Population dynamics of the yew gall midge Taxomyia taxi and its chalcid parasitoids: a 24-year study

Abstract.

• 1 The dynamics of three populations of Taxomyia taxi (Inchbald) (Diptera: Cecidomyiidae) and its chalcid parasitoids have been studied over a 24-year period. Most individuals have a 2-year life cycle but some develop in 1 year. Details of within-generation mortalities in T.taxi are used for life table analyses.
• 2 Mortality in the period between emergence of adult T.taxi and larval infestation of buds appears to be density-dependent and is the largest component of overall mortality. In 2-year life cycles, this mortality and that caused by Torymus nigritarsus (Walker) contribute equally to variance in overall mortality. In 1-year cycles, mortality caused by Mesopolobus diffinis (Walker) is density-dependent and accounts for most within-gall losses.
• 3 T. nigritarsus, which attacks only 2-year galls, is absent from all populations for a number of years in the middle of the study period. Its searching efficiency declines as its density and that of its host increase.
• 4 Densities of M.diffinis are strongly correlated with those of available hosts. Percentage parasitism of 2-year galls is less than that of 1-year galls, suggesting the occurrence of enemy-free space.
• 5 Although there are some correlations in densities and mortality between trees, the dynamics of populations on each are frequently different.
• 6 An earlier analysis of 10 years' data failed to demonstrate density-dependent effects. The extension to a 24-year run has revealed such effects, but also demonstrates long-term fluctuations in population densities, with corresponding changes in the balance of advantage between 1- and 2-year life cycles of T.taxi.

     

Formation of long‐lasting galls by overwintered nymphs in the Japanese aphid Quadrartus yoshinomiyai (Hemiptera: Aphididae: Hormaphidinae)

This paper examines the life history of a generation of galls created by the aphid Quadrartus yoshinomiyai (Hormaphidinae: Nipponaphidini) on its primary host plant, Distylium racemosum. First‐instar fundatrix nymphs of Q. yoshinomiyai initiated galls on stems of developing shoots in early April and incipient enclosed galls were found from later the same month. The galls lasted for up to 14 months, during which they grew to maturity, opened in early or mid‐April of the following year and dried up by the end of June. First‐instar fundatrix nymphs were found on winter buds, indicating that they hatched from eggs in autumn and overwintered as nymphs. These results suggest that Q. yoshinomiyai has a three‐year life cycle.     

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.

     

Red galls: the different stories of two gall types on the same host

• Several studies have suggested reasons why galls have conspicuous colours, but none of the ideas have been confirmed. However, what if the vibrant colours of some galls are explained simply by the effect of light exposure? This may lead to anthocyanin accumulation, functioning as a defence mechanism against the effects of high light.
• We studied the globoid galls induced by Cecidomyiidae (Diptera) on Qualea parviflora (Vochysiaceae), relating anthocyanin accumulation and chlorophyll fluorescence parameters to light incidence in abaxial and adaxial galls. We also tested if the anthocyanin accumulation patterns apply to another Cecidomyiidae‐induced gall morphotype (intralaminar) within the same plant.
• Adaxial galls are exposed to higher incident light, with more anthocyanin accumulation and therefore red coloration. In galls from angled leaves, the greater the angle of the leaf, the higher the difference between anthocyanins on the sun and shade sides of galls. Photosynthetic pigment concentrations did not differ between abaxial and adaxial galls. However, we found higher (F_m′ ? F′)/F_m′ and F_v/F_m in the abaxial galls. Conversely, NPQ and R_fd were higher in adaxial galls. Finally, the pattern of anthocyanin accumulation was not found in the intralaminar gall.
• Anthocyanin accumulation in galls functions as a photoprotective strategy, maintaining tissue vitality in regions exposed to high light conditions. However, this mechanism may vary even among galls within the same host, indicating idiosyncrasy when it comes to coloration in galls. To date, this is the first study to demonstrate quantitatively why the galls of a specific species may be coloured: the variation in light regimes creates differential anthocyanin accumulation, influencing coloration.

     

A plant pathogen causes extensive mortality in an invasive insect herbivore

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Alien herbivores and native parasitoids: rapid developments and structure of the parasitoid and inquiline complex in an invading gall wasp Andricus quercuscalicis (Hymenoptera: Cynipidae)

Abstract.

• 1 Rapid and substantial changes have occurred in the parasitoid and inquiline community associated with the agamic galls of Andricus quercuscalicis since it invaded Britain in the late 1950s. The number of parasitoid and inquiline species has risen from one to thirteen over a 15-year period. Although the number of species has been relatively consistent over the last 8 years, the species composition has changed considerably and in a highly characteristic way during this period.
• 2 The parasitoid complex can be divided into two broadly distinct sets of parasitoid species; one set attacks only the gall former whereas the other set concentrates on the inquilines living in the wall of the gall.
• 3 The most dramatic change, however, is in the abundance of inquilines which were reported to be virtually absent in earlier studies on this community in Britain. Over a period of only 5 years, between 1988 and 1993, inquiline attack rose from less than 0.01 to an average of 0.26 inquilines per gall. The intensity of inquiline attack is geographically heterogenous, with high inquiline numbers restricted to south-east England. Because of the relatively high specificity of the parasitoids, high inquiline abundance is positively correlated with parasitoid species richness in knopper galls.
• 4 Parasitism rates, particularly on the gall former, were generally low (<10%). Over the last 5 years, however, seven parasitoid species have been consistently recorded and the mortality caused by these species has increased continuously. The species composition of the community associated with this alien gall wasp in Britain has quickly converged to the community known from its native range in continental Europe. Parasitoid species known to attack the galls of A.quercuscalisis on the continent have been recorded from it in Britain for the first time mainly in areas where inquilines have recently become abundant.
• 5 Since rates of parasitism of the gall former are still low, parasitoids are unlikely to play a major role in the population dynamics of this invading gall wasp at present, but the rapidly increasing inquiline and parasitoid attack could be a source of increased mortality for native cynipid species which are the alternative hosts of those parasitoid species.

     

Effects of large‐scale host plant addition and removal on parasitoid‐mediated associational resistance in the gall midge Asphondylia borrichiae

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Enzyme‐mediated metabolism in nutritive tissues of galls induced by Ditylenchus gallaeformans (Nematoda: Anguinidae)

• The galls induced by Ditylenchus gallaeformans (Nematoda) on leaves of Miconia albicans have unique features when compared to other galls. The nematode colonies are surrounded by nutritive tissues with promeristematic cells, capable of originating new emergences facing the larval chamber, and providing indeterminate growth to these galls. Considering enzyme activity as essential for the translocation of energetic molecules from the common storage tissue (CST) to the typical nutritive tissue (TNT), and the major occurrence of carbohydrates in nematode galls, it was expected that hormones would mediate sink strength relationships by activating enzymes in indeterminate growth regions of the galls.
• Histochemical, immunocytochemical and quantitative analyses were made in order to demonstrate sites of enzyme activity and hormones, and comparative levels of total soluble sugars, water soluble polysaccharides and starch.
• The source–sink status, via carbohydrate metabolism, is controlled by the major accumulation of cytokinins in totipotent nutritive cells and new emergences. Thus, reducing sugars, such as glucose and fructose, accumulate in the TNT, where they supply the energy for successive cycles of cell division and for nematode feeding. The histochemical detection of phosphorylase and invertase activities indicates the occurrence of starch catabolism and sucrose transformation into reducing sugars, respectively, in the establishment of a gradient from the CST towards the TNT. Reducing sugars in the TNT are important for the production of new cell walls during the indeterminate growth of the galls, which have increased levels of water‐soluble polysaccharides that corroborate such a hypothesis.
• Functional relationship between plant hormone accumulation, carbohydrate metabolism and cell differentiation in D. gallaeformans‐induced galls is attested, providing new insights on cell development and plant metabolism.

     

The life history of a gall‐inducing mite: summer phenology,predation and influence of gall morphology in a sugar maple canopy

• 1 Eriophyoid mites are among the most ubiquitous gall‐inducing arthropods, and are adapted species‐specifically to a broad diversity of plants, although their life histories remain poorly studied outside agricultural systems.
• 2 We examined the seasonal phenology of a leaf‐galling eriophyid mite, the maple spindle gall mite Vasates aceriscrumena (MSGM), in naturally occurring stands of sugar maple Acer saccharum in south‐central Ontario in 2007 and 2008.
• 3 Galls were first induced in spring (mid‐May) and were devoid of mites by late August. In the study region, MSGM appears to have at least two generations, with overwintering, deutogyne females that initiate galls in spring (mid‐May) after leaf flush, giving rise to a generation of protogyne (primary) females and a few morphologically similar males (<1 for every 10 females) and, subsequently, to a new generation of deutogyne females in mid‐July to early August. In July, some galls can be highly crowded, with 50–200 individuals per gall.
• 4 In addition, a tarsonemid mite, Tarsonemus acerbilis, was found in approximately 40% of MSGM galls examined. As much as 95.4% of galls in 2007 and 97.4% in 2008 that contained tarsonemid larvae did not contain MSGM eggs (by contrast, only 2.3% of tarsonemid‐free galls contained no MSGM eggs), suggesting that these juveniles feed, at least opportunistically, on MSGM eggs.
• 5 Gall ostiole morphology appeared to influence both MSGM and Tarsonemus densities within galls, with ‘open’ ostioles (versus ‘closed’) being much more susceptible to invasion by the tarsonemid. The latter is likely to be an important regulator of MSGM populations. We hypothesize that the two ostiole types are the result of selection pressures on the gall inducer, favouring closed gall entrances for increased protection, and possibly also on the host tree, favouring open galls to increase predator access.

     

Survivorship of the sexual and agamic generations of Andricus quercuscalicis on Quercus cerris and Q.robur

ABSTRACT.

• 1 Survivorship within the galls of the sexual and agamic generations of Andricus quercuscalicis (Burgsdorf) is estimated from field populations.
• 2 The sexual generation on Turkey oak suffers a 20% death rate through parasitism by Mesopolobus spp. The emerging parasitoids are virtually all males.
• 3 The agamic generation on the acorn cups of English oak is not attacked by any parasites or inquilines at our Berkshire site despite the fact that most of the species which parasitize the gall in continental Europe are on the British list.
• 4 Agamic galls cause average losses of between 30% and 50% of the total acorn crop in different years. Individual trees differ greatly in their rates of loss, many losing over 90%, others losing virtually none.

     

Structure,biochemistry and ecology of entomogenous galls in Selaginella Pal. Beauv. (Selaginellaceae) from India

Abstract

We studied the structure of newly found entomogenous galls in three Indian species of Selaginella, and the biochemical changes during gall development as well as their seasonal abundances in 10 squares of 1.5 m² under two differing habitat conditions, open and covered. Adult wasps (Cynipidae) initiate two types of gall formation by oviposition, spherical galls on vegetative shoots and elongated, club-shaped strobilar galls on reproductive shoots. Galls are anatomically and biochemically different from the unaffected shoots. Vegetative shoot apices bear more galls (84.6%) than reproductive shoot apices (15.4%). Gall frequency is significantly higher in covered (94.8%) than in open habitat (5.2%), and its seasonal peak occurs earlier in the former (43.1% during the rainy season) than in latter habitat (52.8% in autumn). We discuss the relationship between relative species density and gall seasonality as well as the possible role of certain environmental factors that make covered habitats more favorable to the gall inducer.     

The distribution of risk and density-dependent mortality in the galls of Eurosta solidaginis, the goldenrod gall fly

ABSTRACT.

• 1 This paper explores the net effect of a suite of mortality factors on a sedentary prey, the larvae of the goldenrod gall fly, Eurosta solidaginis Fitch (Diptera: Tephritidae).
• 2 Mortality is caused by unknown factors early in larval development, two species of parasitoid wasp (Hymenoptera: Eurytomidae), an inquiline beetle larva (Coleoptera: Mordellidae), and during the winter months downy woodpeckers Picoides pubescens (L.).
• 3 Distribution of mortality among galls relative to prey (gall) distribution was measured and discussed with respect to the distribution of relative risk of predation.
• 4 Galls are by and large contagiously distributed among quadrats, and mortality is distributed in a comparable pattern to that of galls.
• 5 The pattern of mortality on Eurosta larvae is neither density-dependent nor aggregated independently of gall distribution. Persistence in the system is probably a result of a combination of other factors such as adult mortality and early larval death which may have intergenerational density-dependent effects, and the linkage of locally unstable sub-populations via migration.

     

Resource partitioning and exploitative competition in stem-galling midges mediated by the host plant

Izeniola obesula Dorchin and Stefaniola defoliata Dorchin (Diptera, Cecidomyiidae) are two cecidomyiid species inducing multichambered galls in young shoots of the salt marsh plant, Suaeda monoica Gmelin (Chenopodiaceae). The purpose of this work was to study the relationship between the two species, which co‐occur in time and space, and may therefore potentially compete for plant resources. Characterization of the resources available to ovipositing females revealed that shoots in apical positions are more vigorous than shoots in basal positions, but that the two species of gall midges react to this variability of resources in different ways, as reflected by the patterns of gall distribution. Izeniola obesula galls were primarily induced in the younger, more rapidly growing shoots, whereas those of S. defoliata were predominantly induced in older, less vigorous shoots. Nevertheless, despite the general partitioning of resources, a certain amount of niche overlap was found, resulting in the occasional induction of an S. defoliata gall proximal to that of I. obesula in the same shoot. Co‐occurrence of the two types of galls in time and space did not have apparent effect on S. defoliata galls, but did have a strong adverse impact on young I. obesula galls, 60% of which were killed without producing adult midges, and the remaining 40% produced a much smaller number of adults. These effects are attributed to asymmetric interspecific competition that is mediated by the plant, resulting from the difference in stem tissues utilized by the larvae of the two species. It is concluded that preponderance of vacant niches, low densities of galls, and small niche overlap between the species do not preclude the occurrence of interspecific exploitative competition in this system, although its role in shaping the populations of the species is limited.     

Seasonal flight activity and genetic relatedness of Torymus species in Italy

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Oviposition choice and larval survival of Dasineura marginemtorquens (Diptera: Cecidomyiidae) on resistant and susceptible Salix viminalis

Abstract.

• 1 Substantial intraspecific variation exists in Salix viminalis resistance to the gall midge Dasineura marginemtorquens. Earlier work has found this variation to have a large genetic component. Willow clones are stable in their resistances between midge generations and different nutrient levels in both field and laboratory culture.
• 2 This study reports the results of laboratory experiments on female oviposition choice and larval survival on potted plants from clones that are very different in resistance as determined in field studies.
• 3 In choice experiments using pairs of plants, the average female midge did not prefer susceptible willow clones over resistant ones for oviposition. In about one third of the replicates, midges actually laid more eggs on the resistant clone. Further work is necessary to examine the nature of variation among midges in discrimination of these plant types.
• 4 Resistance is manifested as great differences in larval survival. Six days after oviposition survival was 92% on susceptible plants but only 6% on resistant ones. Galls developed on all of the susceptible plants, while in 73% of the resistant plants galls were not even initiated.
• 5 The plant traits causing resistance are enigmatic. Larval behaviour suggests that resistant plants interfere with feeding behaviour. On resistant plants, most larvae wander for more than 24 h without initiating any galls before dying. On susceptible plants many first instar larvae begin feeding and initiate galls within this period.

     

The functional resource of a gall-forming adelgid

Interactions among shoots within plant modules could allow gall-insects to acquire resources from other plant parts near the feeding sites. As a result, nearby plant parts may act as a functional resource, or extended resource base. We tested for functional interconections between galls and adjacent ungalled shoots in Adelges cooleyi Gil. (Homoptera: Adelgidae) on Picea engelmanni, Engelmann spruce. Observations of gall and surrounding shoot weights showed that gall weights were twice normal shoot weights, but that surrounding shoot weights were unaffected. Reducing photosynthate availability by covering galls or surrounding shoots with opaque cloth suggested that functional interconnections exist among them; covering galls reduced surrounding current-year ungalled shoot weight, and covering surrounding shoots reduced their weight even further, but neither covering treatment affected gall or gall-insect weight. These patterns suggest that surrounding shoots constitute an extended and flexibly utilized resource base for adelgid galls. Resources made available via functional interconnections appeared to be equally available throughout adelgid galls. No differences were found in gall-insect performance in parts of the gall closer to connections with surrounding shoots compared to more distantly-located gall-insects. Further studies are required to examine patterns of resource movement among unmanipulated galls and shoots. Functional resources may be common features of plant/gall-insect interactions, potentially playing an important role in gall-insect reproductive success and habitat selection.     

How detrimental are seed galls to their hosts? Plant performance,germination, developmental instability and tolerance to herbivory in Inga laurina,a leguminous tree

• Gall inducers use these structures as shelters and sources of nutrition. Consequently, they cause multiple physiological changes in host plants.
• We studied the impact caused by seed coat galls of a braconid wasp on the performance of fruits, seeds and seedlings of tree Inga laurina. We tested whether these seed galls are ‘nutrient sinks’ with respect to the fruit/seed of host plant, and so constrain the reproductive ability and reduce seedling longevity. We measured the influence of such galls on the secondary compounds, fruit and seed parameters, seed viability and germination and seedling performance.
• Inga laurina has indehiscent legumes with polyembryonic seeds surrounded by a fleshy sarcotesta rich in sugars. The galls formed inside the seed coat and galled tissues presented higher phenol concentrations, around 7‐fold that of ungalled tissues. Galls caused a significant reduction in parameters such as fruit and seed size, seed weight and the number of embryos. Fluctuating asymmetry (a stress indicator) was 31% higher in leaves of galled seed plants in comparison to ungalled seed plants. However, the negative effects on fruit and seed parameters were not sufficient to reduce seed germination (except the synchronization index) or seedling performance (except leaf area and chlorophyll content).
• We attributed these results to the ability of I. laurina to tolerate gall attack on seeds without a marked influence on seedling performance. Moreover, because of the intensity of seed galling on host plant, we suggest that polyembryony may play a role in I. laurina reproduction increasing tolerance to seed damage.

     

Host plant effects on the development and survivorship of the galling insect Neopelma baccharidis (Homoptera: Psyllidae)

In this study, the mortality factors acting upon the galling psyllid Neopelma baccharidis Burckhardt (Homoptera) caused by its host plant, Baccharis dracunculifolia De Candole (Asteraceae) were analysed. In March 1999, 982 galls of the same cohort were randomly marked on 109 individuals of B. dracunculifolia in the field. Galls were censused each month during their development, from April to August, and dead galls were collected and analysed for mortality factors. Gall dehiscence rates were calculated for each month. The major mortality source of N. baccharidis was gall dropping (13.2% of the original cohort), which is probably a normal outcome of previous mortality caused by the other factors observed in this study. Unknown factors killed 11.7% of this gall population and were ascribed to plant resistance during gall development. Empty galls represented 7.7% of the observed mortality and may be a consequence of egg retention or egg mortality/abortion related to variations in plant quality. Shoot mortality was high during the dry season and killed 7.5% of the galls, but this impact was minimized after the third month from gall formation due to the ability of nymphs to accelerate development and emerge from galls on dying shoots. However, the size of dehisced galls on dead shoots tended to be smaller, possibly affecting adult performance. Mortality of N. baccharidis attributed to B. dracunculifolia strongly controlled the galling insect population, killing 40.7% of the original cohort of galls. Plant‐mediated mortality was caused by often neglected factors acting predominantly during the first 3 months of development, which are critical to gall survivorship. These results reinforce the importance of bottom‐up forces in plant‐insect systems.     

Invasion by the chestnut gall wasp in Italy causes significant yield loss in Castanea sativa nut production

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Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher , g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and , but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO₂ concentration relationships were consistent with the explanation that increased allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low- limitations on non-stomatal components of A. Our working hypothesis is that galls increase and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.