We tested whether ant-provided enemy-free space was a possible mechanism to explain the distribution of myrmecophilous aphids and soft scales in jack pine forests of north-central Michigan. Populations of the aphid Cinara banksiana Pepper and Tissot decreased significantly when the Allegheny mound ant, Formica exsectoides Forel, was excluded from their colonies. A combination of inclusion/exclusion experiments indicated that mound ants provided enemy-free space for aphids against mirid predators. In the absence of ants, mirids were a significant source of mortality to aphids. When ants were present, significantly more aphids survived in the presence of mirids than when ants were not present. Additional direct benefits to aphids provided by ant tending were not detected. In contrast, the soft scale, Toumeyella parvicornis (Cockerell), did not gain enemy-free space from the lady-beetle Hyperaspis binotata (Say) in the presence of ants during this study. Lady-beetle larvae increased in number over 2 yr, and consumed large numbers of scales, which resulted in similar decreases of scale populations between ant-absent and ant-present treatments. This study indicates that an ant species can show differential effectiveness in providing enemy-free space to myrmecophilous homopterans.
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