Food web engineering to enhance biological control of tetranychus urticae by phytoseiid mites (Tetranychidae: Phytoseiidae) in citrus

Abstract

The highly polyphagous Tetranychus urticae Koch is a key pest of Spanish clementine mandarins. This mite may be effortlessly found in clementine orchards, on the trees and also on the associated flora. In this chapter we discuss the suitability of a cover of the grass Festuca arundinacea Schreber (Poaceae) to enhance biological control of T. urticae relative to bare soil or a wild soil cover in citrus orchards. On the one hand, we hypothesize that the natural selection of a local strain of this mite specialized in F. arundinacea could result in an unsuccessful colonization of the associated trees. On the other hand, phytoseiid predatory mites specialized in T. urticae are regularly found in that cover and this presence may account for the better regulation of T. urticae populations in citrus orchards grown using this grass as a cover crop. Contrarily, regular provision of highly nutritious pollen for omnivorous phytoseiid mites in a wild cover relative to a F. arundinacea cover (which produces low quality pollen for phytoseiids and only in the spring), could support the higher abundance of phytoseiids with generalist pollen feeding habits in orchards grown in association with a wild cover. As a consequence, increased competition and/or intraguild predation from these generalists on more efficient T. urticae-specialized phytoseiids could seriously reduce their numbers or even make them disappear from the system. This fact, in combination with periods of prey scarcity in the orchard, could result in the permanent disappearance of T. urticae-specialized phytoseiids, leading to a deficient control of tetranychids in orchards with a wild cover.