Pest status of leafminers in cider-apples: The case of orchards in Asturias (NW Spain)

Abstract Microlepidopterous leafminers are a potential threat to Spanish cider-apple orchards. For pest management purposes it is important 1) to know the species composition, their incidence and their spatial distribution and 2) to have an efficient and accurate sampling method. To fill this gap of knowledge, the occurrence of leafminers was followed in four cider-apple orchards at Asturias (NW Spain) from summer to harvest during 2003–2005. A total of 5120 mines from six different leafminer species were identified. Phyllonorycter blancardella (Fabricius) mines were the most abundant, followed by those of Leucoptera scitella (Zeller). These two species accounted for more than half of the total mines counted (58%). The other four species detected were Stigmella malella (Stainton), Lyonetia clerkella (L.), Phyllonorycter corylifoliella (Hubner) and Callisto denticulella (Thunberg). Abundance and incidence of all leafminer species taken as a whole were similar among orchards, although year-to-year oscillations were recorded. Based on the low densities observed (maximum abundance for a given orchard and sampling date was 0.548 mines per leaf), leafminers could not be considered as a key pest in our apple orchards. Taylor’s coefficient b obtained for the different leafminer species were indicative of aggregated distributions of both infested leaves and mines within the tree. Enumerative and binomial sampling plans for apple leafminers were developed. Although binomial plans are usually faster than enumerative ones, the latter was recommended because of the lower size required for practical usage (the sampling of 20 leaves would be enough for estimating populations around the economic threshold of one mine per leaf).