We investigated the influence of the presence of large males on the mating tactics of small males of the stink bug Megacopta punctatissima (Montandon) (Hemiptera: Plataspidae) by using laboratory experiments. In the presence of large males, the mating acceptance rate of females (the proportion of copulating males to the number of courting males) and the mating success (the proportion of copulating males to the total number of males) of large males were significantly higher than those of small males. Therefore, sexual selection favors larger male body size at mating. Although only 15.4% of small males courted females in the presence of large males, 57.5% of small males courted females in the absence of large males. Consequently, the mating success of small males was conspicuously higher in the absence of large males than in the presence of large males. We suggest that small males adopt an alternative mating strategy in which their courtship behavior is the same as that of large males, but their decision to court females depends on the presence or absence of large rival males.