Predicting the Impact of the Biocontrol Agent Aulacidea subterminalis (Cynipidae) on Growth of Hieracium pilosella (Asteraceae) under Differing Environmental Conditions in New Zealand

A gall wasp, Aulacidea subterminalis, was released into the field in New Zealand in 1999 for biocontrol of the grassland weed Hieracium pilosella. To predict its likely impact in the field, the effect of the gall wasp on plant growth was measured under stress-free conditions in a shade house trial with potted plants. Plants with galls showed a reduction in stolon length of 75%. They had slightly lower total dry matter and reduced root weight. In a glasshouse experiment to measure the impact of water stress, nutrient stress, and plant competition on growth of H. pilosella and performance of the gall wasp, the number and mean diameter of gall clusters were not significantly different between treatment and control plants. However, galled plants produced more, but shorter, stolons in all stress treatments, and stolons that were more branched in nutrient- and water-stressed plants, than ungalled plants. Under the plant competition treatment, total stolon length relative to biomass was greater with, than without, wasps, indicating that stolons were thinner. Galling by A. subterminalis is likely to reduce vegetative reproduction of H. pilosella whether or not the plants are stressed, indicating that the wasp may be a successful biocontrol agent.     

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.     

The applicability of the plant vigor and resource regulation hypotheses in explaining Epiblema gall moth–Parthenium weed interactions

A basic question in insect–plant interactions is whether the insects respond to, or regulate plant traits, or a complex mixture of the two. The relative importance of the directions of the influence in insect–plant interactions has therefore been articulated through both the plant vigor hypothesis (PVH) and the resource regulation hypothesis (RRH). This study tested the applicability of these hypotheses in explaining the interactions between Parthenium hysterophorus L. (Asteraceae) and its stem‐galling moth, Epiblema strenuana Walker (Lepidoptera: Tortricidae). Parthenium plants exposed to galling were sampled at three sites in north Queensland, Australia, over a 2‐year period, and the relationship between gall abundance and plant vigor (plant height, biomass, flowers per plant, and branches per plant) was studied. To test the predictions of PVH and RRH, the vigor of parthenium plants protected from galling using insecticides was compared to galled plants and plants that escaped from galling. The vigor of ungalled plants was less than the vigor of galled plants. The higher plant vigor in galled plants was not due to galling, as was evident from insecticide exclusion trials. The insect seemed to preferentially gall the more vigorous plants. These findings support the predictions of the PVH and are contrary to those of RRH. Since gall abundance is linked to plant vigor, galling may have only a limited impact on the vigor of parthenium. This has implications for weed biological control. If the objective of biological control is to regulate the population of a plant by a galling insect, a preference for more vigorous plants by the insect is likely to limit its ability to regulate plant populations. This may explain the paucity of successes against biocontrol of annual weeds using gall insects.     

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

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

The interplay between plant traits and herbivore attack: a study of a stem galling midge in the neotropics

1. The relationship between plant traits and the frequency of attack by a stem galling midge, Neolasioptera sp. (Diptera: Cecidomyiidae), on Eremanthus erythropappus (Asteraceae) was studied. The morphological changes of the host after a galler attack and the potential effects of these changes on attacks by the next generation of gallers were analysed. The study was conducted in the Serra do Japi, São Paulo, south-eastern Brazil. 2. Galled branches were significantly longer, thicker, and had more leaves than ungalled branches. Accordingly, gall establishment was higher in the longer and more foliose branches. Hence, it is suggested that ovipositing females are maximizing their performance by selecting larger branches. 3. Galled branches were larger than ungalled branches of the same age. Two hypotheses, not necessarily exclusive, can explain this pattern: (1) the plant vigour hypothesis that the females are choosing the more vigorous, fast-growing branches, which still remain more vigorous after galling; or (2) the resource regulation hypothesis that galling increases branch growth rates, thus increasing resource quality for forthcoming conspecifics. 4. Co-occurrence frequencies of current and past generation galls showed that the likelihood of a branch being galled increased when it, or the branch from which it stemmed, had been galled before. The data indicated that this preference was conditioned by the number of previous attacks. Heavier attack intensities, such as one gall in the same branch and another in the branch from which it stemmed, decreased the probability of further galling. 5. The suggested links between herbivore attack and plant traits indicate that studies on host selection by phytophagous insects must take into account that herbivory itself may change the plant traits that are postulated to be selected by the insects.     

Willow regrowth after galling increases bud production through increased shoot survival

Insect herbivory can negatively or positively affect plant performance. We examined how a stem gall midge Rabdophaga rigidae affects the survival, growth, and bud production of current year shoots of the willow Salix eriocarpa. In mid-May, the gall midge initiates stem galls on the apical regions of shoots. The following spring, galled shoots had thicker basal diameters and more lateral shoots than ungalled shoots. Although galled shoots were on average 1.6 times longer than ungalled shoots, there were no significant differences in shoot length or in the numbers of reproductive, vegetative, and dormant buds per shoot. However, the subsequent survival of galled shoots was significantly higher than that of ungalled shoots, probably because of the thicker basal diameter. This increased shoot survival resulted in approximately two times greater reproductive, vegetative, and dormant bud production on galled shoots compared with ungalled shoots in the following spring. These results suggest that the willow regrowth induced by galling can lead to an increase in bud production through increased shoot survival.     

Alteration of free amino acid concentrations in insect galls induced by Andricus mukaigawae (Hymenoptera; Cynipidae)

1. Insect galls are abnormal plant growths that develop in response to a stimulus provided by a galling insect. The nutrition hypothesis suggests that the concentrations of nutritive compounds in galls are changed to provide optimum nutrition for the larvae and adults of galling insects. 2. To test the nutrition hypothesis, we determined the concentrations of 20 free amino acids in galls and in galled and ungalled twigs of Quercus fabri during the larval and adult stages of Andricus mukaigawae using high-performance liquid chromatography with ultraviolet–visible detection. 3. At the larval stage, the concentrations of 12 out of 20 amino acids in A. mukaigawae galls were significantly higher than those in galled and ungalled twigs. Asparagine and tryptophan were the most abundant nonessential and essential amino acids, respectively, in A. mukaigawae galls. 4. At the adult stage, the concentrations of most amino acids, except proline, were significantly lower in A. mukaigawae galls than in galled and ungalled twigs. The A. mukaigawae adults may not manipulate amino acid levels because the adults do not feed on galls. The decrease of amino acid levels in adult galls may be viewed as a depletion. 5. The composition of free amino acids in A. mukaigawae galls was significantly different from the composition in galled and ungalled twigs in both the larval and adult stages. 6. Our results may support the nutrition hypothesis. We suggest that a high concentration of proline in A. mukaigawae galls may protect larvae and adults from plant defense responses.     

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     

Limited field establishment of a weed biocontrol agent, Floracarus perrepae (Acariformes: Eriophyidae), against Old World climbing fern in Florida--a possible role of mite resistant plant genotypes

The leaflet galling mite Floracarus perrepae Knihinicki & Boczek was released on Lygodium microphyllum (Cav.) in 63 plots in Florida from 2008 to 2009. Mites transferred onto field plants in 34 plots, but failed to establish populations in the majority of plots. Leaflet galls were observed in only six plots, and in only two plots did mite populations persist for >12 mo. Rates of mite transfer onto field plants were similar for methods using direct transfer of galls versus approaches using passive transfer of mites from infested plants. Often leaflets on some L. microphyllum plants were heavily galled by F. perrepae, whereas leaflets on intertwined stems of other L. microphyllum plants were ungalled but exhibited a characteristic browning and scorching of the leaflet tips. Living mites were consistently present on the undersurface of scorched leaflet tips on ungalled plants, suggesting that this damage might be caused by mite feeding on L. microphyllum genotypes that did not support induction of leaflet galls. Plant nutritional status did not account for differences in galling response, because there were no differences in leaflet nitrogen between galled and ungalled stems. We review those factors known to affect the colonization of biological control agents, and discuss how they may have contributed to the lower than expected rate of F. perrepae establishment.     

Qualitative and Quantitative Evaluation of Gall Induced by <Emphasis Type="Italic">Pseudophacopteron alstonium</Emphasis> Yang et Li 1983 (Hemiptera: Psyllidae: Phacopteronidae) as Plant Parasite,in <Emphasis Type="Italic">Alstonia scholaris</Emphasis> Leaves

Alstonia scholaris (Dr C. Alston, 1685–1760) (Family Apocynaceae) (Chattim tree), commonly known as devil tree, is an evergreen tropical tree. The tree is native to India and also found in Sri Lanka, Southern China, throughout Malaysia to northern Australia. This plant is seriously damaged by formation of tumor like galls across the Kolkata city,West Bengal which affects its ornamental and medicinal value. Gall is formed by ovipositing adults of Pseudophacopteron alstonium Yang et Li 1983 (Hemiptera: Psyllidae: Phacopteronidae) and results in destruction of host plant. The nymphal stage undergoes moulting through first instar to third instar to reach the adult within galls. It is observed that highly infested leaves can bear 60–80 galls. The gallmaker Pseudophacopteron sp. stresses the host organ, and the host counters it with physiological activities supplemented by newly differentiated tissues. In infested leaves, chlorophyll and carbohydrate contents decreased sequentially with the age of the gall. There were no significant changes in protein and total amino acid content in gall tissue. But total lipid content was highest in mature galled leaves. Increased phenolic content after psylloid herbivory, which exerted oxidative stress on the host plants, was observed in gall infested leaves as compared to fresh ungalled leaves of Alstonia scholaris. Moisture content was highest in ungalled healthy leaves than the young galled, mature galled and perforated galled leaves.     

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

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

     

一种花瘿蚊(双翅目:瘿蚊科)与其寄主植物对萼猕猴桃Actinidia valvata的相互关系(英文)

猕猴桃属植物Actinidia spp,自然分布于中国的亚热带地区。对萼猕猴桃A.valvata的花芽受Pseudasphond ylia 瘿蚊属一未知种的寄生而形成花芽虫瘿,这种花芽瘿由于近年来在制药工业上的应用而受到重视。在中国中南地区研究对萼猕猴桃—瘿蚊的相互关系过程中,我们的记载表明该造瘿昆虫在一定情形下可能改变其寄主植物的雌雄异株之生殖模式。野外调查与实验证明寄主植物受寄生而形成虫瘿的比例很高。但虫瘿密度却于不同沟谷间,或同一沟谷内不同植株间有异。在二条沟谷内,92％和75％植株分别被寄生,而在第3条沟谷内没有植株受寄生。受寄生的雄性植株只产生虫瘿,而受寄生的雌性植株则产生正常果与虫瘿。有迹象显示当寄主植株有虫瘿形成时其正常果数量也更多。作者认为这可能是昆虫诱导功能上雌雄异株植物形成雄性异株雌雄同株之生殖模式的少数例子     

Plant stress and larval performance of a dipterous gall former

According to the plant vigour hypothesis, galling insects should respond positively and perform better on vigorous plants or plant parts, the opposite of the predictions of the plant stress hypothesis. I carried out field experiments to analyse the effects of sustained abiotic stress on the interactions between the common reed (Phragmites australis) and a gall-forming fly (Lipara lucens). The reed shoot diameter (a measure of plant vigour) is strongly affected by environmental conditions, where dry and/or nutrient-poor habitats produce thinner (stressed) shoots. L. lucens gall density is negatively correlated with shoot diameter. In a survival experiment with a wide range of shoot diameters, larval mortality was also highly correlated with shoot quality. Gall formation was higher on thinner, stressed shoots. An analysis of the gall tissues revealed that galls induced by L. lucens contain a high amount of a nutrient-rich feeding tissue. The impact of L. lucens is higher on thinner shoots. The results of this study showed that L. lucens performs better on stressed hosts, which contradicts the plant vigour hypothesis for galling insects. The low nutrient availability in the stressed shoots can be compensated by the production of galls with a nutrient-rich feeding tissue.     

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.     

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

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

Modification of tree architecture by a gall-forming aphid

Herbivory may substantially alter the architectural structure of plants. Among insects, gall-formers that substantially manipulate host traits may have a profound effect on the plants even at low densities. The aphid, Baizongia pistaciae induces banana-like large galls on the terminal buds of Pistacia palaestina. We hypothesized that these large galls are associated with the shape of the plant which may grow as a tree or a bush. In the natural Mediterranean forest, we monitored the effects of the galls on infested branches. In the year of gall formation, usually (~95%) there is neither elongation nor branching beyond the position of the gall. However, in the following years, galled branches produced more lateral branches (branching) than ungalled branches. This effect persists for at least 2 years. Consequently, galled branches carried more leaves and tended to gain more biomass than ungalled branches. Galling did not affect fruit yield. We suggest that repeated galling by B. pistaciae may promote bush-like architecture in P. palaestina.     

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

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

Parasitic Relationships between a Gall-forming Insect Tomoplagia rudolphi (Diptera: Tephritidae) and its Host Plant (Vernonia polyanthes, Asteraceae)

We studied the impact caused byTomoplagia rudolphi Lutz &Costa Lima (Diptera: Tephritidae) on the architecture, flowerproduction, seed germination and seed viability of its hostplantVernonia polyanthes Less (Asteraceae). We measured thegall influence on the number of lateral shoots, on the productionof flower heads and on seed viability. Gall formation induceda significant increase in the number of lateral shoots modifyingthe host plant normal growth pattern. Galled stems producedan average of 7.3±0.45 shoots, whereas ungalled stemsproduced 5.6±0.60 shoots. Galling also caused a significantreduction in seed viability. Seeds of galled stems had a totalpercent seed germination mean of 28.4%, while seeds of ungalledstems had a mean of 33.8%. However, flower head production andseed germination velocity were not affected by the gall. Thisstudy showed thatT. rudolphi galls influence the architectureand seed viability of its host plant. Insect galls; herbivory; herbivore impact; plant growth; Tomoplagia rudolphi ; Vernonia polyanthes     

Arthropod-induced neoplastic formations on trees change photosynthetic pigment levels and oxidative enzyme activities

Abstract

In order to test the hypothesis that arthropod-induced neoplastic formations on trees affect biochemical characteristics of both the newly formed galls and host plant tissues, biochemical characteristics with a possible adaptive role were determined in nine gall-former–host tree combinations. Photosynthetic pigments, extractable protein content, and oxidative enzyme activities were determined in gall tissues, leaf tissues of galled leaves, and leaves on ungalled tree branches. Neoplastic tissues were characterized by a low content of photosynthetic pigments, decreased chlorophyll a/b ratio, lower extractable protein content, and decreased activities of peroxidase and polyphenol oxidase as compared with ungalled host leaf tissues. In galled leaves or in leaves adjacent to galls, increased level of peroxidase activity was found. In several gall-inducer–host plant combinations, galled host plant tissues contained increased activity of polyphenol oxidase as well. The presented data reflect long-term systemic effects of neoplastic formation on host tree physiology suggesting that gall inducers affect potential adaptive responses of host plants.     

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.