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Gall Diameter (gall + diameter)

Distribution by Scientific Domains

Life Sciences	100%

Selected Abstracts

Non-random distribution among a guild of parasitoids: implications for community structure and host survival

ECOLOGICAL ENTOMOLOGY, Issue 6 2006
ANTHONY M. ROSSI
Abstract 1.,Immature stages of the gall midge, Asphondylia borrichiae, are attacked by four species of parasitoids, which vary in size and relative abundance within patches of the gall midge's primary host plant, sea oxeye daisy (Borrichia frutescens). 2.,In the current study, a bagging experiment found that the smallest wasp, Galeopsomyia haemon, was most abundant in galls exposed to natural enemies early in the experiment, when gall diameter is smallest, while the wasp with the longest ovipositor, Torymus umbilicatus, dominated the parasitoid community in galls that were not exposed until the 5th and 6th weeks when gall diameter is maximal. 3.,Moreover, the mean number of parasitoids captured using large artificial galls were 70% and 150% higher compared with medium and small galls respectively, while stem height of artificial galls significantly affected parasitoid distribution. Galls that were level with the top of the sea oxeye canopy captured 60% more parasitoids compared with those below the canopy and 50% more than galls higher than the plant canopy. 4.,These non-random patterns were driven primarily by the differential distribution of the largest parasitoid, T. umbilicatus, which was found significantly more often than expected on large galls and the smallest parasitoid of the guild, G. haemon, which tended to be more common on stems level with the top of the plant canopy. 5.,Large Asphondylia galls, especially those located near the top of the Borrichia canopy, were more likely to be discovered by searching parasitoids. Results using artificial galls were consistent with rates of parasitism of Asphondylia galls in native patches of sea oxeye daisy. Gall diameter was 19% greater and the rate of parasitism was reduced by almost 50% on short stems; as a result, gall abundance was 24% higher on short stems compared with ones located near the top of the plant canopy. 6.,These results suggest that parasitoid community composition within galls is regulated by both interspecific differences in ovipositor length and preferences for specific gall size and/or stem length classes. [source]

GEOGRAPHIC VARIATION IN THE EVOLUTION AND COEVOLUTION OF A TRITROPHIC INTERACTION

EVOLUTION, Issue 5 2007
Timothy P. Craig
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. [source]