Trichoribates trimaculatus, (Koch)

Desurmont, Gaylord A., Kerdellant, Elven, Pfingstl, Tobias, Auger, Phillipe, Tixier, Marie-Stéphane & Kreiter, Serge, 2019, Mites associated with egg masses of the viburnum leaf beetle Pyrrhalta viburni (Paykull) on Viburnum tinus L., Acarologia 59 (1), pp. 57-72 : 60

Impact of T. trimaculatus on overwintering success of P. viburni eggs

Trichoribates trimaculatus was the most common mite observed within P. viburni egg masses (see results section). Although T. trimaculatus is a detritivorous species unlikely to predate directly on P. viburni eggs, its presence might deteriorate the secretion protecting the eggs, reduce the quality of the cavity as a protection against desiccation and natural enemies, and ultimately decrease egg overwintering success. To test this hypothesis, we enclosed V. tinus twigs with newly-laid egg masses in nylon sleeve nets with and without an addition T. of trimaculatus adults in September 2017. We then left the twigs in the sleeve nets under field conditions during the overwintering period and then measured P. viburni egg survivorship after the overwintering period in March-April 2018. The process for twig infestation with P. viburni egg masses went as follows: in July 2017, 10 young V. tinus shrubs (25–50 cm tall) with no sign of P. viburni infestation were transplanted from a field in the Montpellier area and potted in plastic pots (22.5 cm diameter × 18 cm height) with potting soil kept in a growth chamber with a 22 °C constant temperature and a 12/12 (l:d) photoperiod. In August 2017, these shrubs were placed in large mesh cages (90 × 60 × 60 cm, 500μm mesh diameter) with 25 P. viburni individuals (20 females and 5 males) per cage for a period of one week. After a week, P. viburni individuals were removed from the cages and the number of twigs infested and the number of egg masses per twig were counted on each shrub (without damaging/removing the twigs from the shrubs). A total of 54 twigs containing between 1 and 20 egg masses were selected for the experiment and each twig was enclosed in a sleeve net (50 cm long, 200μm mesh diameter, Diatex Co): 26 twigs each received an addition of T. 30 trimaculatus adults (mite treatment), and 28 did not receive any mites (control treatment). A thin metal wire was used to close the extremities of each net. The number of twigs selected for the experiment varied between 2 and 10 per shrub and the number of twigs allocated to each treatment was equally distributed within each shrub (i.e. each shrub had an equal number of twigs with mites and twigs without mites). All T. trimaculatus adults used for infesting sleeve nets were collected locally during the days preceding the infestations. They were kept V. on tinus twigs with a moistened piece of cotton at 17 °C and a 12:12 (l:d) photoperiod until needed for the experiment. Mite addition in the sleeve nets was done as a two-step process. Step 1, on September 1 2017, 20 T. trimaculatus adults were added to the sleeve nets enclosing the twigs selected for the mite treatment. All shrubs were kept in a growth chamber with a 22 °C constant temperature and a 12/12 (l:d) photoperiod until the second mite addition. Step 2, on October 10 2017, 10 more T. trimaculatus adults were added to the same sleeve nets. At the time of the second mite 5 twigs from each treatment were cut and inspected using a dissection microscope to check for mite presence and survival: these twigs were discarded from the rest of the analyses. After the second mite addition, shrubs were moved outside in a shaded location near naturally-growing V. tinus shrubs, and were left undisturbed until February 2018. The number T. of trimaculatus individuals added to twigs of the mite treatment (30) was purposely higher than the natural densities of mites we observed in egg masses during the observational study, in order to account for potential mite mortality during the transfer and early establishment of the mites on the twigs. On February 5 2018, all twigs with sleeve nets were cut and brought back to the laboratory. Portions of each twig containing egg masses were transferred to petri dishes (9 cm diameter) with a thin layer of agar (0.5 cm) covered with a filter paper. Portions of each twig containing egg masses were placed on top of the filter paper. Each petri dish was then sealed with a strip of Parafilm ® (American National Can Co), and all petri dishes were monitored until late April 2018 for P. viburni larval emergence. The number of larvae emerging from each twig was recorded. The sleeve nets that enclosed the twigs and the twigs themselves were carefully inspected for mite presence when twigs were transferred to petri dishes, and all twigs and egg masses were examined again for mite presence at the end of the experiment after P. viburni larval emergence. The total numbers of dead and live T. trimaculatus adults and nymphs that were recovered from the sleeve nets and from the examination of egg masses were recorded.