Climate change and cereal aphids: the relative effects of increasing CO2 and temperature on aphid population dynamics

Experimental evidence regarding the responses of cereal aphids to rising atmospheric CO₂ has been ambiguous. Some studies suggest increased population sizes under future CO₂ levels, others suggest decreased population sizes, and still others suggest little or no difference. Recently, Newman et al. (2003) constructed a general mathematical model of the aphid–grass interaction to investigate whether or not we should, in fact, expect a general aphid response to rising CO₂. They concluded that aphid populations are likely to be larger under future CO₂ concentrations if soil N levels are high, the aphid species' nitrogen requirement is low and the aphid species' density‐dependent response in winged morph production is weak. In that model, and in field experiments, CO₂ concentration influences aphid population dynamics through the effect it has on plant quality. However, future CO₂ concentrations are also likely to be accompanied by higher ambient temperatures, a combination that has received little focus to date. In the present paper, the Newman et al. model is used to consider the combined effects of increased CO₂ concentrations and temperature on aphid population sizes. It is concluded that, when both factors are elevated, aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone. This result has important implications for future experimentation.     

Location and distribution of symbiotic bacteria during floral development in Ardisia crispa

Abstract. The location and distribution of symbiotic bacteria during floral development in Ardisia crispa (Thunb.) A.DC., a species characterized by bacterial leaf nodules, has been studied using light, scanning and transmission electron microscopy. During early floral development, bacteria in mucilage derived from host plant trichomes, become enclosed in a small conical chamber on top of the placenta, as a result of the closure and fusion of the carpel initials. The placental epidermal cells, which appear to be secretory in nature, become detached apically in places forming a network of grooves which traverse the placental surface. The symbiotic bacteria are preferentially located in these grooves. As growth and development of the placenta proceed, the grooves widen and deepen to form channels. The cells lining these channels secrete a mucilaginous material. The network of channels covers the entire placental surface and terminates at the placental margins surrounding the ovules. Bacteria are found within the channels, at the ends of the channels near the margin of the placenta, on the surface of the ovules and in the micropyle. It is suggested that these mucilage-filled channels are responsible for, and a prerequisite of, ensuring that the bacterial partner is efficiently transmitted from one host generation to the next by providing a mechanism by which the bacteria arc accurately placed within the developing seed.     

Reassembly of brome mosaic virus from dissociated virus

The reconstitution of Brome Mosaic Virus (BMV) has been studied using neutron scattering. Experiments were performed on disassembled virus without subsequent separation of components. Phase diagrams of the disassembly and subsequent reassembly of BMV were established as a function of pH and LiCl molarity by analytical centrifugation and quasi-elastic light scattering. Disassembly occurs at a pH above 6.5 and above 0.8 M LiCl. On reassembly, if the pH is lowered first, capsids are formed without subsequent incorporation of RNA. Neutron scattering was used to investigate the formation of virus particles, when the ionic strength was lowered from 1.4 to 0.1 M LiCl at pH 7.8. The reconstitution was followed continuously. As it was driven by a lowering of the ionic strength the kinetics of the process cannot be studied for short times. However the fact that at any given ionic strength no evolution of the scattering was observed with time implies that the reconstitution is complete within a few minutes. The observations in buffers with various amounts of D₂O lead to the conclusion that the reassembly is achieved by co-condensation of the RNA and of the capsid proteins.     

Parameter estimation techniques for interaction and redistribution models: a predator-prey example

Summary The use of parameter estimation techniques for partial differential equations is illustrated using a predatorprey model. Whereas ecologists have often estimated parameters in models, they have not previously been able to do so for models that describe interactions in heterogeneous environments. The techniques we describe for partial differential equations will be generally useful for models of interacting species in spatially complex environments and for models that include the movement of organisms. We demonstrate our methods using field data from a ladybird beetle (Coccinella septempunctata) and aphid (Uroleucon nigrotuberculatum) interaction. Our parameter estimation algorithms can be employed to identify models that explain better than 80% of the observed variance in aphid and ladybird densities. Such parameter estimation techniques can bridge the gap between detail-rich experimental studies and abstract mathematical models. By relating the particular bestfit models identified from our experimental data to other information on Coccinella behavior, we conclude that a term describing local taxis of ladybirds towards prey (aphids in this case) is needed in the model.     

The effect of simulated acid rain on performance of the aphid Euceraphis betulae (Koch) on silver birch

Summary The effect of artificial acid rain on the reproduction and survival of the aphid Euceraphis betulae on silver birch was studied in Turku, southern Finland. Eight bioassays were done during 1984–1986. In four of the bioassays the aphids produced 40% to over 100% more progeny on birches watered with dilute sulphuric acid (pH 3.5) than on control trees. In four other cases the performance of aphids did not differ between the treatments. An index of aphid reproduction pooled over the whole study was significantly higher on acid-treated than on control birches. The reproduction of aphids on acid-treated birches was enhanced when precipitation was below long term average, suggesting an interaction between the stress caused by acid treatment and dry periods.     

Relationships between the effects of insect herbivory and sheep grazing on seasonal changes in an early successional plant community

Summary The effects of spring grazing by sheep and of natural levels of insect herbivory were studied in 1985 on a limestone field abandoned from arable land for four years. A split-plot design was adopted in which paddocks, arranged in Latin squares, were either left ungrazed or heavily grazed by sheep for ten days in April. Within each paddock plots were either sprayed regularly with Malathion-60 or untreated.Natural levels of insect herbivory, compared to the reduced levels in insecticide-treated plots, had effects of similar magnitude to those from the short burst of spring grazing. Many attributes of the grazed/insecticide-treated sward were either increased or decreased by a factor of two within a season. Both types of herbivore caused changes in the direction of plant succession as well as in its rate. Effects on early successional species were large and similar when caused by either type of herbivore. Effects on later successional species were often smaller, but also showed differences in the action of the two herbivore types, as did effects on sward height, species richness and total cover. The effects of sheep and insect herbivory were not always additive or in the same direction.The results suggest that manipulations of both mammal and insect herbivores may be powerful tools for directing changes in plant community composition.     

Why does the bud-galling sawfly,Euura mucronata,attack long shoots?

Summary The bud-galling sawfly, Euura mucronata, attacked longer shoot length classes on its host, Salix cinerea, more frequently than shorter shoots. Shoot length accounted for 76 to 93 percent of the variance in number of galls per 100 shoots in three habitats: forest, watermeadow, and lakeside. The reasons for this pattern were addressed with studies on shoot length in relation to: 1. Number of resources (buds) per shoot; 2. Success in establishment of larvae in galls; 3. Gall size and resources per gall; and 4. Survival of larvae after establishment as influenced by plant resistance and natural enemy attack. The most important factors proved to be success in establishment of larvae, with percent of variance accounted for ranging from 57 to 77 percent in three of four sites where relationships were significant, and survival after establishment of larvae, with variance accounted for ranging from 40 to 54 percent in the same three sites. The pattern of survival was dictated by plant resistance and not by natural enemies. These two additive factors resulted in a general relationship across all sites of increasing emergence of fully developed larvae per cohort as shoot length increased, accounting for 78 percent of the variance. These adaptive advantages to attacking longer shoots are sufficient to account for the pattern of increased probability of shoots being attacked as they increase in length.     

Optimizing the expression of chimeric genes in plant cells

Summary The Serratia marcescens chiA gene encodes a secreted chitinase activity which contributes to the fungal growth inhibition exhibited by this bacterium. The coding region from the chiA gene was fused to the promoter and 3 polyadenylation region of the Agrobacterium nopaline synthase gene. Site-directed mutagenesis of specific nucleotides surrounding the initiating AUG of the coding sequence of this chimeric gene resulted in up to an eight-fold increase in the amount of chitinase protein detected in transformed plant tissue. Analysis of the chiA mRNA indicated that these nucleotides also affected mRNA levels. At least 50% of the chitinase protein produced in transformed tobacco cells was the same molecular weight as the S. marcescen secreted protein.     

Plant-water relations in wheat as influenced by root pruning

Summary A greenhouse study in which 24, 54 and 71 per cent roots of wheat (Triticum aestivum L.) were pruned on the 73rd day from the date of planting (anthesis stage) showed that during a 7-day period following root pruning, total transpiration and leaf water potential were significantly lower (P=0.05) and the stomatal resistance was significantly higher (P=0.05) where 54 and 71 per cent roots were pruned, as compared to no root pruning or 24 per cent root pruning. The leaf relative water content, however, showed no significant differences. Thus about one-fourth root sytem could be reduced without adversely affecting the plant-water status.