A novel method for assessing the threat to oak stands from geometrid defoliators

Abstract

Oak forests are of great ecological and socioeconomic values but are declining in many regions due to abiotic and biotic factors, including crown damage by defoliating insects. In Europe, several geometrid species reach outbreak densities and are then able to cause severe oak defoliation. Prior to control measures, the threat posed by defoliators must be reliably assessed to limit the treated area and consequently to minimize the negative environmental impacts and the costs of the insecticide application. Few methods have been developed so far to assess the threat to oak forests from geometrids, most of which have proven to be unreliable and/or labour-consuming. In this paper, we present a new method for predicting crown damage from geometrid defoliators based on the density of their flightless females assessed with the use of collar traps installed on tree trunks. The studies were conducted in 2014–2020 in 68 oak stands in Poland. Using beta regression, we estimated the relationship between the density of geometrid females (from genera: Operophtera, Erannis, Agriopis, Alsophila and Apocheima) in collar traps and subsequent defoliation caused by their progeny (i.e. larvae). We then proposed threshold numbers of female density to predict different levels of defoliation and calculated the numbers of collar traps that would be required to detect the threshold density of geometrid females indicating expected defoliation >30%. The new method allows forest managers to monitor populations of different species and phenological groups of geometrids and predict the threat (expected defoliation) to oak forests from these insects.