Relative abundance of the Wild silkmoth, Argema mimosae boisduval on different Host plants and host selection Behaviour of parasitoids, at Arabuko Sokoke forest

Abstract

Sustainable utilization of natural resources and biodiversity conservation form the basis of the livelihood and existence of communities living in Africa and in other developing countries. Overexploitation of natural resources and forest destruction for better livelihood by local communities has led to decline in the African biodiversity. Beneficial insects like wild silkmoths exist in the eco-system, and can be integrated in biodiversity conservation and income generating micro enterprises. Hence, there is need to conserve wild silkmoths and increase their distribution for their economic use in silk industry. The general purpose of this study was to survey the relative abundance of wild silkmoth Argema mimosae Boisduval (Lepidoptera: Saturniidae) on different indigenous food plants and explore food plant volatiles responsible for parasitism/predation of the developmental stages of the moth in Arabuko Sokoke forest, Kenya. The distribution of the food plants was studied through Global Position System and mapped with Geographical Information System. Larvae were reared in semi-captivity using net sleeves on the branches of Lannea schweinfurthii Engl. (Anacardiaceae) and Ozoroa obavata Oliv. Anacardiaceae). The mass and size of cocoons were measured using weighing scale and vernier calipers (0-15cm) respectively. A scanning electronic microscope was used to examine the grains and the filament structure of the cocoons. The cocoons were boiled in a solution of 5gms/ltr of sodium carbonate at different time intervals to soften them for silk extraction and in distilled water as a control. Chemical volatiles emitted by growing A.mimosae food plants viz L. shweinfurthii and O. obovata were collected using a portable volatile collection system with super Q as the adsorbent trap.Volatiles were collected for 2 hr in the morning between 0900 and 1500 hr with two replication in each set up. Characterization of the volatile compounds was done by Gas ChromatographyMass Spectrometer and analysis was carried out on a HP 7890A model series GC coupled to a 5975C mass spectrometer and a Triple Axis Detector. Components of the volatiles were identified by comparing their mass spectral data with those in the library of mass spectrometer and by retention time analysis. The distribution of the food plants was significantly different between the forest and the farm land. Among the seven food plants recorded, O. obovata has not been reported elsewhere in East Africa as food plant for A. mimosae. Most of the cocoons recorded belonged to A. mimosae moth and occurred more in the farmland in smaller clusters of 10-20 on the L. schweinfurthii food plant while few cocoons were found in O. obavata in clusters of 1-5. This shows that L. schweinfurthii is probably more preferred by the A. mimosae larvae than O. obovata. Larvae passed through six instars and the developmental period was not significantly different between the two food plants. The instars expressed cryptic colouration of black, orange and green in different instar as a defense mechanism against natural enemies. Silvery brown floss was extracted from cocoons boiled in sodium carbonate solution but not in those boiled in distilled water only. Several terpenes such as (E)-β-farnesene, (E)-β-Caryophyllene, (E)-β-ocimene were identified. (E) beta-Farnesene was distinctively trapped from L. shweinfurthii leaves at higher peaks when the A. mimosae larvae were feeding on the leaves. The abundance of the wild silkmoth food plants discussed under this study provides a basic planning strategy to initiate wild silkmoth farming in Arabuko sokoke forest and conserve the forest sustainably.