Hormaphis hamamelidis and gall size: a test of the plant vigor hypothesis

We tested the Plant Vigor Hypothesis by determining the distribution of galls formed on leaves of witch hazel, Hamamelis virginiana , by the aphid Hormaphis hamamelidis , and by determining various factors that affect the fecundity of the gall-forming fundatrices. We also studied the role of the fundatrix in host plant manipulation. While the mean number of galls per leaf was low, galls had an aggregated distribution among leaves. Among trees, the average number of galls per leaf was not related to the mean leaf size, contrary to the preference prediction of the Plant Vigor Hypothesis. While fundatrices preferred the distal leaves of buds, which grew more than the proximal leaves, being on distal leaves conferred no increase in fecundity for fundatrices, contrary to predictions of the Plant Vigor Hypothesis. Gall size was the factor that explained the largest proportion of variation in fundatrix fecundity; fundatrix size explained somewhat less of the variation. Also, gall position on the leaf, number of aphid galls on the leaf, and on which leaf of the bud the gall was located all played small, statistically significant roles in explaining fundatrix fecundity, but their effects were variable between experiments. Removal of fundatrices shortly after galls had enclosed them limited the growth of galls, indicating the role of the fundatrices in gall growth. We compare and contrast this system versus other gall-forming insects, as well as discuss the adaptive significance of the aphid manipulation of the host plant. Much of the data contradict predictions of the Plant Vigor Hypothesis, and we discuss how gall size, as a measure of plant growth caused by insect manipulation, explains the observed patterns.     

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.     

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.     

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.     

Histopathogenesis of the Galls Induced by Nothanguina phyllobia in Solanum elaeagnifolium

The histopathogenesis of the foliar galls induced by Nothanguina phyllobia Thorne in Solanum elaeagnilolium Cav. was examined via serial sections prepared from plant shoots at 11 time intervals (0.5-30 days) following inoculation. Nematodes infected the blades and petioles of young leaves surrounding the shoot apex. Hypertrophy and hyperplasia of the palisade, pith, cortical, and vascular parenchyma resulted in the formation of confluent leaf, petiole, and stem galls up to 25 cm³ in volume. Externally, leaf galls were irregular, light-green, convoluted spheroid bulges distending the abaxial surface. Mature galls contained a cavity lined with parenchymogenous nutritive tissue comprising intercellular spaces and actively dividing hypertrophied cells. These cells contained granular cytoplasm, hypertrophied nuclei, and brightly stained large nucleoli. Vascular tissues were not discernibly affected during the early stages of gall development. As gall development progressed, however, vascular elements were often displaced and disoriented. The histopathology of this nematode indicates that N. phyllobia is a highly specialized parasite and, for that reason, is suitable as a biological control agent.     

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 number of cecidomyiid insect galls affects the photosynthesis of Machilus thunbergii host leaves

Previous studies of the impacts of galls on host leaf photosynthesis do not suggest any general trends, with a reported range of effects from negative to positive. In this study, photosynthetic characteristics such as chlorophyll fluorescence (Fv/Fm), photosynthetic capacity, and stomata conductance were determined in two types of fruit-like galls (red ovoid and green obovate galls) induced by Daphnephila taiwanensis and Daphnephila sueyenae, respectively, in order to investigate whether the number of galls affects the photosynthesis of galled leaves of Machilus thunbergii. In 2008, chlorophyll fluorescence and photosynthetic capacity were negatively correlated with gall numbers, non-significantly and significantly, respectively, whereas stomata conductance was positively but non-significantly correlated with gall numbers. In 2009, photosynthesis capacity and stomata conductance were negatively, but non-significantly, correlated with gall numbers. Results imply that photosynthesis in M. thunbergii leaves is slightly affected by the number of cecidomyiid insect galls, and that the higher the gall number, the greater the negative effect that galls have on host leaf photosynthesis and subsequent infection.     

Histocytological aspects of four types of ambrosia galls on Machilus zuihoensis Hayata (Lauraceae)

Four types of prosoplasmatic galls induced by Daphnephila midges are found on leaves of Machilus zuihoensis, a species endemic to Taiwan: urn- and small urn-shaped, obovate, and hairy oblong galls. In addition to containing nutritive tissues, these galls are lined with fungal hyphae. The objective of this study was to describe and compare the structural organization of the various gall morphologies and to examine the ultrastructure of the nutritive and fungal cells lining the gall chambers. The morphology and ultrastructure of mature-stage galls were examined by light, scanning electron, and transmission electron microscopy. Diverse epidermal cell shapes and wax textures were observed in the leaves and galls of M. zuihoensis. In small urn-shaped, obovate, and hairy oblong galls vascular bundles extend from the gall base to near the centre of the gall top. In contrast, vascular bundles in urn-shaped galls are distributed in the gall wall and extend to close to the outer gall top. Trichomes were present only abaxially on leaves and on hairy oblong gall surfaces. Starch granules, tannins, and mucilage were distributed differently among the four gall types. Further, fungal mycelia spread in the interior gall wall and partially passed through the intercellular spaces of nutritive cells and reached the sclerenchyma. Histological analyses revealed that the surface structure of galls differs from that of the leaf and that the epidermal organization differs among the four gall types. Different types of leaf galls on the same plant have different patterns of tissue stratification and contain different ergastic substances. The results of this study will contribute to the understanding of tritrophic relationships and the complex interactions among parasitic gall-inducing insects, mutualistic fungi, and host plants.     

Plant module size and dose of gall induction stimulus influence gall induction and galler performance

Several different hypotheses attribute large variations in gall abundance and galler performance to variations in plant growth rate and/or module size. The plant vigor hypothesis (PVH) predicts that galler performance will be greatest on large, fast‐growing plant modules due to their large, rapidly dividing cells that allow them to make large galls, where survival and size of survivors are usually greater. The optimal module size hypothesis (OMSH) predicts that galler performance will be greatest on intermediate‐sized modules, based on a tradeoff between ease of gall induction on small modules and increased resource availability in large modules. Here, we evaluate predictions of the PVH and OMSH during a two‐year field study by examining gall induction and full gall development success, as well as subsequent galler performance in successfully developed galls, using a shoot galling adelgid, Adelges abietis, on white spruce Picea glauca. In addition, we tested whether increased dose of gall induction stimulus on different sized modules, achieved by observing differing A. abietis densities per bud, could increase the rate of successful gall induction, as predicted by the OMSH. Galls of A. abietis appeared to be induced by a dose‐dependent stimulus from fundatrices in spring. Furthermore, the critical stimulus dose required to induce a gall appeared to increase with shoot length. These findings support the OMSH and not the PVH. Galler performance (gall volume and the number of gallicolae that emerged from each gall) in successfully developed galls was positively related to shoot length, supporting both the PVH and OMSH. We conclude that the PVH represents one component of the more inclusive OMSH, which considers both ease of gall induction on different‐sized modules and the effect of plant module size on gall size.     

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

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

Histological Comparisons of Fergusobia/Fergusonina-Induced Galls on Different Myrtaceous Hosts

The putative mutualism between different host-specific Fergusobia nematodes and Fergusonina flies is manifested in a variety of gall types involving shoot or inflorescence buds, individual flower buds, stems, or young leaves in the plant family Myrtaceae. Different types of galls in the early-to-middle stages of development, with host-specific species of Fergusobia/Fergusonina, were collected from Australian members of the subfamily Leptospermoideae (six species of Eucalyptus, two species of Corymbia, and seven species of broad-leaved Melaleuca). Galls were sectioned and histologically examined to assess morphological changes induced by nematode/fly mutualism. The different gall forms were characterized into four broad categories: (i) individual flower bud, (ii) terminal and axial bud, (iii) ''basal rosette'' stem, and (iv) flat leaf. Gall morphology in all four types appeared to result from species-specific selection of the oviposition site and timing and number of eggs deposited in a particular plant host. In all cases, early parasitism by Fergusobia/Fergusonina involved several layers of uninucleate, hypertrophied cells lining the lumen of each locule (gall chamber where each fly larva and accompanying nematodes develop). Hypertrophied cells in galls were larger than normal epidermal cells, and each had an enlarged nucleus, nucleolus, and granular cytoplasm that resembled shoot bud gall cells induced by nematodes in the Anguinidae.     

Spatial Distribution of Galls Caused by <Emphasis Type="Italic">Aculus tetanothrix</Emphasis> (Acari: Eriophyoidea) on Arctic Willows

The distribution of galls caused by Aculus tetanothrix (Acari: Eriophyoidea) on three Salix species was studied. The factors influencing this distribution were analysed, i.e. willow species, study area and shoot length. Spatial pattern of gall distribution within the shoot was also examined. The study was conducted in Russia, Kola Peninsula. Densities of galls caused by A. tetanothrix differed significantly among willow species. Considerably higher gall density was recorded in the White Sea coast than in the Khibiny Mountains. This may be explained by the influence of a milder maritime climate that favors mite occurrence compared to a harsh and variable mountain climate that limits mite abundance. There was no relationship between the gall density and the shoot length. The highest density of galls was recorded on the inner offshoots; within the offshoot, there was a maximum density on the fifth leaf. This pattern was repeatable for all shoots studied, independent of the study area, willow species and length of shoots, suggesting the optimal conditions for A. tetanothrix exist on leaves in the middle part of a shoot. This distribution pattern may be an effect of the trade-off between the costs and benefits resulting from leaf quality and mite movement along the shoot. This hypothesis, however, needs to be tested experimentally.     

Nematode‐induced galls in Miconia albicans: effect of host plant density and correlations with performance

The study of nematodes parasitizing native plants plays a crucial role in understanding plant–pathogen interactions. In the present study we describe the patterns of attack by an undescribed species of Ditylenchus occurring in Miconia albicans (Melastomataceae), a widespread, native shrub from the Brazilian cerrado. We also tested the hypothesis that nematode‐induced leaf galls negatively correlate to host plant performance and that gall density is a function of host plant density. We collected paired healthy and attacked shoots from 28 individuals of M. albicans and estimated the leaf area lost to nematode‐induced galls in up to 10 leaves per shoot. We analyzed the relationships between leaf area lost to nematode galls and reproductive traits. Nematode attack levels were also compared to the spatial distribution of the host plant. Inflorescence length and fruit production were significantly reduced in attacked shoots compared with healthy shoots. Seeds from attacked shoots showed no significant reduction in germinability or germination time when compared with seeds collected from healthy shoots. Gall density was positively correlated with host density. Despite being seldom studied in tropical ecosystems, nematodes may play an important role in plant fitness and in structuring tropical communities.     

Distribution and frequency of galls induced by Anisodiplosis waltheriae Maia (Diptera: Cecidomyiidae) on the invasive plant Waltheria indica L. (Sterculiaceae)

The frequency of galls induced by Anisodiplosis waltheriae Maia, a recently described species, on Waltheria indica L. was studied. W indica is an invasive weed in regeneration areas of Atlantic Forest in southeastern Brazil. Plants were collected in May 2004 and above-ground biomass, main stem length, number of leaves, number of galls per leaf and leaf area of each individual were recorded. Nearly 90% of all plants and 25% of all leaves were attacked by the gall midge, with an average of 0.67 galls/leaf. Leaf area had a weak effect on gall abundance while the number of leaves had no effect on gall abundance. Only 31% of the variation in gall abundance was explained by plant biomass. Natural enemies killed one third of the sampled galls. Predation accounted for 22.9% of gall mortality, unknown factors killed 7.6%, microhymenopteran parasitoids killed 2.5% and fungi only 1%. Mortality factors were not influenced by leaf area or gall density.     

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

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

Galling: the prevalent form of insect folivory in the latest Neogene monsoon-influenced tropical forests of the Chotanagpur Plateau,eastern India

Indian Cenozoic deposits contain well-preserved diverse angiosperm leaf assemblages, but galling, a common form of angiosperm leaf damage in modern tropical forests, has not been well-documented. Here we report insect herbivory on diverse angiosperm fossil leaf specimens from Pliocene (Rajdanda Formation) sediments of the Chotanagpur Plateau, eastern India, revealing that galling was the most common form of folivory in the latest Neogene monsoon-adapted tropical forests of eastern India. Diverse well-preserved galls are described on the basis of their size, shape and position on the host angiosperm leaf remains. Nine gall damage types (DT 32, DT 33, DT 34, DT 80, DT 83, DT 85, DT 110, DT 120, and DT 144) are identified. They are compared with extant galls and the probable gall inducers making morphologically similar galls on related host tropical plant species of Ficus, Mangifera, Albizia, Galactia, Ziziphus, Hylodesmum, Adina, and Psidium. The gall producers belong to the insect orders Orthoptera, Hemiptera, Hymenoptera, Coleoptera, Lepidoptera, and Diptera (Cecidomyiidae). Although the detailed morphology of the phytophagous insects associated with the recovered fossil leaves is unknown, our findings indicate that many modern plant-insect relationships were likely established by the Pliocene. The reconstructed warm, humid Pliocene climate with a weak monsoon seasonality was conducive to extensive galling activity during this time.     

Leaf‐derived cecidomyiid galls are sinks in Machilus thunbergii (Lauraceae) leaves

Three relevant hypotheses – nutrition, environment and the enemies hypothesis – often invoked to explore source and sink relationships between galls and their host plants are still under dispute. In this research, chlorophyll fluorescence, gas exchange capacity, stomatal conductance, total carbon and nitrogen, total soluble sugars and starches, and scanning and transmission electron microscopy of two types of galls were used to investigate source–sink relationships. Compared with host leaves, these galls demonstrated slightly lower chlorophyll fluorescence; however, gas exchange capacity and stomatal conductance were not detected at all. Scanning electron micrographs demonstrated that the abaxial epidermis of host leaves contain normal amounts of stomata, whereas no stomata were observed on the exterior and interior surfaces of both types of galls. In addition, gall inner surfaces were covered with many kinds of fungal hyphae. Gall total carbon (C) and nitrogen (N) levels were lower but the C/N ratio was higher in galls than host leaves. Both types of galls accumulated higher total soluble sugars and starches than host leaves. Transmission electron micrographs also revealed that both types of galls contain plastoglobuli and giant starch granules during gall development. Results strongly indicate that leaf‐derived cecidomyiid galls are sinks in Machilus thunbergii leaves. However, it is perplexing how larvae cycle and balance CO₂ and O₂ in gall growth chambers without stomata.     

Characterisation of the effects of leaf galls of Clusiamyia nitida (Cecidomyiidae) on Clusia lanceolata Cambess. (Clusiaceae): Anatomical aspects and chemical analysis of essential oil

Galls develop in different plant organs and are induced by the activity of various organisms. Some studies have investigated the ecological interactions between species of Clusia and gall-inducing insects. The goal of our study is to characterise changes in leaf anatomy caused by the activity of gall insects in Clusia lanceolata. Additionally, we also investigated the chemical composition of volatile compounds of normal leaves and those with galls to detect possible effects on the host plants. For anatomical studies, we used botanical material fixed in FAA₅₀. Transversal sections of the leaf blade were obtained from samples of leaves located on the third and fourth nodes from both male and female individuals. Material was studied from both sexes both with unaffected leaves and leaves containing galls. Fresh leaves of C. lanceolata were used for the extraction of volatile compounds, which were submitted to stem distillation using a modified Clevenger apparatus determining the oil yields subsequently (w/w). The unaffected leaves of female and male individuals of C. lanceolata exhibit similar anatomical structures. However, galls on leaves of both sexes show anatomical differences. The activity of the gall insect Clusiamyia nitida induces several changes in the foliar anatomy and the distribution of metabolic compounds in new tissues during gall development. However, the larvae are not able to induce significant changes in the volatile compounds of inflected leaves from male and female individuals.     

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.     

Preference–performance relationship in the gall midge Rabdophaga rosaria: insights from a common‐garden experiment with nine willow clones

1. Oviposition preferences of herbivorous insects are predicted to match offspring performance on different host taxa or on conspecific host genotypes. In gall‐inducing insects, host‐plant properties such as growth rate and gall size, which are determined by plant genotype and growing conditions, may have a significant impact on offspring performance and, hence, should influence oviposition site selection. 2. The present study investigated host preference of the European rosette willow gall midge Rabdophaga rosaria (Loew) in relation to offspring success on seven clones of Salix myrsinifolia Salisb. and two naturally hybridised S. myrsinifolia × phylicifolia L. clones growing in a replicated design in an experimental field under two fertilisation regimes. For each clone, the average growth rate, number of shoot tips, and leaf and gall size were determined, and their effects on midge preference and larval survival were examined. 3. Main shoot height, number of shoot tips, and gall size were significantly affected by clone. The midges clearly preferred certain clones over the others, but preferences were not related to willow growth traits or to gall size. Survival probability was higher in large than in small galls, but females did not prefer large‐leaved clones that produced the biggest rosette galls. Midge oviposition was also uncorrelated with prior rates of leaf‐rust infection and with feeding preferences of voles and folivorous insects. 4. The weak preference–performance relationship of R. rosaria within S. myrsinifolia is probably explained by evolutionary constraints that prevent generalist insects from achieving an ability to discriminate among conspecific hosts of variable quality.