Insights into a novel three-partner interaction between ants,coreids (Hemiptera: Coreidae) and extrafloral nectaries: implications for the study of protective mutualisms

Extrafloral nectar of plants and honeydew of hemipterans are the common mediators of facultative interactions that involve ants as a mobile strategy of defence. The outcome of these interactions can vary from mutualistic to commensalistic or even antagonistic, depending on the ecological context and the interacting species. Here, we explore a novel, three-partner interaction involving ants, the coreid Dersagrena subfoveolata (Hemiptera) and the extrafloral nectaries (EFNs) bearing plant Senna aphylla (Fabaceae) in semi-arid Northwest Argentina. We surveyed natural areas and conducted ant exclusion experiments, to understand how each pairwise interaction influences the overall outcome among the three interacting parts. The outcome of the interactions was assessed for experimental plants as the reproductive output and herbivore abundances and for coreids as predator abundances. We found that the coreids occurred exclusively on S. aphylla plants and that at least nine ant species interacted with the EFNs as well as with the coreids. Coreid occurrence and abundance depended on ant densities, which in turn, was determined by the presence of actively secreting EFNs. Coreid and ant presence did not influence plant reproductive success, and ants provided to coreids some biotic defence, mainly against vespid wasp predators, but had no effect on non-coreid herbivores. We conclude that the interaction outcome is commensalistic between ants and plants (assuming that EF nectar is not costly for the plant), antagonistic between coreids and plants, and mutualistic between coreids and ants. The sum of all outcomes is net positive effect for ants and coreids, and net slightly negative to neutral for plants.     

Ant–plant interactions: the importance of extrafloral nectaries versus hemipteran honeydew on plant defense against herbivores

The two most important ant–plant attractions are extrafloral nectaries (EFNs) and hemipteran honeydew. In both cases, ants may offer an effective protection against natural enemies of plants, in exchange for its sugar-rich exudates. The aim of this paper was to compare the efficiency of ant protection between plants with EFNs and with hemipteran honeydew. The study was carried out in the Amazonian Rain Forest Reserve at km 41 (02º 24′S, 59º 44′W), located 80 km from Manaus. We recorded 24 ant species in 25 plants species in the forest understory along two line transects of 5 km. The efficiency of ants in protecting plants was measured by an experiment of prey removal using isopteran workers. It was found that ants are more effective and faster in attacking termites when using honeydew rather than EFNs, probably due to the larger resource monopolization. This study further underlines the importance of experimental studies to elucidate the ecological and evolutionary importance of EFNs and honeydew in ant–plant defense against herbivores.     

Mutualisms and population regulation: mechanism matters

For both applied and theoretical ecological science, the mutualism between ants and their hemipteran partners is iconic. In this well-studied interaction, ants are assumed to provide hemipterans protection from natural enemies in exchange for nutritive honeydew. Despite decades of research and the potential importance in pest control, the precise mechanism producing this mutualism remains contested. By analyzing maximum likelihood parameter estimates of a hemipteran population model, we show that the mechanism of the mutualism is direct, via improved hemipteran growth rates, as opposed to the frequently assumed indirect mechanism, via harassment of the specialist parasites and predators of the hemipterans. Broadly, this study demonstrates that the management of mutualism-based ecosystem services requires a mechanistic understanding of mutualistic interactions. A consequence of this finding is the counter intuitive demonstration that preserving ant participation in the ant-hemipteran mutualism may be the best way of insuring pest control.     

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.     

Does the availability of arboreal honeydew determine the prevalence of ecologically dominant ants in restored habitats?

Ants are extensive users of arboreal sugars, but little is known about how ecological dominance or habitat succession influences this interaction. We investigated how the availability and use of arboreal sugar resources by ants changes across a restoration chronosequence. We surveyed the use and availability of hemipteran honeydew and floral nectar on the two dominant plant genera, Eucalyptus and Acacia, in study sites in south eastern Australia. Sugars used by ants are likely to drive their role as ecosystem engineers, while sugars not used by ants remain available to other organisms. We also tested whether the use of sugars differed between ecologically dominant and non-dominant ants; taxa likely to perform different functions in ecosystems. No floral nectar was available on Acacia, but later successional eucalypts supported more floral resources and fewer mutualist hemiptera. Successional stage significantly affected how much sugar remained unexploited by ants, with similar trends for ant use of sugars. Non-dominant ants used mainly floral nectar, while hemipteran honeydew resources were used disproportionately by dominant ants, consistent with the prediction that this group monopolises persistent carbohydrate resources. This pattern was similar across successional stages, but the difference was least in habitats with the greatest availability of floral nectar, suggesting that high sugar availability may reduce the incentive to defend honeydew. Across habitat types, the proportion of dominant ants increased with the availability of hemipteran honeydew. This suggests that honeydew availability may regulate ecological dominance, thus affecting ant-driven ecosystem processes.     

How plants shape the ant community in the Amazonian rainforest canopy: the key role of extrafloral nectaries and homopteran honeydew

Ant-plant interactions in the canopy of a lowland Amazonian rainforest of the upper Orinoco, Venezuela, were studied using a modified commercial crane on rails (Surumoni project). Our observations show a strong correlation between plant sap exudates and both abundance of ants and co-occurrence of ant species in tree canopies. Two types of plant sap sources were compared: extrafloral nectaries (EFNs) and honeydew secretions by homopterans. EFNs were a frequent food source for ants on epiphytes (Philodendron spp., Araceae) and lianas (Dioclea, Fabaceae), but rare on canopy trees in the study area, whereas the majority of trees were host to aggregations of homopterans tended by honeydew-seeking ants (on 62% of the trees examined). These aggregations rarely occurred on epiphytes. Baited ant traps were installed on plants with EFNs and in the crowns of trees from three common genera, including trees with and without ant-tended homopterans: Goupia glabra (Celastraceae), Vochysia spp. (Vochysiaceae), and Xylopia spp. (Annonaceae). The number of ant workers per trap was significantly higher on plants offering one of the two plant sap sources than on trees without such resources. Extrafloral nectaries were used by a much broader spectrum of ant species and genera than honeydew, and co-occurrence of ant species (in traps) was significantly higher on plants bearing EFNs than on trees. Homopteran honeydew (Coccidae and Membracidae), on the other hand, was mostly monopolised by a single ant colony per tree. Homopteran-tending ants were generally among the most dominant ants in the canopy. The most prominent genera were Azteca, Dolichoderus (both Dolichoderinae), Cephalotes, Pheidole, Crematogaster (all Myrmicinae), and Ectatomma (Ponerinae). Potential preferences were recorded between ant and homopteran species, and also between ant-homopteran associations and tree genera. We hypothesize that the high availability of homopteran honeydew provides a key resource for ant mosaics, where dominant ant colonies and species maintain mutually exclusive territories on trees. In turn, we propose that for nourishment of numerous ants of lower competitive capacity, Philodendron and other sources of EFNs might be particularly important.     

Ecological effects of multi‐species,ant–hemipteran mutualisms in citrus

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Ecological consequences of interactions between ants and honeydew-producing insects

Interactions between ants and honeydew-producing hemipteran insects are abundant and widespread in arthropod food webs, yet their ecological consequences are very poorly known. Ant-hemipteran interactions have potentially broad ecological effects, because the presence of honeydew-producing hemipterans dramatically alters the abundance and predatory behaviour of ants on plants. We review several studies that investigate the consequences of ant-hemipteran interactions as 'keystone interactions' on arthropod communities and their host plants. Ant-hemipteran interactions have mostly negative effects on the local abundance and species richness of several guilds of herbivores and predators. In contrast, out of the 30 studies that document the effects of ant-hemipteran interactions on plants, the majority (73%) shows that plants actually benefit indirectly from these interactions. In these studies, increased predation or harassment of other, more damaging, herbivores by hemipteran-tending ants resulted in decreased plant damage and/or increased plant growth and reproduction. The ecological consequences of mutualistic interactions between honeydew-producing hemipterans and invasive ants relative to native ants have rarely been studied, but they may be of particular importance owing to the greater abundance, aggressiveness and extreme omnivory of invasive ants. We argue that ant-hemipteran interactions are largely overlooked and underappreciated interspecific interactions that have strong and pervasive effects on the communities in which they are embedded.     

Foliage-dwelling ants in a neotropical savanna: effects of plant and insect exudates on ant communities

Ant dominance in tropical ecosystems can be explained by a capacity to exploit liquid foods such as extrafloral nectaries (EFNs) and secretions from honeydew-producing hemipterans (HPHs). Such nutritious exudates may determine ant distribution in space and shape specialization in ant–plant interactions. We provide a first assessment of how EFNs and HPHs mediate the structure of ant assemblages, ant visitation intensity, and characteristics of ant–plant interaction networks across space in Brazilian “cerrado” savanna. We used arboreal pitfall traps to sample visiting ants in four cerrado localities and recorded the presence of lepidopteran larvae to determine their possible response to ant visitation. Ant species composition and richness did not differ regardless of the presence of liquid rewards on plants, and most network patterns did not show consistent differences. However, in two of the four sites, ant densities were higher on plants with HPHs or EFNs due to increased activity by Camponotus and Pseudomyrmex ants. At these two sites, plants with liquid food sources had a more specific ant assemblage (higher specialization d′) than did plants without resources, and caterpillars were more frequently found on plants with fewer workers of Camponotus and Pseudomyrmex. Plants with HPHs had increased ant visitation and accumulated more ant species than did plants with EFNs or without liquid foods. Ant response to such food sources may thus depend on local conditions and identity of ant species, and may determine how ant assemblages are structured. Results highlight how different patterns of ant visitation to liquid resources can produce distinctive effects on herbivore infestation.     

Extrafloral‐nectaries and interspecific aggressiveness regulate day/night turnover of ant species foraging for nectar on Bionia coriacea

Plants bearing extrafloral nectaries (EFNs) vary the secretion of nectar between day and night, which creates turnover in the composition of interacting ant species. Daily variation in the composition of ant species foraging on vegetation is commonly observed, but its mechanisms are poorly understood. We evaluated the daily variation in nectar availability and interspecific aggressiveness between ants as possible regulatory mechanisms of the turnover in ant–plant interactions. We hypothesized that (i) plants would interact with more ant species during periods of higher secretion of nectar and that (ii) aggressive ant species would compete for nectar, creating a daily turnover of species collecting nectar. We tested this hypothesis by measuring the production of nectar during the day and night and by experimentally removing EFNs of Bionia coriacea (=Camptosema coriaceum) (Nees & Mart.) Benth. (Fabaceae: Faboideae) plants in a Brazilian savanna (Cerrado). We then compared the abundance and composition of ant species between those treatments and during the day. Our results indicate that more ant workers forage on plants during the day, when nectar was sugary, while more ant species forage at night, when aggressiveness between ant species was lower. We also detected a day/night turnover in ant species composition. Ant species foraging for nectar during the day were not the same at night, and this turnover did not occur on plants without EFNs. Both dominant ant species, diurnal Camponotus crassus (Hymenoptera: Formicidae) and nocturnal Camponotus rufipes (Hymenoptera: Formicidae), were the most aggressive species, attacking other ants in their specific periods of forage while also being very aggressive toward each other. However, this aggressiveness did not occur in the absence of nectar, which allowed non‐aggressive nocturnal ant species to forage only during the daytime, disrupting the turnover. We conclude that extrafloral‐nectar presence and interspecific aggressiveness between ants, along with other environmental factors, are important mechanisms creating turnovers in ants foraging on plants.     

Plant‐derived differences in the composition of aphid honeydew and their effects on colonies of aphid‐tending ants

In plant–ant–hemipteran interactions, ants visit plants to consume the honeydew produced by phloem‐feeding hemipterans. If genetically based differences in plant phloem chemistry change the chemical composition of hemipteran honeydew, then the plant's genetic constitution could have indirect effects on ants via the hemipterans. If such effects change ant behavior, they could feed back to affect the plant itself. We compared the chemical composition of honeydews produced by Aphis nerii aphid clones on two milkweed congeners, Asclepias curassavica and Asclepias incarnata, and we measured the responses of experimental Linepithema humile ant colonies to these honeydews. The compositions of secondary metabolites, sugars, and amino acids differed significantly in the honeydews from the two plant species. Ant colonies feeding on honeydew derived from A. incarnata recruited in higher numbers to artificial diet, maintained higher queen and worker dry weight, and sustained marginally more workers than ants feeding on honeydew derived from A. curassavica. Ants feeding on honeydew from A. incarnata were also more exploratory in behavioral assays than ants feeding from A. curassavica. Despite performing better when feeding on the A. incarnata honeydew, ant workers marginally preferred honeydew from A. curassavica to honeydew from A. incarnata when given a choice. Our results demonstrate that plant congeners can exert strong indirect effects on ant colonies by means of plant‐species‐specific differences in aphid honeydew chemistry. Moreover, these effects changed ant behavior and thus could feed back to affect plant performance in the field.     

Population growth of Aphis gossypii and Myzus persicae (Hemiptera: Aphididae) in the presence of Linepithema humile and Tapinoma sessile (Hymenoptera: Formicidae)

Invasive ant species can have dramatic impacts on native ants, through direct predation and by usurping common resources. Most invasive ants and many native ants use honeydew, produced by phloem-sucking hemipterans. Because colonies of invasive ants can become very large after establishment, these ants may facilitate greater hemipteran trophobiont population growth compared with their sympatric native ant counterparts. We examined the population growth of an aphid mutualist, Aphis gossypii, and a nonmutualist, Myzus persicae, exposed to two Dolichoderine ants, Linepithema humile, a globally widespread invasive species, and Tapinoma sessile, a widespread co-occurring native ant, in North America in an enemy-free laboratory study. L. humile worker foraging activity was at least twice that of T. sessile, and populations of the myrmecophile, A. gossypii, were greater when exposed to L. humile than T. sessile, possibly caused, in part, by more frequent encounters with L. humile. L. humile ignored M. persicae when A. gossypii was absent, whereas T. sessile preyed on it. Both ant species preyed on M. persicae when A. gossypii was also present. This suggested that both ants may assess nutritional gains from aphid species (i.e., honeydew versus body tissue), eliminating less productive aphids competing for host plant space. Through their impact on populations of hemipteran mutualists, we suggest that colonies of L. humile and perhaps other invasive ants may acquire more honeydew than native ants, thereby fueling colony growth that leads to numerical dominance and widespread success in introduced environments.     

Herbivory-induced extrafloral nectar increases native and invasive ant worker survival

Ascertaining the costs and benefits of mutualistic interactions is important for predicting their stability and effect on community dynamics. Despite widespread designation of the interaction between ants and extrafloral nectaries (EFNs) as a mutualism and over 100 years of studies on ant benefits to plants, the benefits to ants have never been experimentally quantified. The success of invasive ants is thought to be linked to the availability of carbohydrate-rich resources, though reports of invasive ant visits to EFNs are mixed. In two laboratory experiments, we compared worker survival of one native (Iridomyrmex chasei) and two invasive ant species (Linepithema humile and Pheidole megacephala) exposed to herbivorized or non-herbivorized EFN-bearing plants (Acacia saligna) or positive and negative controls. We found that non-herbivorized plants did not produce any measurable extrafloral nectar, and ants with access to non-herbivorized plants had the same survival as ants with access to an artificial plant and water (unfed ants). Ants given herbivorized plants had 7–11 times greater worker survival relative to unfed ants, but there were no differences in survival between native and invasive ants exposed to herbivorized plants. Our results reveal that ants cannot induce A. saligna extrafloral nectar production, but workers of both native and invasive ant species can benefit from extrafloral nectar as much as they benefit from sucrose.     

An analysis of mutualistic interactions between exotic ants and honeydew producers in the Yanbaru district of Okinawa Island,Japan

Invasive exotic ants often have a mutualistic relationship with other insects excreting honeydew, and this is considered to play a key role in their invasion success. We investigated the multispecies association patterns between ants and hemipteran insects in the Yanbaru forests, Okinawa, Japan, an Asian biodiversity hotspot. We especially focused on roadside environments, which are the frontlines of invasion for exotic ants. We found that only a small number of herbaceous and pioneer plants were predominant on the roadsides. Four honeydew producers, Melanaphis formosana, Dysmicoccus sp. A, Heteropsylla cubana, and Sogata hakonensis, living on these roadside plants accounted for 94.9% of the total honeydew-producer aggregations observed. Only a few exotic ants, such as Technomyrmex brunneus and Anoplolepis gracilipes, were observed with these honeydew-producer aggregations, and densities of these ants and honeydew producers were often positively correlated. An ant exclusion experiment showed that exotic ant occurrence improved the survival of some of the hemipteran colonies. Interestingly, the abundance of native ants was not correlated with the abundance of honeydew producers, and the local density of Pheidole noda was negatively correlated with that of M. formosana. These findings, i.e., only a few ants, all exotic, tended to hemipteran honeydew producers despite the existence of many native ants, and the abundances of those exotic ants and those hemipteran insects had positive correlations, provide some insights into the mechanism of biological invasion and provide information for the management of exotic ants.     

The response of an aphid tending ant to artificial extra-floral nectaries on different host plants

Sap-feeding homopterans, which reduce the fitness of their host plants, are often tended by ants that feed on their honeydew. The composition of the honeydew varies with both the aphid and the host plant. Extra-floral nectaries (EFNs) are believed to have evolved to attract attending ants, protecting the hosts, but it is unknown if EFNs on different plants have the same impact on the relations between an aphid species feeding on those plants and its tending ant. Experimental research was conducted to examine the attraction of Tapinoma erraticum scout ants to honeydew from the aphid Aphis gossypii feeding on two different plants, Prunus amygdalus and Mentha piperita, negligence of tending the aphids, and survival of the aphids in the presence of artificial EFNs. The scout ants were significantly more attracted to artificial nectar dispensed on P. amygdalus leaves than on M. piperita, or aphids on both plants and water. They neglected aphids in the presence of artificial EFNs on M. piperita but not on P. amygdalus. The aphid population on M. piperita did not statistically change in the presence of artificial EFNs during the 8 days of the third experiment. On P. amygdalus, the aphids succeeded in developing fully to winged form. In conclusion, the responses of the ants tending aphids to the presence of artificial EFNs were influenced by the host plant.     

Influence of macronutrient imbalance on native ant foraging and interspecific interactions in the field

1. Ants interact with a diversity of organisms. These interactions, coupled with their abundance, cause ants to have ecologically important effects across multiple trophic levels. 2. Empirical study of ant nutritional ecology has led to the prediction that a macronutrient imbalance will affect ant behaviour and interspecific interactions that underlie these broad‐scale effects. Excess carbohydrate relative to protein is predicted to increase ant aggressiveness, predatory tendency and foraging activity, and to decrease collection of hemipteran honeydew and plant nectar. 3. In field experiments conducted in 2009 and 2010, captive colony fragments of a native ant, Formica podzolica (Hymenoptera: Formicidae), were provided with either simulated prey or carbohydrate solution ad libitum. Foraging behaviours and interactions with flowers, myrmecophilous aphids and aphid natural enemies on wild‐grown plants were documented. 4. Strong effects of macronutrient imbalance on foraging manifested quickly and consistently across colonies; in accordance with predictions, prey‐fed foragers collected both honeydew and floral nectar, whereas carbohydrate‐fed ants ceased collecting these resources. Counter to predictions, carbohydrate‐fed ants dramatically lowered their activity levels and did not prey upon aphids. 5. Ants had no effect on aphid enemies in 2009, when the latter were relatively rare, but decreased their abundance in 2010. Despite this protection, the net effect of ants on aphids was negative (measured only in 2009). Prey‐fed ants demonstrated a strong preference for honeydew over floral nectar, thus demonstrating that a macronutrient imbalance may lead to different interactions with similar resources. 6. This study links ant nutrition and community ecology by demonstrating the rapid, asymmetric and multitrophic consequences of nutritionally mediated behaviour.     

Bottom-up control and co-occurrence in complex communities: honeydew and nectar determine a rainforest ant mosaic

Complex distribution patterns of species-rich insect communities in tropical rainforests have been intensively studied, and yet we know very little about processes that generate these patterns. We provide evidence for the key role of homopteran honeydew and plant nectar in structuring ant communities in an Australian tropical rainforest canopy and understorey. We also test the ant visitation of these resources against predictions derived from the 'ant-mosaic' hypothesis. Two ant species were highly dominant in terms of territorial behaviour and abundance: Oecophylla smaragdina and Anonychomyrma gilberti . Both dominant ant species monopolised large aggregations of honeydew-producing homopterans. Attended homopteran species were highly segregated between these two ant species. For the use of extrafloral and floral nectar (involving 43 ant species on 48 plant species), partitioning of ant species among plant species and between canopy and understorey was also significant, but less pronounced. In contrast to trophobioses, simultaneous co-occurrence of different nectar foraging ant species on the same plant individuals was frequent (23% of all surveys). While both dominant ant species were mutually exclusive on honeydew and nectar sources, co-occurrence with non-dominant ant species on nectaries was common. The proportion of visits with co-occurrences was low for dominant ants and high for many sub-ordinate species. These findings support the ant mosaic theory. The differential role of honeydew (as a specialised resource for dominant ants) and nectar (as an opportunistic resource for all ants including the co-occurring non-dominant species) provides a plausible structuring mechanism for the Australian canopy ant community studied.     

Ant–hemipteran association decreases parasitism of Phenacoccus solenopsis by endoparasitoid Aenasius bambawalei

1. Mutualism between ants and honeydew-excreting hemipterans is ubiquitous in the ecosystem. It is widely accepted that ant tending facilitates the colony growth of hemipterans by protecting them from predators and parasitoids. However, few studies have explored how ant tending helps defend against natural enemies. 2. Ghost ant Tapinoma melanocephalum and the invasive mealybug Phenacoccus solenopsis have a close mutual relationship. Previous studies have shown that ghost ant tending can definitely reduce parasitism and visit frequency of Aenasius bambawalei, the dominant endoparasitoid of P. solenopsis. However, the ghost ant workers seldom attack the parasitoids. It is still unclear how the ghost ant adversely affects parasitoids. This study explored the mechanism underlying the impacts of ants on natural enemies of the mealybugs. 3. Honeydew produced by P. solenopsis was an attractant to A. bambawalei. Parasitoids exhibited less searching activity, shorter longevity and lower parasitism when supplied with less honeydew. Aenasius bambawalei showed significant avoidance of pygidial gland secretions and visual cues of ghost ants. Parasitism in plants treated with 6-methyl-5-hepten-2-one, actinidine, and gland extracts was significantly lower than that in plants treated only with solvents (paraffin oil or double-distilled water). 4. It is concluded that honeydew consumption by ghost ants could negatively influence the performance of parasitoids. The pygidial gland secretions and visual cues of ghost ants also significantly inhibit the parasitism. These results may contribute to a better understanding of the regulation mechanism in ant–hemipteran–enemy interactions.     

Does the host plant affect the benefits from mutualisms? The invasive mealybug and ghost ant association

1. The benefits to trophobionte hemipterans are affected by the ant tending level, which is a widely accepted statement. The ant tending level is closely related to multiple factors. It is clear that the ant tending level can be affected by the temporal factor, age‐specific, the density of the hemipterans, and quantity and quality of honeydew produced by hemipterans. 2. Few studies of ant–hemipteran mutualisms have reported the patterns of host plants‐dependent effects, and whether host plants influence the ant tending level that is also unclear. As such, laboratory experiments were conducted to test whether the colony growth rate of an invasive mealybug Phenacoccus solenopsis Tinsley, parasitism of Aenasius bambawalei Hayat, an dominant parasitoid of P. solenopsis, are affected by tending by ghost ants (Tapinoma melanocephalum(Fabricius)], host plants (tomato and cotton), and interactions between the two factors. The difference in the ant tending level between the host plants was also determined. 3. The results showed that mealybug colony growth and parasitism were significantly affected by ant tending and host plant separately. There were significant interactions between the independent factors on the mealybug colony growth rate and percentage parasitism. These results suggest that benefits to mealybugs are host plant‐dependent.     

Ant-hemipteran trophobioses in a Bornean rainforest – diversity, specificity and monopolisation

Trophobiotic interactions between ants, hemipterans and plants play an important role for all three partners. This study compared a broad spectrum of trophobiotic associations in a tropical rainforest in Sabah, Borneo. We studied partner specificity, ant recruitment, temporal continuity and monopolisation in 218 trophobioses, comprising 58 ant species, 62 hemipteran and over 31 plant species. The most common associations involved Dinochloa trichogona (Poaceae) with coreids and delphacids in the forest understorey, and the invasive weed Chromolaena odorata (Asteraceae) with Aphis gossypii and A. spiraeola in the open vegetation; both associations were attended by a broad spectrum of ant species. In general, associations between hemipterans and plants were highly and significantly specialised, while ants were more opportunistic in their choices of partners, although partitioning was also significant between ant versus hemipteran species and consequently between ant versus plant species. The number of ant workers increased significantly, but at a declining rate, with the number of hemipterans at a trophobiosis. Most trophobioses (96%) were only tended by a single ant species at a time and thus effectively monopolised. Occasionally these guards were replaced by another ant species after a few weeks (11%) or during the night (34%). In order to test whether other sugar-seeking ants as potential competitors occurred in the vicinity of trophobioses, sugar baits were placed next to the trophobioses, on a different branch of the same plant, and on a neighbouring plant. While the hemipteran-tending ant colony mostly monopolised the nearest sugar bait, the number of ant species on more distant baits was significantly higher. Our results show that ant associations with honeydew-producing hemipterans may be relatively opportunistic at the community level, but highly predictable on a smaller spatio-temporal scale in respect to recruitment to, and long term securing of this important resource. Received 23 August 2005; revised 22 November 2005; accepted 19 December 2005.