Exploration and Host Specificity Tests for Potential Natural Enemies of Dioscorea bulbifera L. (Dioscoreaceae) in Ghana.

Dioscorea bulbifera L. (aerial yam) belongs to the yam family (Dioscoreaceae) and produces numerous aerial tubers in the leaf axils. Dioscorea bulbifera is considered the most serious environmental threat. Currently, it is the widespread weed throughout many parts of the southern United States of America where it threatens the stability and biodiversity of native communities. Control interventions such as chemical and manual methods have been ineffective, expensive and laborious, hence the decision to explore for natural enemies to control the weed. A survey to establish the presence of D. bulbifera and its associated phytophagous insects was conducted in Ghana in 2004 and 2005. Field exclusion and laboratory experiments were also carried out to assess the abundance, diversity and herbivory of insects in 2006 and 2007. Rearing and host range studies were undertaken on potential candidates in 2007 and 2008. A total of 40 phytophagous species in 9 orders were encountered on the plant with 24 species attacking D. bulbifera in two feeding guilds in Ghana; foliage and bulbil feeders. The impact of foliage feeding varied from species to species. The mean percent defoliation of aerial yam over all plots was 30%. Coleopterous species fed on only foliage and nearly all the leaves showed damage from these species. Anomala sp. and Adoretus sp. were the most important Coleoptera (Scarabaeidae) found feeding on the plant. Anomala species exhibited a very narrow host range, attacking only D. alata together with D. bulbifera. Even though the damage it caused to aerial yam was relatively small, it appears a potential biological control agent. Lepidopterous species, mainly the Arctiid moths Diacrisia and Estigmene species attack aerial yam leaves and bulbils. They caused considerable damage to the bulbils the principal planting material. They however exhibited a wide host range attacking other Diocorea species. In view of the poor representation of Diocorea species of economic importance in the United States of America, the results of the study will have favorable implications for research programs aimed at searching for long-term intervention of the ecological risks posed by D. bulbifera in the US.
Thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science Technology. Kumasi, in Partial Fulfillment of the Requirement for the Award of Master of Science Crop Protection (Entomology).