Spatial distribution and biology of Gonometa postica Walker (Lepidoptera: Lasiocampidae) with reference to its key parasitoids on Acacia species in Mwingi, Kenya

Abstract

The African wild silkmoth, Gonometa postica Walker produces silk of high quality. A study on the spatial distribution and biological and biology of G. postica on host and non-host plants and the parasitism rates and reproductive strategy of its parasitoids was conducted during the long and short rainy seasons in 2006 and 2007. Three sites, each in the Imba and Mumoni forests of Mwingi eastern Kenya, were selected for sampling. One hundred trees of the host plants of G. postica were sampled at each site, in additional to the non-host plant species having G. postica pupae. In order of decreasing abundance, the host plants in Imba forest were Acacia tortilis, A. elatior and A. nilotica; and in the Mumoni forest, A. tortilis, A. nilotica, A. mellifera, and A. brevispica. Host plant species richness did not differ between the two forests but their evenness was significantly higher in Imba than in Mumoni. At Imba, the distribution of A. tortilis, A.nilotica and A.elaitor was clumped while the non-host plants were random. A. nilotica and A. brevispica were clumped in Mumoni, whereas A. tortilis, A. mellifera and the non-host plants were randomly distributed. The distribution of G. postica larvae was clumped on all host plants in Imba, except on the non-host plants, where they were randomly distributed. In Mumoni forest, larval distribution was clumped on A. tortilis but random on the other host plants. A. elatior had significantly more larvae than other host plants in Imba. In Mumoni, A. tortilis and A. mellifera had significantly more larvae, followed by nilotica and A. brevispica. The pupae of G. postica were randomly distributed on all host plants in Imba and Mumoni forests. Interestingly, the non-host plants harboured significantly more pupae than the host plants in both forests. Frequently, Imba had a significantly higher abundance of larvae and pupae than Mumoni. Generally, the female moth laid more eggs on the net sleeves, followed by the wooden board, plastic container and the twigs. The developmental periods for egg hatching, larva, pupa and the adult moth lifespan ranged between 11-12, 55-72, 101-126 and 3-10 days respectively. Larval developmental period and quality of cocoons differed according to the larval food plant, season and site, for those reared in semi-captivity. However, the quality of cocoons was similar on the different larval food plants from the wild habitat, though it varied according to season and site. Larvae reared on A. elaitor had the shortest developmental period and produced cocoons of the highest quality than those raised on A. tortilis and A. nilotica. Larval development was generally shorter in the sites and seasons where rainfall was high. Trapping of adult moths revealed that there were more males than females and that two districts peak periods occurred during a year. Six parasitoids, vis., four hymenopterans and two dipterans, were collected during the study. The most common parasitoids were Palexorista sp. (Diptera: Tachinidae) and Goryphus sp. (Hymenoptera: Ichneumonidae) with parasitism rate ranging from 1.8-32.7% and 2.2 - 7.5%, respectively. Parasitism rates on G. postica in relation to its host plants in the two forests of Mwingi. This information would be crucial in the monitoring, sustainable utilization and the conservation of this economically important silkmoth and its host plants species. The data on the bionomics and reproductive strategy of the key parasitoids will be a prerequisite in devising any management programme to boost cocoon production.