Variation in pea aphid population development in three different habitats

Abstract.

• 1 Seasonal population growth rates for the pea aphid, Acyrthosiphon pisum Harris, were determined in three different host plant habitats; alfalfa, Medicago sativa (L.), clover, Trifolium pratense (L.), and peas, Pisum sativum (L.); over four years and eight places. It was possible to estimate a common intrinsic rate of increase for each host plant habitat.
• 2 An analysis of the relative influence of temporal, spatial and host plant habitat variation showed that the host plant habitat was most important in determining the growth rates of the populations, both in rate of build-up and decline.
• 3 Patterns of alate production in the three different habitats differed substantially between the annual peas and the two perennial legumes. During the summer, alate production was large and rapid in peas and remained low and constant in clover and alfalfa
• 4 Parasitism was highest in peas. The species composition of parasitoids differed between crops.
• 5 Aphids in annual peas had a higher intrinsic rate of increase and a faster rate of decline than in the two perennial legumes. This explains the presence of both migratory and sedentary forms among pea aphids.

     

A note on combining two methods of dispersal-for-distance

We ask under what circumstances two methods of dispersal-for-distance should be combined, given that the second method may carry the diaspore back towards its point of origin. The combination is made possible when the morphological adaptations of the diaspore are compatible. It is advantageous when the return on investment in the first method of dispersal declines sharply beyond some level of investment. The median seed achieves the best net distance when the two methods achieve similar distances; the upper decile of seeds achieve nearly a simple sum of the two distances. The first two conditions apply to the combination of ballistic with ant-dispersal, which is widespread in Australian sclerophyll shrubs. Mother plants’fitnesses could well be determined by the upper decile of distances their seeds achieve. It remains an open question whether the addition of ant-dispersal to ballistic dispersal achieves the selective advantage of distance or of placement.     

Thermoregulation in orb-web spiders: new descriptions of thermoregulatory postures and experiments on the effects of posture and coloration

Ideally, diurnal orb-web spiders should be able to ignore problems of insolation when siting their webs and should be able to operate such webs without the necessity of retiring into shade. Postures that minimize the surface area of the spider exposed to insolation may help to free the spider from the danger of overheating. Such postures are here described for the first time for Argiope argentata and three species of Gasteracantha . Experiments with dead Nephila clavipes show that postures described as thermoregulatory do, in fact, reduce absolute temperatures (and the rate of temperature increase) compared to normal predatory postures.
Metalic or other reflectant coloration occurs in many species of diurnal orb-web spiders. These are partly listed herein. Experiments with one such species, Argiope argentata , show that overpainting the silver parts increases both the rate of temperature increase and the absolute temperatures reached when naturally coloured and black-painted spiders are exposed to the same radiant heat sources.
It is suggested that these results on the probable thermoregulatory function of metallic coloration can provide insights into the probable habitat distributions of species whose coloration is known but whose ecology is presently unknown. The general question of adaptive coloration in spiders is discussed in the light of these results.     

Mycoviruses of filamentous fungi and their relevance to plant pathology

Mycoviruses (fungal viruses) are reviewed with emphasis on plant pathogenic fungi. Based on the presence of virus-like particles and unencapsidated dsRNAs, mycoviruses are common in all major fungal groups. Over 80 mycovirus species have been officially recognized from ten virus families, but a paucity of nucleic acid sequence data makes assignment of many reported mycoviruses difficult. Although most of the particle types recognized to date are isometric, a variety of morphologies have been found and, additionally, many apparently unencapsidated dsRNAs have been reported. Until recently, most characterized mycoviruses have dsRNA genomes, but ssRNA mycoviruses now constitute about one-third of the total. Two hypotheses for the origin of mycoviruses of plant pathogens are discussed: the first that they are of unknown but ancient origin and have coevolved along with their hosts, the second that they have relatively recently moved from a fungal plant host into the fungus. Although mycoviruses are typically readily transmitted through asexual spores, transmission through sexual spores varies with the host fungus. Evidence for natural horizontal transmission has been found. Typically, mycoviruses are apparently symptomless (cryptic) but beneficial effects on the host fungus have been reported. Of more practical interest to plant pathologists are those viruses that confer a hypovirulent phenotype, and the scope for using such viruses as biocontrol agents is reviewed. New tools are being developed based on host genome studies that will help to address the intellectual challenge of understanding the fungal–virus interactions and the practical challenge of manipulating this relationship to develop novel biocontrol agents for important plant pathogens.