Towards the development of sustainable control options for the African root and tuber scale on cassava in Central Africa : understanding the biology and ecology of the tending ant "Anoplolepis tenella" (Hymenoptera: Formicidae)

Abstract

Anoplolepis tenella Santschi is an afrotropical ant widely distributed in the forest zones of Central Africa. It is the most frequent species associated with the African root and tuber scale (ARTS) Stictococcus vayssierei (Hemiptera: Stictococcidae), an afrotropical insect that infests a wide range of root and tuber plants, particularly cassava. Present evidence supports the hypothesis that the scale needs the ant to remove its honeydew secretions, as the scale would otherwise drown in its own secretions. Therefore developing effective management options to reduce ARTS infestations, requires adequate understanding of the biology and ecology of A. tenella and its impact on ARTS life cycle and ecology. Therefore, we carried out study on the distribution and abundance of A. tenella and thereby co-occurring ant species in mixed-crop fields, fallow and forest which are the dominant vegetation type in southern Cameroon. We also assessed some of the factors that could explain the observed distribution. Results indicated highest species richness in the forest. Anoplolepis tenella abundance, density and activity was affected by vegetation type, season and a set of abiotic parameters among which are air temperature and relative humidity. Its abundance was higher in mixed-crop fields, while it was and either absent or rare in the lowland where coincidently S. vayssierei was also absent. These results indicate that human activities reduce ant diversity and offer new nesting niches or food sources to ants which could explain the proliferation of species like A. tenella. Among other factors affecting the ant’s distribution are plants producing nectar and extrafloral nectaries and honeydew producers among which S. vayssierei. The distribution of these plants and honeydew-producing insects significantly differed between vegetation types, hence the difference in ant composition. Network analysis of the ant-hemipteran matrix showed high specialization index at individual level for A. tenella and S. vayssierei. We couldn’t demonstrate the dispersal of the scale by air currents or through phoresis on A. tenella founding queens. However, through a dispersal experiment in a screenhouse, we demonstrated the active role of A. tenella workers in the transport and dissemination of scale crawlers within and between cassava plants. In an effort to control ant colonies under laboratories condition, Boric acid-sucrose solution showed a significant reduction of A. tenella population within six weeks. These results demonstrate that control of S. vayssierei in crop fields is contingent on reductions in the densities of its associated ant A. tenella.