Fitness costs associated with milbemectin resistance in the two-spotted spider mite Tetranychus urticae

Abstract

The two-spotted spider mite Tetranychus urticae Koch is one of the most important pests of a wide range of crops worldwide. Its control is still largely based on the use of acaricides. However, due to its short life-cycle, high fecundity and arrhenotokous reproduction, it is able to develop resistance to these compounds very rapidly. Preliminary studies for milbemectin resistance in T. urticae showed that, under laboratory conditions, such resistance was unstable in the absence of a selection pressure. The aim of this study was to evaluate the possible fitness costs associated with milbemectin resistance in T. urticae. Comparison of biological traits between resistant and susceptible strains indicated the occurrence of fitness costs associated with milbemectin resistance. The resistant strain showed a longer juvenile development period for females, lower fecundity and a higher proportion of males among the progeny. The figures for net reproductive rate (R ₀), intrinsic rate of natural increase (r _m) and finite rate of increase (λ) were higher in the susceptible strain. The instability of milbemectin resistance can be exploited in resistance management programmes.

Keywords:

• two-spotted spider mite
• resistance management
• instability of resistance
• reproductive rate
• sex ratio

Acknowledgements

The authors would like to thank FAPESP (São Paulo Research Foundation) for the funding received for this research (Processes # 2007/08612-4), the first author's scholarship (Process # 2007/02748-1) and the third author's post-doctoral fellowship (Process # 2009/08789-7). Our thanks also go to CNPq-Brazil (National Council for Scientific and Technological Development – Brazil) for funding Mario E. Sato's research fellowship.