Aphids Facing Their Parasitoids: A First Look at How Chemical Signals May Make Higher Densities of the Pea Aphid Acyrthosiphon pisum Less Attractive to the Parasitoid Aphidius ervi.

Herbivore-induced plant volatiles constitute the first indicators of insect host presence, and these can affect the foraging behavior of their natural enemies. The density of insect hosts may affect the nature and concentration of these plant-induced volatiles. We tested the impact of infestation density (low, intermediate, and high) of the pea aphid, Acyrthosiphon pisum (Homoptera: Aphididae), feeding on the broad bean Vicia faba, on the attractiveness of the parasitoid Aphidius ervi (Hymenoptera: Braconidae), using a Y-tube olfactometer (infested vs. non-infested plants). The emitted volatile compounds from both infested and non-infested plants were collected and identified. In addition, two series of experiments were carried out to test the impact of the presence of a conspecific female parasitoid within the aphid/plant complex on the attractiveness to other females. Parasitoids were significantly more attracted to the plants with low and intermediate aphid infestation levels. The volatile blend composition of the infested plants changed in relation to aphid density and may explain the low attraction of parasitoids toward high aphid density. The presence of conspecific females on the aphid patch had no apparent impact on the behavioral choices of other parasitoid females. Our study adds a new aspect to understanding plant-aphid-parasitoid interactions, including the possibility that aphids may manipulate chemical cues of host plants affecting the orientation of parasitoids.

Fitness trade-offs explain low levels of persister cells in the opportunistic pathogen Pseudomonas aeruginosa.

Microbial populations often contain a fraction of slow-growing persister cells that withstand antibiotics and other stress factors. Current theoretical models predict that persistence levels should reflect a stable state in which the survival advantage of persisters under adverse conditions is balanced with the direct growth cost impaired under favourable growth conditions, caused by the nonreplication of persister cells. Based on this direct growth cost alone, however, it remains challenging to explain the observed low levels of persistence (<<1%) seen in the populations of many species. Here, we present data from the opportunistic human pathogen Pseudomonas aeruginosa that can explain this discrepancy by revealing various previously unknown costs of persistence. In particular, we show that in the absence of antibiotic stress, increased persistence is traded off against a lengthened lag phase as well as a reduced survival ability during stationary phase. We argue that these pleiotropic costs contribute to the very low proportions of persister cells observed among natural P. aeruginosa isolates (3 × 10(-8) -3 × 10(-4)) and that they can explain why strains with higher proportions of persister cells lose out very quickly in competition assays under favourable growth conditions, despite a negligible difference in maximal growth rate. We discuss how incorporating these trade-offs could lead to models that can better explain the evolution of persistence in nature and facilitate the rational design of alternative therapeutic strategies for treating infectious diseases.

Early presence of an enolase in the oviposition injecta of the aphid parasitoid Aphidius ervi analyzed with chitosan beads as artificial hosts.

Maternal factors of female wasps that are injected into hosts with their eggs at oviposition play a major role in strategies used by insect parasitoids to overcome host immunity, and to regulate host physiology during early stages of parasitism. Here, we attempted to precisely determine and compare the protein patterns injected by the endoparasitoid Aphidius ervi into two different host systems. Chitosan beads of aphid size designed as artificial and physiologically inert hosts were used as oviposition medium, to be compared with the natural aphid host as young nymphs of Macrosiphum euphorbiae. Proteins that the A. ervi wasp injects into hosts at oviposition were separated by SDS-PAGE, complemented with proteomic techniques. Analyses confirm the identification of A. ervi γ-glutamyl transpeptidase (γ-GT) as a key component in the venom of the endoparasitoid. Using proteomic techniques, the quantity of γ-GT injected by the A. ervi wasp into aphids along with the egg was estimated as approximately 4ng per oviposition strike. We suggest that similar quantities suffice to explain natural parasitization success in A. ervi, which do not rely on polydnavirus to establish into hosts. Moreover, an enolase that showed a high level of sequence identity with teratocyte A. ervi enolase was detected both in chitosan beads extracts, and in extracts of mature eggs excised from the A. ervi ovaries, but not in its venom glands extracts. Detecting enolase shortly after oviposition in the artificial inert hosts at a stage of parasitism when the A. ervi egg is still in the primary chorionated undifferentiated stage suggests the enolase as a chorionic protein of the mature egg. The possible functions of this enolase enzyme for the establishment and early development of A. ervi in aphid hosts are discussed.

Heterokairy as an anti-predator strategy for parasitic species.

Heterokairy refers to plasticity in the timing of onset of developmental events at the level of an individual. When two developmental stages do not share the same ecological niche, referred to as 'ontogenetic niches', the control of the niche shift through a change in developmental timing can be advantageous for the individual (e.g., when mortality risk is different in the two niches). Heterokairy can arise either from plasticity in developmental rate (ontogenetic shift) or by a purely behavioral decision (behavioral shift). Parasitic species living inside of their hosts often inherit the predators of their hosts. To cope with the predation risk on their hosts, parasites and parasitoids show either host-manipulation abilities or either host-leaving strategies. Nevertheless, leaving the host should be associated with developmental costs, since the parasitic individuals are usually unable to parasitize another host. This process is thus related to the classical tradeoff between size and developmental time. Recent studies provided examples of behavioral heterokairy in invertebrates. The goal of this publication is to review and discuss recent results on developmental plasticity in parasitic species in an evolutionary perspective.

Induced niche shift as an anti-predator response for an endoparasitoid.

When two developmental stages do not share the same ecological niche, the control of the niche shift through a change in developmental timing, referred to as 'heterokairy', can provide an adaptive advantage for the individual (e.g. if mortality risk is higher in the first niche). For endoparasitic species that develop inside another (host) species, mortality of the host may directly induce mortality risk for the parasite. Thus, endoparasitoid larvae should be selected for response to host predation. In this study, aphids previously parasitized by the endoparasitoid Endaphis fugitiva, Gagné and Muratori (Diptera: Cecidomyiidae), were experimentally exposed to increased mortality risks. Both simulated attack and actual predator attacks against aphid hosts induced early emergence of the parasitoid larvae. Parasitoid emergence from the aphids occurred several minutes before the predator finished feeding on the aphid, allowing enough time for the parasitoid larvae to avoid direct predation. Predator-induced emergence produced significantly smaller parasitoid larvae than controls, but, interestingly, no effect on Endaphis adult size was found. To our knowledge, this is the first evidence of induced emergence in an insect parasitoid, but we suggest that this mechanism might be at work in many other species where plasticity in development time allows the individual to perform an adaptive niche shift.

Bad housekeeping: why do aphids leave their exuviae inside the colony?

BACKGROUND: Animals can gain protection against predators and parasites by living in groups. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to group size (i.e. encounter effect), so that predators do not offset the encounter effect by attacking more members of the group (i.e. dilution effect). In this paper, we propose a novel mechanism by which prey insects could gain by producing decoys that act as multiple targets for predators or parasitoids if these decoys are recognised as preys or hosts and negatively affect the patch foraging strategy of these predators and parasitoids. Such a decoy mechanism could be present in aphid colonies in which aphid exuviae are recognised and attacked by Aphidiine wasps. RESULTS: We conducted a behavioural study to evaluate the effect of exuviae on parasitoid patch residence time and egg allocation in experimental aphid patches with or without exuviae. We showed that exuviae are recognised and attacked at the same level as aphids when both are present in the patch. While parasitism rate was not significantly lower in patches with exuviae when the parasitoid left the patch, the time wasted by parasitoids to handle exuviae did influence the patch residence time. As a consequence, the attack rate on the live aphids was lower in patches that contain exuviae. CONCLUSION: Aphids had more time available to flee and thus each individual might gain protection against parasitoids by leaving their exuviae near and within the colony. These results demonstrate that the encounter-dilution effect provided by living in a group can be enhanced by extra-materials that act as decoy for natural enemies.

Epicuticular factors involved in host recognition for the aphid parasitoid Aphidius rhopalosiphi.

In insect parasitoids, fitness is dependent on the host finding and recognition abilities of the female. Host recognition cues have been described for various host-parasitoid systems, but are still under investigation in aphid parasitoids. Our study aimed to clarify the respective role of physical and chemical cues in recognition of the aphid cuticle. Shed aphid exuviae were used as an elicitor in order to avoid any influence of color, movement, or volatiles present in a living aphid. We assessed the effect of chemical and heat treatments on the texture of the cuticle by using scanning electron microscopy and tested the recognition of treated cuticles by the parasitoid. We showed that recognition cues of the cuticle can be removed chemically (using combined treatments with n-hexane and methanol). Moreover, heat treatment destroyed the physical texture of the cuticle without significantly reducing parasitoid recognition. In a second step, we showed that epicuticular extracts deposited on pieces of paper triggered female attack behavior. First results concerning the chemical composition of the active extract are presented. This study shows that chemical compounds extractable by organic solvents mediate cuticle recognition by aphid parasitoids.