Frédéric Muratori

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.

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.

Cold‐induced expression of diapause in Praon volucre: fitness cost and morpho‐physiological characterization

Cold exposure (2°C for 7 days) in constant darkness at mummy stage induces diapause expression in 9% of the Praon volucre Haliday population. Diapausing parasitoids show a significant delay in emergence time compared with nondiapausing counterparts. A diapause‐mediated polyphenism is observed in mummy colour, with diapausing mummies being clearly darker than nondiapausing ones. The diapause status of dark mummies is confirmed by a significant reduction in metabolic rate. Diapausing parasitoids also display specific morphological characteristics: they are heavier (fresh and dry mass) and accumulate larger fat reserves than nondiapausing counterparts. The diapause status is associated with a fitness cost in terms of adult longevity. There is no evidence of diapause‐related change in supercooling ability.

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

BackgroundAnimals 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.ResultsWe 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.ConclusionAphids 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.

Chemical Characterization of Cuticular Extracts of Sitobion avenae (Hemiptera: Aphididae)

Abstract Data on the chemical composition of the aphid epicuticle are scarce and often incongruent depending on the methodological parameters of the extraction and chemical analysis. This study aims to provide a chemical characterization of the epicuticular lipids of the English grain aphid, Sitobion avenae (F.) (Hemiptera: Aphididae). First, our results showed that the adsorption chromatography on a silica gel column is suitable to separate the hydrocarbons from more polar constituents found in the extract (e.g., wax esters, alcohols, and aldehydes). Then, we showed that other compounds not detected in previous analysis are likely to be constituents of the extract. Through a transesterification reaction, we demonstrated that the extracts produced numerous fatty acid methyl esters that can come either from long chain wax esters or fatty acid parts of cuticular acylglycerols. The possible semiochemical value of the wax esters and other long chain compounds is discussed. This study opens new insight on the identification of host recognition chemical cues by the parasitoids.