Effects of cultivar and leaf traits on the abundance of Pacific spider mites in almond orchards

Abstract

A greater understanding of the ecological relationships between Pacific spider mite, Tetranychus pacificus, and its almond host is needed for better management recommendations and reduced need for pesticides. Since populations of T. pacificus are thought to increase with deficit irrigation and fertilization, they may also differ between almond cultivars varying in leaf traits relating to water use and leaf nitrogen (N) content. In the current study, the relationships between T. pacificus populations, almond cultivars and leaf traits were examined at studies in two locations. In the first study, a large-scale cultivar trial with a high level of T. pacificus infestation, spider mite eggs were more common on the cultivar ‘Sweetheart’ than either ‘Nonpareil’ or ‘Marcona,’ and across all cultivars, mites and leaf damage increased with stomatal density, but decreased with stomatal length. Almond cultivars differed in traits, with ‘Sweetheart’ having the highest leaf N and ‘Marcona’ the lowest stomatal density. In the second study, a germplasm repository orchard collection with low levels of T. pacificus infestation, spider mites were not detected on the cultivar ‘Mission’ but were similarly highest on ‘Sweetheart.’ Mites increased with N across all cultivars, and leaf damage due to spider mites increased with more negative δ13C, indicating low water use efficiency. Results suggest that monitoring of spider mite populations should include conditions of high nutrient and water availability, especially for susceptible cultivars, and that spider mite populations are not consistently higher with leaf traits indicating water stress.