Effectiveness of Samia cynthia ricini Boisduval (Lepidoptera: Saturniidae) Cocoon Extract as UV Protectant of Bacillus thuringiensis kurstaki in Controlling Beet Armyworm Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) under Sunl

Abstract

Bacillus thuringiensis (Bt) is a biological agent for insect pest management. Its toxins effectively control Spodoptera exigua Hübner (Lepidoptera: Noctuidae) larvae, but it is sensitive to ultraviolet (UV) radiation from the sunlight. This study aimed to investigate the effect of sericin extract from Samia ricini Boisduval (Lepidoptera: Saturniidae) cocoons as a UV protectant for Bt after exposure under direct sunlight for 1, 2, 3, and 4 weeks. After being exposed to sunlight, the Bt formulae were tested against 20 larvae of 24 hr old, the first larval instar in the laboratory. The larval mortality was observed 72 hr after the treatment. The results indicated that the mortality of S. exigua in Bt + sericin extract treatment was significantly different compared with Bt alone. For the first week, the mortality of S. exigua in exposed Bt + sericin exposed Bt alone, unexposed (Bt + sericin, and unexposed Bt alone were 80, 61, 85, and 97%, respectively. Scanning electron microscopy analysis revealed that Bt + sericin, after being exposed to sunlight, still showed the presence of spore and crystal protein comparable to the unexposed Bt. Based on the results, sericin provides good protection against sunlight and prevents the Bt spores from light-induced damage.