Characterization and separation of indigenous and exotic populations of Diaeretiella rapae (M'intosh) (Hymenoptera : Aphidiidae)

Abstract

Laboratory experiments were designed to identify distinct biological attributes that might separate and help clarify the taxonomic status of one indigenous and six exotic populations of Diaeretiella rapae (M'Intosh). Experiments were conducted to investigate courtship and mating behaviors, host remains characters, host aphid acceptability and suitability, genetic differences, and reproductive isolation among the D. rapae populations. Parasite cultures in this research were selected from those imported and cultured for control of Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko). Observations of mating behavior among D. rapae populations showed a series of distinct components. Qualitative observations and quantitative analyses of the mating behavior components did not distinguish behaviors that could reliably separate the D. rapae populations. Host remains characters including exit hole shape and location, and mummy coloration, could not be used to separate the D. rapae populations. RWA mummy dissections showed no differences among populations regarding shape or color of meconial pellets. Significant differences were determined in the mean number of meconial pellets produced by different parasite populations. However, the magnitude of difference was insufficient for distinct separation of one population from another. Host aphid acceptability and suitability experiments differentiated between three parasite populations based on their unique ability to parasitize a particular host aphid. The Syrian, Spain, and Jordan populations were distinctive because they could develop on cotton aphid, crape myrtle aphid, and yellow pecan aphid, respectively; while the other populations could not. Twenty three enzymes surveyed with starch gel electrophoresis revealed twenty eight distinct loci, and six of these were polymorphic among D. rapae populations. None of the identified loci showed fixed allelic differences between the populations. Analyses of isozyme polymorphism data, using Nei's and Rodger's genetic distance, indicated that the populations were genetically very similar. Reciprocal cross experiments between D. rapae populations showed varying levels of either complete reproductive compatibility, or one- or two-way partial isolation between some D. rapae populations. No cases of complete reproductive isolation were observed. Collectively, these observations indicate that the Syrian, Spain, and Jordan populations of D. rapae are biotypes, and the other populations are geographical strains.