Host plant-mediated effects on eri silkworm fitness and Trichogramma parasitoid efficacy

Abstract

The eri silkworm, Samia ricini, is the world’s third most important silkworm species, valued for its cocoon silk in traditional textiles as well as its diverse by-products, including edible pupae, bioactive materials, and organic fertilizers. Its large eggs also provide a promising factitious host for the mass-rearing of Trichogramma egg parasitoids in biological control programs. This study aimed to determine (i) whether host plants influence the development of S. ricini and (ii) whether such host plant effects cascade to the performance of six Trichogramma species. Larvae were reared on castor (Ricinus communis), Nepalese coriaria (Coriaria nepalensis), or cassava (Manihot esculenta), and key life history traits were assessed across the entire life cycle, including larval growth, developmental time, cocoon/pupal weights, adult longevity, and egg characteristics. Silkworms reared on castor showed faster larval development, higher body weight, greater cocoon/pupal mass, and longer adult longevity. Eggs from castor-fed silkworms were significantly wider, though eggshell thickness did not differ among host plants. Parallelly, the suitability of S. ricini eggs from each host plant was evaluated for Trichogramma parasitism rate, emergence rate, offspring sex ratio, and developmental time. For most Trichogramma species, eggs from castor- or cassava-fed silkworms resulted in higher parasitism and emergence rates than those from Nepalese coriaria-fed hosts. However, parasitoid responses also showed clear species-specific patterns. The developmental time of several Trichogramma species was prolonged on castor-fed silkworm eggs, indicating a trade-off between developmental time and resource acquisition. Host plant identity strongly mediated tri-trophic interactions by shaping S. ricini performance and egg suitability for parasitoids.