Body size-dependent effects on the distribution patterns of phoretic mite species assemblages on Rhynchophorus ferrugineus (Olivier, 1790)

Phoretic mites attach to different body parts of the red palm weevil (RPW), Rhynchophorus ferrugineus (Olivier, 1790), to disperse. However, the question of how the patterns of attachment sites are formed remains intriguing. Here, we examined RPW-associated phoretic mites in four districts in Northern Portugal (macrohabitat), and investigated the patterns of mite distribution on six body parts of RPW (microhabitat). At the macrohabitat level, we detected seven phoretic mite taxa using the RPW host in each of the four studied districts, all documented for the first time in association with this invasive exotic species in Portugal. However, their relative abundance (species evenness) varied between districts, as did species diversity. All examined weevils carried mites, and the prevalence of the different taxa did not differ between districts or sex of weevils. Measured by mean abundance and degree of aggregation, Centrouropoda sp. proved to be the dominant taxon, while Acarus sp. and Curculanoetus rhynchophorus were considered common subordinate taxa and Uroovobella sp., Mesostigmata, Nenteria extremica and Dendrolaelaps sp. sparse taxa. At the microhabitat level, all taxa were present on all body parts of the RPW; the highest abundance was in a region encompassing the inner surface of the elytra and the membranous hind wings (subelytral space). Analysis of niche overlap revealed that the distribution patterns of phoretic mite taxa on the RPW were not randomly structured. In the subelytral space, interspecific coexistence of mites increased as a function of body size difference with the dominant Centrouropoda sp. We found that in the subelytral space the large dominant species Centrouropoda sp. displaced the larger species Uroobovella sp. and the similarly sized species Nenteria extremica, but coexisted with smaller taxa.

Methods
Samples were from eight sites in the Northern Portugal districts of Viana do Castelo, Braga, Porto and Aveiro. The name and geographical localisation of these eight sites are as follows: Moledo do Minho (41.847698, -8.864375) in Viana do Castelo; Famalicão (41.361287, -8.539348) in Braga; Marquês (41.161283, -8.604427), Porto Botanical Garden (41.153008, -8.642730) and Foz do Douro (41.147716, -8.670845) in Porto; Esmoriz (40.955908, -8.650886), Barra Beach (40.632630, -8.748307) and Santiago Campus - University of Aveiro (40.635053, -8.659500) in Aveiro. The captured weevils were transported to the laboratory in plastic containers and fed with apple slices. The specimens were then stored in the refrigerator at 8 degrees Celcius until dissection. The cold-anaesthetised weevils were separated by sex and dissected individually under a stereomicroscope. Phoretic mites were counted on six body parts of the weevils, i.e. neck, head-antenna, thorax, legs, ventral surface (abdominal sterna), and subelytral space (membranous hind wings + inner elytra surface), then removed with a camel hair brush and preserved in 70% ethanol. For identification, selected specimens of each observed mite morphotype were mounted on microscopic slides in lactic acid (90% solution in water) (Helle & Sabelis, 1985) and examined with an Axiophot microscope (Carl Zeiss, Oberkochen, Germany) connected to a computer running Leica Application Suite X (LAS X) image processing software. Mites were identified to the lowest possible taxonomic level based on the original descriptions and illustrations in which these taxa were first described (McGraw & Farrier, 1969; Griffiths, 1970; Fain, 1974; Lindquist, 1975; Kinn, 1984; Wisniewski et al., 1992; Krantz & Walter, 2009; Porcelli et al., 2009; Farahani et al., 2016; Abo-Shnaf & Allam, 2019) and the length of their ventral idiosoma measured.

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