Anthidium manicatum males defend patches of flowering plants as mating territories and copulate with females that enter the territory to obtain nectar or pollen. Territorial males defend their territories not only against conspecific males but also against many other insect species that attempt to utilize the flowers inside the territory. In our study area, a territorial male defended its territory against conspecific males on average only twice per h but it attacked other species of insects on average 70 times per h. Territorial males distinguished between different species of intruders and attacked them unequally. During the hour following experimental removal of the territory holder the number of intruders of some insect species tripled but the number of intruders of those species that were rarely attacked remained approximately the same.

A. manicatum males, but not females, bear long spines on the last two segments of the abdomen. Territorial males rammed intruding insects at high speed and shortly before the moment of impact they curved the abdomen forward to hit with the spines. Intruders could be seriously damaged or killed by these attacks. Abdominal spines of A. manicatum males may have evolved as weapons to increase the effectiveness of interspecific territoriality.