Male demographic constraints to extreme sex ratio in the twospotted spider mite.

Twospotted spider mites, Tetranychus urticae Koch are arrhenotokous. As a result of this genetic structure, primary and secondary sex ratios diverge from the 1:1 female:male ratio commonly found in diploid systems. Ratios vary, but 3:1 is the most common. The influence of life history parameters on spider mite sex ratio is unclear, although maternal genetic effects, resource quality and maternal age are known to play a role. An area that has not been studied is the relationship between male reproductive capabilities and spider mite sex ratio. A prior study on male reproduction in spider mites suggests that males have far too high a reproductive capacity to explain sex ratio patterns, but that study was not conducted under realistic mating conditions. Thus, this study was conducted to determine if there is a link between male reproduction and spider mite sex ratio. This was done by exposing males to various exposure regimes of females and recording the number of copulations, inseminations and daughters fathered by each male. Results include the following: i) males are most virile when one day old; ii) virgin males become nearly completely devoid of sperm (or other copulatory products) after about 15 matings and then take about four days to recover; and iii) the optimal number of matings per day (defined as that which produces the greatest number of daughters in the least number of inseminations) is four females per day. The principle conclusion is that although males have a high potential reproductive output, insemination quality is only high for the first four or five matings each day. Therefore, to ensure full inseminations of all daughters by sons, females are constrained to producing primary sex ratios of about 5:1 or less.