Bioecology of the mango mealybug, Rastrococcus iceryoides green (Hemiptera : Pseudococcidae) and its associated natural enemies in Kenya and Tanzania

Abstract

Rastrococcus iceryoides Green (Hemiptera: Pseudococcidae), an alien invasive mealybug pest of Asian origin was first detected in Tanzania in 1989. This pest rapidly spread by the mid-1990s and was soon present in Coastal Kenya and Northern Malawi, where it has been regarded and remains a major pest of mango. Because of its novelty status, there was no information on its biology, ecology and its natural enemies that could aid development of management efforts. This study, therefore, was initiated to establish the bioecology of R. iceryoides and its natural enemies in Kenya and Tanzania, and to explore for efficient co-evolved natural enemies in the aboriginal home of the pest in India. Based on the exploratory survey data, two correlative approaches, Desktop-GARP (Genetic Algorithm for Rule-set Prediction) and Maxent ( Maximum entropy) were used to identify climatically suitable areas in Africa that are agro-meteorologically similar to the aboriginal home of the pest. The first step was to carry out a countrywide survey in Kenya and Tanzania to establish the distribution, host-plant relationship and natural enemies of this pest. The survey revealed that R. iceryoides infested twenty-nine plant species particularly Mangifera indica L. and Cajanus cajan (L.) Millspaugh, and the wild plants Parkinsonia aculeata L., Caesalpinia sepiaria Roxb, and Deinbollia borbonica Scheft. A total of six primary parasitoid species were recovered from R. iceryoides with Anagyrus pseudococci Girault (Hymenoptera: Encyrtidae) predominating. Thirty-eight species of predators belonging to 14 families were also recorded. Despite the presence of these indigenous natural enemies, their ability to regulate the population of R. iceryoides was inadequate. In laboratory host preference studies, M. indica, Cucurbita moschata Duchesne, P. aculeata and C. cajan were found to be the most preferred host plants in view of improving laboratory mass rearing of this pest and the parasitoid. The impact of O. longinoda on the biological control activities of A. pseudococci in the laboratory revealed that percentage parasitism of R. iceryoides by A. pseudococci was significantly higher on ant-excluded trials than on ant-attended trials. Worker ants were observed to remove mummified mealybugs, which resulted in significantly reduced percentage of adult parasitoid eclosion. Oecophylla longinoda showed aggressive behaviour and caused a significant mortality of A. pseudococci during the exposure period. The spatial and temporal population dynamics of this pest was also studied and revealed that populations of R. iceryoides followed an annual cycle which is synchronized with the mango fruiting season, with a peak incidence occurring during the dry season (December to February) on all plant parts. The population dynamics of R. iceryoides and its natural enemies were significantly and positively influenced by temperature, while it was significantly and negatively correlated with rainfall. The exploratory survey in India showed that R. iceryoides is widely distributed throughout the state of Tamil Nadu and infested ten cultivated and wild plant species with extremely low levels of infestation. Percentage parasitism based on the proportion of mummified R. iceryoides was high on all host plants. Out of eleven primary parasitoid species, Praleurocerus viridis Agarwal (Hymenoptera: Encyrtidae) and Anagyrus chryos Noyes&Hayat (Hymenoptera: Encyrtidae) were the most dominant and widely distributed species. In addition to the parasitoids, 10 predator species from 7 families were recorded. Based on the model established with data from India, it was determined that climatically suitable areas for introduction of promising parasitoids in Africa include the humid tropical coastlines of Kenya and Tanzania, as well as some restricted areas in West and Central Africa. Studies of the potential worldwide distribution of R. iceryoides showed that the pest might poses a serious threat on a worldwide scale as it could narrowly become established in all the mango producing countries in the continents.