Mite counts from manure samples

Pest regulation by natural enemies has a strong potential to reduce the use of synthetic pesticides in agroecosystems. However, the effective role of predation as an ecosystem service remains largely speculative, especially with minute organisms such as mites

Predatory mites are natural enemies for ectoparasites in livestock farms. We tested for an ecosystem-level control of the poultry pest Dermanyssus gallinae by other mites naturally present in manure in poultry farms, and investigated differences among farming practices (conventional, free-range and organic).

We used a multiscale approach involving (i) in-vitro behavioural predation experiments, (ii) arthropod inventories in henhouses with airborne DNA, (iii) a statistical model of covariations in mite abundances comparing farming practices.

Behavioural experiments revealed that three mites are prone to feed on D. gallinae. Accordingly, we observed covariations between the pest and these three taxa only, in airborne DNA at the henhouse level, and in mites sampled from manure. In most situations, covariations in abundances were high in magnitude and their sign was positive.

Predation on a pest happens naturally in livestock farms due to predatory mites. However, the complex dynamics of mite trophic network prevents the emergence of a consistent assemblage-level signal of predation. Based on these results, we suggest perspectives for mite-based pest control and warn against any possible disruption of ignored services through the application of veterinary drugs or pesticides.

Methods
The study was focused on the acarofauna of poultry manure. It was conducted in 20 barn henhouses (no cages, all birds on ground) distributed among 3 types of management practices (6 conventional, 8 free-range and 6 organic) and located half in eastern France down to Jura mounts (Ain region) and half in the South of the Rhône valley (Drôme region). The layout of French barn henhouses is similar in the three farm types. The inside always contains the basic equipment necessary for the hens and placed on the slatted floor (high stratum) under which manure accumulates (low stratum). We collected five standard manure samples from randomly-selected points under the slatted floor, during four sampling campaigns in each henhouse (n= 20 x 4 x 5), to characterize manure mite assemblages following the methodology in Roy et al. (2017).

Each sample consisted of 250 mL of manure taken from an area approximately 40 cm in diameter and 5 cm deep in a glass flask sealed with a rubber stopper. Exposure to saturating ethyl acetate vapours within one hour of sampling killed the arthropods and prevented further development of the assemblage. All arthropods from these samples were identified and counted using the Rapid Biodiversity Assessment method developed by Roy et al. (2017) for sorting out mite morphospecies and counting them using a binocular stereomicroscope.