Abstract:
This study was carried out to assess whether the predacious phytoseiid mite, Euseius
kenyae (Swirski and Ragusa), commonly found in major coffee growing regions in Kenya has
developed resistance to Chlorpyrifos. Mite populations were collected from coffee farms
harbouring E. kenyae and where Chlorpyrifos or other organophosphates were sprayed to
manage the primary coffee insect pests. The mites collected were reared in mass in the
laboratory for bioassays. The findings showed that under coffee agro-ecosystems, levels of
resistance existed among the populations of E. kenyae after their exposure to Chlorpyrifos
or other organophosphates. The population of E. kenyae from a coffee farm (C44) was most
susceptible to Chlorpyrifos with LC50 = 0.044 that was below the lowest concentration of
0.1875 ml per litre of water which was tested. The E. kenyae from coffee farms (C1, C4, C7,
C37, C25 and C119) had resistance ratios more than ten times that of C44. The coffee farms
(C2, C12, C19, C116, C31, C50 and C72) had populations of E. kenyae susceptible to
Chlorpyrifos at concentration of 0.75 ml per litre of water which is the field recommended
rate for control of insect pests in coffee. The population of E. kenyae from C7 was resistant
to the highest field rate of 200% (1.5 ml per litre of water) with LC50 of 1.716 and resistance
ratio of 39 times. The existence of resistance populations of E. kenyae is an aspect that
needs to be considered in the integrated pest control strategies against coffee insect pests.
