n African Entomology - Variability in cocoon size in southern African wild silk moths : implications for sustainable harvesting




In southern Africa, two indigenous silk moth species, and (Lepidoptera : Lasiocampidae), produce commercial-quality silk with fibre that rivals that of the domesticated silkworm and thus has potential for commercial utilization. Although extensive harvesting of cocoons is taking place, little is known of the biology of these two species, including the frequency distribution of cocoon and adult sizes. In both species the sexes differ in size, but the extent of these differences has never been formally quantified. Female cocoons are approximately twice as large as male cocoons, and yield more silk fibre. Thus, sex ratios in natural populations will be important when harvesting cocoons. Our aim was to determine if there are differences in sex-specific cocoon size between : the species; host-plant-specific populations; localities within their distribution range; and between first- and second-generation pupae. The sex ratio and frequency of dwarfism in populations were also determined. Both species were significantly sexually dimorphic in cocoon length, width and shape and they could be sexed based on size and shape. Cocoon length was a suitable alternative measure to occupied cocoon mass, and may be used as a rough estimate of silk yield. The sex ratio of both species was approximately 1 : 1 and did not differ consistently between generations. Dwarfs were only found in populations. Cocoon sizes differed significantly between species, the sexes and localities, but not between generations and host-plant-specific populations. The variability in cocoon sizes found at localities would have implications for silk yields, but sex and species are by far the most important determinants of cocoon size. This information will form the basis of a sustainable harvesting programme for spp. in southern Africa.


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