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1. Uptake of environmental contaminants by lower trophic groups can have negative effects on higher trophic groups. This study tested the ability of selenium, an environmental contaminant found in high concentrations throughout the tissues of certain accumulating plants, to be transferred to ants via aphid tissue and honeydew. 2. Plants of the selenium accumulator, Raphanus sativus (wild radish), were watered with three different selenium treatments (0, 0.25 or 0.5 µg Se ml−1). Aphids, Myzus persicae, and Argentine ant colonies, Linepithema humile, were added to each caged plant and allowed to interact freely. Ant colonies were supplemented with one of three different food options to encourage the consumption of aphids, aphid honeydew, or aphids and honeydew. 3. The accumulation of selenium by each trophic group and a trophic transfer factor (TTF) was calculated. The TTF for plants to aphids was > 1, indicating biomagnification, whereas the TTF for aphids to worker ants was

1. Although plant invasions often reduce insect abundance and diversity, non-native plants that support phytophagous insects can subsidise higher trophic levels via elevated herbivore abundance. 2. Here ant–aphid interactions on non-native fennel on Santa Cruz Island, California are examined. Fennel hosts abundant, honeydew-producing fennel aphids. The patchiness of fennel and the relative lack of honeydew-producing insects on other plants at our study sites suggest that assimilation of fennel-derived honeydew would increase the abundance and decrease the trophic position of the omnivorous, aphid-tending Argentine ant. 3. To assess the strength of the ant–aphid interaction, a comparison of ant abundance on and adjacent to fennel prior to and 3 weeks after experimental aphid removal was performed. Compared with control plants with aphids, ants declined in abundance on and around fennel plants following aphid removal. At the habitat scale, pitfall traps in fennel-dominated habitats captured more ants than in fennel-free scrub habitats. 4. To determine if assimilation of aphid-produced honeydew reduces the ant's trophic position, variation in δ15N values among ants, plants and other arthropods was analysed. Unexpectedly, δ15N values for ants in fennel-dominated habitats were higher than those of arthropod predators from the same sites and also higher than those of ants from fennel-free habitats. 5. Our results illustrate how introduced plants that support phytophagous insects appear to transfer energy to higher trophic levels via elevated herbivore abundance. Although assimilation of fennel-derived honeydew did not appear to reduce consumer trophic position, spatial variation in alternative food resources might obscure contributions from honeydew.

1. 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 pairwisemutualisms. This study investigates how multi-species mutualisms between ants and honeydew-producing insects structure arthropod food webs in citrus.2. 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.3. Mutualism between ants and honeydew-producing hemipterans indirectly andpositively affected red scale. Levels of parasitism decreased with increasing antrecruitment, 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 redscale density in different analyses; this finding suggests that individual hemipteran taxa exhibit distinct and spatially localised effects on ants.4. 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.

Author(s): Yoo, Ho Jung S; Holway, David A | Abstract: 1. In ant–hemipteran mutualisms, ants receive carbohydrates in the form of honeydew, while hemipterans receive protection from natural enemies. In the absence of natural enemies, however, the direct effects of tending are generally less well known. We hypothesised that with increasing tending intensity (ant to aphid ratio), aphid performance would increase initially, then decrease at high tending levels due to the metabolic cost of producing high quality honeydew.2. We tested our hypothesis in a greenhouse experiment by manipulating Argentine ant (Linepithema humile Mayr) colony size while holding constant the initial size of aphid (Chaitophorus populicola Thomas) aggregations. The two parameters associated with survival, aphid survivorship to maturity and longevity, declined with increasing tending intensity, whereas per capita birth rate and time to first reproduction showed no relationship to attendance. The intrinsic rate of increase declined only at relatively high tending levels, suggesting a nonlinearity in the effect of tending intensity.3. Tending intensity measured in the experiment was similar to that observed in free-living aggregations of C. populicola. Furthermore, the per capita recruitment rate of ants to free-living aphid aggregations was negatively density-dependent, indicating that small aggregations tend to experience the highest levels of tending intensity. This finding suggests that the aphid's intrinsic rate of increase may be positively density-dependent, mediated by the aphid's mutualistic interaction with the ant.4. In the Argentine ant–C. populicola interaction, experimental manipulation of colony size revealed a direct cost of ant attendance that was conditional upon tending intensity. Experiments that manipulate only ant presence or absence may yield an incomplete understanding of the mutualistic interaction if underlying nonlinearities exist.