Plant size, latitude, and phylogeny explain within-population variability in herbivory.

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

Genetic variation and constraints on the evolution of defense against spittlebug (Philaenus spumarius) herbivory in Mimulus guttatus.

Plants mediate carbon into most ecosystems and are thus under persistent attack by diverse enemies. The evolution of defense against such assaults will depend on the availability of genetic variation, as well as the costs and constraints on defense. We estimated the magnitude of genetic variation for defense against spittlebug (Philaenus spumarius) herbivory in Mimulus guttatus using a diallel cross-grown in a greenhouse. Except for flowering time, additive genetic variation for the plant traits we measured was negligible, regardless of herbivory environment. In contrast, nonadditive genetic variation contributed significantly to all plant traits measured. We found significant additive genetic variation among plants for biomass of adult spittlebugs, suggesting heritability for resistance to herbivory. The other putative resistance trait measured, spittlebug maturation time, was not significantly heritable. We found no evidence for significant genetic variation for tolerance to herbivory except for a small non-nuclear paternal contribution to tolerance for flower number. Additive genetic correlations indicated that more resistant plant genotypes (in terms of adult spittlebug biomass) were also smaller in the absence of spittlebugs, suggesting a potential cost of resistance to herbivory. We found no other significant genetic correlations indicating a cost of defense, nor did we find evidence for a tradeoff between resistance and tolerance to herbivory. Overall, these results suggest the future adaptive evolution of tolerance to spittlebugs in this population will be limited primarily by available genetic variation, whereas the future evolution of antibiosis resistance may be constrained by allocation costs of resistance.

Efficacy of broadcast and perimeter applications of S-methoprene bait on the red imported fire ant in grazed pastures.

The red imported fire ant, Solenopsis invicta (Buren), is a major pest in the United States because of its painful sting. Toxic bait has been an important management tool against fire ants, but site registrations prohibit applications of most baits on grazed pastures. Extinguish, containing the insect growth regulator methoprene, was selected for this study because it has a broad site registration that includes grazed pastures. The primary objective of this research was to evaluate the efficacy for control of red imported fire ants by using broadcast applications of methoprene bait at a label rate of 1,121 g/ha versus applications around the perimeter of a target area at the reduced rate of 280 g/ha. Grazed pastures in Lee County, Alabama, and Chambers County, Alabama, were selected for this study, with broadcast treatments, perimeter treatments, and controls replicated three times at each site. All mounds were counted and rated using the USDA population index before applications and then at 8 and 16 wk posttreatment. Perimeter applications did not significantly reduce S. invicta mound abundance, but bait treatments significantly reduced mound abundance at 16 wk posttreatment at site 1 where applications were conducted in early evening. However, broadcast applications were not effective at site 2 where treatments were conducted in early morning with warmer temperatures. Emergence of winged alates was observed at 12 wk posttreatment, followed by a high density of incipient mounds that may have masked the full treatment effect of methoprene applications at site 2. Methoprene bait was effective in reducing abundance of S. invicta only when full label rates were applied.

Genetic variation and covariation for resistance and tolerance to Cucumber mosaic virus in Mimulus guttatus (Phrymaceae): a test for costs and constraints.

Genetic variation for resistance and tolerance to pathogens may be maintained by costs represented as genetic tradeoffs between these traits and fitness. The evolution of resistance and tolerance also may be constrained by negative genetic correlations between these defense systems. Using a complete diallel, we measured genetic variation and covariation for and among performance, resistance, and tolerance traits in Mimulus guttatus challenged with a generalist pathogen, Cucumber mosaic virus (CMV). Viral coat protein was detected by enzyme-linked immunosorbent assay (ELISA) in all inoculated plants, indicating that all plants were susceptible to infection, although the ELISA absorbance varied quantitatively across plants. Plants inoculated with CMV had significantly reduced aboveground biomass and flower production relative to controls, although date of first flower was unaffected by infection. All three of these performance traits showed moderate to high narrow-sense heritability (h2 = 0.32-0.62) in both inoculated and control plants. We found phenotypic variation for both tolerance of and resistance to our strain of CMV, but both displayed very low narrow-sense heritability (h2 < 0.03). We found no evidence of a trade-off between resistance and tolerance. We also found no evidence for a cost of resistance or tolerance. In fact, a significant genetic correlation suggested that plants that were large when healthy had the greatest tolerance when infected. Significant, positive genetic correlations found between performance of uninfected and infected plants suggested that selection would likely favor the same M. guttatus genotypes whether CMV is present or not.

Sequential radiation of unrelated organisms: the gall fly Eurosta solidaginis and the tumbling flower beetle Mordellistena convicta.

Host shifts and the formation of insect-host races are likely common processes in the speciation of herbivorous insects. The interactions of goldenrods Solidago (Compositae), the gall fly Eurosta solidaginis (Diptera: Tephritidae) and the beetle Mordellistena convicta (Coleoptera: Mordellidae) provide behavioural, ecological and genetic evidence of host races that may represent incipient species forming via sympatric speciation. We summarize evidence for Eurosta host races and show that M. convicta has radiated from goldenrod stems to Eurosta galls to form host-part races and, having exploited the galler's host shift, has begun to differentiate into host races within galls. Thus, host-race formation has occurred in two interacting, but unrelated organisms representing two trophic levels, resulting in 'sequential radiation' (escalation of biodiversity up the trophic system). Distributions of host races and their behavioural isolating mechanisms suggest sympatric differentiation. Such differentiation suggests host-race formation and subsequent speciation may be an important source of biodiversity.