Cereal Straw Mulching in Strawberry—A Facilitator of Plant Visits by Edaphic Predatory Mites at Night?

In Norway, strawberry producers use cereal straw mulching to prevent berries from contacting the soil and to control weeds. We hypothesized that organic matter such as straw mulch also favors the maintenance of predatory mites which visit strawberry plants at nighttime. We compared mite diversity in cereal straw exposed for different periods in strawberry fields and evaluated their possible migration to plants in two experiments with potted plants in 2019. An ‘Early season’ experiment compared no mulching (T1), oat straw mulch exposed in field since 2018 (T2), or 2017 (T3), while a ‘Mid-season’ experiment compared no mulching (T1), barley straw mulch from 2018 (T2), or a mix from 2017 and 2018 (T3). To provide edaphic predatory mites with a potential source of food, all plants were infested with two-spotted spider mite (Tetranychus urticae Koch). Results suggested that straw mulch facilitates the prevalence of predatory mites in strawberry fields. Most predatory mite visits were at night, confirming our initial hypothesis. Predominant nocturnal mites on leaves belonged to Melicharidae (Proctolaelaps sp.) (‘Early season’, T2), Blattisociidae (Lasioseius sp.) (‘Early and Mid-season’, T3) and Phytoseiidae (‘Mid-season’, T2). Parasitus consanguineus Oudemans & Voigts was the predominant species (‘Early season’, T3) at the base of plants. Anystidae were diurnal visitors only (‘Mid-season’, T2). Future studies should evaluate the predation potential of Proctolaelaps sp. and Lasioseius sp. on two-spotted spider mite and other strawberry pests.