Facteurs de limitation des populations d'Aphis fabae dans l'ouest de la France

Résumé

En Bretagne, les mycoses àEntomophthora ont une incidence sur les populations d'Aphis fabae Scop., non seulement sur Fèverole (Vicia faba L. var.minor), mais également sur Fusain (Evonymus europaeus L.) et Boule-de-neige (Viburnum opulus L. var.sterilis). Alors que sur hôte secondaire elles donnent lieu à des épizooties, elles ne détruisent à la même date qu'une faible partie des aphides sur les hôtes primaires. L'objet de la présente note est de comparer le déroulement de la maladie sur ces deux types de plantes et de discuter de son incidence sur la structure des populations.

Summary

A three years study ofA. fabae in relation withEntomophthora infections, on its primary and secondary host plants, is reported. Tick bean experiments were carried out with two sowing dates in 1969 and 1970, and with only one in 1971. Spring populations were studied from 1969 to 1972 and autumn ones in 1971 and 1972 on the primary host-plants.

Differing from observations in other countries such as England or Czechoslovakia, usually some aphids remain on the primary hosts after migration. They are destroyed late in the season (July 11th in 1972) by predators and parasites (fig. 1, 2, 3).

Three species ofEntomophthora (E. planchoniana Cornu,E. aphidis Hoff. andE. fresenii Now.) were found on these plants, but only two (E. aphidis andE. fresenii) on beans. Every year, aphids begin to die onEvonymus andViburnum at the exact moment of the maximum production of alatiform larvae. Infected aphids are present at the same time on tick beans and on primary host plants, but, whilst epizootics develop on the former plants (Robert, Rabasse & Scheltès, 1973), such an evolution never occurred on the later ones (fig. 4, 5). Climatic factors and spatial distribution of insects are thought to be responsible for this difference. During autumn, alatae and young larvae were the only infected forms recorded (table 2). The authors believe that winged aphids play a major part in disseminating inoculum. Resting spores could be observed only in a single occurrence (July 11th 1972 onViburnum). For practical reasons, it was not possible to have heed to anholocyclic populations ofA. fabae.

Epizootics have a rather different incidence according to the year. As they develop in the same way on fields sown at different dates, the peak populations are higher in the first sown plots. In 1971, the efficiency of the fungi was enhanced, because, from the beginning of June, they developed in the populations more regularly than during the other two years.

As for the proportions of different forms of the aphids in the healthy part of the population and in the infected one, some differences are pointed out (fig. 6, 7). Generally, apterae, which are exposed for a longer period to inoculum, are more numerous in the infected part of the population.

In the same way, apart from 1971, the alatiform larvae are more numerous in the healthy part and the other larvae in the infected part. Such differences in fungi infection are to be investigated further for a deeper understanding of the incidence of these pathogens on the population dynamics of the black bean aphid.