Facteurs de limitation des populationsd'Aphis fabae Scop. dans l'Ouest de la France

Résumé

Le déclenchement dépizooties au sein de populations d'Aphis fabae sur féverole de printemps est largement dû aux facteurs climatiques et dépendant de l'hôte. Trois années d'expérimentations ont permis de préciser ces facteurs en conditions naturelles.

Cette note insiste surtout sur les facteurs dépendant de l'hôte qui interviennent lorsque se prépare une épizootie: en particulier l'accent est mis sur la quantité minimum de pucerons mycosés requise et sur leur répartition au sein des colonies et de la culture.

Summary

A three-year study of the mechanisms involved in initiating epizootics onAphis fabae on field beans is reported. In 1969 and 1970, two field experiments were done at two sowing dates (with a month interval in 1969 and a fortnight in 1970); in 1971, only one could be conducted.

Despite the two widely separated sowing dates, epizootics occurred at only a ten-day interval in 1969, and at the same time in 1970 in the two sowings. In 1971, there was a slow but steady progression of fungus attack, resulting in good control (fig. 1).

Climatic factors, such as relative humidity above 90% for more than 10 hours per day, temperatures above 20°C and rains, are thought to be largely responsible for outbreaks; nevertheless, it is essential to have sufficient inoculum well-distributed among the population. It now seems that this quantity is better expressed as a minimum of 300 dead aphids per stem than as a percentage (table 1). As for inoculum distribution, dead aphids occur in any size of colony but, at the beginning they seem proportionately more numerous in the smaller colonies (fig. 2, 3). When epizootics are about to occur, the proportion of dead aphids is the same in all sizes of colony. Clearly, a well-spread production of inoculum will result from this apparent proportionality and from the distribution of aphid colonies in the field (fig. 3, 4, 5).

Emphasis is laid on the difficulties of determining in field experiments whether the above factors are causal agents. Also, the threshold figure of 300 dead aphids per stem associated with the initiation of outbreaks may well vary under different ecological conditions.