Effect of host-plant manipulation by a gall-inducing insect on abundance of herbivores on chestnut trees

We investigated plant-mediated effects of the stem gall wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), on other herbivores on the chestnut tree Castanea crenata. In the early season, leaves emerged earlier and in greater numbers on galled shoots than on ungalled shoots. On galled shoots the leaf to shoot biomass ratio was lower and the leaves were physically different. In May and June the concentration of nitrogen in leaves was higher on galled shoots than on ungalled shoots. In July, the water content of leaves was lower on galled shoots. In May and June, the number of aphids, Myzocallis kuricola Matsumura (Homoptera: Aphidoidea), on leaves was higher on galled shoots than on ungalled shoots, but the opposite was true at the end of July. Laboratory experiments showed that aphid fecundity and body weight decrease were higher in May and June when aphids fed on leaves on a galled shoot than when they fed on those on ungalled shoots. In contrast, aphid performance in July was greater on ungalled leaves than on galled leaves. Our findings suggest that gall initiation in a chestnut tree affected aphid performance by affecting host plant quality.     

Plant phenology-mediated indirect effects: The gall midge opens the phenological window wider for a leaf beetle

We examined whether larvae of the gall midge Rabdophaga rigidae (Diptera: Cecidomyiidae) can modify the seasonal dynamics of the density of a leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), by modifying the leaf flushing phenology of its host willow species, Salix serissaefolia and Salix eriocarpa (Salicaceae). To test this, we conducted field observations and a laboratory experiment. The field observations demonstrated that the leaf flushing phenology of the willows and the seasonal dynamics of the beetle density differed between shoots with stem galls and shoots without them. On galled shoots of both willow species, secondary shoot growth and secondary leaf production were promoted; consequently, leaf production showed a bimodal pattern and leaf production periods were 1 to 2 months longer than on non‐galled shoots. The adult beetle density on galled shoots was thus enhanced late in the season, and was found to change seasonally, synchronizing with the production of new leaves on the host willow species. From the results of our laboratory experiment, we attributed this synchrony between adult beetle density and willow leaf flush to beetles’ preference to eat new leaves rather than old. Indeed, beetles consumed five times more of the young leaves when they were fed both young and old leaves. These results indicate that stem galls indirectly enhance the adult beetle density by enhancing food quality and quantity late in the beetle‐feeding season. We therefore conclude that midge galls widen the phenological window for leaf beetles by extending the willows’ leaf flush periods.     

Leaf Production and Morphology in the Artichoke Gall of Yew (Taxus baccata L.)

Comparative leaf production rates and leaf morphology studiesfor galled and normal shoots of yew trees have been obtainedthroughout the life cycle of the causative agent, the gall midge,Taxomyia taxi. Normal and galled shoot leaf numbers have beenrelated to those of their parent shoots. It was found that whereasthe annual leaf production of normal shoots was positively relatedto that of the parent shoots, galled shoot leaf production remainedconstant regardless of parent shoot vigour showing leaf stimulationby the midge to be a special case. The midge larva appears tobe determining the rate of leaf production in galled shoots.In galls leaf production continues throughout the winter monthswith no dormant period. From morphological evidence, alternationof leaves and cataphylls is continued in galled buds.     

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.     

Stem galls affect oak foliage with potential consequences for herbivory

Abstract.   1. On two dates, foliar characteristics of pin oak, Quercus palustris , infested with stem galls caused by the horned oak gall, Callirhytis cornigera , were investigated, and the consequences for subsequent herbivory assessed.
2. Second-instar caterpillars of the gypsy moth, Lymantria dispar , preferred foliage from ungalled trees.
3. Ungalled trees broke bud earlier than their galled counterparts.
4. Galled trees produced denser leaves with higher nitrogen and tannin concentrations, but foliar carbohydrates did not differ among galled and ungalled trees.
5. Concentrations of foliar carbohydrates in both galled and ungalled trees increased uniformly between the two assay dates. Nitrogen concentrations were greater in leaves from galled trees, and decreased uniformly in galled and ungalled trees over time. Foliar tannins were also greater in foliage from galled trees early in the season; however, foliar tannins declined seasonally in galled tissue so that by the second assay date there was no difference in tannin concentrations between galled and ungalled foliage.
6. In spite of differences in foliar characteristics, performance of older, fourth instar gypsy moth caterpillars did not differ between galled and ungalled trees.     

Edaphic environment, gall midges, and goldenrod clonal expansion in a mid-successional old-field

During the middle stage of old-field succession, genets of clonal plants vie to take over space from annual and short-lived perennial plants. We studied factors that may influence the relative rates of expansion of Solidago altissima genets in an old-field population attacked by the gall midge Rhopalomyia solidaginis. Genets growing in more clayey soil expanded more slowly, as evidenced by differences in rhizome growth. Edaphic conditions also affected galling frequencies, with genets in more sandy soil having twice as many galls. Gall midges reduced goldenrod stem growth, and stem height was positively correlated with rhizome growth. For a given stem height, galled ramets allocated relatively more biomass to rhizome growth than ungalled ramets. The end result was that galled ramets produced the same number and sizes of rhizomes as ungalled ramets.     

Effects of gall damage by the introduced biocontrol agent Epiblema strenuana (Lep., Tortricidae) on the weed Parthenium hysterophorus (Asteraceae)

Effects of gall damage by the introduced moth Epiblema strenuana on different growth stages of the weed Parthenium hysterophorus was evaluated in a field cage using potted plants with no competition and in naturally regenerated populations with intraspecific competition. Gall damage at early stages of plant growth reduced the plant height, main stem height, flower production, leaf production, and shoot and root biomass. All galled, potted plants with no competition produced flowers irrespective of the growth stage at which the plants were affected by galling, but lesser than in ungalled plants. Gall induction during early growth stages in field plants experiencing competition prevented 30% of the plants reaching flowering. However, 6% of the field plants escaped from gall damage, as their main stems were less vigorous to sustain the development of galls. Flower production per unit total plant biomass was lower in galled plants than in ungalled plants, and the reduction was more intense when gall damage was initiated at early stages of plant growth. In potted plants with no competition, the number of galls increased with the plant vigour, as the gall insects preferred more vigorous plants. But in field plants there were no relationship between gall abundance and plant vigour, as intraspecific competition enhanced the negative effects of galling by reducing the vigour of the weed.     

Galling by Rhopalomyia solidaginis alters Solidago altissima architecture and litter nutrient dynamics in an old-field ecosystem

Plant–insect interactions can alter ecosystem processes, especially if the insects modify plant architecture, quality, or the quantity of leaf litter inputs. In this study, we investigated the interactions between the rosette gall midge Rhopalomyia solidaginis and tall goldenrod, Solidago altissima, to quantify the degree to which the midge alters plant architecture and how the galls affect rates of litter decomposition and nutrient release in an old-field ecosystem. R. solidaginis commonly leads to the formation of a distinct apical rosette gall on S. altissima and approximately 15% of the ramets in a S. altissima patch were galled (range: 3–34%). Aboveground biomass of galled ramets was 60% higher and the leaf area density was four times greater on galled leaf tissue relative to the portions of the plant that were not affected by the gall. Overall decomposition rate constants did not differ between galled and ungalled leaf litter. However, leaf-litter mass loss was lower in galled litter relative to ungalled litter, which was likely driven by modest differences in initial litter chemistry; this effect diminished after 12 weeks of decomposition in the field. The proportion of N remaining was always higher in galled litter than in ungalled litter at each collection date indicating differential release of nitrogen in galled leaf litter. Several studies have shown that plant–insect interactions on woody species can alter ecosystem processes by affecting the quality or quantity of litter inputs. Our results illustrate how plant–insect interactions in an herbaceous species can affect ecosystem processes by altering the quality and quantity of litter inputs. Given that S. altissima dominates fields and that R. solidaginis galls are highly abundant throughout eastern North America, these interactions are likely to be important for both the structure and function of old-field ecosystems.     

Effect of gall formation by a cynipid wasp, Andricus symbioticus, on the development of the leaves and shoots of Quercus dentata

The sexual generation of a cynipid wasp, Andricus symbioticus Kovalev, forms its leaf galls most frequently near and on the leaf petiole of Quercus trees. I examined the effect of gall formation by A. symbioticus on the leaf development of a host plant, Quercus dentata Thunberg, by comparing the size and shape of galled and ungalled leaves. I also examined the effect of gall formation on shoot development by comparing the length of shoots with and without galled leaves. Three of seven Q. dentata trees surveyed were heavily infested with A. symbioticus. Leaf size did not differ between galled and ungalled leaves. However, the ratio of leaf width to length was greater in galled leaves, which is regarded to be a result of gall formation by A. symbioticus inhibiting the growth in length of Q. dentata leaves. Shoot length did not differ significantly between shoots with and without galled leaves. These results suggest that galls of A. symbioticus act as a sink that competes with leaves for reserved photoassimilates.     

The impact of two gall-forming arthropods on the photosynthetic rates of their hosts

The impact of herbivores on host plant photosynthetic rates can range from negative to positive. While defoliation by chewing herbivores can result in increases in photosynthesis followed by compensatory growth, other herbivore guilds, such as mesophyll feeders which damage photosynthetic leaf tissues, almost always reduce photosynthetic rates. The impact of galling herbivores on host photosynthesis has rarely been examined, even though the limited tissue disruption and the strong metabolic sinks induced by gall-forming herbivores could potentially stimulate photosynthetic rates. I examined the hypothesis that gall-inducing herbivores could stimulate photosynthesis in neighboring leaves in response to increased sink-demand by the gall. To address this hypothesis, I measured photosynthetic rates of galled leaves or leaflets, neighboring ungalled leaves or leaflets, and ungalled leaves or leaflets on ungalled shoots on naturally growing Prunus serotina (wild cherry) and Rhus glabra (smooth sumac). The leaves of wild cherry were galled by an eriophyid mite, Phytoptus cerasicrumena; the leaves of smooth sumac by an aphid, Melaphis rhois. I found that both species reduced the photosynthetic rates of the leaves or leaflets they galled from 24 to 52% compared to ungalled leaves in ungalled areas of the plants. Contrary to my hypothesis, mite galls on wild cherry reduced photosynthesis of neighboring ungalled leaves within the same shoot by 24% compared to ungalled leaves on gall-free shoots. Aphid galls on sumac leaflets did not significantly alter the photosynthetic rates of neighboring leaflets relative to ungalled leaves on ungalled shoots. Although gall-formers would appear to have the potential to stimulate photosynthesis in the same manner as defoliating herbivores, i.e., by increasing sink demand relative to source supply, I found only negative impacts on photosynthesis. I suggest that sink competition for nutrients between developing leaves and growing gall tissue may account for the negative impacts of sink-inducing gallers on photosynthesis. Received: 17 October 1997 / Accepted: 2 February 1998     

How does spittlebug oviposition affect shoot growth and bud production in two willow species?

We measured the effects of oviposition by the spittlebug Aphrophora pectoralis on shoot growth and bud production in two willow species, Salix miyabeana and Salix sachalinensis. In autumn, adult females of A.pectoralis insert their ovipositor into the apical region of 1-year-old shoots, resulting in the death of most shoot tips within 1week. Consequently, an increase in the number of dead buds and a decrease in the number of vegetative buds on 1-year-old shoots was recorded. In the following spring, the growth of current-year shoots was greatly increased on 1-year-old shoots damaged by spittlebug oviposition. Furthermore, spittlebug oviposition increased the production rate of vegetative buds in both S.miyabeana and S.sachalinensis. However, no impact on the production rate of reproductive buds was detected in either willow. We conclude that the compensatory growth of current-year shoots and an increase in vegetative buds in the two willow species was caused by oviposition of A.pectoralis.     

Distribution and phenology of Dasineura oxycoccana (Diptera: Cecidomyiidae) in Michigan blueberries

The blueberry gall midge, Dasineura oxycoccana Johnson, is a serious pest of rabbiteye blueberries in Florida, Georgia, and Mississippi, and a potential pest of southern and northern highbush blueberries. Its damage has been observed with increasing frequency in highbush blueberry plantings in the Great Lakes region, including in Wisconsin and in Michigan. Unlike in rabbiteye blueberry plantings, where blueberry gall midge primarily damages flowering buds, it is found to damage only the vegetative shoots of northern highbush blueberry. In this study, farms throughout Michigan were surveyed for the presence of blueberry gall midge and it was found in 43 of 46 sampled farms in 11 counties. From 2009-2011, several monitoring techniques, including yellow sticky traps, emergence traps, observational sampling, and vegetative shoot dissections were used to determine the ecology of this species in blueberry fields in southwest Michigan. Emergence traps were most useful in early detection of blueberry gall midge in April, and observational sampling for damage symptoms and vegetative shoot dissections revealed multiple population peaks throughout July and August. Infestation was detected in vegetative shoot tips in all parts of the bushes, with initial infestation greatest at the base of bushes. Degree day accumulations until first midge detection and peak infestation suggest some potential for predicting key events in the pest's phenology. This information about the distribution and timing of infestation will be useful in developing management strategies for blueberry gall midge infestation.     

Lateral bud development and shoot growth in low-pruned Morus alba as affected by stem orientation

Two phases of bud activity were identified in the new growth of one-year-old erect coppice shoots on 11-year-old low-pruned stumps of mulberry (Morus alba L. cv. Shin-ichinose) in spring, the sprouting phase in which the majority of the buds, including the basal ones, sprout and elongate, and the dominance phase (starting 4–5 weeks after sprouting) during which the upper laterals begin to assert dominance and suppress the growth of lower laterals, becoming new leading shoots. In contrast, arching before sprouting markedly inhibited buds on the under side, leading to poor shoots. By late April, the sprouts on the upper side grew readily into new erect shoots, resulting in considerable dominance over those from the lateral sides. Of these erect shoots, those located closer to the stem base grew more in May and June. The effects of arching made during the sprouting phase (late April) on bud activity and shoot lengths were generally similar to those of earlier archings before spring bud bursting. Separation of the shoots from the upper and under sides by longitudinal, horizontal splitting of the arched stems in late April did not affect the inhibited elongation of the shoots from the under side. These results suggest that in the response to arching before and in late April, the effects are related to spring bud bursting and gravimorphism. In contrast, arching during and after the dominance phase (May) had no gravimorphic effects on growth of the shoots on the upper side, although there was a stimulation of outbreak of the buds on the upper side, which remained dormant during spring bud bursting. Continuous basal applications of abscisic acid in aqueous solution inhibited bud break and shoot growth of the postdormant erect stem segments, and defoliation of the new shoots markedly. In contrast, similar applications of an ethylene-releasing compound, Ethephon, depressed shoot elongation slightly, but enhanced defoliation greatly. Gibberellic acid (GA₃) stimulated shoot elongation, but depressed leaf enlargement.     

Manipulation of host plant development by the gall-midge Rhabdophaga strobiloides

Abstract. 1. The gall-midge Rhabdophaga strobiloides (O.S.) forms a gall in the apical bud of actively growing willow twigs.
2. Galls were not randomly distributed among twigs. Twigs that arose towards the distal end of the branch were much more likely to be galled. Distally located twigs also grew to greater girth than more proximally located twigs.
3. Comparisons of galled twigs with normal twigs in similar locations along their branch showed that the gall causes even greater growth in twig girth than when no gall is present.
4. The hypothesis that galled twigs draw photosynthate produced elsewhere in the plant was tested in an experiment that measured the growth of galled and normal twigs. with their leaves intact, to galled and normal twigs that were manually defoliated. Defoliation caused reduced growth in normal twigs, but galled twigs grew equally well with or without their leaves. Leaf removal had no effect on gall growth.
5. Twig diameter was positively correlated with gall diameter. Call diameter was positively correlated with larval biomass.
6. Patterns of twig and gall growth suggest that the gall-midge manipulates host plant growth and development to provide resources for growth and survival. Manipulation of the host may be an important phenomenon in the evolution of parasitic organisms.     

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.     

Sex ratio in relation to season and host plant quality in a monogenous stem-galling midge (Diptera: Cecidomyiidae)

Abstract.  1. The sex determination mechanism in gall midges is little understood, although it is known that the females of several species primarily or exclusively produce unisexual broods throughout their lifetime.
2. The gall midge Izeniola obesula Dorchin is a multivoltine species, inducing multi-chambered stem galls on the salt-marsh plant Suaeda monoica . Each gall contains 5–70 individuals, all being the progeny of a single female. Sampling of more than 450 galls, from which adult midges were reared, suggested that I .  obesula exhibits strict monogeny, resulting in galls that contain either all female or all male progeny.
3. Characterisation of the growth pattern of young S .  monoica shoots revealed that shoots in apical positions grew more rapidly than shoots in more basal positions. Galls that were induced on such shoots were larger and yielded more adult midges.
4. No difference in the site of gall induction was found between male and female galls, with galls of either sex being mainly induced on more rapidly growing shoots. It is concluded that I .  obesula females cannot control the sex of their progeny, and that both female-producers and male-producers strive to maximise their reproductive success by choosing the faster-growing shoots for gall induction.
5. Female galls were larger and more abundant than male galls at almost all times. The sex ratio among galls fluctuated throughout the year, ranging from 4:1 in spring to 1:1 in winter. The skewed sex ratio among galls possibly results from greater mortality rates among male galls than among female galls, due to either primary or secondary factors. Alternatively, it is possible that the number or fitness of male-producers in the population is reduced relative to female-producers.     

Effect of nitrogen fertilization on growth of Arundo donax and on rearing of a biological control agent,the shoot gall-forming wasp Tetramesa romana

The shoot tip-galling wasp Tetramesa romana Walker (Hymenoptera: Eurytomidae) has been released for biological control of giant reed or arundo (Arundo donax L.) (Poaceae), an invasive grass in the USA and Mexico. The role of urea fertilization to improve plant-based mass-rearing was examined. In a greenhouse study, rhizomes were fertilized with urea pellets at rates equivalent to 1000 kg (low), 2000 kg (moderate), and 4000 (high) kg N per ha^–1. Total nitrogen content of ungalled stems was significantly 0.60–0.65% higher under low and moderate fertilization compared to unfertilized pots, and shoot water content was elevated 3–4% at all urea levels. Moderate fertilization significantly (by 1.4-fold) increased the relative growth rate of all shoots in pots, but did not affect final dry biomass. Fertilization did not affect number and duration of probing events by females. The percentage of shoots colonized by wasps that were galled, progeny production per shoot and per female, and emergent wasp size were not affected. However, average generation time (adult to adult) of emergent wasps was 4–5 days shorter on shoots in pots under moderate and high urea fertilization. After a four-week wasp emergence period, only 3–9% of progeny remained in fertilized shoots, while 21% of progeny remained inside unfertilized shoots. In field plots, fertilization did not affect gall density per m shoot length or per female released. Urea fertilization increased the efficiency of greenhouse rearing of the arundo wasp and availability of adults for release, even without direct effects on gall production.     

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.     

Growth of coyote willow and the attack and survival of a mid-rib galling sawfly,Euura sp.

We studied the relationship between variation in age and shoot characteristics of the host plant Salix exigua Nuttall (coyote or sandbar willow) and the attack and survival of Euura sp. (an unnamed leaf-midrib galling sawfly). Variation in shoot characteristics resulted from reduced growth as willow ramets aged. Mean shoot length per ramet and mean longest leaf length per shoot decreased by 95% and 50% respectively between 1- and 9-year-old willow ramets. All measured shoot characteristics-shoot length, longest leaf length, number of leaves per shoot, and mean internode length-were significantly negatively correlated with ramet age (r ² ranged from –0.23 to –0.41). Correlations between shoot characteristics were highly positive, indicating that plants also grew in a strongly integrated fashion (r ² ranged from 0.54 to 0.85). Four hypotheses were examined to explain sawfly attack patterns. The host-plant hypothesis was supported in explaining enhanced larval sawfly survival through reduced plant resistance. As willow ramets aged, the probability of Euura sp. attack decreased over 10-fold, from 0.315 on 1-year-old ramets to 0.024 on 2- to 9-year-old ramets. As shoot length increased, the probability of sawfly attack increased over 100-fold, from 0.007 on shoots <100 mm, to 0.800 on shoots in the 1001–1100 mm shoot length class. These attack patterns occurred even though 1-year-old ramets and shoots >500 mm each represented less than 2% of the total shoots available for oviposition. Host plant induced mortality of the egg/early instar stage decreased by 50% on longer leaves and was the most important factor determining survival differences between vigorous and non-vigorous hosts. Sawfly attack was not determined by the resource distribution hypothesis. Although shoots <200 mm contained 82% of the total leaves available, they contained only 43% of the galls initiated. The attack pattern also was not explained by the gall volume hypothesis. Although gall volume increased on longer shoots, there was no significant variation in mid or late instar mortality over shoot length, as would be expected if food resources within smaller galls were limited. The natural enemy attack hypothesis could not explain the pattern of oviposition since predation was greater on longer shoots and leaves. In addition, larval survival was related to oviposition behavior. Due to a 69% reduction in late instar death and an 83% reduction in parasitism, survival of progeny in galls initiated close to the petiole base was 2.8 times greater than in galls initiated near the leaf tip. A 75% reduction in gall volume over this range of gall positions may account for the observed increases in late instar mortality and parasitism.     

Impact of a gall-inducing aphid on Pistacia atlantica Desf. trees

The impact of gall-inducing aphids on shoot development was analyzed in 900 shoots from 20 pistachio trees, Pistacia atlantica Desf. (Anacardiaceae): 600 in which the axillary—lateral buds were galled by Slavum wertheimae HRL during the previous growth season, and 300 ungalled shoots. Although P. atlantica is a compensating tree, and the aphids do not attack the apical buds, further development of shoots from the apical buds was stopped in 62% of the galled shoots, while only 8.7% of nongalled shoots stopped their growth. Further development was stopped more often on shoots carrying two or more galls than on shoots supporting only one gall. To assess the hypothesis that bud destruction by the aphids explains this pattern, a field experiment was conducted in 140 shoots, distributed across seven trees. One, two or three axillary buds from five shoots of each tree were removed for each treatment, and five other shoots were marked as controls. Only 14 shoots (10%) of the 140 did not develop. The growth of the other shoots was not very different among the treatments. The colonization of the apical shoots, which developed on previously treated shoots, by three other galling aphid species was monitored. Removing lateral buds considerably reduced the establishment of Geoica sp. galls (70% of them colonized control shoots), but weakly influenced Forda riccobonii (Stefani). It also contributed only 5% of the total variance of the distribution of Smynthurodes betae West. The different results of the survey and the experiment show that the impact of S. wertheimae galls on the future growth of shoots from apical buds is more complex than the simple physical destruction of the axillary buds. Handling editor: Graham Stone