The impact of different densities of Allegheny mound ant, Formica exsectoides Forel, populations on myrmecophilous (tended) and nonmyrmecophilous (untended) aphid and scale species and their potential predators was examined in jack pine forests of north-central Michigan. A summer-long survey indicated that areas with large populations of F. exsectoides had larger populations of the aphid Cinara banksiana Pepper & Tissot and the soft scale Toumeyella parvicornis (Cockerell), two obligate myrmecophiles, than areas with low or no F. exsectoides populations. Cinara ontarioensis Bradley was also obligately tended by ants but did not show a significant response to F. exsectoides densities, having a patchy distribution. In contrast, in areas without F. exsectoides, these homopterans were replaced by the woolly aphid Schizolachnus piniradiatae (Davidson), a species that never forms ant associations. Mound ant exclusion experiments and observations indicated that F. exsectoides preyed upon S. piniradiatae, potentially reducing their numbers in areas with this ant. Aphid and scale predators showed a mixed response to F. exsectoides presence: densities of lacewing larvae and salticid spiders were inversely related to F. exsectoides populations, whereas mirid populations did not show a response to different F. exsectoides densities. This study suggests that the presence of large populations of aggressive, honeydew seeking F. exsectoides can shift the homopteran community from one composed predominantly of nonmyrmecophilous species to one composed of myrmecophilous species.
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Vol. 94 • No. 1