An experimental study of morphological variation in mealybugs (Homoptera: Coccoidea: Pseudococcidae).
Mealybug identification has been traditionally based on the adult female, but the morphological characters may vary considerably within any species. It can be difficult to determine the limits of this variation, impeding the detection of species complexes. The limits of variation can be determined from the examination of large numbers of field-collected individuals, but 4 factors must be taken into consideration: variation induced by the environment of each population, genetic variability within each population, variation caused by genetic differences between populations and genetic differences between species. The results are given of studies in England with species of pseudococcids in which the effects of temperature, relative humidity and the food-plant on the adult female morphology of the progeny of a single female was examined. Temperature had the greatest effect. The lengths of the appendages and setae and the numbers of wax-producing pores and ducts varied over a wide range at different rearing temperatures. There was generally a negative linear correlation between temperature and the size and number of the pores. However, some characters, especially the numbers of tubular ducts, reached their highest values at an intermediate temperature. It is suggested that while variation in size is a direct response to temperature, other characters are influenced by size or by a combination of size and environmental factors acting independently. The usefulness of rearing mealybugs under different environmental conditions to differentiate species is illustrated by the example of Planococcus citri (Risso), P. ficus (Sign.) and P. pacificus Cox.