Explosions demogràfiques de l'eruga peluda del suro, Lymantria dispar (Linnaeus, 1758), als boscos del Montnegre el 2019 i 2020: possibles causes, impactes i idoneïtat dels tractaments per combatre la plaga

Abstract

After years without significant damage, in 2018-2020 an outbreak of the gypsy moth, Lymantria dispar (Linnaeus, 1758), defoliated large areas of cork oaks and holm oaks in the Montnegre mountains. The onset of this outbreak could be related to the collapse of mice and shrew populations that regulate the moth populations when they are at low densities. A literature search reveals the existence of a very extensive complex of natural enemies that prey on the gypsy moth, some of which provoke density-dependent mortality that predictably leads to a return to normal populations levels within a period of 1-4 years. Several species of parasitoids and the beetle Calosoma sycophanta (Linnaeus, 1758) have been detected in the study area. In addition, the monitoring of egg-clusters in 10 defoliated and 10 unaffected localities in 2019 showed that around 50 % of the egg masses suffered predation at the outbreak sites. In May 2020, approximately 2,500 ha of forest were treated with Bacillus thuringiensis (Berliner, 1915) var. kurstaki to combat the pest. To assess the effectiveness of this measure, we designed a systematic survey in 15 areas, whose results show that the use of this insecticide did not reduce defoliation when comparisons were made with untreated areas. Moreover, we noted that defoliated trees, especially cork oaks, recovered well and a month after the peak of the infestation showed defoliations of only 15 %. In Catalonia, we thus consider that there is currently no argument in favour of using this method to combat sporadic outbreaks of L. dispar.