Timing of butterfly parasitization of a plant–ant–scale symbiosis

In the Southeast Asian tropics, Arhopala lycaenid butterflies feed on Macaranga ant-plants inhabited by Crematogaster (subgenus Decacrema) ants tending Coccus-scale insects. A recent phylogenetic study showed that (1) the plants and ants have been codiversifying for the past 20–16 million years (Myr), and that (2) the tripartite symbiosis was formed 9–7 Myr ago, when the scale insects became involved in the plant–ant mutualism. To determine when the lycaenids first parasitized the Macaranga tripartite symbiosis, we constructed a molecular phylogeny of the lycaenids that feed on Macaranga by using mitochondrial and nuclear DNA sequence data and estimated their divergence times based on the cytochrome oxidase I molecular clock. The minimum age of the lycaenids was estimated by the time-calibrated phylogeny to be 2.05 Myr, about one-tenth the age of the plant–ant association, suggesting that the lycaenids are latecomers that associated themselves with the pre-existing symbiosis of plant, ant, and scale insects.     

Various Chemical Strategies to Deceive Ants in Three Arhopala Species (Lepidoptera: Lycaenidae) Exploiting Macaranga Myrmecophytes

Macaranga myrmecophytes (ant-plants) are generally well protected from herbivore attacks by their symbiotic ants (plant-ants). However, larvae of Arhopala (Lepidoptera: Lycaenidae) species survive and develop on specific Macaranga ant-plant species without being attacked by the plant-ants of their host species. We hypothesized that Arhopala larvae chemically mimic or camouflage themselves with the ants on their host plant so that the larvae are accepted by the plant-ant species of their host. Chemical analyses of cuticular hydrocarbons showed that chemical congruency varied among Arhopala species; A. dajagaka matched well the host plant-ants, A. amphimuta did not match, and unexpectedly, A. zylda lacked hydrocarbons. Behaviorally, the larvae and dummies coated with cuticular chemicals of A. dajagaka were well attended by the plant-ants, especially by those of the host. A. amphimuta was often attacked by all plant-ants except for the host plant-ants toward the larvae, and those of A. zylda were ignored by all plant-ants. Our results suggested that conspicuous variations exist in the chemical strategies used by the myrmecophilous butterflies that allow them to avoid ant attack and be accepted by the plant-ant colonies.     

Congruence of Microsatellite and Mitochondrial DNA Variation in Acrobat Ants (Crematogaster Subgenus Decacrema,Formicidae: Myrmicinae) Inhabiting Macaranga (Euphorbiaceae) Myrmecophytes

A previously reported mitochondrial DNA (mtDNA) phylogeny of Crematogaster (subgenus Decacrema) ants inhabiting Macaranga myrmecophytes indicated that the partners diversified synchronously and their specific association has been maintained for 20 million years. However, the mtDNA clades did not exactly match morphological species, probably owing to introgressive hybridization among younger species. In this study, we determined the congruence between nuclear simple sequence repeat (SSR, also called microsatellite) genotyping and mtDNA phylogeny to confirm the suitability of the mtDNA phylogeny for inferring the evolutionary history of Decacrema ants. Analyses of ant samples from Lambir Hills National park, northeastern Borneo, showed overall congruence between the SSR and mtDNA groupings, indicating that mtDNA markers are useful for delimiting species, at least at the local level. We also found overall high host-plant specificity of the SSR genotypes of Decacrema ants, consistent with the specificity based on the mtDNA phylogeny. Further, we detected cryptic genetic assemblages exhibiting high specificity toward particular plant species within a single mtDNA clade. This finding, which may be evidence for rapid ecological and genetic differentiation following a host shift, is a new insight into the previously suggested long-term codiversification of Decacrema ants and Macaranga plants.     

Differences in the fruit maturation stages at which oviposition occurs among insect seed predators feeding on the fruits of five dipterocarp tree species

The seeds of dipterocarp trees are the main food resources for many species of weevils, bark beetles and small moths; however, for most seed‐eating insects on dipterocarp tropical trees, seed utilization patterns remain poorly investigated. This study aimed to determine the fruit maturation stages at which eggs are laid by different insect seed predators feeding on the seeds or fruits of the following five dipterocarp species: Dipterocarpus globosus, Dryobalanops aromatica, Shorea beccariana, S. acuta and S. curtisii, which reproduced during the same period. We investigated the occurrence frequencies of the insect seed predators at various growth stages by collecting both unfallen and fallen fruit on several occasions during the period of seed/fruit maturation in a tropical rainforest in Borneo from September to December 2013. Weevils and bark beetles were the dominant insect seed predators of the five tree species. One or two weevil species of Alcidodes, Damnux and/or Nanophyes preyed on the seeds of each of the five tree species, and one bark beetle species, Coccotrypes gedeanus, preyed on the seeds of all five tree species. Many larvae, pupae and adults of each weevil species were found in pre‐dispersal (unfallen) fruit, whereas bark beetles at various growth stages were found in post‐dispersal (fallen) fruit. These results suggested that, among the dominant insect seed predators of the five dipterocarp species, weevil species oviposit on pre‐dispersal fruit and begin their larval growth before seed dispersal, whereas the oviposition and larval development of bark beetle species occurs in post‐dispersal fruit.     

Reduced ant defenses in <Emphasis Type="Italic">Macaranga</Emphasis> myrmecophytes (Euphorbiaceae) infested with a winged phasmid <Emphasis Type="Italic">Orthomeria cuprinus</Emphasis>

Macaranga is a tree genus that includes many species of myrmecophytes, which are plants that harbor ant colonies within hollow structures known as domatia. The symbiotic ants (plant–ants) protect their host plants against herbivores; this defense mechanism is called ‘ant defense’. A Bornean phasmid species Orthomeria cuprinus feeds on two myrmecophytic Macaranga species, Macaranga beccariana and Macaranga hypoleuca, which are obligately associated with Crematogaster ant species. The phasmids elude the ant defense using specialized behavior. However, the mechanisms used by the phasmid to overcome ant defenses have been insufficiently elucidated. We hypothesized that O. cuprinus only feeds on individual plants with weakened ant defenses. To test the hypothesis, we compared the ant defense intensity in phasmid-infested and non-infested M. beccariana trees. The number of plant–ants on the plant surface, the ratio of plant–ant biomass to tree biomass, and the aggressiveness of plant–ants towards experimentally introduced herbivores were significantly lower on the phasmid-infested trees than on the non-infested trees. The phasmid nymphs experimentally introduced into non-infested trees, compared with those experimentally introduced into phasmid-infested trees, were more active on the plant surface, avoiding the plant–ants. These results support the hypothesis and suggest that ant defenses on non-infested trees effectively prevent the phasmids from remaining on the plants. Thus, we suggest that O. cuprinus feeds only on the individual M. beccariana trees having decreased ant defenses, although the factors that reduce the intensity of the ant defenses remain unclear.     

Host‐plant use by two Orthomeria (Phasmida: Aschiphasmatini) species feeding on Macaranga myrmecophytes

Myrmecophytes depend on symbiotic ants (plant‐ants) to defend against herbivores. Although these defensive mechanisms are highly effective, some herbivorous insects can use myrmecophytes as their host‐plants. The feeding habits of these phytophages on myrmecophytes and the impacts of the plant‐ants on their feeding behavior have been poorly studied. We examined two phasmid species, Orthomeria alexis and O. cuprinus, which are known to feed on Macaranga (Euphorbiaceae) myrmecophytes in a Bornean primary forest. Our observations revealed that: (i) each phasmid species relied on two closely‐related myrmecophytic Macaranga species for its host‐plants in spite of their normal plant‐ant symbioses; and (ii) there was little overlap between their host‐plant preferences. More O. cuprinus adults and nymphs were found on new leaves, which were attended by more plant‐ants than mature leaves, while most adults and nymphs of O. alexis tended to avoid new leaves. In a feeding choice experiment under ant‐excluded conditions, O. alexis adults chose a non‐host Macaranga myrmecophyte that was more intensively defended by plant‐ants and was more palatable than their usual host‐plants almost as frequently as their usual host‐plant, suggesting that the host‐plant range of O. alexis was restricted by the presence of plant‐ants on non‐host‐plants. Phasmid behavior that appeared to minimize plant‐ant attacks is described.     

Increased seed predation in the second fruiting event during an exceptionally long period of community-level masting in Borneo

In Southeast Asian tropical rainforests, community-level masting (CM) occurs at irregular intervals of 2–10 years. During CM periods, many plant species from various families synchronously flower and subsequently undergo community-level fruiting. Seed predation is a key factor in understanding the ecological and evolutionary factors affecting CM. Masting is proposed to decrease seed mortality due to predation in two ways: by depressing predator abundance through extended and unpredictable absences of seeds; and by satiating predators via mass seed production (predator satiation hypothesis). If the hypothesis is valid in these rainforests, the incidence of seed predation will be higher in a fruiting event that occurs soon after a previous fruiting event, because the intervening period of seed absence would be inadequate to starve the predators. In this study, we examined seed predation by insects, focusing on five dipterocarp species that exceptionally reproduced twice during an extended CM period. All of the five species suffered more intense seed predation in the second fruiting event, consistent with the prediction expected from the predator satiation hypothesis. Weevils, bark beetles and mammals were the main cause of increased seed predation in three, one and one plant species, respectively. However, seed predation intensity did not increase during the second fruiting event in a few combinations of predator and plant species. We discuss the possibility that competition for seeds among predators and/or the interspecific differences in life history traits among predators might affect the varying intensities of seed predation among dipterocarp species by different seed predators.