Density-dependent outcomes in a digestive mutualism between carnivorous <Emphasis Type="Italic">Roridula</Emphasis> plants and their associated hemipterans

Recent studies have shown that mutualisms often have variable outcomes in space and time. In particular, the outcomes may be dependent on the density of the partners with unimodal or saturating outcomes providing stability to the mutualism. We examine density-dependent outcomes of an obligate, species-specific mutualism between a South African carnivorous plant (Roridula dentata) and a hemipteran (Pameridea) that facilitates prey digestion, but also sucks plant sap. Plants occur in sandy, leached, nitrogen-poor soils and have no digestive enzymes to digest prey. Instead they rely on obligately dependent hemipterans to supply nitrogen by digesting prey for them and defecating on their leaves. We documented the densities of Pameridea on Roridula in the field. In the greenhouse, we manipulated the hemipteran densities on Roridula and measured the mean relative growth rates of plants with differing hemipteran densities. Plants exhibited a unimodal response to the density of their mutualist partners. Those with no hemipterans had negative growth rates, suggesting that hemipterans are important in facilitating nitrogen absorption. Plants with intermediate hemipteran densities had positive growth rates but growth rates were negative under very high hemipteran densities. Our research provides support for variable and unimodal outcomes in mutualism. Unimodal outcomes may be particularly important in obligate mutualisms and this is one of the few studied outside of pollinating seed parasite mutualisms. In this system, extrinsic factors such as other predators may affect the mutualism by altering the numbers of hemipterans.     

Food source availability and interspecific dominance as structural mechanisms of ant-plant-hemipteran multitrophic networks

Extrafloral nectar of plants and honeydew of hemipterans is a food source extensively explored by ants. Although basically a sugary liquid food, nectar and honeydew are composed of different nutrients and offered in distinct ways; thus, ants must interact differently with plants and hemipterans. In this study we assessed the availability and dominance of nectar of extrafloral nectaries and honeydew of sap-sucking hemipterans (i.e., sugar-based resources) as mechanisms regulating interaction frequency and structuring ant-plant-hemipteran networks. We studied 12 plant species (240 shrubs, 20 per species) and 12 hemipteran species (240 aggregations, 20 per species) that interacted with 26 ant species in an area of Rupestrian Fields (Rocky Montane Savannah), Brazil. We observed that the 7 ant species that collected honeydew were a subset of the 25 ant species feeding on nectar, but the highly interacted species Camponotus crassus was the same for both subnetworks. The ant-plant subnetwork exhibited a nested pattern of interaction with a low degree of specialization, while the ant-hemipteran subnetwork exhibited lower nestedness but higher specialization. We found a positive relationship between the offer of EFNs and the number of interactions with ants, probably resulting from reduced competition in plants with high availability of EFNs. However, hemipteran species that were most abundant did not interact with more species of ants, probably because of the numerical dominance of the species tending all hemipteran aggregations, regardless of size. However, segregation between ant species was higher than expected by chance for both plants and hemipterans, confirming a deterministic factor (i.e., competition between ant species) regulating the frequency of interactions. In summary, the availability of ENFs seems to be an important mechanism regulating ant-plant interactions, while numerical dominance seems to be an important mechanism structuring ant-hemipteran interactions.     

Ant–scale mutualism increases scale infestation,decreases folivory,and disrupts biological control in restored tropical forests

Ant–hemipteran mutualisms can have positive and negative effects on host plants depending on the level of hemipteran infestation and plant protection conferred by ants against folivory. Differential effects of such mutualisms on plant survival are well documented in undisturbed and ant-invaded systems, but few have explored how anthropogenic disturbance affects interactions between hemipterans and native ant species and what the consequences may be for recovering ecosystems. Within a fragmented landscape in Costa Rica, restored tropical forests harbor a mutualism between the native ant Wasmannia auropunctata and the scale insect Alecanochiton marquesi on the abundant, early-successional tree Conostegia xalapensis. I added A. marquesi scales to C. xalapensis seedlings and either allowed or excluded W. auropunctata to investigate if this mutualism leads to increased scale infestation, decreased scale mortality, and decreased folivory. I also examined whether these effects are mediated by the percentage of remnant forest cover in the landscape. I found that seedlings with ants excluded had fewer scale insects and higher herbivory than plants with ants present. I also found evidence that scale mortality due to fungal attack and parasitism was higher on ant-excluded versus ant-allowed seedlings but only at sites with high surrounding landscape forest cover. Together, these results suggest that mutualisms between scale insects and native ants can promote scale infestation, reduce folivory on native plant species, and potentially disrupt biological control of scale insects in recovering tropical forests. Further, my experiment underscores the importance of remnant tropical forests as sources of biological control in anthropogenically disturbed landscapes. Abstract in Spanish is available with online material.     

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

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Comparative population genetic structures and local adaptation of two mutualists

Similar patterns of dispersal and gene flow between closely associated organisms may promote local adaptation and coevolutionary processes. We compare the genetic structures of the two species of a plant genus (Roridula gorgonias and R. dentata) and their respective obligately associated hemipteran mutualists (Pameridea roridulae and P. marlothi) using allozymes. In addition, we determine whether genetic structure is related to differences in host choice by Pameridea. Allozyme variation was found to be very structured among plant populations but less so among hemipteran populations. Strong genetic structuring among hemipteran populations was only evident when large distances isolated the plant populations on which they live. Although genetic distances among plant populations were correlated with genetic distances among hemipteran populations, genetic distances of both plants and hemipterans were better correlated with geographic distance. Because Roridula and Pameridea have different scales of gene flow, adaptation at the local population level is unlikely. However, the restricted gene flow of both plants and hemipterans could enable adaptation to occur at a regional level. In choice experiments, the hemipteran (Pameridea) has a strong preference for its carnivorous host plant (Roridula) above unrelated host plants. Pameridea also prefers its host species to its closely related sister species. Specialization at the specific level is likely to reinforce cospeciation processes in this mutualism. However, Pameridea does not exhibit intraspecific preferences toward plants from their natal populations above plants from isolated, non-natal populations.     

Resource exchange and partner recognition mediate mutualistic interactions between prey and their would-be predators

Animals may develop mutualistic associations with other species, whereby prey offer resources or services in exchange for protection from predators. Alternatively, prey may offer resources or services directly to their would-be predators in exchange for their lives. The latter may be the case of hemipterans that engage in mutualistic interactions with ants by offering a honeydew reward. We test the extent to which a honeydew offering versus partner recognition may play a role as proximate mechanisms deterring ants from predating upon their hemipteran partners. We showed that, when presented with a choice between a hemipteran partner and an alternative prey type, mutualist ants were less likely to attack and more likely to remain probing their hemipteran partners. This occurred even in the absence of an immediate sugary reward, suggesting either an evolved or learned partner recognition response. To a similar extent, however, ants were also less likely to attack the alternative prey type when laced with honey as a proxy for a honeydew reward. This was the case even after the honey had been depleted, suggesting an ability of ants to recognize new potential sources of sugars. Either possibility suggests a degree of innate or learned partner recognition.     

The ecological effects of the ant–hemipteran mutualism: A meta-analysis

Ant–hemipteran mutualism has a variety of ecological effects on the host plants, but the magnitude and moderators of those effects are poorly known. We evaluated this issue by conducting a meta-analysis based on 49 published studies. Results showed that the mutualism had significant protective effects on the host plans, although those effects did not lead to enhanced plant growth or reproductive performance. Both herbivores and predators on plants were strongly suppressed by the mutualism; a similar pattern was also detected for fruit removal. The ecological effects of the mutualism tended to be more consistent and stronger in tropical and subtropical regions, whereas in temperate regions, none of the tested variables was significantly affected by the mutualism. The protective effects of the mutualism on plants were independent of plant life form and the invasiveness of ants. The effect of the mutualism on predators varied with plant life form. The study confirmed that the ant–hemipteran mutualism has a wide range of ecological influences on plants and highlights the significance of a common and wide-spread mutualism.     

Impact of a mutualism between an invasive ant and honeydew-producing insects on a functionally important tree on a tropical island

Mutualisms between invasive ants and honeydew-producing Hemiptera have the potential to result in unusually high population levels of both partners, with subsequent major changes to ecosystem composition and dynamics. We assessed the relationship between the invasive ant, Pheidole megacephala, and its hemipteran mutualists, Dysmicoccus sp. and Pulvinaria urbicola, on Cousine Island, Seychelles. We also assessed the impacts of the mutualism on the condition of the hemipteran host plant, Pisonia grandis, a native and functionally important tree species. There was a strong positive relationship between Ph. megacephala activity and hemipteran abundance, and the exclusion of ants from Pi. grandis resulted in a significant decline in Pu. urbicola abundance. High abundance of the mutualists was strongly associated with damage to the Pi. grandis forest. This indicates that the mutualism is contributing to the massive increase in the population levels of the mutualist species, and is intensifying their impacts on the island. The widespread trophobiosis and its associated high densities of mutualists pose serious threats to the ecosystem, highlighting the need to control the ant and associated hemipteran populations.     

Inferring evolutionary patterns from the biogeographical distributions of mutualists and exploiters

Exploitation may lead to the breakdown of obligate species-specific mutualisms. However, the mutualism between Roridula (plants) and Pameridea (hemipterans) is often exploited by spiders. The aim of the present study was to determine when the exploiters became associated with the Roridula-Pameridea mutualism. The phylogenetic and geographical associations between Roridula and Pameridea are documented and the distribution patterns of Roridula and exploiters are overlaid to see how closely they correlate. A geographical discontinuity in Roridulas ' range divides both the host plants and associated hemipterans into two sister species so that each hemipteran species is associated with a different plant species. This suggests that Roridula was associated with Pameridea before fragmentation/vicariance events split the genus, allowing allopatric speciation. By contrast, Roridula is only associated with exploiters in parts of its current range. This suggests that exploiters are unable to traverse the disjunctions in Roridulas' distribution and that they only developed associations with the mutualism after vicariance events. It is hypothesized that Pameridea and Roridula were closely associated for a long period before the invasion of nonmutualists. The absence of associated nonmutualist species may have helped facilitate the evolution of an obligate interaction between Roridula and Pameridea .  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 541–549.     

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.     

Invasive Argentine ants reduce fitness of red maple via a mutualism with an endemic coccid

Many invasive ant species form mutualisms with honeydew-producing Hemiptera and their aggressive presence deters the natural enemies of the Hemiptera. Invasive ant species like the Argentine ant have often been associated with hemipteran outbreaks in urban, agricultural and natural ecosystems. We investigated the effects of a mutualism between the invasive Argentine ant and the endemic terrapin scale on coccid density and the fitness of the host of this mutualism, the endemic red maple, situated in a commercial park. The terrapin scale has numerous natural enemies and we predicted that the high terrapin scale numbers associated with tending Argentine ants would collapse once Argentine ants were excluded from the host tree canopy. We predicted that excluding the Argentine ant from the tree canopy would result in an indirect net fitness benefit to the host. Terrapin scale numbers collapsed when Argentine ants were excluded from the host tree canopy. Red maples with Argentine ants excluded from their canopy had higher seed mass and larger early leaves indicating that this invasive ant-endemic scale mutualism imposed a net fitness cost to the host tree. The Argentine ant has yet to invade closed-canopy forest within its introduced range. The red maple is common in adjacent closed-canopy forest fragments and recent work has shown that invasion of these forest fragments by the Argentine ant is limited by a steady carbohydrate resource. We discuss the implications to forest invasion posed by a mutualism involving the Argentine ant and an endemic coccid.     

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.     

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.     

It takes two to tango but three is a tangle: mutualists and cheaters on the carnivorous plant <Emphasis Type="Italic">Roridula</Emphasis>

Roridula dentata is associated with hemipterans, which facilitate nitrogen assimmilation from insects. R. dentata is also associated with spiders and their role in digestion is unknown. We quantify approximately how much nitrogen Roridula assimilates from insects through "indirect digestion." Using '¹⁵N we then determine whether nitrogen absorption from hemipteran insects differs with varying spider densities. In this way, we are able to determine their nutritional role. At low spider densities, indirect digestion of prey accounts for approximately 70% of plant nitrogen. These values are comparable to methods of direct prey digestion found in other carnivorous plants. However spiders decrease the numbers of hemipteran individuals inhabiting Roridula plants and also decrease efficiency of indirect prey digestion by up to 30%. We deduce that spiders are cheaters as they exploit plant rewards without offering any rewards in return. However, indirect carnivory is still efficient enough when hemipteran densities are at their lowest, ensuring that the mutualism does not break down.     

Zoophytophagous pentatomids feeding on plants and implications for biological control

Zoophytophagous insects can feed on a variety of prey, plants and plant products. By studying the interactions between predatory hemipterans and plants harbouring the prey of these insects, scientists have started to establish two potential outcomes: (1) positive effects like the enhancement of their life history characteristics by acquiring plant contents; and (2) negative effects mediated by plant resistance to herbivores or prey ingesting secondary plant metabolites. Despite this research, there is a lack of information about the feeding sites of predatory hemipterans on their host plants, what they ingest from plants, and whether they cause damage to their host plants. The results presented here indicate that the xylem is one of the feeding sites of predatory hemipterans on plants. The dissection of predators that fed on plants with marked vessels and testing insects for the presence of Cry protein constitutively expressed in the cytoplasm of plant cells revealed that bugs are not able to acquire cytoplasm contents from the plant cell. In addition, we demonstrate that systemic insecticide circulating inside plants from soil applications contaminates these predators. Our results are discussed in the context of zoophytophagous feeding behaviour exhibited by predatory hemipterans and the use of systemic insecticides for the conservation of natural enemies. This interaction contradicts the concept of ecological selectivity obtained for natural enemies through the placement of systemic insecticide in the soil as a selective method of deploying chemical control and predatory hemipteran conservation within the integrated pest management framework.     

Does Mutualism Drive the Invasion of Two Alien Species? The Case of Solenopsis invicta and Phenacoccus solenopsis

Although mutualism between ants and honeydew-producing hemipterans has been extensively recognized in ecosystem biology, however few attempts to test the hypothesis that mutualism between two alien species leads to the facilitation of the invasion process. To address this problem, we focus on the conditional mutualism between S. invicta and P. solenopsis by field investigations and indoor experiments. In the laboratory, ant colony growth increased significantly when ants had access to P. solenopsis and animal-based food. Honeydew produced by P. solenopsis also improved the survival of ant workers. In the field, colony density of P. solenopsis was significantly greater on plots with ants than on plots without ants. The number of mealybug mummies on plants without fire ants was almost three times that of plants with fire ants, indicating a strong effect of fire ants on mealybug survival. In addition, the presence of S. invicta successfully contributed to the spread of P. solenopsis. The quantity of honeydew consumption by S. invicta was significantly greater than that of a presumptive native ant, Tapinoma melanocephalum. When compared with the case without ant tending, mealybugs tended by ants matured earlier and their lifespan and reproduction increased. T. melanocephalum workers arrived at honeydew more quickly than S. invicta workers, while the number of foraging S. invicta workers on plants steadily increased, eventually exceeding that number of T. melanocephalum foragers. Overall, these results suggest that the conditional mutualism between S. invicta and P. solenopsis facilitates population growth and fitness of both species. S. invicta tends to acquire much more honeydew and drive away native ants, promoting their predominance. These results suggest that the higher foraging tempo of S. invicta may provide more effective protection of P. solenopsis than native ants. Thus mutualism between these two alien species may facilitate the invasion success of both species.     

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.     

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.     

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.     

Facilitated selfing offers reproductive assurance: a mutualism between a hemipteran and carnivorous plant

Reproductive assurance is frequently used to explain the evolution of selfing but has become controversial from lack of evidence. We studied the pollination system of the near carnivorous plant genus Roridula and showed that reproductive assurance is important in this system. Hemipterans have a digestive mutualism with Roridula and have been implicated in pollination but flowers show adaptations to hymenopteran pollination. We deduce that hemipterans are the primary pollinators of Roridula because seed set is significantly reduced when hemipterans are excluded from the flowers. Using allozyme electrophoresis, we show that hemipterans are responsible for mostly selfed progeny. Although bees still pollinate Roridula on very rare occasions, their exclusion does not affect seed set. The complicated floral structures that occur in Roridula most likely evolved as adaptations for bee pollination. Resident hemipterans facilitate selfing by Roridula, and this acts as a reproductive assurance mechanism because it increases seed production and ensures that plants still reproduce in the absence of more motile, outcrossing pollinators.