Protection mutualisms between ants and honeydew-producing insects influence arthropod food webs by altering the behaviour, activity and local abundance of ants on plants. Ants often tend multiple species of honeydew-producing insects; however, studies that examine such effects typically consider only pairwise mutualisms. This study investigates how multi-species mutualisms between ants and honeydew-producing insects structure arthropod food webs in citrus.
In an organic lemon orchard in San Diego County, California, U.S.A., ants or honeydew-producing insects (or neither) were experimentally removed from individual, mature lemon trees and then abundances of the following were estimated over a 2-year period: the Argentine ant, five species of honeydew-producing hemipterans, and California red scale. Red scale produces no honeydew but indirectly benefits from the presence of ants, which disrupt parasitism by Aphytis wasps.
Mutualism between ants and honeydew-producing hemipterans indirectly and positively affected red scale. Levels of parasitism decreased with increasing ant recruitment, and red scale density increased with both increasing ant recruitment and increasing honeydew-producing hemipteran abundance. Moreover, abundances of each of three honeydew-producing hemipteran species emerged as positive predictors of red scale density in different analyses; this finding suggests that individual hemipteran taxa exhibit distinct and spatially localised effects on ants.
Evidence of positive, indirect effects between two focal species of honeydew-producing Hemiptera was also detected. Guilds of honeydew-producing hemipterans may provide ants with honeydew more consistently or for longer than that produced by any single species. These results argue for an appreciation of how mutualist diversity affects the ecological consequences of mutualisms.