On the distribution and ecology of the African wild silk moth, Gonometa

Abstract

The African wild silk moths, Gonometa postica and G. rufobrunnea are a valuable natural resource in southern and East Africa because of the high quality silk fibre derived from their cocoons. However, using these species for commercial silk production has proven problematic because of the sporadic and unpredictable nature of Gonometa outbreaks. Moreover, little is known about the ecology and distribution of the Gonometa species. The first part of this study focussed on the importance of moth-host plant interactions, in addition to climatic variables, in determining the species’ current regional distributions, using species distribution modelling (SDM). Based on recent studies which have documented the importance of biotic interactions in shaping broad-scale species distributions, I expected moth-host plant interactions to be an important predictor of Gonometa species distributions because of the species’ dependencies on host plants for survival. Contrary to expectation, my results showed that models’ predictive power did not improve following the inclusion of moth-host plant interactions. Biotic and abiotic models predicting G. postica distribution performed fairly well, but all models predicting G. rufobrunnea distribution performed poorly. Nonetheless, host plant distributions contributed significantly to SDMs of large-scale Gonometa distributions, along with annual mean temperature, annual precipitation and precipitation seasonality. Host plant distribution also appeared to limit Gonometa species distributions, suggesting that the effect of host tree occurrence patterns on Gonometa species distributions should not be ignored. The idea that climate drives large-scale species distributions was supported, but host plant occurrence also had an effect on Gonometa species distributions. The second focus of this study was the potential ecosystem engineering effect created by G. postica (via its cocoons) for other invertebrates. Arboreal ants are known to use a wide variety of plant structures as potential nesting sites, including structures created by arboreal ecosystem engineers. However, ant nest site selection in arboreal systems is poorly understood. I observed, for the first time, ants using empty G. postica cocoons as shelter and nesting sites on the myrmecophilic tree Vachellia erioloba. I examined cocoon selection by these ants and tested whether selection was based on cocoon characteristics. Cocoons were predominantly occupied by one dominant ant species, but in some cases simultaneously by two ant species. Ant abundance and occurrence were positively related to cocoon size, the presence of scale insects on branches of the trees and cocoon entrance hole type (i.e. cocoons containing only small parasite holes), and negatively influenced by cocoon occupancy by other invertebrates. This suggested that ants select G. postica cocoons based on specific cocoon characteristics, and revealed a novel ecological role of the moth-host plant interaction at alocal-scale, where the product of this interaction (i.e. cocoon production) appeared to benefit arboreal invertebrates. Empty G. postica cocoons probably create a new, favourable habitat for ants. Thus, G. postica acts as autogenic ecosystem engineer in arboreal habitats. This study highlighted the importance of Gonometa-host plant interactions in shaping large-scale Gonometa species distributions, but also showed that these interactions may play a role in the abundance and distribution of arboreal species at local scales.