<b>Scrutinizing </b><b><i>Acalitus simplex </i></b><b>Flechtmann et Etienne (Eriophyidae): an eriophyoid mite associated with </b><b><i>Ruellia</i></b><b> spp.</b><b><i> </i></b><b>(</b><b>Acanthaceae</b><b>)</b>

<i>Acalitus simplex</i> is an erinose eriophyoid mite that feeds exclusively on plants in the genus <i>Ruellia,</i> which are frequently sold as ornamentals for landscape plantings. This study presents a comprehensive examination of <i>A. simplex</i> collected from Anguilla, Brazil, Dominican Republic, Florida, and Hawaii through sequencing of the mitochondrial cytochrome oxidase I gene (COI), coupled with morphological examination by several microscopy techniques. The collected <i>A. simplex</i> specimens were similar in morphology across geographic locations and populations clustered together in a phylogenetic analysis based on COI sequences. In addition to <i>Ruellia simplex</i>, new host plant associations are reported in Florida, including <i>R. blechum</i>, <i>R. squarrosa</i> and <i>R. caroliniensis</i>. We defined the different erineum stages, determined the number of days after infestation on an uninfested plant required for erinea development, and estimated population densities in each erineum stage. Four erineum stages were observed on <i>R. simplex</i> - hyaline (stage 1), white (stage 2), purple (stage 3) and beige (stage 4). At 25 ± 2 ^oC, 12:12/L:D and 50 ± 10% RH, the purple stage did not develop and the duration of the hyaline, white and beige erinea was 12.4 ± 0.7 (mean ± SE), 44.4 ± 1.3, and 20.5 ± 0.9 days, respectively. Population densities varied significantly across the erineum stages, with most individuals in the white and purple stages and fewer in the hyaline and beige stages. This study improves our understanding of interactions between <i>A. simplex</i> and <i>R. simplex</i>, aiding the development of IPM strategies to mitigate this mite.