Lack of kin discrimination in the eusocial aphid pseudoregmabambucicola (Homoptera: Aphididae)

The significance of kin discrimination for the evolution of sociality is theoretically important. The behavioural response ofPseudoregma bambucicola soldiers to aphids from other colonies was studied to examine the possibility of context-specific kin discrimination. When non-kin aphids were artificially introduced into experimental colonies, they were never killed or removed by soldiers. Field experiments revealed that the soldiers frequently attacked distantly related species such asAstegopteryx bambucifoliae andCeratoglyphina styracicola, but rarely conspecific aphids or congeneric species such asP. koshunensis andP. alexanderi. Another experiment showed that the soldiers showed altruistic defence even when coexisting with non-kin reproductives. These results agree with and extend the results from previous studies of kin discrimination in social aphids. The absence of kin discrimination suggests that some other factors besides higher genetic relatedness may play a prominent role in the evolutionary maintenance of soldier behaviour in aphids.     

Life history characteristics and behaviour of the bamboo aphid,Pseudoregma bambucicola (Hemiptera: Pemphigidae), having sterile soldiers

Summary Life history and behavioural characteristics of the bamboo aphid,Pseudoregma bambucicola (Takahashi), which has sterile soldiers, were studied in the laboratory. The stadium of normal (fertile) first instar larvae was two times longer than that of second instar larvae, and the stadium of soldier-type (sterile) first instar larvae was much longer (max. 116 days) than the stadium of normal first instar, suggesting that soldiers are able to take nutrition from bamboo. Stimulation of larvae with breath, vibration of bamboo shoots or disturbing the larvae with the tip of a fine brush induced significantly more defensive acts by soldiers than those by normal larvae — the latter usually fled. Soldiers did not attack non-kin conspecific intruders or even aphids of different species, suggesting that, in this species, kin-recognition ability is low.     

Social structure and the defensive role of soldiers in a eusocial bamboo aphid,Pseudoregma bambucicola (Homoptera: Aphididae): A test of the defence-optimization hypothesis

A complete understanding of the evolution of sociality in aphids requires a detailed knowledge of the patterns of soldier investment in their ecology. The eusocial bamboo aphidPseudoregma bambucicola has a morphologically specialized first-instar soldier caste. The proportion of soldiers was positively correlated with colony size. Within a colony, soldiers were evenly distributed among subcolonies; within each subcolony, however, their distribution was biased toward peripheries which were exposed to many predators. Field experiments introducing natural enemies such asEupeodes confrater (Diptera: Syrphidae) andSynonycha grandis (Coleoptera: Coccinellidae) revealed that the survival rate of these predators was negatively correlated with the density of soldiers, suggesting that soldiers can more or less defend their colonies by killing or removing a range of natural enemies. Observations suggest that large mature colonies attract more predators than newly established small colonies and that, within a colony, the predators attack each subcolony regardless of its position on bamboo shoots. This implies the presence of a positive correlation between colony size and predation risk. Thus, the investment in soldiers seems to reflect the attacking pattern of predators within a colony. These results agree with the defence-optimization hypothesis in soldier investment ofP. bambucicola colonies.     

Host range and host preference of a flea weevil, Orchestes hustachei, parasitizing aphid galls

Although larvae of flea weevils (Curculioninae: Rhamphini) have been known to be leaf miners, larvae of the rhamphine weevil Orchestes hustachei have been found in aphid galls of four Tetraneura species on Ulmus davidiana and in galls of Paracolopha morissoni on Zelkova serrata. This study clarified the feeding habits of O. hustachei larvae and evaluated gall selection by ovipositing females to test the hypothesis of host race formation on their hosts, Tetraneura and Paracolopha galls. When weevil larvae were placed in half‐cut galls, they always fed on aphids rather than on gall tissue. When given gall tissue only, all larvae failed to reach adulthood. The number of aphids surviving in a parasitized gall decreased significantly with the development of the weevil larvae. These results suggest that O. hustachei larvae use aphids as their major food source. In the field, ovipositing females did not discern between four Tetraneura species on U. davidiana, in spite of a large difference in suitability as food. Paracolopha morrisoni was introduced into Hokkaido approximately 100 years ago, together with the host plant Z. serrata. It is probable that P. morrisoni has recently come to be used as a host by O. hustachei in Hokkaido. Host choice experiments using Tetraneura sp. O and P. morrisoni galls indicated that female weevils from Z. serrata preferred P. morrisoni to Tetraneura sp. O galls, while females from U. davidiana selected the two types of gall randomly. On Z. serrata, female weevils selected larger P. morrisoni galls, while on U. davidiana, females did not show a preference for gall size. These results suggest that a host shift to P. morrisoni galls may have led to an initial stage of host race formation between the weevil population using Tetraneura galls on U. davidiana, and that using P. morrisoni galls on Z. serrata.     

Astegopteryx spinocephala (Hemiptera: Aphididae), a new aphid species producing sterile soldiers that guard eggs laid in their gall

Astegopteryx spinocephala sp. nov., a soldier‐producing aphid species forming banana‐bunch shaped galls on Styrax benzoides in northern Thailand, is described. We found that galls of the species are formed in approximately June and last for almost 1 year, and that the aphid completes its life cycle without migrating to secondary hosts. Many alate sexuparae appeared in March/April, when many sexuals (arostrate males and rostrate females) and eggs were found in live subgalls guarded by soldiers that plugged the ostiole with their sclerotized, spiny heads. Two healthy galls sampled in April contained 2799 and 2659 eggs, respectively. Many live galls were still found in May, and some at the beginning of June. These galls contained both active soldiers and eggs, some of which had already hatched. This indicates that soldiers of A. spinocephala guard eggs until they hatch in at least some galls.     

Morphological observation and characterization of the Pseudoregma bambucicola with the scanning electron microscope

Both leica microscopic camera system and scanning electron microscopy was used to observe and characterize the feet, back, abdomen, antennae and mouthparts of the Pseudoregma bambucicola from the bamboo, Bambusa multiplex. The possible functions of all the external morphological characteristics of the P. bambucicola were described and discussed in detail, which offers a basis for further enriching the biology, phylogeny and ecological niche of the P. bambucicola. Moreover, the morphological results should contribute to morphological identification and differentiation of the P. bambucicola from other aphids in the same family.     

The aphid Ceratovacuna nekoashi (Hemiptera: Aphididae: Hormaphidinae) and its allied species in Korea,Japan and Taiwan

The aphid Ceratovacuna nekoashi and its allied species have been a taxonomically difficult group. They form peculiar “cat's‐paw” galls (called “Nekoashi” in Japanese) on Styrax trees and also use Microstegium grasses as their secondary hosts. Through sampling aphids from both Styrax galls and Microstegium grasses in South Korea, Japan and Taiwan, and sequencing their DNA, we made it clear that four distinct species occur in these regions: C. nekoashi (Sasaki), C. oplismeni (Takahashi), C. orientalis (Takahashi) and C. subtropicana sp. nov. In Korea, C. nekoashi forms galls on both S. japonicus and S. obassia, whereas in Japan the species forms galls on the former but not on the latter; our molecular analyses unequivocally indicated the occurrence of a single species in South Korea and mainland Japan. Aphids of the four species on the secondary host were morphologically discriminated from one another. The identity of the primary‐ and secondary‐host generations was also clarified for each species. All four species were found to produce second‐instar sterile soldiers in their Styrax galls, and first‐instar soldiers were found in colonies of C. subtropicana on the secondary host.     

Evolutionary relationships of Pemphigus and allied genera (Hemiptera: Aphididae: Eriosomatinae) and their primary endosymbiont,Buchnera aphidicola

Aphids harbor primary endosymbionts, Buchnera aphidicola, in specialized cells within their body cavities. Aphids and Buchnera have strict mutualistic relationships in nutrition exchange. This ancient association has received much attention from researchers who are interested in endosymbiotic evolution. Previous studies have found parallel phylogenetic relationships between non‐galling aphids and Buchnera at lower taxonomic levels (genus, species). To understand whether relatively isolated habitats such as galls have effect on the parallel relationships between aphids and Buchnera, the present paper investigated the phylogenetic relationships of gall aphids from Pemphigus and allied genera, which induce pseudo‐galls or galls on Populus spp. (poplar) and Buchnera. The molecular phylogenies inferred from three aphid genes (COI, COII and EF‐1α) and two Buchnera genes (gnd, 16S rRNA gene) indicated significant congruence between aphids and Buchnera at generic as well as interspecific levels. Interestingly, both aphid and Buchnera phylogenies supported three main clades corresponding to the galling locations of aphids, namely leaf, the joint of leaf blade and petiole, and branch of the host plant. The results suggest phylogenetic conservatism of gall characters, which indicates gall characters are more strongly affected by aphid phylogeny, rather than host plants.     

Clonal mixing in the soldier-producing aphid Pemphigus spyrothecae (Hemiptera: Aphididae)

Illuminating the genetic relationships within soldier-producing aphid colonies is an essential element of any attempt to explain the evolution of the altruistic soldier caste. Pemphigus spyrothecae is a soldier-producing aphid that induces galls on the leaf petioles of its host (trees of the genus Populus). At least a quarter of the aphids within the clonally produced gall population are morphologically and behaviourally distinct first-instar soldiers that defend the gall population from predation. Using field trapping and microsatellites, we investigated the degree of clonal mixing within natural gall populations. Field trapping in the UK showed that all the migrants of P. spyrothecae and of two other Pemphigus species were wingless first-instar soldiers. The average degree of mixing estimated from trapping P. spyrothecae migrants was 0.68% (range = 0-15%). Microsatellite genotyping of 277 aphids from 13 galls collected in Italy revealed an average mixing level of 10.4% (range = 0-59%). Six galls contained more than one clone (range = 2-5 clones). Non-kin aphids were not restricted to the soldier caste but were evenly distributed across instars. An additional gall, from which 527 occupants were genotyped, contained 12 non-kin aphids distributed among nine clones, showing that clonal diversity can be high even when mixing is very low. These observations suggest that although soldiers migrate regularly and can moult and reproduce within foreign galls, clonal mixing in this species is generally low and is unlikely to provide a barrier to the evolution of investment by the aphid clones in an altruistic soldier caste.     

Competition and niche relationships among Eriosoma aphids occurring on the Japanese elm

Summary Galls of more than one species of Eriosoma (Aphidoidea) are found sympatrically even on single trees. Incipient galls are frequently invaded by conspecific and/or allospecific fundatrices. Eriosoma yangi, a component of Eriosoma communities, does not form its own galls but obligatorily usurps those of other species. There were interspecific differences in the timing of gall formation and the spatial distribution of galls. Nevertheless E. yangi fundatrices randomly invaded galls of any Eriosoma species and occupied 33%–41% of galls of each species. When more than one E. yangi fundatrix invaded one gall, mortal fights sometimes occurred. Fundatrices of gall-forming species also seemed to take part in such fights. Fundatrices of gall-forming species had a significant tendency to invade galls of a particular species. However, taking account of niche differences among species, invaders apparently entered available galls at random. Apparently E. yangi fundatrices search an extensive range within a branch for galls, while invaders of gall-forming species search a restricted speciesspecific range. The niche relation of gall-forming species in a northern community containing E. yangi were compared with those in a southern community lacking E. yangi. No obvious difference was found between them, suggesting that parasitism by E. yangi has not influenced niche divergence within the Eriosoma community.     

Tuberaphis owadai (Homoptera), a new aphid species forming a large gall on Styrax tonkinensis in northern Vietnam

Tuberaphis owadai sp. nov., an aphid species forming coral‐shaped galls on Styrax tonkinensis in northern Vietnam, is described. We found that the species produces many sterile second‐instar soldiers in the gall. The colony size of a large gall was estimated to be 178 000, approximately half of which were soldiers. Alates emerging from galls contained sexual embryos, indicating that the life cycle is monoecious (non‐host‐alternating). Predaceous larvae of the pyralid moth Assara seminivalis were found in several galls.     

Genetically Distinct Populations in an Asian Soldier-Producing Aphid, Pseudoregma bambucicola (Homoptera: Aphididae), Identified by DNA Fingerprinting and Molecular Phylogenetic Analysis

To estimate genetic structure of a soldier-producing aphid, Pseudoregma bambucicola, samples from natural populations throughout southeastern Asia were analyzed by a DNA fingerprinting technique. We unexpectedly found that P. bambucicola comprises two geographic groups, the northern group and the southern group, which are genetically distinct from each other but morphologically almost indistinguishable. Molecular phylogenetic and statistical analyses based on mitochondrial ribosomal DNA sequences demonstrated that the northern and southern groups of P. bambucicola are not closely related but constitute distinct lineages in the genus Pseudoregma. Detailed morphological reexamination revealed that the two groups could be distinguished by the number of setae on the 8th abdominal tergite of 1st instar nymphs and soldiers. From these results, it was suggested that P. bambucicola should be divided into two species. The northern group from Japan, Taiwan, Hong Kong, and northern Vietnam retains the name P. bambucicola, whereas we suggest that the name P. carolinensis (R. Takahashi, 1941, Tenthredo 3, 208–220) should be used for the southern group from Thailand, Malay Peninsula, Java, Irian Jaya, and Micronesia. The morphological resemblance between P. bambucicola and P. carolinensis might be due to shared ancestral characters of the genus Pseudoregma.     

Maternal death relaxes developmental inhibition in nymphal aphid defenders

In certain aphids, first-instar nymphs defend their gall by attacking intruding arthropod predators. One correlate of such defensive behaviour is a lengthened duration of the first nymphal stadium during the galling phase of the life cycle. A prolonged first stadium allows a large army of first-instar defenders to accumulate, which may be advantageous for gall defence. The factors determining developmental delay have been unclear, however. Our field experiment with Pemphigus obesinymphae, a North American gall-forming aphid with defensive first-instar nymphs, tests whether first-stadium duration is influenced by the death of the colony''s fundatrix (mother). We killed fundatrices in certain galls, left those in control galls alive, and counted aphids in each stadium in each gall. Galls in which fundatrices were killed contained a lower proportion of first-instar defenders and more late-instar nymphs than did galls with living fundatrices, indicating that maternal death dramatically increased developmental rate of nymphs. Possibly nymphal aphids respond adaptively to environmental cues that signal a threat to the colony''s welfare. Alternatively, the fundatrix actively suppresses offspring development in order to maintain a large army of soldiers to protect her gall. The results add a new layer of complexity to our understanding of social aphid systems.     

Soldier behaviour and division of labour in the aphid genus Pemphigus (Hemiptera, Aphididae)

Summary. We devised a test for quantitatively measuring the defensive behaviour of individual aphids of the gall generations of the genus Pemphigus. Five species (P. spyrothecae, P. gairi, P. bursarius, P. phenax, and P. protospirae) were shown to have soldiers, but the early instars of Pemphigus populi showed no aggressive behaviour. A higher proportion of the first instars of P. spyrothecae showed aggressive behaviour, compared with the other four soldier-producing species. Although the first instars were always the most likely to be aggressive, later instars also showed significant levels of aggression in all the soldier-producing species. Soldier behaviour was elicited by a range of stimuli. A live, restrained, predator was the most effective stimulus tested, but physical stimulation (with a paintbrush) enhanced the attack rate. The aphids could distinguish between lures soaked in water and those soaked in aphid haemolymph, which elicited a higher level of response. First instars of the early and late gall generations of P. spyrothecae were equally likely to attack predators. Although the species with the most specialized soldiers (P. spyrothecae) also had the most prolonged gall phase, this is clearly not the only factor that is important in the evolution of soldiers in this genus.     

Social aphids use their antennae to perceive density cue for soldier production

In social insects, local interactions among colony members facilitate information transfer, and allow the whole colony to regulate division of labor and task allocation in an integrated and coordinated manner. In particular, regulation of caste differentiation in response to external cues is important for sustaining social insect colonies. The social aphid Tuberaphis styraci exhibits a caste polyphenism, producing second‐instar soldiers and non‐soldiers. Previous studies using an artificial diet rearing system identified high aphid density as a crucial cue for soldier production, which acts on embryos in the maternal body and newborn first‐instar nymphs to induce soldier differentiation. While direct contact stimuli from live non‐soldiers were suggested to mediate the density effect, how the aphids perceive the stimuli has been unknown. Here we investigated how antennal removal of adult females affects the soldier production in T. styraci. Under a high density condition, intact females produced the highest percentage of soldiers, females deprived of both antennae produced the lowest percentage of soldiers, and females deprived of one antenna exhibited an intermediate percentage of soldiers. Scanning electron microscopic observations of the aphids revealed the existence of sensory organs for chemoreception and tactile sensation on the antennae of the mother aphids. These results indicate that T. styraci females use their antennae to perceive soldier‐inducing density cue, suggesting that maternal perception of density cue is involved in regulation over caste phenotype of their offspring.     

Quantitative differences detected in the histology of galls induced by the same aphid species in different varieties of the same host

• Plant galls are abnormal growths caused by an inducer that determines their morphology and anatomy. We qualitatively and quantitatively compared the histological anatomy of five aphid species (Paracletus cimiciformis, Forda marginata, Forda formicaria, Baizongia pistaciae and Geoica wertheimae) that induce galls in Pistacia terebinthus shrubs growing in Israel. We also quantitatively compared these galls to those that the aphids create on the same host in Spain.
• Histological study was conducted following methods described previously by the authors.
• Quantitative differences among the galls were found in five of 12 common anatomical traits: gall thickness, stomatal number in the epidermis‐air, size of vascular bundles, distance of phloem ducts from the lumen and number of intraphloematic schizogenous ducts. Other structures were particular to one or some species: number of cracks in the epidermis–lumen, a sclereid layer, trichomes and microcrystal inclusions. Fisher's tests of combined probabilities showed that the galls induced in Israel were statistically different from those in Spain. In particular, the number of intraphloematic schizogenous ducts was higher in the galls induced in P. terebinthus in Israel. Such differences were also found in other traits related to defence of the gall inhabitant.
• In conclusion, while the gall shape and size are determined mainly by the cecidogenic insect, it seems that the host plant also plays an important role in determining the number/size of quantitative traits, in this case mainly protective structures.

     

Cover Caption

The social aphid Pseudoregma bambucicola can produce morphologically and behaviorally specialized sterile soldiers and reproductively normal nymphs, and represents a good model system for studying social evolution. The cover photo shows a colony of this species on bamboo, in which several parthenogenetic viviparous mothers are producing offspring. Remarkable reproductive plasticity in parthenogenetic mothers and a novel strategy of regulating caste ratio via maternal reproductive differentiation were found in this social hemipteran (see pages 582-586). Photo provided by Xiaolei Huang.     

Defensive Behavior in Primary- and Secondary-Host Generations of the Soldier-Producing Aphid, Pemphigus bursarius (Hemiptera: Aphididae)

The genus Pemphigus comprises several species that produce soldiers (defensive morphs) in galls on the primary host. At the moment, it is unclear if host-alternating species also produce defenders on their secondary host. We therefore examined how P. bursarius morphs of the secondary host generations respond to predators to test whether they show defensive behavior. We further examined how this response compares with the antipredator behavior of soldiers in P. bursarius and P. spyrothecae occurring on the primary host. We performed two manipulative experiments using two predatory species to quantify the behavior of the different morphs in response to predators. In both experiments, secondary host morphs of P. bursarius showed no attacking behavior and antipredator behavior in these morphs was limited to escaping natural enemies by walking away. In contrast, the first instars of the primary host generations in both Pemphigus species showed attacking behavior and were capable of killing the predators.     

Molecular phylogeny of Iberian Fordini (Aphididae: Eriosomatinae): implications for the taxonomy of genera Forda and Paracletus

Abstract Mediterranean representatives of the galling aphid tribe Fordini (Hemiptera: Aphididae: Eriosomatinae) are usually grouped under the subtribe term Fordina. Aphids within Fordina display two‐year life cycles, alternating between Pistacia shrubs, where they induce conspicuous galls, and roots of Poaceae species. The high number of morphs present in a given species, the lack of knowledge of the complete cycle in some species, and the similarity between homologous morphs observed in different species pose many taxonomic problems in this group. We present results of a survey to elucidate the phylogenetic relationships among Fordini species present in the Iberian Peninsula and the Canary Islands. Sequences from the nuclear long‐wavelength opsin (LWO) and translation elongation factor 1α (EF‐1α) genes and from a region of the mitochondrial DNA containing the genes encoding the subunits six and eight of the F‐ATPase were used to infer relationships among 10 Fordina species, namely Smynthurodes betae, Aploneura lentisci, Baizongia pistaciae, two Geoica species (G. utricularia and G. setulosa), three species of Forda and two of Paracletus. Relationships between and within representatives of the genera Forda and Paracletus, both exemplifying taxonomic and ecological problems, were investigated in greater depth through extensive sampling and morphometric analysis. A total of eight, six and six samples from F. formicaria, F. marginata and F. riccobonii, respectively, were included in the survey, along with 40 and 4 samples classified initially as P. cimiciformis and P. donisthorpei, respectively. Our results are relevant to current studies on the evolution of host selection by aphids and on the evolution of gall morphology. Our phylogeny suggests that the group can be divided into two main clades. One clade is composed of aphids inducing small, low‐capacity galls on either P. atlantica or P. terebinthus (Smynthurodes betae and genera Forda and Paracletus). The second clade is composed of species inducing larger galls on P. lentiscus and P. terebinthus (Aploneura lentisci, Baizongia pistaciae and Geoica species). Our results suggest that revision of diagnostic characters used in the taxonomy of Paracletus is needed, and suggest Forda rotunda as a new synonym of Paracletus cimiciformis syn.n.     

The impact of kleptoparasitic invasions on the evolution of gall-size in social and solitary Australian Acacia thrips

Many species of gall-inducing Acacia thrips are attacked by kleptoparasitic thrips who enter the gall, destroy the occupants, and then use the gall for producing their own offspring. The hypothesis tested here is that pressure exerted by ldeptoparasites （genus Koptothrips） not only provoked the evolution of soldiers in the gall-inducing clade, but have also influenced the evolution of gall size and morphology. Various size dimensions of invaded galls were compared to those of uninvaded galls using data from six gall-inducing species and their kleptoparasites. For the non-social gall-inducing species （K. ellobus and K. nicholsoni） invaded galls showed no significant size differences from galls that had not been invaded. For the four social gall-inducingspecies （K. habrus, K. intermedius, K. waterhousei and K. morrisi） invaded galls were significantly narrower and/or shorter than uninvaded galls. Galls of social species that had not been invaded and contained adult soldiers were significantly larger than galls where soldiers were still at a larval stage, suggesting that gall size is related to gall age in these species. An hypothesis is proposed that links the timing of invasion by kleptoparasites to size of the host gall： induction of a smaller gall by host founders will reduce the period of vulnerability to invasion （before soldiers become adults） for social thrips by allowing foundresses in these smaller galls to begin laying soldierdestined eggs relatively sooner.