Winter Geometrid Moths in Oak Forests: Is Monitoring a Single Species Reliable to Predict Defoliation Risk?

Abstract

Species within the group of winter moths (Geometridae) are important oak defoliators in European forests. Adults of these species emerge either in late autumn (‘autumn species’) or in early spring (‘spring species’), and caterpillars of both ‘autumn’ and ‘spring’ species appear in spring. The abundance of adults assessed by regular monitoring allows the prediction of the defoliation intensity in trees by caterpillars in the following spring. ‘Autumn species’ (mostly a single one, Operophtera brumata) are monitored by forestry practices as pests, whereas ‘spring species’ are often not paid any attention. We hypothesised that ‘spring species’ could also have an important share in caterpillar assemblages in oak forests. We aimed to study the proportions between ‘autumn’ and ‘spring’ species in adult and larval stages. In a xeric thermophilous oak forest in southern Slovakia, Central Europe, we collected adult moths using sticky bands set up on trunks of Quercus pubescens during the winter of 2014–2015 and caterpillars from other trees belonging to the same oak species over the following spring. We also captured caterpillars from several oak species in various areas and over different years in Slovakia and Bulgaria, and we compiled relevant literature data. ‘Spring species’ recorded from a unique forest as adults during winter and as caterpillars in the following spring were significantly more abundant than ‘autumn species.’ Moreover, ‘spring species’ from Slovakian forests, regardless of the locality, the oak species and the year of sampling, reached high proportions, mostly over 50% of individuals in caterpillar assemblages. The ‘spring species’ Agriopis leucophaearia was the most abundant, followed by the ‘autumn species’ Operophtera brumata. ‘Spring species’ accounted for more than 50% of individuals in caterpillar assemblages in the Balkan Peninsula (Bulgaria) concerning one case, and they were of little importance in northern Greece. We recommend monitoring all winter moth adults (‘autumn’ and ‘spring’ species together) continuously in forestry practices, using sticky bands on oak trees from late autumn to early spring.