorter periods of intensive grazing.

Insecticide resistance in RLEM populations in parts of the grainbelt has renewed interest in alternative control methods, such as intensive spring grazing around the Timerite® period. Grazing is an attractive option as it possibly offers multiple benefits such as improved yields in following grain crops and better weed control in spring pasture.

While any producer with a mixed farming enterprise could benefit from a spring grazing package aimed at controlling RLEM we expect the largest benefits to be for those in the medium to high rainfall zones with more pasture feed on offer (FOO) and larger RLEM populations. The cost to implement this spring grazing package in most cases would be zero however it would require some additional labour.

In the absence of chemical control options, resistant RLEMs can reduce crop yields by as much as 30% at a cost of about $74/ha. A spring grazing package could recover a substantial proportion of these losses.

Method

In 2019, on-farm demonstrations were established at Boyup Brook, Cranbrook and Kalgan to compare the effectiveness of intensive grazing for two and four weeks in spring, around the Timerite® date, to an ungrazed control. Intensive grazing involved maintaining pasture feed on offer (FOO) at about 1.4t DM/ha. Pastures were assessed weekly and RLEM were sampled fortnightly during the 2019 spring and following the break of the growing season in 2020.

RLEM were suction sampled using a Stihl™ blowervac BG55. The nozzle of the blowervac had a sieve with fine mesh (holes at ~10µm) placed 5cm into the aperture of the blowervac. The nozzle of the blowervac was held on the ground for two seconds and 10 suction samples were taken per sample jar containing 10mL of 70% ethanol. A total of 10 sample jars were collected per treatment. In spring 2019, collections were taken at fortnightly intervals starting from around the Timerite® date; while in 2020 collections were taken following the break of the growing season and again one month later. Mites were counted under a dissecting microscope.

Pasture FOO was determined using the calibrated visual assessment technique of Campbell & Arnold (1973). At each sampling, 60 individual visual assessments were made for each treatment using a quadrat (0.1m²) and these estimates were related to actual FOO by taking 7 to 10 calibration cuts.

Results and Discussion

Boyup Brook

In 2019, the intensively grazed plots were maintained at about 2t DM/ha of dry matter (Figure 1). By the end of two weeks grazing (2 Oct 2019), there were 95% fewer RLEM compared to the control (Figure 2). Two weeks after livestock had been removed from this treatment, the numbers of RLEM had increased five-fold (84%), whereas FOO only increased by 0.5t DM/ha (Figures 1, 2). Four weeks of grazing led to a 98% reduction in RLEM compared to the ungrazed control and a fortnight later (6 November 2019) RLEM populations had crashed, indicating the population was undergoing summer diapause.

Sampling in early autumn 2020 found RLEM only in the ungrazed treatment, but by early winter only the four-week grazed treatment had fewer than 500 RLEM per square metre (P<0.05) (Figure 2). This is ten times less than the threshold for mites in cereals (equivalent to 5000 mites per square metre) and five times less than the threshold for canola (equivalent to 1000 mites per square metre).

Figure 1. Feed on offer (t DM/ha) of clover-based pasture at Boyup Brook in 2019 that was either ungrazed, or grazed for two or four weeks

Figure 2. Number of RLEM in pasture at Boyup Brook that was either ungrazed, grazed for two weeks or grazed for four weeks ± standard error of mean (SEM). Arrows indicate when livestock were removed, colour indicates treatment.

Kalgan

Within a week of imposing the intensive grazing treatment pasture FOO had fallen to 1.6t DM/ha, however by the end of the four-week grazing period FOO had risen to 3t DM/ha due to late rain lifting pasture growth rates (Figure 3).

Compared to the ungrazed control, grazing for two and four weeks resulted in a 90% reduction in RLEM numbers. When livestock were removed after two weeks grazing, RLEM numbers increased by 10% (Figure 4). By early November, RLEM numbers had decreased to less than 100 per square metre in the grazed treatments. When resampled in June 2020 the ungrazed treatment had three times as many mites as the grazed treatments. Both grazed treatments had an average of 1400 mites per square metre, which is below the threshold for growing a cereal crop.

Figure 3. Feed on offer (t DM/ha) of kikuyu-based pasture at Kalgan in 2019 that was either ungrazed, or grazed for two or four weeks

Figure 4. Number of RLEM in pasture at Kalgan that was either ungrazed or grazed for two or four weeks ± standard error of mean (SEM). Arrows indicate when livestock were removed, colour indicates treatment.

Cranbrook

At the commencement of the demonstration, pasture FOO was low for a spring pasture at 2.5t DM/ha. Grazing for either two or four weeks reduced FOO only to about 2t DM/ha. At the end of this four-week period, FOO in the ungrazed treatment had increased by about 0.7t DM/ha FOO indicating the pasture growth rates were relatively low compared to the Boyup Brook and Kalgan sites (Figures 1, 3, 5).

Grazing did not cause a significant reduction in RLEM numbers as the population, unlike at the other sites, had declined in the ungrazed plot due to the dry conditions. By mid-October, RLEM numbers had crashed (Figure 6). However, by the end of May 2020, the ungrazed plot had 40% more mites than grazed plots. By mid-June RLEM numbers across all treatments had dropped below 500 mites per square metre.

Figure 5. Feed on offer (t DM/ha) of clover, ryegrass-based pasture at Cranbrook in 2019 that was either left ungrazed or grazed for two or four weeks

Figure 6. Number of RLEM in pasture at Cranbrook that was either left ungrazed or grazed for two or four weeks ± standard error of mean (SEM). Arrows indicate when livestock were removed, colour indicates treatment.

Conclusion

Intensive grazing in spring appears promising as a tactic for suppressing RLEM for the following season. However, it should only be used when the population of mites is above 5000 per square metre and FOO is more than 3t DM/ha. To achieve a significant reduction in mites the following season pasture needs to be grazed to a FOO of 2t DM/ha or less for four weeks around the Timerite® period. The aim is to have fewer than 1000 mites per square metre in the following season which is below the damage threshold for cereals. Control measures for canola, such as seed dressings, may still be required to protect the crop from RLEM damage.

Acknowledgments

This research has been made possible by funding from the Small Farms, Small grants, Department of Agriculture and Water Resources. The authors wish to thank Fitzgerald Biosphere Group, Southern Dirt, Gillami and the farmers on whose properties these demonstrations were conducted.

References

Campbell NA, Arnold GW 1973 The visual assessment of pasture yield. Australian Journal of Experimental Agriculture and Animal Husbandry13, 263-267.

Contact details

Paul Sanford
Department of Primary Industries and Regional Development
444 Albany Hwy, Albany, WA 6330
Phone: 08 9892 8475
Email: paul.sanford@dpird.wa.gov.au