Influence of environmental conditions on patch exploitation strategies of parasitoids

In patchy environments, patch-leaving decision rules are a keycomponent of the foraging behavior of parasitoids that haveto share their searching time between the different host patchesto optimize the number of ovipositions. It has been experimentallyshown that one of the proximate cues involved in patch-leavingdecision is the number of ovipositions performed by the parasitoidwhile in the patch. Ovipositions can have either a decrementalor an incremental effect on the patch residence time dependingon parasitoid species. Previous theoretical studies have suggestedthat environmental conditions and the ability of parasitoidsto reliably estimate the number of available hosts on a patchwhen entering it should influence how patch departure dependson ovipositions. We developed an individual-based model to testa large variety of patch-leaving decision rules in differentenvironmental conditions. This model includes a more realisticrepresentation of individual acquisition of information thanprevious theoretical work. In accordance with previous results,we show that the best use of information depends on environmentalconditions. Moreover, we identify the environmental factorsthat have a decisive influence on the most appropriate typeof rule (incremental or decremental). Decremental mechanismsare most efficient only in restricted conditions characterizedby a large number of patches and few parasitoids. The sensitivityof decision rules to frequency-dependent processes was testedby numerical invasibility experiments. Rare mutants using anyalternative rule never outperform populations using a high-performancerule. Incremental rules seem to be more sensitive than decrementalones to frequency-dependent processes.     

Kin recognition by the predatory mite Iphiseius degenerans: discrimination among own, conspecific, and heterospecific eggs

1. Kin recognition is important in many social insects, but has also been found in several nonsocial insects such as parasitoids, where it plays an important role in oviposition behaviour. In nonparasitic arthropods, however, the fitness of ovipositing females also depends on the oviposition behaviour of related and unrelated females, especially when eggs are oviposited in clusters by several females. 2. In this paper, kin recognition in a predatory mite, Iphiseius degenerans, is studied. Mothers are capable of determining offspring sex ratio, and cannibalism on juvenile stages is a common phenomenon. Therefore, kin recognition is expected to occur in this predator. 3. Oviposition behaviour of this species is particularly interesting because it alternates foraging bouts in flowers with deposition of a single egg at a time on a leaf, where predation risk is lower. The eggs are not scattered but are deposited in clusters. After feeding in a flower, females therefore have to locate clusters of eggs. 4. Experiments on two‐choice arenas showed that females prefer to oviposit close to conspecific eggs rather than close to heterospecific eggs. Females also showed a preference for ovipositing near closely related conspecific eggs rather than more distantly related eggs. 5. Females tended to displace eggs of heterospecifics more frequently than eggs of conspecifics. 6. These behavioural observations show that females can discriminate not only between conspecific and heterospecific eggs but also between eggs that vary in degree of relatedness. This enables females to oviposit in clusters containing related eggs and thus avoid cannibalism by non‐kin and/or produce adaptive sex ratios despite the fact that the adults commute between flowers and leaves.     

Habitat selection in a large orb‐weaving spider: vegetational complexity determines site selection and distribution

1. The distribution of the large orb‐weaving spider Argiope trifasciata in old field habitats of North America and the habitat selection process this species used was studied for 2 years. 2. Because web spiders have limited dispersal abilities and an energetically costly prey capture device, they do not have the ability to sample potential foraging sites. Structural complexity of the vegetation to which the web must be attached is relatively easy to assess. The hypothesis that the structural complexity is a primary factor in determining initial web site selection was tested both by relating the natural distribution of the spiders across habitats to vegetational complexity and by manipulating the complexity of the habitats in a series of experiments. 3. Argiope trifasciata was not distributed evenly among three old field vegetation types. Habitat complexity was related to spider density in both years although no measure of insect activity, prey capture, or prey consumption was correlated with spider distribution. 4. Three experimental manipulations were conducted to test the impact of habitat structure on spider establishment: (1) the amount of natural vegetation was reduced, (2) structures were added to a simple habitat, and (3) the complexity of the structures added was varied. In each case, spiders were introduced and establishment of webs was monitored. In all manipulations, spider establishment was related to the complexity of the substrate available. 5. These results are important for understanding the cues that influence foraging site selection and therefore provide insight into the distribution of species with limited dispersal abilities and high site investment requirements.     

Risk-sensitive decisions during nesting may increase maternal provisioning capacity in the subsocial shield bug Parastrachia japonensis

Abstract 1. Females of the monophagous shield bug Parastrachia japonensis Scott provision their nymph-containing nests with high-quality drupes of the single host tree, Schoepfia jasminodora , a resource that is poor and unstable, under an array of variable environmental constraints.
2. Although the number of drupes provisioned is correlated positively with enhanced nymphal development and survival, there is great variability in the number of drupes that females provide.
3. The ability to adjust behaviour and/or physiology, i.e. make risk-sensitive decisions, should be adaptive for organisms such as P. japonensis that are confronted with extreme variability in food availability and in the conditions under which they must forage. To assess whether females of this species use risk-sensitivity to improve their provisioning success, data on the parameters of provisioning activity obtained during a long-term field study were analysed. The relationships between variation in three female-controlled factors during nesting (nest site, active stage during the provisioning season, duration of provisioning activity) and the conditions of three environmental factors (drupe availability, intraspecific competition, weather) were examined. The relationships between all of these factors and provisioning capacity of females were investigated.
5. Nest site, intraspecific competition, and timing of provisioning activity only affected provisioning capacity when drupe availability was extremely good or bad or weather was particularly bad.
6. The findings suggest that females use risk-sensitive decisions to increase provisioning capacity under extreme conditions of low and high drupe availability and inclement weather.     

GATA3 Mediates a Fast,Irreversible Commitment to BMP4-Driven Differentiation in Human Embryonic Stem Cells

1. Download : Download high-res image (137KB)
2. Download : Download full-size image

     

Just When You Thought the Euthanasia Debate Had Died

The death by assisted suicide in Switzerland of Australian Dr. John Elliott, in early 2007 has highlighted the inadequacy of the law pertaining to medical decisions at end-of-life, both from a legal as well as ethical perspective. Despite being illegal in most jurisdictions around the world, physician-assisted death is a reality, in part because of the flexibility, inconsistent application and, at times, invisibility, of laws surrounding it. The appropriate response to this should be greater transparency by a reform of the law.