Timothy P. Craig

A strong relationship between oviposition preference and larval performance in a shoot-galling sawfly

To investigate the relationship between oviposition preference and offspring performance we tested three hypotheses about the oviposition behavior of Euura lasiolepis: (1) the preference hierarchy hypothesis that Euura had an oviposition preference based on shoot lengths; (2) the larval survival hypothesis that the preference hierarchy corresponded to the suitability of shoots for larval survival; and (3) the flexibility hypothesis that the preference hierarchy changed as resource availability changed. The oviposition preference hierarchy of Euura lasiolepis, a shoot—galling sawfly, for plant shoots of different lengths strongly corresponded with the rank of shoot lengths for larval survival. The oviposition preference hierarchy was related to shoot growth; the more rapidly a shoot was growing the higher the probability that an oviposition site on a shoot would be attacked. Larval survival was highest on long shoots on young ramets. The probability of shoot abscission was negatively related to shoot length, and Euura in galls on abscised shoots invariably died. Intraspecific interactions and willow clone identity, independent of shoot length and ramet age, had relatively small influences on larval survival. The oviposition hierarchy was flexible; if long shoots were not available shorter shoots were accepted for oviposition. Shoots were always attacked in order of length, so that a strong relationship between preference and performance was always maintained.

Resource Regulation by a Stem‐Galling Sawfly on the Arroyo Willow

We studied the impact of the stem—galling tenthredinid sawfly, Euura lasiolepis, on the growth and branch age structure of the arroyo willow, Salix lasiolepis. Normally, as willows age they become less susceptible to galling, but heavy Euura galling maintains willows at a young, relatively susceptible juvenile stage. The Euura form more galls and long shoots. Long shoots are found on clones with young branches. Heavy galling stunts or kills growth distal to the gall, stimulating sprouting by indefinitely dormant buds located near branch bases. The resulting young branches keep the clone susceptible to further galling. In contrast, increasing branch age of lightly galled clones confers resistance to galling. We term this model of gall—induced resource maintenance the resource regulation hypothesis. Resource regulation is the maintenance or increase of high—quality resources by an herbivore species that impacts immediately subsequent generations of the same herbivore species on the same plant. See full-text...

Ecological communities: Plant mediation in indirect interaction webs

Preface Part I. Introduction: 1. Indirect interaction webs: an introduction Takayuki Ohgushi, Timothy P. Craig and Peter W. Price Part II. Interaction Linkages Produced by Plant-mediated Indirect Effects: 2. Plant-mediated interactions in herbivorous insects: mechanisms, symmetry, and challenging the paradigms of competition past Robert F. Denno and Ian Kaplan 3. Going with the flow: plant vascular systems mediate indirect interactions between plants, insect herbivores and hemi-parasitic plants Susan E. Hartley, Kathy A. Bass and Scott N. Johnson 4. Plant-mediated effects linking herbivory and pollination Judith L. Bronstein, Travis E. Huxman and Goggy Davidowitz 5. Trait-mediated indirect interactions, density-mediated indirect interactions and direct interactions between mammalian and insect herbivores Jose M. Gomez and Adela Gonzales-Megias 6. Insect-mycorrhizal interactions: patterns, processes and consequences Alan C. Gange Part III. Plant-mediated Indirect Effects in Multitrophic Systems: 7. Plant-mediated interactions between below- and aboveground processes: decomposition, herbivory, parasitism and pollination Katja Poveda, Ingolf Steffan-Dewenter, Stefan Scheu and Teja Tscharntke 8. Bottom-up cascades induced by fungal endophytes in multitrophic systems Enrique J. Chaneton and Marina Omacini 9. Ecology meets plant physiology: herbivore-induced plant responses and their indirect effects on arthropod communities Maurice W. Sabelis, Junji Takabayashi, Arne Janssen, Merijn Kant, Michiel van Wijk, Beata Sznajder, Nayanie Aratchige, Izabela Lesna, Belen Belliure and Robert C. Schuurink Part IV. Plant-mediated Indirect Effects on Communities and Biodiversity: 10. Nontrophic, indirect interaction webs of herbivorous insects Takayuki Ohgushi 11. Effects of arthropods as physical ecosystem engineers on plant-based trophic interaction webs Robert J. Marquis and John T. Lill 12. Host plants mediated ant-aphid mutualisms and their effects on community structure and diversity Gina M. Wimp and Thomas G. Whitham 13. Biodiversity is related to indirect interactions among species of large effect Joseph K. Bailey and Thomas G. Whitham Part V. Evolutionary Consequences of Plant-mediated Indirect Effects: 14. Evolution of plant-mediated interactions among natural enemies Timothy P. Craig 15. Linking ecological and evolutionary change in multitrophic interactions: assessing the evolutionary consequences of herbivore-induced changes in plant traits David M. Althoff Part VI. Synthesis: 16. Indirect interaction webs propagated by herbivore-induced changes in plant traits Takayuki Ohgushi, Timothy P. Craig and Peter W. Price.To gain a more complete understanding of plant-based ecological community structure requires knowledge of the integration of direct and indirect effects in plant–herbivore systems. Trait modification of plants as a result of herbivory is very common and widespread in terrestrial plants, and this initiates indirect interactions between organisms that utilize the same host plant. This book argues that food webs by themselves are inadequate models for understanding ecological communities, because they ignore important indirect, nontrophic links. This subject is of great importance in understanding not only community organization but also in identifying the underlying mechanisms of maintenance of biodiversity in nature. This book will be an invaluable resource for researchers and graduate students interested in community and population ecology, evolutionary biology, biodiversity, botany, and entomology.

Carbon-nutrient balance hypothesis in within-species phytochemical variation ofSalix lasiolepis.

Predictions of the carbon-nutrient balance hypothesis were tested using a study of within-species phytochemical variation in the arroyo willow,Salix lasiolepis. The prediction that a balance between nutrients (total protein) and carbon-based secondary metabolites (total phenols) should exist was supported using water treatment and fertilizer experiments and wild willow clones. Leaf nitrogen content and net photosynthetic rates of plants potted in soil in which parental plants grew was low, indicating that wild plants exist under relatively low nutrient status-high carbon balance conditions. The hypothesis also correctly predicted positive relationships between shoot length and phenols in glasshouse plants, wild plants, and plants in the water treatment experiment and negative relationships between shoot length and phenols in the fertilizer treatment experiment. Total phenolic glycosides, fragilin, picein, salicortin, tremulacin, and tremuloidin all correlated positively with shoot length in glasshouse plants on a carbon-biased balance, and male willows had generally lower levels of phenolic glycosides than females. Salicortin and tremulacin showed the strongest positive relationships with shoot length.

The Window of Vulnerability of a Shoot-Galling Sawfly to Attack by a Parasitoid

We studied the susceptibility of a shoot-galling sawfly, Euura lasiolepis, to attack by the parasitoid Lathrostizus euurae. Each Euura had a window of vulnerability: a limited time period when the parasitoid attacked it. Lathrostizus attacked a limited range of gall sizes and ages during each sample period. Gall age was related to its position on a shoot, and Lathrostizus shifted the gall position it attacked as it tracked vulnerable-aged galls. The parasitoid emergence period matched the window of vulnerability of its host. Lathrostizus attacked and successfully parasitized a low proportion of hosts encountered. A low interval-parasitism rate uniformly distributed among Eulura galls formed at different times is consistent with the pattern expected if the window of vulnerability influenced attack. The window of vulnerability was formed by the interaction of gall and sawfly devel- opment rates. Two hypotheses that defined the window were supported: (1) the host de- velopment hypothesis, that Lathrostizus is a larval parasite and does not attack galls containing eggs and (2) the gall toughness hypothesis, that large, old galls that are tough are not attacked by Lathrostizus. Our data did not support (3) the ovipositor limitation hypothesis, that some galls were too large for the larva inside to be reached by the Lath- rostizus ovipositor. We observed that if these galls were soft, they were attacked. Lathrosti- zus parasitism rates on Euura varied widely among willow clones. Willows differed in the rate of gall toughening, which produced different-sized windows and thus contributed to variation in parasitism rates among Euura populations on different willow clones.

Impact of Shoot Galler Attack on Sexual Reproduction in the Arroyo Willow

The assumption that herbivores cause reduction of plant fitness is central to plant herbivore theory, but empirical support from field studies is scant. We tested the hypothesis that the shoot-galling sawfly Euura lasiolepis significantly influences the fitness of its obligate host, the arroyo willow (Salix lasiolepis). Surveys of reproductive attributes on field willow clones at different elevations indicated that the presence of shoot galls led to a significant reduction in reproductive buds, and thereby inflorescences, on individual shoots on both male and female plants. Shoots with galls suffered a loss of 43% of their reproductive buds relative to shoots without galls. Inflorescence size, measured as flowers per inflorescence, seed mass, germination success, and pollen germination success were not significantly affected by gall presence. Two sawfly exclosure-enclosure experiments on plants that had been established from cuttings from clones showed reduced reproductive bud production on plants with galls at both the whole-plant and individual-shoot levels; whole-plant losses of 34-56% and per- shoot losses of 17-55% of reproductive buds on different clones were coupled with densities that varied from 0.56 to 2.70 galls per shoot. Most plants suffered relatively small repro- ductive losses because of low sawfly densities. However, 20% of all plants suffered losses of 10%, while 7.5% experienced losses of 20% or more of their reproductive potential. A small willow clone attacked over the range of 0.5-2.0 galls per shoot would lose 381 000- 1 500 000 seeds of a potential 3 760 000 seeds in the year of attack. Genetic variation among plants in susceptibility to sawfly attack and subsequent herbivore-related repro- ductive loss was suggested by a common garden study; some clones had 12 times as many galls as others.

GEOGRAPHIC VARIATION IN THE EVOLUTION AND COEVOLUTION OF A TRITROPHIC INTERACTION

Abstract The geographic mosaic theory of coevolution predicts that geographic variation in species interactions will lead to differing selective pressures on interacting species, producing geographic variation in the traits of interacting species (Thompson 2005). We supported this hypothesis in a study of the geographic variation in the interactions among Eurosta solidaginis and its natural enemies. Eurosta solidaginis is a fly (Diptera: Tephritidae) that induces galls on subspecies of tall goldenrod, Solidago altissima altissima and S. a. gilvocanescens. We measured selection on E. solidaginis gall size and shape in the prairie and forest biomes in Minnesota and North Dakota over an 11-year period. Galls were larger and more spherical in the prairie than in the forest. We supported the hypothesis that the divergence in gall morphology in the two biomes is due to different selection regimes exerted by natural enemies of E. solidaginis. Each natural enemy exerted similar selection on gall diameter in both biomes, but differences in the frequency of natural enemy attack created strong differences in overall selection between the prairie and forest. Bird predation increased with gall diameter, creating selection for smaller-diameter galls. A parasitic wasp, Eurytoma gigantea, and Mordellistena convicta, an inquiline beetle, both caused higher E. solidaginis mortality in smaller galls, exerting selection for increased gall diameter. In the forest there was stabilizing selection on gall diameter due to a combination of bird predation on larvae in large galls, and M. convicta- and E. gigantea-induced mortality on larvae in small galls. In the prairie there was directional selection for larger galls due to M. convicta and E. gigantea mortality on larvae in small galls. Mordellistena convicta-induced mortality was consistently higher in the prairie than in the forest, whereas there was no significant difference in E. gigantea-induced mortality between biomes. Bird predation was nonexistent in the prairie so the selection against large galls found in the forest was absent. We supported the hypothesis that natural enemies of E. solidaginis exerted selection for spherical galls in both biomes. In the prairie M. convicta exerts stabilizing selection to maintain spherical galls. In the forest there was directional selection for more spherical galls. Eurytoma gigantea exerted selection on gall shape in the forest in a complex manner that varied among years. We also supported the hypothesis that E. gigantea is coevolving with E. solidaginis. The parasitoid had significantly longer ovipositors in the prairie than in the forest, indicating the possibility that it has evolved in response to selection to reach larvae in the larger-diameter prairie galls.

Preference and performance are correlated in the spittlebug Aphrophora pectoralis on four species of willow

Abstract 1. There was a positive correlation between oviposition and feeding preferences and offspring performance in the spittlebug Aphrophora pectoralis Matsumura (Homoptera: Cercopoidea, Aphrophoridae) on four species of willow Salix sp. (Salicaceae) growing near Sapporo, Japan. Spittlebugs preferred rapidly growing shoots where performance was highest.

Fearful behavior by caged hens of two genetic stocks

Abstract Two White Leghorn stocks (Y 1 and Y 2 ) and sire families within each were compared for nervousness scores and for latencies to feed and to return to normal activity after being subjected to fear-inducing stimuli, involving either a metronome or the cage being struck by a human observer. Information on feather damage and loss, age at sexual maturity, and part-year egg-mass production was also collected. Strain differences were clearly evident for nervousness score, duration of fearful behavior, and feather loss. The strain having more nervous and fearful hens also suffered greater feather damage and loss. Sire-family differences were erratically present for the same traits. Correlation coefficients calculated within strains for latency data obtained by 2 observers, working independently, indicated significant repeatability in 5 of 8 comparisons ( r values from 0.50 to 0.91). Correlations of latencies within strains obtained by the use of 2 kinds of fear-inducing stimuli also indicated significant repeatability in 4 of 8 comparisons ( r values from 0.35 to 0.88). Further intra-strain correlation analyses indicated that fearful behavior, as measured by latencies, tended to be associated with number of hens per cage, but was relatively independent of neighbors behavior. Greater nervous and fearful behavior of caged groups tended to be significantly associated with greater feather loss and non-significantly associated with earlier sexual maturity and lower egg-mass production of 30 – 40-week-old pullets.

Plant genotypic diversity increases population size of a herbivorous insect

It is critical to incorporate the process of population dynamics into community genetics studies to identify the mechanisms of the linkage between host plant genetics and associated communities. We studied the effects of plant genotypic diversity of tall goldenrod Solidago altissima on the population dynamics of the aphid Uroleucon nigrotuberculatum. We found genotypic variation in plant resistance to the aphid in our experiments. To determine the impact of plant genotypic diversity on aphid population dynamics, we compared aphid densities under conditions of three treatments: single-genotype plots, mixed-genotype plots and mixed-genotype-with-cages plots. In the latter treatment plants were individually caged to prevent natural enemy attack and aphid movement among plants. The synergistic effects of genotypes on population size were demonstrated by the greater aphid population size in the mixed-genotype treatment than expected from additive effects alone. Two non-exclusive hypotheses are proposed to explain this pattern. First, there is a source–sink relationship among plant genotypes: aphids move from plant genotypes where their reproduction is high to genotypes where their reproduction is low. Second, natural enemy mortality is reduced in mixed plots in a matrix of diverse plant genotypes.