Efficacy of low-dose combinations of diatomaceous earth, spinosad and Trichoderma harzianum for the control of Callosobruchus maculatus and Callosobruchus chinensis on stored cowpea seeds

Abstract

The insecticidal efficacy of diatomaceous earth (DE), spinosad (SP) and Trichoderma harzianum (TH), and their combinations was evaluated against the adults of Callosobruchus maculatus (F.) and Callosobruchus chinensis L. (Coleoptera: Chrysomelidae: Bruchinae) on cowpea seeds. The application rates used in this study were 100 and 500 mg/kg (ppm) for DE, 0.1 and 0.5 mg/kg (ppm) for SP, and 1 × 104 and 1 × 106 spores/kg for TH. The adult mortality of both insect species was counted after 7 days. Progeny reduction after 40 and 80 days, and cowpea seeds weight loss after 80 days were determined for all tested individual and combined treatments. In individual treatments, DE caused higher mortality of both insects than SP and TH. DE induced 76.4 and 78.7% mortality of C. maculatus and C. chinensis, respectively, at 500 mg/kg. The ternary combined treatments of SP, TH and DE showed higher insecticidal activity than binary combination and individual treatments. Complete adult mortality of both insect species was achieved using a ternary mixture at application rate of 500 mg/kg of DE, 0.5 mg/kg of SP and 1 × 106 spores/kg of T. harzianum. All individual and combined treatments significantly reduced progeny production of both insect species after 40 and 80 days. However, the ternary mixture treatments strongly suppressed progeny production. Similarly, ternary mixture treatments protected cowpea seeds from damage and weight loss caused by both insect species for 80 days. These results indicate that the combinations of DE, SP and TH could be successfully used for the management of C. maculatus and C. chinensis in stored cowpea seeds.