Kairomone gel formulations enhance biocontrol efficacy of Trichogramma japonicum Ashmead on rice yellow stem borer, Scirpophaga incertulas Walker

Abstract

The egg parasitoid, Trichogramma japonicum (Hymenoptera:Trichogrammatidae) is a documented successful biocontrol agent to reduce rice yellow stem borer (YSB), Scirpophaga incertulas (Lepidoptera: Crambidae). In the present study, three kairomone gel formulations: T1: n-hexadecanoic acid (200 ppm); T2: n-octadecanoic acid (500 ppm) and; T3: octadecane (500 ppm); were applied 24 h after each release of adult T. japonicum in low land rice field plots during the rainy season (2018) and summer season (2019). The study evaluated whether the combination of the kairomone gel treatments with parasitoid could improve suppression of YSB. The best results were from T3: application of octadecane gel, 24 h after each T. japonicum release (@ 50,000 wasps/ha × 4 releases) on the 32nd, 39th, 46th, 53rd days after transplanting (DAT) at weekly intervals during the rainy season (2018). This regime reduced incidence of YSB induced damage called: ‘dead heart’ and ‘white ear’ by 30.01%–37.06% compared to T5: control. During the summer season (2019) again the best results were observed in T3: octadecane augmented wasps releases, the reduction in damages by YSB ranged from 17.86% to 24.47% below control. The grain yields in T3, increased from 14.61% to 15.73% across both seasons measured. These results support augmenting T. japonicum releases with the kairomone gel containing octadecane provide an affordable treatment to increase parasitic activity and to reduce damage in rice crops caused by YSB.