A Morphometric Study of the Cassava Green Mite, Mononychellus Spp. (Acari: Tetranychide) in Africa

Abstract

The study deals with investigations undertaken to assess and identify characters other than the dorso-central body setal lengths (D1-D3) in the adult females for better morphological understanding of the Mononychellus spp. in Africa. Twenty-seven characters were measured on 200 adult female specimens from 10 African countries. Fifteen characters each, on 150 immature stages (larva, protonymph and deutonymph) from Kenya were also examined. The ontogeny of body setae in all the instars was also studied. Measured characters were subjected to principal components analysis (PCA), cluster analysis and other basic statistical analysis. Characters assessed, using the PCA were mostly intercorrelated. The few uncorrelated characters, such as all the ratios, particularly RL2 in the adult, P1 ,P2,P3 and P5 in the larva and P1, L2 and D4 in the protonymph could not effectively indicate i dices of variation for classification. The pattern of the scatter plots of the "Operational Taxonomic Units" (OTUs) for all the instars and the results obtained from the cluster analysis of adult females inferred a single species. Location effect was significant for all the variables with analysis of variance (ANOVA) tests but the percentage variances due to location were very low {except for variable (RL2) with 50%), suggesting in the non statistical sense, an insignificant role of geographical influence in the species distribution using the intercorrelated characters. Mean setal lengths for the instars were not all directly proportional to the increases in body sizes. Seta! counts on the life stages from both the laboratory cultures of immatures and preserved adult specimens showed a constant number of 13 pairs of setae on the dorsal idiosoma from larval to adult instars, while there was a progressive addition of setae on both the ventral idiosoma and leg segments. Complete setal formulae for the leg segments for all the instars, including mean setal lengths and body sizes of the larva , protonymph and deutonymph are presented. Finally, the results of the study, based on the variables used, indicated no real evidence for describing the species as more than a single species.