Costs and constraints in aphid-ant mutualism

While many studies have demonstrated that ants provide beneficial services to aphids, Bristow (Ant-plant interactions, Oxford University Press, Oxford, 104–119, 1991) first questioned why so few aphid species are ant-attended. Phylogenetic trees have demonstrated multiple gains and loss of ant-attendance in the course of aphid-ant interactions, implying that mutualisms easily form and dissolve. Several studies have reported the factors that influence the formation and maintenance of aphid-ant interactions. Examples include the physiological costs of ant attendance, competition for mutualistic ants, ant predation on aphids, the influence of host plants, and parasitoid wasps. Recent physiological techniques have also revealed the chemical component of aphid-ant mutualisms. The honeydew of ant-attended aphids contains melezitose (a trisaccharide), which has an important role in aphid-ant interactions. Studies of cuticular hydrocarbons on aphids and ants have clarified the underlying mechanisms of ant predation on aphids. Attending ants also reduce aphid dispersal ability, causing the formation of fragmented aphid populations with low genetic diversity in each population. The reduced aphid dispersal could be partly explained by higher wing loading and reduction of flight apparatus due to ant attendance. Whether ant attendance is associated with the range of host plants of aphids or genetic variation in microorganism in aphids remain to be explored.     

Ant attendance reduces flight muscle and wing size in the aphid Tuberculatus quercicola

In otherwise mutualistic relationships between aphids and ants, attendance by ants often has negative impacts on aphids. For example, in a previous study using traps in the field, the aphid Tuberculatus quercicola, which exhibits mutualistic interactions with ants, showed extremely low dispersal rates, despite having long wings. This study investigates whether components of the flight apparatus (mesonotum length, flight muscle and wings) differ between aphids attended by ants and not attended by ants. Randomized block analysis of variance, using body length as a covariate, showed that ant attendance has a negative influence on aphid flight apparatus. This result indicates that aphids produce honeydew at the expense of resource investment in flight apparatus. Since the dispersal of T. quercicola is limited under ant attendance, the reduction in flight apparatus could precede a decrease in body size. This study also showed that flight apparatus was more developed in aphids under ant-exclusion conditions. This may imply that T. quercicola fly when ants are not available. The maintenance of flight apparatus in T. quercicola might therefore be partly explained by gene flow on the rare occasions that this aphid species disperses.     

Ant–aphid mutualism: the influence of ants on the aphid summer cycle

There are few longtime studies on the effects on aphids of being tended by ants. The aim of this study is to investigate how the presence of ants influences settling decisions by colonizing aphids and the post‐settlement growth and survival of aphid colonies. We conducted a field experiment using the facultative myrmecophile Aphis fabae and the ant Lasius niger. The experiment relied on natural aphid colonization of potted plants of scentless mayweed Tripleurospermum perforatum placed outdoors. Ants occurred naturally at the field site and had access to half of the pots and were prevented from accessing the remainder. The presence of winged, dispersing aphids, the growth and survival of establishing aphid colonies, and the presence of parasitoids were measured in relation to presence or absence of ants, over a period of five weeks. The presence of ants did not significantly influence the pattern of initial host plant colonization or the initial colony growth, but ant‐tended aphids were subject to higher parasitism by hymenopteran parasitoids. The net result over the experimental period was that the presence of ants decreased aphid colony productivity, measured as the number of winged summer migrants produced from the colonized host plants. This implies that aphids do not always benefit from the presence of ants, but under some conditions rather pay a cost in the form of reduced dispersal.     

The effect size of aphid‐tending ants in an agricultural tri‐trophic system

Most studies regarding ant–aphid interactions focus only on the direct effects of ants on tended aphids and aphidophagous predators, or the indirect effects on the host plant. Studies evaluating the effects of aphid‐tending ants on more than one trophic level are rare and evaluate only the presence or absence of such effects. Here we assessed the effect sizes of ants in a tri‐trophic system (common bean plants, aphids and lacewing larvae). We tested if the presence of aphid‐tending ants has positive effects on aphid abundance and host‐plant production and negative effects on aphid predator abundance. We also hypothesized that aphid‐tending ants affect more intensely trophic levels that are more directly related to them (i.e., first aphids, then aphid predators and then host plants). We tested these hypotheses in field mesocosms experiments using the presence and absence of ants. We found that aphid‐tending ants have great positive effects on final aphid abundance. Ants also positively affected the number of seeds; however, it was not possible to measure the effect size for this trophic level. Furthermore, ants had negative effects on lacewing larvae only at first release. The effect size of ants was greater for aphids, followed by lacewing larvae, and with no effects on the number of seeds produced. Ants positively affect aphids and host‐plant production, probably by way of honeydew collection preventing the development of entomophagous/saprophytic fungi. On the other hand, ants negatively affect lacewing larvae by excluding them from the host plant. In natural systems, several ant species may attend aphids, differently affecting the organisms of the various trophic levels within the ant–aphid interaction, thereby obscuring the real effect size of ants. Assessing the effect size of aphid‐tending ants on the organisms involved in ant–aphid interactions provides more realistic information about the effects of this interaction on natural systems.     

Alate production in an aphid in relation to ant tending and alarm pheromone

1. Winged dispersal is vital for aphids as predation pressure and host plant conditions fluctuate. 2. Ant‐tended aphids also need to disperse, but this may represent a cost for the ants, resulting in an evolutionary conflict of interest over aphid dispersal. 3. The combined effects of aphid alarm pheromone, indicating predation risk, and ant attendance on the production of winged aphids were examined in an experiment with Aphis fabae (Homoptera: Aphididae) (Scopoli 1763) aphids and Lasius niger (Formicidae: Formicinae) (Linné, 1758) ants. 4. This study is the first to investigate the joint effects of alarm pheromone and ant attendance, and also the first to detect an influence of alarm pheromone on the production of winged morphs in A. fabae. 5. After a period of 2 weeks, it was found that aphid colonies exposed to intermittent doses of alarm pheromone produced more winged individuals, whereas ant tending had the opposite effect. The effects were additive on a log scale, and ant attendance had a greater proportional influence than exposure to alarm pheromone. A tentative conclusion is that ants have gained the upper hand in an evolutionary conflict about aphid dispersal.     

A facultative mutualism between aphids and an invasive ant increases plant reproduction

1. The consequences to plants of ant–aphid mutualisms, particularly those involving invasive ants, are poorly studied. Ant–aphid mutualisms may increase or decrease plant fitness depending on the relative cost of herbivory by ant‐tended aphids versus the relative benefit of increased ant suppression of other (non‐aphid) herbivores. 2. We conducted field and greenhouse experiments in which we manipulated the presence and absence of cotton aphids (Aphis gossypii) on cotton plants to test the hypothesis that a mutualism between cotton aphids and an invasive ant, the red imported fire ant (Solenopsis invicta), benefits cotton plants by increasing fire ant suppression of caterpillars. We also manipulated caterpillar abundance to test whether the benefit of the mutualism varied with caterpillar density. 3. We found that more fire ants foraged on plants with cotton aphids than on plants without cotton aphids, which resulted in a significant reduction in caterpillar survival and caterpillar herbivory of leaves, flower buds, and bolls on plants with aphids. Consequently, cotton aphids indirectly increased cotton reproduction: plants with cotton aphids produced 16% more bolls, 25% more seeds, and 10% greater seedcotton mass than plants without aphids. The indirect benefit of cotton aphids, however, varied with caterpillar density: the number of bolls per plant at harvest was 32% greater on plants with aphids than on plants without aphids at high caterpillar density, versus just 3% greater at low caterpillar density. 4. Our results highlight the potential benefit to plants that host ant–hemipteran mutualisms and provide the first experimental evidence that the consequences to plants of an ant–aphid mutualism vary at different densities of non‐aphid herbivores.     

Ants indirectly reduce the reproductive performance of a leafless shrub by benefiting aphids through predator deterrence

Ant–aphid mutualisms can generate cascade effects on the host plants, but these impacts depend on the ecological context. We studied the consequences of ant–aphid interactions on the reproductive performance of a Mediterranean leafless shrub (Retama sphaerocarpa), through direct and indirect effects on the arthropod community. By manipulating the presence of ants and aphids in the field, we found that ants increased aphid abundance and their persistence on the plant and reduced aphid predators by nearly half. However, the presence of ants did not affect the abundance of other plant herbivores, which were relatively scarce in the studied plants. Aphids, and particularly those tended by ants, had a negative impact on the plant reproductive performance by significantly reducing the number of fruits produced. However, fruit and seed traits were not changed by the presence of aphids or those tended by ants. We show that ants favoured aphids by protecting them from their natural enemies but did not indirectly benefit plants through herbivory suppression, resulting in a net negative impact on the plant reproductive performance. Our study suggests that the benefits obtained by plants from hosting ant–aphid mutualisms are dependent on the arthropod community and plant traits.

     

Why do ants shift their foraging from extrafloral nectar to aphid honeydew?

When aphids parasitize plants with extrafloral nectaries (EFNs) and aphid colony size is small, ants frequently use EFNs but hardly tend aphids. However, as the aphid colony size increases, ants stop using EFNs and strengthen their associations with aphids. Although the shift in ant behavior is important for determining the dynamics of the ant–plant–aphid interaction, it is not known why this shift occurs. Here, we test two hypotheses to explain the mechanism responsible for this behavioral shift: (1) Extrafloral nectar secretion changes in response to aphid herbivory, or (2) plants do not change extrafloral nectar secretion, but the total reward to ants from aphids will exceed that from EFNs above a certain aphid colony size. To judge which mechanism is plausible, we investigated secretion patterns of extrafloral nectar produced by plants with and without aphids, compared the amount of sugar supplied by EFNs and aphids, and examined whether extrafloral nectar or honeydew was more attractive to ants. Our results show that there was no inducible extrafloral secretion in response to aphid herbivory, but the sugar concentration in extrafloral nectar was higher than in honeydew, and more ant workers were attracted to an artificial extrafloral nectar solution than to an artificial aphid honeydew solution. These results indicate that extrafloral nectar is a more attractive reward than aphid honeydew per unit volume. However, even an aphid colony containing only two individuals can supply a greater reward to ants than EFNs. This suggests that the ant behavioral shift may be explained by the second hypothesis.     

Do aphids actively search for ant partners?

The aphid–ant mutualistic relationships are not necessarily obligate for neither partners but evidence is that such interactions provide them strong advantages in terms of global fitness. While it is largely assumed that ants actively search for their mutualistic partners namely using volatile cues; whether winged aphids (i.e., aphids’ most mobile form) are able to select ant‐frequented areas had not been investigated so far. Ant‐frequented sites would indeed offer several advantages for these aphids including a lower predation pressure through ant presence and enhanced chances of establishing mutuaslistic interactions with neighbor ant colonies. In the field, aphid colonies are often observed in higher densities around ant nests, which is probably linked to a better survival ensured by ants’ services. Nevertheless, this could also result from a preferential establishment of winged aphids in ant‐frequented areas. We tested this last hypothesis through different ethological assays and show that the facultative myrmecophilous black bean aphid, Aphis fabae L., does not orientate its search for a host plant preferentially toward ant‐frequented plants. However, our results suggest that ants reduce the number of winged aphids leaving the newly colonized plant. Thus, ants involved in facultative myrmecophilous interactions with aphids appear to contribute to structure aphid populations in the field by ensuring a better establishment and survival of newly established colonies rather than by inducing a deliberate plant selection by aphid partners based on the proximity of ant colonies.     

Ant attendance of the cotton aphid is beneficial for okra plants: deciphering multitrophic interactions

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Myrmecophilous aphids produce cuticular hydrocarbons that resemble those of their tending ants

Aphid-tending ants protect aphids from natural enemies and collect honeydew secreted by the aphids. However, ants also often prey on the aphids they attend. Aphids, therefore, like social parasites of ants, may well have evolved chemical mimicry as an anti-predation strategy. In this study, we aimed to determine whether the aphid Stomaphis yanonis actively produces cuticular hydrocarbons (CHCs) that resemble those of the tending ant Lasius fuji. In the wild, ants put their CHCs on the aphids that they are tending, so in this study we analyzed “ant-free” aphids. Mature aphids that exuviated in the absence of ant attendance had almost all of the hydrocarbon components that the ants’ CHCs had. Moreover, hydrocarbons artificially applied to the aphids’ body surface were lost by exuviation. Taken together, these findings indicate that mature aphids actively produced ant-like CHCs, and they constitute the first documentation of a chemical resemblance between aphids and ants in a specific aphid–ant association.     

Role of ants in structuring the aphid community on apple

1. The aphids Dysaphis plantaginea Passerini, Aphis spp. (Aphis pomi De Geer and Aphis spiraecola Patch), and Eriosoma lanigerum Hausmann are commonly found together in apple orchards. Ants establish a mutualistic relationship with the myrmecophilous aphids D. plantaginea and Aphis spp. but not with E. lanigerum. 2. Field surveys and one experiment manipulating the presence of ants and the aphid species were conducted to test the hypothesis that ants play a role in structuring the community of these aphids on apple. 3. Ants tended D. plantaginea and Aphis spp. but not E. lanigerum colonies. In the field, D. plantaginea performed better in the presence of ants while no effect was observed in Aphis spp. Contrarily, populations of Aphis spp. in the manipulative experiment performed better in the presence of ants while no differences were observed for D. plantaginea. Such differences between field and manipulative conditions could be related to thermal tolerance, phenology, and life cycles. In contrast, populations of E. lanigerum were reduced in the presence of ants. 4. Ants also had a significant negative effect on the abundance of natural enemies, which could partially explain the benefits to the tended aphids. However, while ants did not provide a benefit to Aphis spp. when it was reared alone, in the presence of other species ant attendance increased Aphis abundance by 256% and simultaneously reduced E. lanigerum abundance by 63%. Therefore, ants benefited Aphis by reducing competition with other aphid species, which involves a different mechanism, explaining the benefit of ant attendance. Considering all the aphid species together, ants had a net positive effect on aphid abundance, which was consequently considered harmful for the plant. 5. Our results highlighted the role that ants play in structuring apple aphid communities and give support to the observed pattern that ants can benefit tended aphids while simultaneously reducing the abundance of untended herbivores.     

The aphid-tending ant Lasius fuji exhibits reduced aggression toward aphids marked with ant cuticular hydrocarbons

Some aphid species are attended by ants, which protect aphids against enemies, but ants sometimes prey on the aphids they are attending depending on the resource conditions. A previous study indicated that the ant Lasius niger preys less on the aphid individuals that experienced ant attendance than on those that did not. This observation leads to the hypothesis that ants transfer some substances to the aphids they attend and selectively prey on the aphids without the substances. In this study, we focus on cuticular hydrocarbons (CHCs), which are used by ants as nestmate recognition substances, and test whether ants discriminate the aphids on the basis of CHCs. We confirmed that the ant Lasius fuji preyed less on the aphids that were attended by their nestmates than those that were not attended. Glass dummies treated with CHCs from attended aphids were attacked less by ants than those treated with CHCs from non-attended aphids. The CHC profiles of ant attended aphids resembled those of the ants, suggesting that ants’ CHCs are transferred to the aphids’ body surface through ant attendance. These results support the hypothesis that ants “mark” their attended aphids with their CHCs and the CHCs reduce ant predation intensity.     

Avoidance responses of an aphidophagous ladybird, Adalia bipunctata, to aphid-tending ants

Abstract.  1. Insect predators often aggregrate to patches of high prey density and use prey chemicals as cues for oviposition. If prey have mutualistic guardians such as ants, however, then these patches may be less suitable for predators.
2. Ants often tend aphids and defend them against predators such as ladybirds. Here, we show that ants can reduce ladybird performance by destroying eggs and physically attacking larvae and adults.
3. Unless ladybirds are able to defend against ant attacks they are likely to have adaptations to avoid ants. We show that Adalia bipunctata ladybirds not only move away from patches with Lasius niger ants, but also avoid laying eggs in these patches. Furthermore, ladybirds not only respond to ant presence, but also detect ant semiochemicals and alter oviposition strategy accordingly.
4. Ant semiochemicals may signal the extent of ant territories allowing aphid predators to effectively navigate a mosaic landscape of sub-optimal patches in search of less well-defended prey. Such avoidance probably benefits both ants and ladybirds, and the semiochemicals could be regarded as a means of cooperative communication between enemies.
5. Overall, ladybirds respond to a wide range of positive and negative oviposition cues that may trade-off with each other and internal motivation to determine the overall oviposition strategy.     

Costs and benefits of ant attendance to the drepanosiphid aphid Tuberculatus quercicola

The defensive effects of ants against aphid predators have been well documented in the mutualistic relationship of aphids and their attending ants. However, it is not clear whether ant attendance has any direct effect on the aphids' growth and reproduction. Through field experiments, this study evaluates the benefits and, in particular, the costs of ant attendance to aphid colonies, focusing on the drepanosiphid aphid Tuberculatus quercicola which is associated with the Daimyo oak, Quercus dentata , and which is always attended by the red wood ant Formica yessensis . Ant attendance was clearly beneficial to the aphid; the exclusion of ants led to a significant increase in the extinction rate of aphid colonies. However, MANOVA and randomized block ANOVA indicated that in colonies continuously attended by ants, aphids had significantly smaller body size and produced a smaller number of embryos than in colonies isolated from ants when they were reared under homogeneous host conditions free from natural enemies. Thus, ant attendance had a negative influence on the growth and reproduction of the aphids, even though it contributed to the greater longevity of the aphid colonies. We hypothesize that ant-attended aphids are under intense selective pressures that act against aphid clones which fail to attract many ants, so that aphids have developed an adaptive mechanism to allocate a larger fraction of resources to the honeydew when they are requested to do so by the ants in order to ensure the ants' consistent visitation.     

Elevated temperatures alter an ant‐aphid mutualism

Ant‐hemipteran mutualisms are keystone interactions that can be variously affected by warming: these mutualisms can be strengthened or weakened, or the species can transition to new mutualist partners. We examined the effects of elevated temperatures on an ant‐aphid mutualism in the subalpine zone of the Rocky Mountains in Colorado, USA. In this system, inflorescences of the host plant, Ligusticum porteri Coult. & Rose (Apiaceae), are colonized by the ant‐tended aphid Aphis asclepiadis Fitch or less frequently by the non‐ant tended aphid Cavariella aegopodii (Scopoli) (both Hemiptera: Aphididae). Using an 8‐year observational study, we tested for two key mechanisms by which ant‐hemipteran mutualisms may be altered by climate change: shifts in species identity and phenological mismatch. Whereas the aphid species colonizing the host plant is not changing in response to year‐to‐year variation in temperature, we found evidence that a phenological mismatch between ants and aphids could occur. In warmer years, colonization of host plant inflorescences by ants is decreased, whereas for A. asclepiadis aphids, host plant colonization is mostly responsive to date of snowmelt. We also experimentally established A. asclepiadis colonies on replicate host plants at ambient and elevated temperatures. Ant abundance did not differ between aphid colonies at ambient vs. elevated temperatures, but ants were less likely to engage in tending behaviors on aphid colonies at elevated temperatures. Sugar composition of aphid honeydew was also altered by experimental warming. Despite reduced tending by ants, aphid colonies at elevated temperatures had fewer intraguild predators. Altogether, our results suggest that higher temperatures may disrupt this ant‐aphid mutualism through both phenological mismatch and by altering benefits exchanged in the interaction.     

Colony growth and dispersal in the ant-tended aphid, <Emphasis Type="Italic">Aphis craccivora</Emphasis> Koch,and the non-ant-tended aphid, <Emphasis Type="Italic">Acyrthosiphon pisum</Emphasis> Harris,under the absence of predators and ants

To elucidate the potential for colony growth and the dispersal of aphids in relation to the ant attendance, the mobility, tolerance of starvation, colony growth, and dispersal were examined in the ant-tended Aphis craccivora Koch and the non-ant-tended Acyrthosiphon pisum Harris under the absence of predators and ants. The increase of the dispersal rate with density was more conspicuous in Ac. pisum than Ap. craccivora. The success rate of dispersal was higher in Ac. pisum than Ap. craccivora. These results would be derived from the higher ability of dispersal by walking in Ac. pisum than Ap. craccivora. The longer legs, higher walking speed, and stronger tolerance of starvation in Ac. pisum might result in a higher ability of dispersal by walking. These traits may have developed in relation to non-ant attendance, because Ac. pisum aphids frequently escape from natural enemies by dropping from the host plant. On the other hand, Ap. craccivora have not developed morphological and behavioral traits concerning dispersal by walking, because increasing colony size without dispersal is likely to be advantageous for ant attraction. Escaping behaviors of Ap. craccivora from natural enemies have not been well developed due to the ant’s defense against natural enemies. The proportion of alatae in Ap. craccivora was higher than in Ac. pisum instead of a lesser ability for dispersal by walking in Ap. craccivora. The differences in ant attendance have been influenced in the development of morphological and behavioral traits concerning the dispersal ability, and then, different potentials for reproduction and dispersal have developed in ant-tended aphids and non-ant-tended aphids.     

The influence of Lasius neoniger (Hymenoptera: Formicidae) on population growth and biomass of Aphis glycines (Hemiptera: Aphididae) in soybeans

In the United States, the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), are often tended by the aphid-tending ant, Lasius neoniger Emery (Hymenoptera: Formicidae). In this study, we examined the effects of tending by ants on the density and biomass of soybean aphids on soybeans in Kentucky. We performed cage studies that limited access by ants and/or natural enemies. We used a split-plot design with natural enemy access as the main plot and ant attendance as the sub plot. We found that natural enemy access negatively affected aphid population density in the presence of tending ants, seen as a three- to four-fold increase in aphid density when natural enemies were excluded. In addition, we found that ant tending positively affected aphid biomass, both when natural enemies were given access to aphids or when natural enemies were excluded, seen by a two-fold increase in aphid biomass when ants tended aphids, both in the presence or absence of natural enemies. Biomass accumulation is seen as an important measurement for assessing aphid performance, and we argue that aphid-tending by ants can have an influence on natural field populations of soybean aphids. Agronomic practices that affect ant abundance in soybeans may influence the performance and hence pest outbreaks for this economically important pest.     

Ants farm subterranean aphids mostly in single clone groups - an example of prudent husbandry for carbohydrates and proteins?

ABSTRACT: BACKGROUND: Mutualistic interactions are wide-spread but the mechanisms underlying their evolutionary stability and ecological dynamics remain poorly understood. Cultivation mutualisms in which hosts consume symbionts occur in phylogenetically diverse groups, but often have symbiont monocultures for each host. This is consistent with the prediction that symbionts should avoid coexistence with other strains so that host services continue to benefit relatives, but it is less clear whether hosts should always favor monocultures and what mechanisms they might have to manipulate symbiont diversity. Few mutualisms have been studied in sufficient genetic detail to address these issues, so we decided to characterize symbiont diversity in the complex mutualism between multiple root aphid species and Lasius flavus ants. After showing elsewhere that three of these aphid species have low dispersal and mostly if not exclusively asexual reproduction, we here investigate aphid diversity within and between ant nest mounds. RESULTS: The three focal species (Geoica utricularia, Forda marginata and Tetraneura ulmi) had considerable clonal diversity at the population level. Yet more than half of the ant mounds contained just a single aphid species, a significantly higher percentage than expected from a random distribution. Over 60% of these single-species mounds had a single aphid clone, and clones tended to persist across subsequent years. Whenever multiple species/clones cooccurred in the same mound, they were spatially separated with more than 95% of the aphid chambers containing individuals of a single clone. CONCLUSIONS: L. flavus "husbandry" is characterized by low aphid "livestock" diversity per colony, especially at the nest-chamber level, but it lacks the exclusive monocultures known from other cultivation mutualisms. The ants appear to eat most of the early instar aphids, so that adult aphids are unlikely to face limited phloem resources and scramble competition with other aphids. We suggest that such culling of carbohydrate-providing symbionts for protein ingestion may maintain maximal host yield per aphid while also benefitting the domesticated aphids as long as their clone-mates reproduce successfully. The cost-benefit logic of this type of polyculture husbandry has striking analogies with human farming practices based on slaughtering young animals for meat to maximize milk-production by a carefully regulated adult livestock population.     

Timing of dispersal: effect of ants on aphids

Mutualists can affect many life history traits of their partners, but it is unclear how this translates into population dynamics of the latter. Ant–aphid associations are ideal for studying this question, as ants affect aphids, both positively (e.g., protection against natural enemies) and negatively (e.g., reduction of potential growth rates). The unresolved question is whether these effects, which have been observed at the level of individuals and under controlled environmental conditions, have consequences at the population level. On estimating aerial aphid populations by using weekly suction trap data spanning up to 22 years from different locations in France, we show that in ant-attended aphid species long-distance dispersal occurs significantly later, but that the year-to-year changes in the peak number of migrants are not significantly lower than for non-attended aphids. Host alternation had the same retarding effect on dispersal as ant attendance. We discuss the delay in the timing of dispersal in ant-attended aphids, and potential costs that arise in mutualistic systems.