Morphometric Comparisons of Citrus Rust Mite, Phyllocoptruta oleivora (Ashmead), Populations in Texas and Kenya

The citrus rust mite, Phyllocoptruta oleivora (Ashmead) (Acari: Eriophyidae), is a serious pest of citrus worldwide, but is especially important in areas where citrus is produced for fresh markets. Kenya and Texas produce fruit for fresh markets; however, Texas fruit is marketed commercially and globally, while fruit in Kenya is sold locally. Development of an effective pest management program to control P. oleivora requires proper identification to species. However, diminutive size and lack of taxonomic characteristics make proper identification of P. oleivora difficult without the aid of a powerful microscope, and therefore cryptic species can occur. Cryptic species might occur in a citrus tree, grove, or variety, or geographical area, and different species might respond differently to insecticides that can severely impede management strategies. The purpose of this study was to use morphological characteristics to assess populations of P. oleivora in Texas and Kenya. Morphometrics is statistical analysis of characteristics or a combination of characteristics that can detect morphological variation. 300 P. oleivora individuals were collected; 90 individuals were collected from oranges (Citrus sinensis (Linnaeus)) and 60 from tangerines (Citrus tangerina (Tanaka)) in Kenya. In Texas, 90 individuals were collected from oranges and 60 individuals were collected from grapefruit (Citrus x paradisi (MacFadanyen)). Ten morphological traits and four biometric ratios were compared between each population using a pooled t-test. Principal component analysis was used to analyze the morphometric measurements and stepwise discriminant analysis was used to distinguish further any morphological trait that determined population grouping. Morphological compairsons revealed all to morphological traits and three out of 4 bioratios were significantly different from each population. Principal component 1 and principal component 2 represented 86% of morphological variation, and Kenyan and Texan populations were distinguished by relation to principal component 1. Stepwise discriminant analysis revealed three morphological traits that strongly contributed to differentiation between P. oleivora from Kenya and Texas: width of prodorsal shield (F = 97.18, P < 0.0001), tail end (F = 30.67, P < 0.0001), and body length (F = 29.64, P < 0.0001). Morphological differences could be due to differences in geography or complete or partial genetic separation.