Loss of sex-allocation plasticity in the predatory mite Metaseiulus occidentalis and possible triggering cues

In two recent studies, we showed that females from a long-established Metaseiulus (Typhlodromus or Galendromus) occidentalis laboratory colony reared under crowded conditions for approximately 27 years produced equal numbers of male and female progeny. This contrasts with our previous results that showed the females from the same colony produced progeny with a female-biased sex ratio. These discrepant findings raised the possibility that dense rearing conditions can affect the sex-allocation plasticity in this important biological control agent. In the current study, we show that females from a newly collected M. occidentalis colony initially were capable of producing progeny with a female-biased sex ratio, but lost this ability over a 12-month period (43 generations) when reared under dense rearing conditions and provided with damaged prey. We demonstrate that females from this newly collected colony initially could adjust offspring sex ratio in response to both conspecific cues and cues produced by their damaged prey. The results suggest that dense rearing conditions, which are often used in commercial rearing facilities, may have detrimental effects on the reproductive fitness of M. occidentalis and, possibly, other predatory mites.