The alternation of mutualistic ant species affects the population growth of their trophobiont mealybug

Abstract

Many species of honeydew-producing Homopteran insects have mutualistic relationships with ants which defend them against their natural enemies in return for honeydew. In many cases of this type of mutualism (trophobiosis), a particular homopteran can associate with a range of mutualistic ant species. We studied the influence of alternation of two ant species, Lasius niger and Pristomyrmex pungens, both of which are mutualistic partners of the citrus mealybug Pseudococcus citriculus, on population growth of their trophobiont mealybug. The growth rate of the mealybug population was significantly higher with attendance by either ant species than without it throughout the year. However, in the field, the mealybug population was significantly larger on trees where a L. niger colony made its territory than on trees where a P. pungens colony made its territory. When the attending ant species changed from L. niger to P. pungens, the mealybug population significantly decreased compared with the population that L. niger continued to attend. In contrast, the mealybug population increased following the reverse shift, from P. pungens to L. niger. These results indicate that, for the mealybug, L. niger is a more effective mutualistic partner than P. pungens. This deviation was due to a difference in attack behavior by the two ant species. Behavioral responses by attending ants to artificial models of natural enemies were more aggressive in L. niger than in P. pungens. The territories of the two ant species were completely separated, and territory takeovers of one ant species by the other were occasionally observed. These facts suggest that interspecific competition between the two ant species is an important factor in determining the spatial distribution of the two species. Thus, we conclude that interspecific competition between the two ant species indirectly affects the population growth of their common trophobiont mealybug.