Mate-guarding strategies are known to evolve in response to changes in the environment, but little is known about the genetic and plastic components of this source of variation. Here, we investigated how risk associated with aggression shapes postcopulatory association time between mates in the bulb mite, Rhizoglyphus robini, a species in which aggressive, armoured fighters often coexist with unarmoured scramblers. In some populations, scramblers have been reported to prevent females remating by remaining in copula for over 6 h. In this study, we investigated whether mate guarding by scramblers is affected by the presence of aggressive fighters in populations. We investigated whether guarding is riskier in the presence of fighters and found that guarding males were more likely to be attacked. Our data allowed us to determine whether the presence of fighters can affect mate-guarding duration, by comparing guarding duration between populations (both natural and artificially selected). We found that in both types of population, males guarded longer when fighters were absent. Comparisons between lines selected for the presence of fighters, lines selected for the presence of scramblers and their source populations indicated that scrambler morphs evolved prolonged guarding. We also investigated whether males show plasticity and shorten guarding in response to the presence of fighters in a social group. Surprisingly, we found that males in a mixed-morph context copulated significantly longer than males from single-morph groups. Our results demonstrate that mate guarding may evolve in response to the presence or absence of fighters in populations, but males are not able to adjust guarding behaviour to the risk of being attacked by fighters. The study provides insight into the role of genetics and plasticity in guarding strategies.