Resistant redlegged earth mite update
Redlegged earth mites (RLEM) are showing increasing resistance to available insecticides.
These insecticides include synthetic pyrethroids (SP) (Group 2A), the organophosphates (Group 1B) omethoate and chlorpyrifos.
SP resistance was first detected in Western Australia in 2006 and there are now more than 65 WA properties with SP resistant RLEM.
SP resistance is heritable and can persist in RLEM populations with resistant populations identified in 2006 in Esperance found to still be resistant 11 years later.
There are currently more than 12 properties with omethoate resistant RLEM, however these mites are still susceptible to SP’s and other insecticides in the OP range.
Omethoate resistance tends to be found in paddocks with a history of pasture manipulation and the use of omethoate as a spring time spray to decrease RLEM numbers for the following year.
Dual resistance to SP’s and the OP omethoate was first detected in the Boyup Brook area in 2014. These mites were 30 times more resistant to omethoate than a susceptible population.
To date just over 10 properties with dual resistance have been found, however, RLEM in these populations have only shown resistance to omethoate and have been controlled by other insecticides in the OP group.
Farmers with SP resistant RLEM have switched to using chlorpyrifos as an alternative to bifenthrin. However, there has been one such property tested that had RLEM with chlorpyrifos resistance.
These RLEM survived field rate applications of chlorpyrifos but were controlled using other insecticides in the OP and SP groups.
Locations of RLEM with resistance within WA are geographically quite distinct, suggesting that the resistance develops in isolated populations within each property.
Resistant RLEM have been found on properties from Esperance to Dandaragan, making it unlikely that resistant RLEM have spread between locations.
All properties with resistant RLEM have one common factor, the regular application of insecticides of the same group.
What to do this season
This year, the dry start to the season has delayed the hatching of RLEM.
Mites hatch when there are at least seven days of daytime temperatures below 20oC.
It is likely that peak mite hatchings will occur when crops are emerging or at the seedling stage.
The only control option will be to apply insecticides if RLEM are causing damage.
Consider using the following tactics to decrease the likelihood of resistance developing:
- If using a pesticide mix, ensure the full rate of each chemical is used and reserve these sprays for situations where RLEM and other pests are present.
- If spraying winter pastures, aim to control the first generation of mites before adults start laying eggs around three weeks after their appearance.
- Do not re-spray a paddock in the same season if there has been a spray failure using the same product or another product from the same insecticide group.
If you have RLEM that survive registered rates of insecticide treatments or suspect that you have mites resistant to chemicals, please contact Svetlana Micic, Research Officer, Albany on 0427 772 051 or Alan Lord, Technical Officer, South Perth on 0409 689 468 and arrangements can be made to have the mites tested for their level of resistance.
This testing is part of a national Grains Research and Development Corporation (GRDC) funded project, led by the Centre for Environmental Stress and Adaptation Research, with collaborators CSIRO, University of Melbourne and Department of Primary Industries and Regional Development (DPIRD).
For more information contact Svetlana Micic, Research Officer, Albany on +61(0)8 9892 8444.