Bernard Chaubet

Analysis of wheat resistance to the cereal aphid Sitobion avenae using electrical penetration graphs and flow charts combined with correspondence analysis

The behaviour of Sitobion avenae (F.) was compared on resistant wheat lines of Triticum monococcum (L.) and a susceptible variety of Triticum aestivum (L.). Firstly, stylet penetration activities were monitored with the Electrical Penetration Graph (EPG) technique and subsequently analysed using flow charts combined with correspondence analysis. Plant resistance was shown to be associated with repeated penetrations without access to either the xylem or the phloem, and with numerous failures in starting a sustained sap ingestion (as represented by pattern E2). Access to sieve elements of the phloem did not seem to be much affected on resistant plants but it took the aphid three times as long to produce a sap ingestion pattern when maintained on the resistant line T. monococcum n ° 44 (Tm44) as compared with aphids maintained on susceptible plants. As a result the total time spent in ingesting from sieve elements was reduced by 72% on Tm44. Secondly, direct observations of freely‐moving apterous adults were performed. Aphids did not discriminate between resistant and susceptible wheat during the first 30 min of access to test leaves, but only 4 out of 25 aphids were still probing after eight hours on resistant Tm44.

Superparasitism limitation in an aphid parasitoid: cornicle secretion avoidance and host discrimination ability.

Superparasitism avoidance by the endoparasitoid Aphidius rhopalosiphi De Stefani Perez on the grain aphid, Sitobion avenae was studied. Experiments were carried out in which aphids were exposed to two consecutive attacks by parasitoids. Results showed that superparasitism avoidance in A. rhopalosiphi was mediated by two successive stimuli whose effectiveness depended on the time interval between attacks. For short time intervals (<16 h), host rejections were mainly associated with the presence of dried cornicle secretion on the hosts body which was exuded during the first attack. The repellency of this secretion declined with the time interval between attacks, becoming ineffective 2 days after the first parasitization, and allowed females to reject up to 30% of parasitized hosts. For longer time intervals (>/=16 h), host rejection behavior was a response of parasitoid females to internal changes in host quality associated with parasite development. This response gradually increased with an increase in time interval, reaching no more than 60%, 96 h after initial parasitization. This host discrimination ability did not allow females to distinguish between hosts parasitized by themselves or by conspecifics. Consequently, these findings suggest that superparasitism is a common event in A. rhopalosiphi and especially on recently parasitized hosts.

Clonal variability in the response of Sitobion avenae (Homoptera: Aphididae) to resistant and susceptible wheat

The development and reproduction of 60 clones of Sitobion avenae (Fabricius), collected in the Rennes Basin, were compared on a resistant Triticum monococcum (Linnaeus) line (Tm44) and a susceptible Triticum aestivum (Linnaeus) cultivar (Arminda). All clones had lower larval survival and mean fecundity when reared on Tm44 in comparison with Arminda. They all performed equally well on Arminda whereas there was a marked and continuously distributed variation in performance parameters among clones maintained on Tm44. The plant species, from which clones originated, significantly affected aphid performances on resistant Tm44. A more detailed experiment was carried out with four clones, shown to differ in their level of fitness on Tm44. Their performances were compared on resistant (Tm44 and Tm46) and susceptible (Tm47 and Arminda) wheat genotypes. On the basis of larval development time and intrinsic rate of natural increase (r m ), two types of response were distinguished among the four clones: clones Sa1 and Sa39 appeared to be less affected by Tm44 and Tm46 resistance than Sar2 and Sa48. Moreover, Tm46 was much less resistant to clone Sal than was Tm44. These results are discussed in relation to the origins of the variation in performance of S. avenae on resistant wheat, and their implications in plant breeding for resistance to S. avenae .

The influence of ant‐attendance on aphid behaviour investigated with the electrical penetration graph technique

For the mutualistic interaction between the aphid Metopeurum fuscoviride Stroyan (Homoptera: Aphididae) and the ant Lasius niger L. (Hymenoptera: Formicidae) it has been shown that ant‐tended aphids develop faster, reproduce at a higher rate, and live longer than aphids not tended by ants. We used electrical penetration graphs (EPG) to investigate if behavioural patterns differ between ant‐tended and untended M. fuscoviride during 8 h experiments. Measurements were made on adult aphids from four different ant‐tended colonies that continued to be tended by L. niger during the experiments, and from four different colonies where ant workers were excluded several days before the start of the experiment and that were also not tended by ants during the experiments. Ants readily tended wired aphids and ant tending did not interfere with the EPG measurements. There were no significant differences in the duration of sieve element penetration or in any other analysed feeding‐related EPG parameters between ant‐tended and untended individuals. However, the quality of the EPG recordings did not allow the distinction between the EPG‐waveform E1 (salivation only) and E2 (salivation and ingestion). These results suggest that the changes in life‐history traits of ant‐tended aphids do not result from changes in time of sieve element penetration waveforms. Alternative mechanisms may involve an increase in the rate of sap uptake or a higher effectiveness in nutrient uptake in the presence of ants. Our study demonstrates that the EPG technique is a useful tool to investigate the feeding behaviour of aphids during interactions with ants.

Larval Hitch-Hiking and Adult Flight are Two Ways of Aphidiinae Parasitoids Long-Range Dispersal

ABSTRACT Dispersal strategies and success of pests’ natural enemies widely influence the efficiency of biological control. In this study, we compare two dispersal strategies among Aphidiinae parasitoids: eggs and larvae dispersal through winged aphid flight and active dispersal by adult parasitoids. Using a molecular method applied to a sample of >2,000 winged migratory aphids captured in a suction trap situated in Western France, we assessed the proportion of winged aphids carrying an aphidiine larva. In the six most abundant aphid species, we found an average parasitism rate of migrating aphids close to 1% and identified seven different, mainly generalist, parasitoid species. We also identified the species and the sex of adult Aphidiinae captured by the suction trap based on morphological criteria. We found that dispersing adult parasitoids were almost exclusively female. Parasitoid dispersal strategy seems to be species-dependant but this result needs to be confirmed by an exhaustive analysis of winged aphids captured. We discuss the possible impact of the low parasitism rate of winged aphids on parasitoid population dynamics and the importance of these results in the context of biological control and of the study of food webs between aphids and their natural enemies.

Aphids and associated natural enemies on Ile Amsterdam and Ile Saint-Paul, Southern Indian Ocean

Abstract Ile Amsterdam (37°50′S, 77°30′E, 55 km2) and Ile Saint-Paul (38°43′S, 77°31′E, 7 km2) are very isolated volcanic islands which were originally colonized by a few invertebrate fauna and flora. Invasive species richness has then increased along with human activity. A three-year monitoring programme (1997, 2000, 2001) and a summer campaign (2007) allowed species diversity, host plants, abundance and phenology of introduced aphids and natural enemies to be described. Seven cosmopolitan aphid species have been found on Ile Amsterdam (Aulacorthum solani, A. circumflexum, Macrosiphum euphorbiae, Myzus ascalonicus, M. cymbalariae, M. ornatus and Rhopalosiphum padi) and three on Ile Saint-Paul (A. solani, M. cymbalariae and R. padi). On Ile Amsterdam, these aphids were found on 28 host plants (out of 57 sampled plants), mainly introduced species. Phylica arborea was the only native plant much colonized by one aphid species, A. circumflexum. Aphids were mainly present on the base or in this vicinity. One Hymenopteran parasitoid, Aphidius matricariae, and two hyperparasites (Dendrocerus aphidum and Phaenoglyphis villosa), probably introduced along with their host, were collected. Aphid activity is very low during the autumn (March–May) and at a maximum in spring and summer. Their density and diversity decrease with distance from the research station. From these results, the possible impact of aphids on native plants is discussed.