Mechanisms of hydraulic conductivity in the leaf galls of Meunieriella sp. (Cecidomyiidae) in Avicennia schaueriana (Acanthaceae): does vascularization explain the preferred sites of induction?

• Intralaminar galls of Meunieriella result from ground tissue proliferation in leaves of Avicennia schaueriana, a typical halophytic mangrove. We tested if the preferred sites of gall induction were the midribs and secondary veins (SV) at the basal leaf portion, where the galls were expected to be largest; and if the vascular system in galls and adjacent regions was altered to favour water supply in galls, thus increasing their growth.
• Gall induction sites and gall sizes were quantified according to leaf portions and regions. Anatomical and histometric analyses in vascular and ground tissues of galls and adjacent regions were compared to equivalent regions of non-galled leaves.
• The galls were largest at basal sites on leaves, the midrib and SV. More galls occurred on the apical portion of the leaf, and on the leaf blade and secondary vein regions. Changes in shape and vascular system area, number and diameter of vessel elements were detected in both galls and adjacent regions. Fewer and smaller-sized vessel elements were observed in regions proximal to the galls and inside them.
• Gall size is not related with preferred induction sites, which could be explained by factors such as thermal balance. Alterations in the vascular system indicate reduced hydraulic conductivity in the xylem in the proximal region and inside galls, leading to water leakage to gall parenchyma cells. This compensatory mechanism explains the expansion and proliferation of water storage and spongy parenchyma cells in the galls, explaining the higher growth in more vascularized regions.

     

The role of pectic composition of cell walls in the determination of the new shape-functional design in galls of Baccharis reticularia (Asteraceae)

The pectic composition of cell wall is altered during the processes of cell differentiation, plant growth, and development. These alterations may be time-dependent, and fluctuate in distinct regions of the same cell or tissue layer, due to the biotic stress caused by the activity of the gall inducer. Among the roles of the pectins in cell wall, elasticity, rigidity, porosity, and control of cell death may be crucial during gall development. Galls on Baccharis reticularia present species-specific patterns of development leading to related morphotypes where pectins were widely detected by Ruthenium red, and the pectic epitopes were labeled with specific monoclonal antibodies (LM1, LM2, LM5, LM6, JIM5, and JIM7) in distinct sites of the non-galled and the galled tissues. In the studied system B. reticularia, the epitopes for extensins were not labeled in the non-galled tissues, as well as in those of the rolling and kidney-shaped galls. The high methyl-esterified homogalacturonans (HGA) were labeled all over the tissues either of non-galled leaves or of the three gall morphotypes, while the intense labeling for arabinogalactans was obtained just in the rolling galls. The pectic composition of non-galled leaves denotes their maturity. The kidney-shaped gall was the most similar to the non-galled leaves. The pectic dynamics in the gall tissues was particularly altered in relation to low methyl-esterified HGA, which confers elasticity and expansion, as well as porosity and adhesion to cell walls, and are related to the homogenization and hypertrophy of gall cortex, and to translocation of solutes to the larval chamber. Herein, the importance of the pectic dynamics of cell walls to the new functional design established during gall development is discussed for the first time. The repetitive developmental patterns in galls are elegant models for studies on cell differentiation.     

Stem galls drain nutrients and decrease shoot performance in Diplusodon orbicularis (Lythraceae)

Insect galls are important nutrient sinks in the plant, ultimately affecting its reproductive success. We assessed the influence of a stem galler on the survival of plant shoots and whether these are able to concentrate nutrients in the gall’s tissues; thus testing the nutrient sink hypothesis. We measured gall sizes and nutrient concentrations in a Brazilian Cerrado plant, Diplusodon orbicularis, and its Cecidomyiidae stem galler. Galls grew larger on thicker shoots. Also, these shoots suffered smaller mortality from gall attack, while thinner shoots were commonly associated with the death of the shoot distal to the gall. Apical shoots suffered higher mortality, while basal shoots suffered lower mortality and were proportionally less attacked. Galled tissues were more nutrient rich than non-galled tissues. The nutrients P, Ca, and Mg were more concentrated inside the galls when compared to tissues in non-galled stems. Gallers also seem to be able to reduce toxic Al concentration in the plant tissues, as Al occurred in smaller concentrations inside the galls than out of them. Although stem gallers feed on tissues of low nutritional value, these are in a favourable position to intercept flowing nutrients and water. The death of small galled shoots possibly is due to the lack of essential nutrients and energy drained, or water restriction in them.     

Phenotypic plasticity and similarity among gall morphotypes on a superhost,Baccharis reticularia (Asteraceae)

Understanding factors that modulate plant development is still a challenging task in plant biology. Although research has highlighted the role of abiotic and biotic factors in determining final plant structure, we know little of how these factors combine to produce specific developmental patterns. Here, we studied patterns of cell and tissue organisation in galled and non‐galled organs of Baccharis reticularia, a Neotropical shrub that hosts over ten species of galling insects. We employed qualitative and quantitative approaches to understand patterns of growth and differentiation in its four most abundant gall morphotypes. We compared two leaf galls induced by sap‐sucking Hemiptera and stem galls induced by a Lepidopteran and a Dipteran, Cecidomyiidae. The hypotheses tested were: (i) the more complex the galls, the more distinct they are from their non‐galled host; (ii) galls induced on less plastic host organs, e.g. stems, develop under more morphogenetic constraints and, therefore, should be more similar among themselves than galls induced on more plastic organs. We also evaluated the plant sex preference of gall‐inducing insects for oviposition. Simple galls were qualitative and quantitatively more similar to non‐galled organs than complex galls, thereby supporting the first hypothesis. Unexpectedly, stem galls had more similarities between them than to their host organ, hence only partially supporting the second hypothesis. Similarity among stem galls may be caused by the restrictive pattern of host stems. The opposite trend was observed for host leaves, which generate either similar or distinct gall morphotypes due to their higher phenotypic plasticity. The Relative Distance of Plasticity Index for non‐galled stems and stem galls ranged from 0.02 to 0.42. Our results strongly suggest that both tissue plasticity and gall inducer identity interact to determine plant developmental patterns, and therefore, final gall structure.     

Patterns of cell elongation in the determination of the final shape in galls of Baccharopelma dracunculifoliae (Psyllidae) on Baccharis dracunculifolia DC (Asteraceae)

Cell redifferentiation, division, and elongation are recurrent processes, which occur during gall development, and are dependent on the cellulose microfibrils reorientation. We hypothesized that changes in the microfibrils orientation from non-galled tissues to galled ones occur and determine the final gall shape. This determination is caused by a new tissue zonation, its hyperplasia, and relative cell hypertrophy. The impact of the insect’s activity on these patterns of cell development was herein tested in Baccharopelma dracunculifoliae—Baccharis dracunculifolia system. In this system, the microfibrils are oriented perpendicularly to the longest cell axis in elongated cells and randomly in isodiametric ones, either in non-galled or in galled tissues. The isodiametric cells of the abaxial epidermis in non-galled tissues divided and elongated periclinally, forming the outer gall epidermis. The anticlinally elongated cells of the abaxial palisade layer and the isodiametric cells of the spongy parenchyma originated the gall outer cortex with hypertrophied and periclinally elongated cells. The anticlinally elongated cells of the adaxial palisade layer originated the inner cortex with hypertrophied and periclinally elongated cells in young and mature galls and isodiametric cells in senescent galls. The isodiametric cells of the adaxial epidermis elongated periclinally in the inner gall epidermis. The current investigation demonstrates the role of cellulose microfibril reorientation for gall development. Once many factors other than this reorientation act on gall development, it should be interesting to check the possible relationship of the new cell elongation patterns with the pectic composition of the cell walls.     

Sexually dimorphic gall structures correspond to differential phytohormone contents in male and female wasp larvae

Sexually dimorphic galls are rare among gall‐inducing insects and the reason for their occurrence is unknown. The pteromalid wasp Trichilogaster acaciaelongifoliae, which induces galls on Acacia longifolia, is one such species. In the present study, the anatomical and physiological attributes of male and female galls of T. acaciaelongifoliae are examined and compared. Histological preparations are used to characterize anatomical differences between male and female gall chambers. Bioassays, high‐performance liquid chromatography‐mass spectrometry and an enzyme immunoassay are used to measure concentrations of auxin and cytokinin in normal buds, galled tissues, and larvae of both sexes. Female chambers are found to be 3.3‐fold larger, and are associated with 1.5‐fold more storage tissue and 3.5‐fold more vascular tissues than male chambers. Tissues from female chambers induce stronger cytokinin‐like bioactivity than tissues from male chambers. Female larvae have considerably higher concentrations of cytokinin free bases, ribosides, glucosides and monophosphates than male larvae; higher auxin‐like bioactivity than in normal or galled plant tissues; and almost twice the concentration of auxin than male larvae. Both male and female larvae contain much higher auxin concentrations than either galled or normal plant tissues. These findings suggest that differing levels of phytohormones are involved in the development of sexual dimorphism of gall structures in this species.     

Oxycephalomyia, gen. nov., and life history strategy of O. styraci comb. nov. (Diptera: Cecidomyiidae) on Styrax japonicus (Styracaceae)

A new genus Oxycephalomyia is described to contain the gall midge that was previously known as Asteralobia styraci (Shinji). Oxycephalomyia styraci, comb. nov., produces leaf vein galls on Styrax japonicus (Styracaceae). The adult of O. styraci is redescribed, and its full‐grown larva and pupa are described for the first time. The annual life cycle of the gall midge in northern Kyushu was clarified; the first instars overwinter in the galls on the host plant. However, the galls of O. styraci mature much later in the season than those of other gall midges with a similar life history pattern, and the durations of second and third larval instars are remarkably short. Such a life history pattern is considered to have an adaptive significance in avoiding larval parasitism, particularly by early attackers. The number of host axillary buds as oviposition sites decreased in bearing years and increased in off years, but there was no sign of oviposition site shortage even in bearing years, probably due to the low population density of the gall midge. An unidentified lepidopteran that feeds on galled and ungalled host buds and a Torymus sp. that attacks pupae of O. styraci were recognized as mortality factors of the gall midge.     

Why do the galls induced by Calophya duvauae Scott on Schinus polygamus (Cav.) Cabrera (Anacardiaceae) change colors?

One of the galling herbivores associated to the superhost Schinus polygamus (Cav.) Cabrera (Anacardiaceae) is Calophya duvauae Scott (Hemiptera: Calophyidae). The galls are located on the adaxial surface of leaves and vary from red to green. The levels of their pigments were herein investigated in relation to age. Samples were collected between June 2008 and March 2009, in a population of S. polygamus at Canguçu municipality, Rio Grande do Sul, Brazil. Galls were separated by color, measured, dissected, and the instar of the inducer was determined. The levels of photosynthetic (chlorophyll a and b, total chlorophylls, and carotenoids) and protective pigments (anthocyanins) were also evaluated. Red galls were more numerous than the green ones, and the induction should occur preferentially on young leaves, but may also occur on mature leaves. Immature stages of C. duvauae were observed in all samples throughout the year, characterizing its life cycle as multivoltine. There was a significant correlation between the instar of the inducer and the size of the gall (r = 0.675, p < 0.001), with larger galls corresponding to more advanced instars. The ratio between red and green galls varied over the samples, with the highest (96%) and lowest (38%) frequency of red galls observed in September, and December 2008, respectively. The average amounts of chlorophyll b and total chlorophylls were 70% lower in the gall tissues when compared to non-galled portions of galled leaves. There is a notable reduction in the contents of all pigments in the galls when compared to non-galled leaves, especially for total anthocyanins in green galls. The red galls presumably had the constant stimuli of healthy gall inducers, whereas in green galls they died prematurely, due to the interference of parasitoids and inquilines. The alterations in gall structure and color were related to the gradual decreasing in galling stimuli.     

Influence of galls of Phanacis taraxaci on carbon partitioning within common dandelion, Taraxacum officinale

We examined how leaf galls, induced by the cynipid wasp Phanacis taraxaci, influence the partitioning of photoassimilates within the host, the common dandelion, Taraxacum officinale. Galled and ungalled plants were exposed to ¹⁴CO₂ and the labelled photoassimilates accumulating within galls and other parts of the host were measured. During the growth phase of the gall they were physiological sinks for photoassimilates, accumulating 9% to 70% of total carbon produced by the host, depending upon the number of galls per plant. High levels of ¹⁴C assimilation in the leaves of galled plants compared to controls, suggest that galls actively redirect carbon resources from unattacked leaves of their host plant. This represents a significant drain on the carbon resources of the host, which increases with the number and size of galls per plant. Active assimilation of ¹⁴C by the gall is greatest in the growth phase and is several orders of magnitude lower in the maturation phase. This finding is consistent with physiological and anatomical changes that occur during the two phases of gall development and represents a key developmental strategy by cynipids to ensure adequate food resources before larval growth begins.     

Six new species of Asphondylia (Diptera: Cecidomyiidae) damaging flower buds and fruit of Australian Acacia (Mimosaceae)

Six species of gall midge are described from Australian acacias. Asphondylia bursicola Kolesik sp.n. and A. occidentalis Kolesik sp.n . form galls on fruit; A. germinis Kolesik sp.n ., A. pilogerminis Kolesik sp.n . and A. glabrigerminis Kolesik sp.n . induce severe deformation of flower buds; and A. acaciae Kolesik sp.n . causes galls on both fruit and flower buds. Galled flower buds do not produce flowers, and galled fruit produce no or undeveloped seeds. Host ranges of the new species comprise between two and eight acacia hosts. Larval, pupal and male morphology, together with phylogenetic analyses of a 410‐bp fragment of the mitochondrial cytochrome b gene, were used to characterize the new species. For A. bursicola, A. germinis, A. pilogerminis, A. glabrigerminis and A. acaciae, the intraspecific divergence values were between 0.2 and 3.4%, and the interspecific divergence values ranged between 5.1 and 10.5%. For A. occidentalis, the only species with geographical distribution confined solely to Western Australia, the intraspecific divergence was between 6.6 and 10.3%, and the interspecific difference from the other five new species was between 9.3 and 13.9%. In contrast to Dasineura spp. from Acacia, for which the morphology was more informative in species recognition than the cytochrome b sequence, in Asphondylia spp. treated here the partial cytochrome b sequence data provided better species recognition than did the morphology. Several of the new Asphondylia have potential as biological control agents in ecosystems in which Australian acacias are invasive and their sexual reproduction needs to be restricted. A list of Australian acacias whose reproductive organs are destroyed by known gall midges, all belonging to Dasineura and Asphondylia, is provided.     

Why a gall former can be a good biocontrol agent: the gall wasp Trichilogaster acaciaelongifoliae and the weed Acacia longifolia

ABSTRACT.

• 1 The pteromalid Trichilogaster acaciaelongifoliae (Froggatt) causes galls to develop in the place of inflorescences during the reproductive phase of its host Acacia longifolia (Andr.) Willd. (Fabaceae). After being introduced to South Africa during 1982 and 1983, this wasp reduced the reproductive potential of A. longifolia by >89% when only 50% of the branches on a tree were galled. In addition, vegetative growth of galled branches was reduced by 53% when the vacated galls had desiccated.
• 2 The wasp is able to manipulate its host so effectively because: (a) the dry mass of developing galls was significantly greater than that of the corresponding reproductive organs, peaking in spring (September) when galls were 25 times the mass of unfertilized inflorescences. (b) The large biomass diverted to gall production is accounted for by a high proportion (66–73%) of multiple-chambered galls. The dry mass of multiple-chambered galls was significantly greater than that of both single-chambered galls and pods. (c) The extent of biomass diversion to gall production was relatively greater during the earlier part of the reproductive season, enhancing stress on the host. (d) The wasp sometimes forced the plant to produce up to 200% more galls per branch than the normal quota of inflorescences. This phenomenon, called forced commitment, further increases stress on the host plant. (e) Galls constituted up to 21% and 40% of the dry and wet biomass of above-ground parts of infested trees, respectively. This caused breakage and mortality of large branches and stems. (f) Reproduction in A. longifolia has been shown elsewhere to be so energy consuming that vegetative growth is strongly inversely related to pod production. Because the stress from galling by T. acaciaelongifoliae coincides with and replaces reproduction with a greater stress, successful reduction of both reproduction and vegetative growth are achieved.
• 3 This biological control programme is the first in which a gall-forming hymenopteran has been used to control a weed. Since the effects of gall-formers are indirect compared with those of insects attacking vegetative plant parts, their potential as biocontrol agents has been underrated. This research provides ecologically based guidelines for the future selection of such agents.

     

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.

     

THE RELATIONSHIP BETWEEN A FLOWER GALL MIDGE PSEUDAS-PHONDYLIA SP. (DIPTERA: CECIDOMYIIDAE) AND ITS HOST PLANT ACTINIDIA VALVATA*

Abstract Kiwifruit plants, Actinidia sp., are native to subtropical China. The flower-bud gall of A. valvata, which is induced by an undescribed gall midge in the genus Pseud as phond ylia, is valued by the pharmaceutical industry. When studying the biology of the Actinid ia/Pseud as phond ylia interaction in Central-south China we found evidence suggesting that under certain circumstances the gall insect modifies the reproductive mode of the dioecious host plant. Surveys and field experiments in the National Hupingshan Natural Reserve showed a high frequency of galled trees. The density of galled trees varied among valleys and among trees within the valleys. In two valleys, 92% and 75%, respectively, of all trees were attacked, while in a third valley no trees were attacked. When infested, staminate tree only produced galls, whereas pistillate plants produced normal fruits as well as galls. Gall shape differed between male and female trees. Trees with galls tended to produce more fruits than treea without galls. We speculate that this is one of a few documented examples of an insect that induces androdioecy in an otherwise functionally dioecious plant.     

The chemical composition of plant galls: are levels of nutrients and secondary compounds controlled by the gall-former?

The chemical composition of galled and ungalled plant tissue was compared in a series of experiments. Gall and adjacent plant tissue was analysed for 20 species of gall-former on 11 different plant species. There were clear differences between galled and ungalled tissue in levels of nutrients and secondary compounds. Gall tissue generally contained lower levels of nitrogen and higher levels of phenolic compounds than ungalled plant tissue. The gall tissue produced by the same plant in response to different species of gall-former differed in chemical composition, as did the gall-tissue from young and mature galls of the same species. The chemical differences between gall and plant tissues were studied in more detail in two field manipulations. Firstly, the seasonal changes in phenolic biosynthesis in Pontania proxima and P. pedunculi (Hymenoptera: Tenthredinidae) gall tissue were compared to those of their host plants, Salix alba and S. caprea. In both types of gall tissue, phenolic levels declined as the season progressed, but levels in the surrounding plant tissue increased. When the gall insects were killed with insecticide, phenolic levels in the galled tissue dropped to the same level as those in adjacent plant tissue. Secondly, the density of Cynips divisa (Hymenoptera: Cynipidae) galls on Quercus robur leaves was reduced by removing half the galls present, either those from the central region of the leaf or those from the edge. Decreasing gall density increased the size of the remaining galls and the weight of the insects, but these effects were most marked when the galls remaining were growing centrally on the leaf, i.e. when the galls from the edge had been removed. Decreasing gall density increased the nitrogen content of the remaining galls, again to a greater extent in galls growing centrally on the leaf. The results of these studies suggest that the levels of nutrients and secondary compounds in gall tissue are usually markedly different to those of surrounding plant tissue, and that gall-formers may produce species-specific and temporally variable changes in the chemical composition of gall tissue. Received: 7 July 1997 / Accepted: 29 September 1997     

Manipulation of food resources by a gall-forming aphid: the physiology of sink-source interactions

Summary We examined the capacity of the galling aphid, Pemphigus betae, to manipulate the sink-source translocation patterns of its host, narrowleaf cottonwood (Populus angustifolia). A series of ¹⁴C-labeling experiments and a biomass allocation experiment showed that P. betae galls functioned as physiologic sinks, drawing in resources from surrounding plant sources. Early gall development was dependent on aphid sinks increasing allocation from storage reserves of the stem, and later development of the progeny within the gall was dependent on resources from the galled leaf blade and from neighboring leaves. Regardless of gall position within a leaf, aphids intercepted ¹⁴C exported from the galled leaf (a non-mobilized source). However, only aphid galls at the most basal site of the leaf were strong sinks for ¹⁴C fixed in neighboring leaves (a mobilized source). Drawing resources from neighboring leaves represents active herbivore manipulation of normal host transport patterns. Neighboring leaves supplied 29% of the ¹⁴C accumulating in aphids in basal galls, while only supplying 7% to aphids in distal galls. This additional resource available to aphids in basal galls can account for the 65% increase in progeny produced in basal galls compared to galls located more distally on the leaf and limited to the galled leaf as a food resource. Developing furits also act as skins and compete with aphid-induced sinks for food supply. Aphid success in producing galls was increased 31% when surrounding female catkins were removed.     

Cytological and histochemical gradients on two <Emphasis Type="BoldItalic">Copaifera langsdorffii</Emphasis> Desf. (Fabaceae)—Cecidomyiidae gall systems

Previous ultrastructural and histochemical analysis proposed patterns in the accumulation of substances in galls of Diptera: Cecidomyiidae in some plant species of the temperate region. Similar analyses were done to verify the conservativeness of these patterns in the Neotropical region, where a great number of species of Cecidomyiidae is responsible for a wide diversity of morphotypes. Two gall morphotypes induced by Cecidomyiidae in a unique host plant, Copaifera langsdorffii, were studied. The gradients of carbohydrates and the activity of invertases and acid phosphatases were similar, but the cytological gradients and distribution of proteins evidenced that the sites of the induction as well as the amount of neoformed tissues may be peculiar to each gall system. The production of lipids just in the secretory cavities either in the non-galled or galled tissues indicated a potentiality of the host plant which could not be manipulated by the galling insects. Further, the absence of nucleus in the nutritive tissue, an exclusive feature of the horn-shaped galls, indicates cell death attributed to the feeding habit of the galling herbivore.     

Changes in Clonal Poplar Leaf Chemistry Caused by Stem Galls Alter Herbivory and Leaf Litter Decomposition

Gall-inducing insects are highly specialized herbivores that modify the phenotype of their host plants. Beyond the direct manipulation of plant morphology and physiology in the immediate environment of the gall, there is also evidence of plant-mediated effects of gall-inducing insects on other species of the assemblages and ecosystem processes associated with the host plant. We analysed the impact of gall infestation by the aphid Pemphigus spirothecae on chemical leaf traits of clonal Lombardy poplars (Populus nigra var. italica) and the subsequent effects on intensity of herbivory and decomposition of leaves across five sites. We measured the herbivory of two feeding guilds: leaf-chewing insects that feed on the blade (e.g. caterpillars and sawfly larvae) and skeletonising insects that feed on the mesophyll of the leaves (e.g. larvae of beetles). Galled leaves had higher phenol (35%) and lower nitrogen and cholorophyll contents (35% respectively 37%) than non-galled leaves, and these differences were stronger in August than in June. Total herbivory intensity was 27% higher on galled than on non-galled leaves; damage by leaf chewers was on average 61% higher on gall infested leaves, whereas damage by skeletonising insects was on average 39% higher on non-galled leaves. After nine months the decomposition rate of galled leaf litter was 15% lower than that of non-galled leaf litter presumably because of the lower nitrogen content of the galled leaf litter. This indicated after-life effects of gall infestation on the decomposers. We found no evidence for galling x environment interactions.     

Distribution of metabolites in galled and non-galled foliar tissues of Tibouchina pulchra

This paper reports the contents of foliar metabolites of Tibouchina pulchra (Melastomataceae) in (a) galls induced by a lepidopteran, (b) remaining parts of the galled leaf after gall removal, (c) leaves opposite to the galled leaf, and (d) leaves of non-infested stem branches (control). The parameters assayed were soluble phenols, flavonoids, tannins, lignins, fibers, soluble carbohydrates, lipids and organic nitrogen. Differences in the parameters assayed were evaluated using Principle Components Analysis. Compared to other tissues, galls showed significantly higher contents of soluble phenols, tannins, lignins, fibers, soluble carbohydrates and lipids, and significantly lower contents of flavonoids and organic nitrogen. Apart from gall tissues, in most cases no significant differences were detected in the quantitative analyses among the leaf tissues assayed. Flavonols and flavones were not detected in galls. Other tissues revealed a similar flavonoid pattern, characterized by 3-O-monoglycosides of kaempferol, myricetin and quercetin. A luteolin glycoside was obtained exclusively from control leaves. Carbohydrate amounts are lower in the foliar tissues closer to the galls than in non-galled tissues. Palmitic acid was essentially the sole fatty acid found in all tissues analysed. The high lipid content of the galls suggests that such substances represent the main energy source for the insect, and suggests that the studied galls could be classified as cynipid galls. The observed metabolic changes taking place in the galls strengthen the hypotheses that galls behave as new organs, operating a metabolic machinery of their own.     

Nutrition as a facilitator of host‐race formation: the shift of a stem‐boring beetle to a gall host

1. The importance of host‐race formation to herbivorous insect diversity depends on the likelihood that successful populations can be established on a new plant host. A previously unexplored ecological aid to success on a novel host is better nutritional quality. The role of nutrition was examined in the shift of the stem‐boring beetle Mordellistena convicta to fly‐induced galls on goldenrod and the establishment there of a genetically distinct gall host race. 2. First, larvae of the host race inhabiting stems of Solidago gigantea were transplanted into stems and galls of greenhouse‐grown S. gigantea plants. At the end of larval development, the mean mass of larvae transplanted to galls was significantly greater than the mass of larvae transplanted to stems, indicating a likely nutritional benefit during the shift. This advantage was slightly but significantly diminished when the gall‐inducing fly feeding at the centre of the gall died early in the season. Additionally, there was a suggestion of a trade‐off in the increased mortality of smaller beetle larvae transplanted into galls. 3. In a companion experiment, S. gigantea gall‐race beetle larvae were likewise transplanted to S. gigantea stems and galls. Besides the expected greater mass in galls, the larvae also exhibited adaptations to the gall nutritional environment: larger inherent size, altered tunnelling behaviour, and no diminution of mass pursuant to gall‐inducer mortality. 4. In a third line of inquiry, chemical analyses of field‐collected S. gigantea plants revealed higher levels of mineral elements important to insect nutrition in galls as compared with stems.     

Aphid galls accumulate high concentrations of amino acids: a support for the nutrition hypothesis for gall formation

The nutrition hypothesis for the adaptive significance of insect gall formation postulates that galls accumulate higher concentrations of nutritive compounds than uninfested plant tissue, resulting in a high performance of the gall former. This hypothesis has been supported by some taxa of gall insects, but not by taxa such as cynipid wasps. Aphid galls are expected to require higher levels of nitrogen than other insects’ galls with a single inhabitant, because aphid galls are required to sustain a number of aphids reproducing parthenogenetically over two generations. The present study tested this hypothesis by evaluating aphid performance and amino acid concentration in phloem sap, using the aphid Rhopalosiphum insertum (Walker) (Homoptera: Aphididae), which establishes colonies on leaves of Sorbus commixta Hedlund or in galls of the aphid Sorbaphis chaetosiphon Shaposhnikov (Homoptera: Aphididae). We prepared the gall and non‐gall treatments on trees of S. commixta, in which R. insertum fundatrices were reared and allowed to reproduce. In S. chaetosiphon galls, R. insertum colonies propagated more rapidly, and the second generation grew larger and more fecund than on ungalled leaves. The amount of amino acids exuding from cut galled leaves was fivefold that in ungalled leaves; however, there was no significant difference in the amino acid composition between galled and ungalled leaves. In the intact leaves, total amino acid concentration in the phloem sap declined rapidly from late April to late May; however, the galls retained this high amino acid concentration in developing leaves for 1 month. These results indicate that the improved performance in R. insertum is ascribed to the increased concentration of amino acids in galled leaves. We suggest that S. chaetosiphon galls function to promote the breakdown of leaf protein, leading to an increased performance of gall‐inhabiting aphids.