Patterns of interspecific associations of stem gallers on willows

Abstract.  The pattern of interspecific associations of three stem-galling sawfly species ( Euura atra , E. elaeagnos , E. purpureae ) and three stem-galling gallmidge species ( Rabdophaga sp. 3–5) was investigated on five willow taxa ( Salix alba , S. fragilis , S.  ×  rubens , S. elaeagnos , S. purpurea ) at five natural sites in Central Europe. The willow species harboured specific species associations of two stem gallers, each pair consisting of one Euura and one Rabdophaga species. The stem gallers were patchily distributed and their densities varied significantly among willow host plant species, host plant individuals, and host plant sexes. Four of the six species showed a significant increase in galling rate with shoot length. The other two species were the sawfly and cecidomyiid pair that induce galls on S. purpurea . The preference of stem gallers to longer shoots was generally not related to higher larval performance in terms of survival. Only one species, Rabdophaga sp. 5, was found to be more abundant on male plants. The correlation of densities of the species pairs of stem gallers was independent of willow sexes. Species pairs of stem gallers co-occurring on the same willow species tended to attack different shoots within the same host plant individual. When species pairs co-occurred on shoots they were usually found in similar densities as when occurring alone on shoots. The stem-galling sawflies usually formed galls at the basal part of a shoot, whereas the gallmidge R . sp. 5 ( R . sp. 3 and R . sp. 4 showed no clear tendency) preferred the middle or distal part of a shoot. This is interpreted with differences of their phenology and oviposition period.     

Long term oviposition preference and larval performance of Schizomyia macrocapillata (Diptera: Cecidomyiidae) on larger shoots of its host plant Bauhinia brevipes (Fabaceae)

The Plant Vigor Hypothesis (PVH) proposes that natural selection on female oviposition choice results from higher fitness of larvae on more vigorous and larger plant modules. For six consecutive years we tested the PVH predictions by investigating the effect of shoot size of Bauhinia brevipes (Fabaceae) on the oviposition preference and offspring survival of the gall-midge Schizomyia macrocapillata (Diptera: Cecidomyiidae). Additionally, we analyzed the effects of bottom-up and top-down mortality forces on the system. The hypersensitive reaction (bottom-up effect) accounted for more than 90% larval mortality of S. macrocapillata, making available few galls to be found and killed by natural enemies (top-down effect). Smaller shoots were always more abundant while longer shoots were rare. Nevertheless, the percent number of galls induced by S. macrocapillata was up to 10-fold greater on the largest shoots, corroborating the preference prediction of the PVH. Schizomyia macrocapillata should use over-exploit larger shoots to maximize the preference for, and consequently increase the performance on these shoots. Our results partially support the performance prediction of the PVH: (1) the observed survival was higher than expected on longer shoots, and (2) the ratio of survival per shoot was positively related with shoot length only in 2 years. Thus, we found a link between female preference and larval performance on large-sized shoots, at least in some years. The gall-midge attack pattern in this study might be an evolved response to maximize the female preference and increased larval performance on longer shoots of the host plant.     

Preference and performance of a gall-inducing sawfly: a test of the plant vigor hypothesis

To test the Plant Vigor Hypothesis, we determined female oviposition preference of Phyllocolpa leavitii (Hymenoptera: Tenthredinidae) for shoot lengths on 20 clones of Salix discolor (Salicaceae) and examined larval performance by determining larval survival based on clone, shoot length, and leaf length for each Phyllocolpa gall observed. Sawfly galls were found on significantly longer shoots compared to shoots without galls, and shoots with 2, 3, 4, and 5 galls were successively longer than shoots with fewer galls. Furthermore, a much higher proportion of longer shoots had galls that did shorter shoots. These data demonstrate strong sawfly oviposition preference for long shoots. However, when shoot length was adjusted for numbers of available leaves per shoot, the number of galls per available leaf showed a weaker, but still positive relationship with shoot size. Using a logistic regression on survival of Phyllocolpa larvae, we detected highly significant effects of clone, shoot length, leaf length, and the square of leaf length on survival. Additionally, much higher sawfly survival on the long shoots of one clone caused a significant clone by shoot-length interaction. Survival was positively correlated with shoot length, indicating directional selection favoring oviposition on longer shoots; still, shoot length explained only 11.8% of the variation in survival. The directional component of selection favored oviposition on smaller leaves as survival was inversely related to leaf length; however, disruptive selection for leaf length oviposition preference was also detected, with survival of Phyllocolpa galls lowest on intermediate leaf lengths. This study provided evidence supporting the preference prediction of the Plant Vigor Hypothesis. However, much of the data contradicted the performance prediction of the Plant Vigor Hypothesis, with only a modest amount of evidence supporting the performance prediction.     

Release of phylogenetic constraints through low resource heterogeneity: the case of gall-inducing sawflies

Abstract. 1. A group of six unusual sawfly species, which do not conform to the phylogenetic constraints hypothesis as it has been applied to sawflies, was examined in natural populations. All species were in the genus Pontania (Hymenoptera: Tenthredinidae), which induce galls on leaves of willow species (Salicaceae). An understanding of these non‐conformist species was important as a test of the validity of the general hypothesis. 2. The six species of sawfly, Pontania mandshurica, P. cf. arcticornis, P. aestiva, P. arcticornis, P. pacifica, and P. nr. pacifica, showed no oviposition preference for long, vigorous shoots, in contrast to 37 documented tenthredinid species that have demonstrated such a preference. Rather, the non‐conformist species attacked the shortest shoot length classes more frequently and larval establishment in galls was successful. 3. The evident escape from the phylogenetic constraint, which commonly limits sawfly attack to the most vigorous shoots in a willow population, resulted from low apparent heterogeneity of the resources exploited by these Pontania species. At the time of female oviposition, shoots and leaves were too uniform to allow discrimination by females among shoot length classes, resulting in random, or near random attack of shoots. 4. The unusual relative uniformity of resources to which sawflies were exposed resulted from several characteristics. (1) Females emerged early relative to shoot growth phenology, making discrimination among shoot length and vigour difficult or impossible. (2) Low heterogeneity in leaf length resulted in resource similarity independent of shoot length. (3) Abscission of leaves occurred after emergence of larvae from leaf galls so that differential abscission of leaves in relation to shoot length became irrelevant. (4) In some cases, low variance in shoot lengths was evident in old ramets lacking long, vigorous shoots. Probably as a result of low resource heterogeneity, larvae survived well across all shoot length classes, revealing no ovipositional preference and larval performance linkage related to the exploitation of the longest shoot length classes in a population of willows, as in the conformist species. Therefore, larval survival did not provide positive feedback on female preferential behaviour for long shoots, as in the conformist species studied.     

Evidence for long-distance, chemical gall induction by an insect

Abstract We report that a chemical stimulus from a herbivore, a galling insect, changes plant morphology and physiology to benefit the herbivore. Previous studies could not determine whether insect galls are induced by mechanical or chemical stimuli because feeding and oviposition both occurred at the site of gall formation. We report that the mouthparts of a spruce‐galling insect, Adelges cooleyi, were inserted in stem phloem cells far from induced galls, that tissues between mouthparts and galls appeared normal, and that the ability to initiate galls was inversely correlated with distance from buds (potential gall sites). Thus the effects of chemical stimuli were unambiguously separated from any mechanical influence of probing stylets or ovipositors. Our results strongly suggest that galls were induced by a chemical stimulus transported to buds via vascular tissue and that its efficacy was dose‐dependent.     

Preference and performance linkage in aPhyllocolpa sawfly on the willow,Salix miyabeana,on Hokkaido

A leaf-folding sawfly in the genusPhyllocolpa (Hymenoptera: Tenthredinidae) attackingSalix miyabeana (Salicaceae) was studied near Sapporo, Hokkaido, along the Ishikari River in 1993. Host plant individuals were young trees 4–7 years old which were growing rapidly, producing some long shoots with large leaves. On a gradient of shoot length classes from 0–5 cm long to over 80 cm long, shoots were much more abundant in the shorter shoot length classes. However, attacks by ovipositing females increased as shoot length increase from 0 attacks on the shortest shoots to 5.17 attacks per shoot on the longest shoots. The frequency of attack per leaf increased from 0 to 0.13 over the same range of shoot lengths. This pattern of attack resulted in a high frequency of larval establishment in feeding sites, between 0.96 and 1.00, in all attacked shoot length categories. However, probability of survival to a late instar larva increased with shoot length and corresponded to the attack pattern, indicating a preference-performance linkage between female ovipositional decisions and larval survival. The patterns found for thisPhyllocolpa species are similar for galling sawflies in North America and Europe, especially in the genusEuura, members of which make stem, bud and leaf midrib galls. Extending the pattern to aPhyllocolpa species broadens identification of pattern and ultimately the generality of the emerging theory on populations of galling sawflies.     

EVOLUTION OF DIFFERENT GALL TYPES IN WILLOW-FEEDING SAWFLIES (HYMENOPTERA: TENTHREDINIDAE)

The sawflies that feed on the plant family Salicaceae can be divided into eight informal groups based on larval feeding habit or gall type: (1) species with free-living larvae; (2) leaf folders; (3) leaf blade gallers; (4) apical leaf gallers; (5) basal leaf gallers; (6) midrib and petiole gallers; (7) stem gallers; and (8) bud gallers. It has been proposed that the galling habit evolved from free-living larvae via leaf folders, and that the different gall types evolved gradually in the sequence mentioned above. Thus, the galling site would have “wandered” from the leaf margin toward the stem as a result of gradual changes in oviposition site preference. Allozyme data from eight informative loci were used to reconstruct the phylogeny of 18 representative sawfly species. The results suggest that indeed leaf folders seem to be a basal group; leaf blade gallers evolved independently of the other true gallers; apical and basal leaf gallers are not the ancestors of petiole and bud gallers, but they may share a common galling ancestor; bud gallers evolved from midrib/petiole gallers; and stem gallers are polyphyletic. The cause for the observed wandering of the galling site could be intraspecific competition due to a possible “nutrient shading effect” of galls situated closer to the host plant's main vascular system.     

Community structure, survival and mortality factors in arctic populations of Eupontania leaf gallers

We studied survival, mortality factors, and community structure of nine species of leaf-galling sawflies, Eupontania spp., living on ten willow species (Salix spp.) at six sites on the Russian arctic tundra. The sawfly species represented two different gall types: the viminalis-type, which forms pea-shaped galls on the underside of leaf blades, and the vesicator-type, which forms bean-shaped galls on both sides of the leaf blade. Gall communities in the northernmost site had only one parasitoid species, but up to six parasitoids were found at the southernmost site. Inquiline parasitoids were encountered only in the two southern sites. Survival of the larvae varied between 20.0 and 82.8% among galler species at different sites. Parasitoids were the most important mortality factor for the sawflies. They caused mortality of 7.8-65.4%, depending on galler species and site, and it was highest in the northernmost site. Plant-specific mortality varied from 1.7 to 28.4% by galler species and it tended to decrease towards the north. Mortality from parasitoids was greater in the vesicator-type gallers than in the viminalis-type gallers. The total mortality caused by parasitoids in the arctic communities does not appear to differ from that in the diverse southern communities of Eupontania in Middle Europe, Scandinavia and North America, despite the assemblage having only a few members in the Arctic. The largest difference between the southern and the northern communities was the lack of inquiline parasitoids in the north. Our data do not support the hypothesis that abiotic, rather than biotic, factors would be more important in determining the abundance of populations of herbivorous insects in the harsh arctic environment.     

Mixed significance of plant vigor: two species of galling Pontania in a hybridizing willow complex

Distributions of leaf galls and offspring performance of two Pontania sawfly species were explored in individual willows of the subarctic Salix caprea – starkeana hybrid complex. The more common sawfly, an undescribed species near the dolichura group (P1), had the highest gall numbers in trees with long shoots both in S. caprea and hybrids. While numbers were high on vigorously growing hybrids, offspring of P1 were aborted significantly more often on hybrids than on pure hosts. Further, non-aborted galls were smaller on hybrids. Fast shoot growth may be important for P1 sawflies, because females oviposit early in summer and larvae develop rapidly compared with the other species, Pontania pedunculi (P2). Distributions of P2 galls were related to tree height and not to shoot length in both parental and hybrid groups of willows. Like P1, also P2 offspring were frequently aborted on hybrids, but not significantly more often than on pure hosts, and P2 galls were equally large among the host groups. Survival of both species was related to abortion rates, while larvae were parasitized equally in all host groups. This study demonstrates that the significance of plant vigor may vary even for closely related galling sawflies exploiting the same hosts, or for the same species on different host plants. Vigorous growth may mislead gallers to oviposit on suboptimal plants.     

Hypersensitivity of Fagus sylvatica L. against leaf galling insects

Hypersensitivity is known as a localized resistance of plants against pathogens. It also can be detected in response to galling insects, i.e., in the area immediately adjacent to the site of oviposition and attempted penetration by the galling larva. This host response includes morphological and histological changes that cause the death of the attacked tissue. It is observed as a rounded dark brown halo around the gall induction site. We provide the first observation on the occurrence and possible relevance of this induced mechanism by which one of the most common tree species in Germany, Fagus sylvatica L., resists attack by two of its most common galling insects, Mikiola fagi and Hartigiola annulipes (Diptera: Cecidomyiidae). Galls induced by these cecidomyiids were extremely common in the studied area in beech forests around Darmstadt, Germany. The availability of resources (leaves on a stem) was a poor predictor of attack by the galling insects as well as for gall abundance (galls successfully formed). Hypersensitive reaction was the most important factor acting against the galling population studied. More than 77% of the attempts of the insects to induce galls on F. sylvatica resulted in failure and consequently the death of the galling larvae. Therefore, few live galls remained to be found and destroyed by natural enemies. This corroborates the view that in galling insect–host plant system interactions plant-driven factors may play a major role in determining herbivore failure and success, and perhaps the resulting community structure.     

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.     

Hibernation adaptation and eclosion synchrony in leaf-galling sawflies in subarctic Norway

The winter dormancy adaptation in gall-inducing sawflies is poorly known. Diapause termination and the following post-diapause quiescence enhance synchronous eclosion in spring. This is probably the most critical part in the life history in gall-inducing sawflies, as there is only a short phenological window of opportunity for mating and oviposition. In a 2?years’ study, diapause duration, termination, survival and eclosion synchrony were experimentally investigated for three gall-inducing sawfly species (Symphyta: Tenthredinidae: Pontania nivalis, P. glabrifrons and P. arcticornis). Field-collected galls, sampled from willows (Salix spp.) in early autumn, were kept under natural temperatures outdoors until next spring. Subsamples were successively transferred to the laboratory at 14-day intervals from October to April in a test of development time to eclosion and survival ratio. The time to eclosion decreased throughout the experiment, whereas the proportion successfully eclosed and eclosion synchronicity increased, all indicative of prepupae entering a diapause in early autumn. The diapause terminates midwinter, and the prepupae enter a post-diapause quiescence until the temperature in spring allows a direct development and contributes to a nearly synchronous eclosion. In all three species, males eclosed 1–2?days prior to females (protandry). We hypothesize that synchronous eclosion as well as protandry enhance mating and oviposition success. Our finding indicates that gall-inducing sawflies are well adapted to its harsh subarctic and arctic environment.     

Mortality factors and resource use of the bud-galling sawfly, Euura mucronata (Hartig), on willows (Salix spp.) in arctic Eurasia

We studied egg and larval mortality factors in arctic populations of the bud-galling sawfly, Euura mucronata (Hartig) on three willow species (Salix glauca L., S. phylicifolia L., and S. lapponum L.) and the quality of resources (shoot length of willows) required for egg-laying. The survival was independent of latitude. There was a positive correlation in survival on different willow species among sampling sites, indicating that similar, locally operating factors affected survival. Host plant-based mortality factors were dominant and caused 17.9–48.0% mortality in eggs and 6.6–44.1% mortality in larvae. Parasitoids and inquilines caused relatively low and variable rates of mortality. Parasitoids were absent from several of the northernmost populations, but caused up to 11.0% mortality at southern sites. Mortality caused by inquilines was minor in other areas except in some sites in the Taymyr Peninsula, where it varied from 0 to 23.1%. E. mucronata laid eggs on the longest shoots available. Plant vigour as measured by shoot length decreased toward the north, and densities of galls were positively correlated with plant vigour. The difference in length between galled and ungalled shoots was 2.0- to 4.6-fold. Absence of E. mucronata in the most northern populations of willows was apparently caused by insufficient shoot growth. Short shoots failed to provide sufficient resources for successful development of galls. Our results suggest that the quality of host plants is the main factor determining abundance and distribution of E. mucronata in arctic areas. Received: 10 February 1997 / Accepted: 2 May 1997     

Intra‐plant heterogeneity influences the preference and performance of juveniles and adults of a gall midge

Abstract 1. Field studies were conducted to evaluate the preference and performance of a gall‐inducing midge (Harmandia tremulae) within the crown of trembling aspen (Populus tremuloides). Females did not select oviposition sites preferentially within leaves, but did lay preferentially on young leaves. 2. Larvae were the only life stage involved in gall site selection within leaves and in gall initiation and development. Gall size, which was positively related to survival, was highest for galls on mid veins that were located close to the petiole. However, one‐third of galls were located on lateral veins and most galls were not adjacent to the petiole, indicating that many larvae choose sub‐optimal gall initiation sites. 3. Gall density was positively associated with leaf length, and leaf length, was positively associated with gall size. However, gall density per leaf was not related to larval survival in galls. This latter result may be a result of an observed inverse relationship between gall size and gall density for similar‐sized leaves. 4. The results partially support the plant vigour and optimal plant module size hypotheses, which predict that galler fitness in successfully induced galls should be highest on large, fast‐growing plant modules. The lack of a strong preference‐performance link supports the confusion hypothesis, which predicts that oviposition and gall site selection may often be suboptimal in systems where galler lifespan is short. This study suggests that small‐scale variations in plant quality within leaves, can render gall site selection by juveniles as important as that previously reported for adult females.     

Testing a new hypothesis: plant vigor and phylloxera distribution on wild grape in Arizona

Summary Longer, meaning more vigorous, shoots of a wild grape clone (Vitis arizonica) were more susceptible to attack by second and third generations of leaf-galling grape phylloxera,Daktulopsphaira vitifoliae, as the growing season progressed. Although there was no significant difference in mean shoot length between attacked and unattacked shoots within a clone at the beginning of shoot elongation, attacked shoots were significantly longer than unattacked shoots when elongation had ceased (P<0.01). Also, long attacked shoots had a significantly greater population of phylloxera galls than short attacked shoots (P<0.01) as the season progressed. The phylloxera population on long shoots increased rapidly while the population on short shoots remained the same. Longer shoots also produced significantly more axillary shoots than shorter shoots as the season progressed (P<0.001), and the number of axillary shoots accounted for 66 percent of the variance in number of attacked leaves on a shoot. Experimental evidence showed that there was a significantly greater percentage of available leaves attacked on long shoots than on short shoots (P<0.05) and the leaves on long shoots generally had a greater number of galls per leaf. The relationship between shoot length and probability of attack was also tested by comparing shoots lengths of 10 attacked clones and 10 unattacked clones at a second location. Mean shoot lengths of attacked clones were significantly longer than mean shoot lengths of unattacked clones (P<0.05), and mean shoot lengths of attacked shoots within a clone were significantly longer than unattacked shoots (P<0.001). Longer shoot length accounted for 81 percent of the variance in probability of attack. The reason for this pattern of attack was that long shoots produced newly expanding leaves over a longer time during the growing season and multivoltine phylloxera require undifferentiated tissue to initiate gall formation. Patterns of attack within a shoot were characterized by an uneven distribution of galls among leaves. This was due to development time between generations and the current availability of undifferentiated tissue at times of colonization. This study supports the hypothesis that some herbivore species are favored more by vigorous plants than by stressed plants.     

Interactions among elk, aspen, galling sawflies and insectivorous birds

Using two years of observational and experimental data, we examined the hypothesis that browsing by elk on aspen indirectly affects the distribution of a leaf-galling sawfly, which in turn affects insect diversity and foraging patterns of insectivorous birds. We found that: i) in an analyses of 33 arthropod species, the presence of sawflies significantly increased arthropod richness and abundance by 2 X and 2.5 X, respectively. ii) browsing by elk reduced sawfly gall abundance such that 90% of the galls were found on unbrowsed aspen ramets. iii) insectivorous birds attacked 60–74% of the galls on unbrowsed shoots compared to 11% on browsed shoots. When leaf-galler abundance was experimentally held constant on browsed and unbrowsed shoots, predation by insectivorous birds did not differ significantly. This result suggests that browsing affects the patterns of avian predation by altering the distribution of a galling insect. These data argue that bottom-up, top-down, and lateral factors can act in concert to affect the distribution of a galler, structure arthropod communities and affect predation by insectivorous birds.     

Patterns of gall-forming in Ossaea confertiflora (Melastomataceae) by Lopesia brasiliensis (Diptera: Cecidomyiidae) in an area of Atlantic Rainforest in southeastern Brazil

Patterns of galling by the gall midge Lopesia brasiliensis (Diptera: Cecidomyiidae) were studied in Ossaea confertiflora (Melastomataceae) in an Atlantic forest site at Ilha Grande, RJ. Out of the 81 plants surveyed, 55 (67.9%) bore galls. The number of galls per galled individual ranged from 1 to 261 and 94.4% of the galls were in leaves. The number of galls per galled leaf varied from 1 to 25. Total gall number was positively correlated with plant height. Larger and more ramified plants tended to have a smaller percentage of their leaves with galls and a lower density of galls per leaf than smaller plants. Plants that were close to other individuals of the same species tended to have more galls per leaf than relatively isolated plants. The observed patterns may be linked to strategies of optimization in the use of resources (i.e. oviposition sites) and predation avoidance by the gall midges.     

Gall aphids do not select optimal galling sites (Smynthurodes betae; Pemphigidae)

Abstract. 1. In studies of insect-host plant interaction it is often suggested that insects preferentially colonize host plants (or sites within plants) on which their fitness is maximized (a positive covariance of preference and performance). This suggestion stems from the assumption that natural selection has driven the system toward optimal use of resources.
2. Our study of the galling aphid Smynthurodes betae Westw. demonstrates that the distribution of galls on leaves is not due to preference, and can be altered by manipulating the aphid arrival time or the shoot growth rate.
3. We found no correlation between gall density and performance (aphid clone size) at different positions along the shoot.
4. Because leaves on the growing shoot are not equally responsive to aphid stimulation, the colonizers have no choice but to settle on leaves that are at the right stage when they arrive.
5. S.betae colonizers did not discriminate between shoots of their host and a congeneric non-host, on which their fitness is invariably zero.
6. Synchronization between galler and host plant phenologies seems to be the key to the observed distribution of galls on the tree. The data give no support to the preference-performance hypothesis.     

Vigour of a dioecious shrub and attack by a galling herbivore

1. The pattern of attack by the leaf‐galling insect Neopelma baccharidis (Homoptera: Psyllidae) was studied in three populations of the dioecious shrub Baccharis dracunculifolia (Asteraceae) in south‐eastern Brazil. The plant vigour hypothesis, which predicts higher rates of attack and increased herbivore performance on the longest plant shoots, was tested. This work also provides further information for the study of differential herbivory in dioecious plants. 2. In total, 9200 shoots were collected randomly from 46 male and 47 female plants belonging to the three populations. Shoot length, number of leaves per shoot, rate of galling, and survival of psyllids did not differ between male and female plants. Another population on the Campus of the Federal University of Minas Gerais was used only to determine the pattern of shoot growth. 3. The hypothesis of sex‐mediated herbivory was not corroborated in this study. 4. The frequency of galling increased with increasing shoot length, as predicted by the plant vigour hypothesis. Nevertheless, the number of oviposition sites (leaf buds) increased with shoot length. 5. The performance of the galling herbivore was not related to shoot length in the plant populations studied. 6. In conclusion, Neopelma baccharidis did not select shoots based on length only.     

The role of olfactory cues in the sequential radiation of a gall‐boring beetle,Mordellistena convicta

1. Herbivorous insects often have close associations with specific host plants, and their preferences for mating and ovipositing on a specific host‐plant species can reproductively isolate populations, facilitating ecological speciation. Volatile emissions from host plants can play a major role in assisting herbivores to locate their natal host plants and thus facilitate assortative mating and host‐specific oviposition. 2. The present study investigated the role of host‐plant volatiles in host fidelity and oviposition preference of the gall‐boring, inquiline beetle, Mordellistena convicta LeConte (Coleoptera: Mordellidae), using Y‐tube olfactometers. Previous studies suggest that the gall‐boring beetle is undergoing sequential host‐associated divergence by utilising the resources that are created by the diverging populations of the gall fly, Eurosta solidaginis Fitch (Diptera: Tephritidae), which induces galls on the stems of goldenrods including Solidago altissima L. (Asteraceae) and Solidago gigantea Ait. 3. Our results show that M. convicta adults are attracted to galls on their natal host plant, avoid the alternate host galls, and do not respond to volatile emissions from their host‐plant stems. 4. These findings suggest that the gall‐boring beetles can orient to the volatile chemicals from host galls, and that beetles can use them to identify suitable sites for mating and/or oviposition. Host‐associated mating and oviposition likely play a role in the sequential radiation of the gall‐boring beetle.