pullulation

CONT: [...] la pullulation, c'est l'agrégation de plusieurs centaines ou milliers d'individus qui persistent à forte densité pendant des mois voire des années [...
OBS: Le terme pullulation, contrairement au terme anglais «outbreak», désigne une explosion démographique qui peut, sans distinction, avoir un effet négatif ou non pour l'environnement dans lequel elle se produit.

CONT: Under certain conditions, insects may attack a very large number of trees over a wide area. This is known as an outbreak. An outbreak often occurs in a vulnerable or ageing forest. An outbreak is more likely to occur in large stands of the same vulnerable species when populations of insect predators are too small.
OBS: outbreak: An outbreak generally relates to a negative event (pest outbreak); otherwise the term ""irruption"" is used to describe an abrupt increase in the number of a species.