COSTS AND BENEFITS OF TERRITORIALITY - BEHAVIORAL AND REPRODUCTIVE RELEASE BY COMPETING APHIDS

Colonizing stem mothers of the gall-forming aphid Pemphigus betae defend gall sites on narrowleaf cottonwood, Populus angustifolia. Kicking-shoving contests are used to establish microterritories 3-5 mm in length. The largest stem mothers usually win in competitive bouts. Small differences in body size (i.e., a 3.4% difference in prothorax widths) are more importantly associated with territorial interactions than with fecundity. Stem mothers that win the superior basal position of a leaf produce an average of 56% more progeny than stem mothers displaced to inferior distal positions. Differences in reproductive performance within a leaf result from microhabitat variation in leaf quality. A change of only a few millimetres in the position of the gall on the leaf blade affects aphid reproduction. Experimental removal of either member of a competing pair allows the remaining stem mother to cross the former territorial boundary and enlarge her own territory. The impact of competitive release on reproductive success is asymmetrical. The distal stem mother of a competing pair achieves an average 48.5% increase in number of progeny, whereas the basal stem mother achieves an 18.5% increase in number of progeny. Because both members of a competing pair suffer reduced success from competitive interactions, selection favors stem mothers that occupy leaves singly. The largest stem mothers are solitary; their despotic behavior prevents smaller competitors from settling.