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 to this chemical group.
The repeated cumulative exposure of RLEM to synthetic pyrethroids (SP) is the main factor behind resistance developing. Even if a SP insecticide 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.
All SP have the same molecular mode of action once RLEM develop resistance to one insecticide then they are resistant to all insecticides in this chemical group (Group 3A).
RLEM that are highly tolerant to the organophosphate (OP) chemical omethoate have also been found. At these properties, omethoate was used regularly for the control of RLEM in pastures in spring. There were a number of mite populations that were also resistant to SP as well but were controlled by other insecticides in the OP group.
Populations of RLEM that are highly tolerant to the OP chemical chlorpyrifos has been found at single locality. Regular applications of chlopryrifos for the control of RLEM has contributed to this tolerance developing. These mites were susceptible to SP and other insecticides in the OP group.
Chemicals in the Group 1B, OP have slightly different molecular modes of action. RLEM are tolerant to one insecticide in this group have been found to be susceptible to other insecticides in the Group 1B.
Chemical control options
Farmers with SP resistant RLEM have been able to control these mites by using insecticides from the OP group (Group 1B), for example, dimethoate or omethoate.
RLEM with omethoate tolerance or chlorpyrifos can not be controlled by using registered rates of these insecticides. As populations have been found with omethoate tolerance and SP resistance is showing that reliance on insecticides for the control of RLEM is starting to break down.
In all cases residual populations of resistant or tolerant RLEM were found on weeds along fencelines and re-infested paddocks.
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.
Tolerance in omethoate and chlorpyrifos 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. 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 fencelines.
Chlorpyrifos tolerant populations have only been found at a single locality at Capel, where as omethoate tolerant populations have been found in the southern agricultural regions.