How does resistance occur?
Repeated use of the same insecticide groups, within seasons and between seasons, selects for redlegged earth mites (RLEM) to develop resistance.
The repeated cumulative exposure of RLEM to the same chemical group is the main factor behind resistance developing. For instance, even if a SP insecticide (Group 3A) is used against pests such as weevils or aphids, RLEM also receive a dose of the chemical, even though they are not the direct target.
Chemical control options
Farmers with SP resistant RLEM are in some cases able to control these mites by using insecticides from the organophosphate (OP) group (Group 1B) for example, dimethoate or omethoate.
However, RLEM that are highly tolerant to the OP's have been found in multiple paddocks. At some localities mites were susceptible to SP's, whereas at other sites mites, were resistant to SP's and OP's.
The development of dual resistance means insecticides cannot be relied upon for long-term RLEM control.
How long does resistance last?
SP resistance in RLEM is heritable and mechanisms to switch it off have not been found. RLEM from one site have been tested first found to be resistant in 2006 were re-tested in 2015 and were found to still be resistant to SP. This indicates that resistance to SP, once established, is likely to persist in RLEM populations over many years.
OP resistance has been found to be heritable and it is unknown how long it will persist for.
We need to prevent further development of resistance by decreasing overall use of insecticides.
Spread of resistance
Locations of resistance within southern WA are geographically quite distinct, suggesting that the resistance develops in isolated RLEM populations within each property. For instance, SP resistant RLEM have been found on properties from Esperance to Dandaragan, making it unlikely that resistant RLEM have spread between locations. However, resistant RLEM can move into adjacent paddocks from weeds on fence lines.
Managing resistance
Identify your mites
RLEM are often found with other mites, such as blue oat mite, bryobia (clover) mite or balaustium mite, but resistance has only been found in RLEM. In situations where spray failures have occurred, it is important to correctly identify the mite. Blue oat mites are controlled by all chemicals registered for RLEM control, whereas chemical controls for bryobia mite and balaustium mites differ.
Plan ahead to reduce mite numbers
If you prepare paddocks in the preceding season, there will be lower numbers of pests on your crops. Consider the following to reduce pest numbers:
Control weeds in the crop and along fence lines
Weeds provide habitat for mites. Controlling weeds with herbicides, cultivation or heavy grazing will decrease mite numbers. A weed free crop will have few mites and over-summering eggs to carry through to the following season.
Controlled grazing
Controlled grazing of pasture paddocks in the year prior to a cropping year will reduce RLEM numbers to levels similar to chemical sprays. Sustained grazing of pastures throughout spring to maintain feed on offer (FOO) levels below two tonnes per hectare (dry weight) will restrict mite numbers to low levels.
Spring insecticide application
Apply insecticides to some paddocks during spring to prevent RLEM populations producing over-summering eggs will decrease the pest population in the following autumn. Only specific paddocks should be selected for spring spraying based on FOO levels, future grazing management options, seed production requirements and intended paddock use next season. The routine spraying of all pasture paddocks in spring using Timerite® dates to prevent a build-up of mites is not sustainable. Timerite® is a free package that provides a date in spring for a spraying to stop RLEM from producing over-summering eggs. For further information see the Links section.
Cropping rotations
Use cropping rotations to decrease reliance on pesticides. Some paddocks will have a higher or lower risk of RLEM damage depending on previous crop rotations. The risk is generally highest if paddocks have been in long term pasture (with high levels of broad-leafed plants) where mite populations have been uncontrolled. Lower risk paddocks that generally do not require mite control are often those which follow a cereal or canola weed free crop where conditions are less favourable for mite increase.
What you can do this season
Spray only if you need to
Farmers that currently have populations of resistant RLEM have mostly used repeated applications of the same insecticide or insecticide group. To decrease the likelihood of resistance developing on your property apply insecticides only on paddocks that have damaging numbers of pests.
Where spraying is needed, rotate chemical groups
For example, rotate between insecticide groups in and between seasons, as this will help to reduce resistance occurring. If spraying other pests, such as aphids, consider using chemical options such as pirimicarb that only target aphid.
Predict hatchings of RLEM on your property to target your control strategy
Knowing approximately when the first autumn hatchings of RLEM is occurring on your property will help to determine if they will coincide with seedling crops. RLEM hatch in autumn from their over-summering egg stage, after adequate rainfall and at least seven days of average temperatures below 20°C. Crops sown in seasons with 'early breaks' with maximum temperatures well above 20°C (for example, canola sown in April) will not coincide with RLEM.
A new tool that predicts hatching dates for RLEM, based on weather station data, is also available. Refer to the links section.
Use insecticide seed treatments
Use insecticide seed treatments for crops and sown pastures with moderate pest pressure rather than spraying whole paddocks. Seed treatments allow smaller quantities of pesticide to be used that directly target plant feeding pests, allowing any predatory insects to continue their important beneficial role.
Do you suspect you have resistant RLEM?
If you have RLEM that survive registered rates of insecticide treatments or suspect that you have mites resistant to chemicals, please contact the Department of Agriculture and Food, Western Australia’s broadacre entomologists. Arrangements can be made to have mites sampled and tested for their level of resistance.
Acknowledgements
Testing for resistant RLEM has been possible by funding from the Grains Research and Development Corporation.