Increased grass cover height in the alleys of apple orchards does not promote Cydia pomonella biocontrol
Résumé
Habitat management such as ground cover implementation in orchards is considered a promising approach to augment natural enemy abundance and diversity. However, the actual effects of grass cover management and specifically mowing frequency, on the abundance of tree natural enemies and thus on the biocontrol of major pests, are still largely unknown. We compared the effect of three heights, tall (no cutting), medium (mean height of 20 cm) and short (mean height of 5 cm), of a spontaneous grass cover in an experimental orchard on natural enemy abundance and predation rates from April to August. Early in the season (from April to May) grass cover height had no effect on codling moth egg predation. However later in the season (July and August) the predation rate increased more in the short grass cover than in the tall grass (66 vs. 38 %. respectively). The abundance of the earwig Forficula pubescens (Gen,) (Dermaptera: Forficulidae) was significantly positively correlated with egg predation in the plots with short grass but negatively correlated in the tall and medium plots suggesting that these predators could find an alternative resource in the taller grass cover.