Efficacy of the parasitic wasp, Dinarmus basalis Rondani (Hymenoptera: Pteromalidae), in reducing infestations by the cowpea beetle, Callosobruchus maculatus (L.) (Coleoptera: Chrysomelidae: Bruchinae)

Abstract

Background Cowpea seed beetle, Callosobruchus maculatus (L.) (Coleoptera: Chrysomelidae: Bruchinae), is the most important pest of stored cowpea in tropical regions. This study was designed to determine the presence of parasitoids associated with C. maculatus, investigate the efficacy of the parasitic wasp, Dinarmus basalis Rondani (Hymenoptera: Pteromalidae), in controlling C. maculatus as influenced by time and number of applications and ascertain the use of olfactory cues by D. basalis in host searching. Three markets in Ibadan, Southwest Nigeria, namely: Bodija, Mapo and Ojoo, were purposively surveyed for parasitoids associated with C. maculatus. Two pairs of D. basalis were released at 3-day intervals into cowpea seeds previously infested with C. maculatus. Treatments included: four times of parasitoid applications (4-TPA), three applications (3-TPA), two applications (2-TPA), 1 application (1-TPA) and a control without parasitoid application (0-TPA). All treatments were replicated four times in a completely randomized design to determine F1 progeny of C. maculatus and seed damage. Olfactory bioassay was carried out with D. basalis adults placed in a Y-tube olfactometer; and their preference for infested or uninfested three cowpea varieties, namely: Ife Brown, Ife BPC (Branching Peduncle) and Oloyin, as well as infested cowpea grains or pure air was evaluated. Results Among the previously known parasitoids associated with C. maculatus, only D. basalis was found in the sampled markets. F1 progeny of adult C. maculatus reduced from 4.75 individuals (0-TPA) to 1.25 (2-TPA), 0.25 (3-TPA) and 0 (4-TPA). Concurrently, the number of exit holes on cowpea seeds significantly (p < 0.05) ranged from 5.25 (0-TPA) > 3.21 (1-TPA) > 2.20 (2-TPA) > 2.18 (4-RAP) > 1.39 (3-TPA). Adults D. basalis were more attracted to infested grains of Ife Brown (χ2 = 4, df = 1, p = 0.0455) and infested grains of Ife BPC (χ2 = 4, df = 1, p = 0.0455) than clean air. Similarly, adults D. basalis were more attracted to infested Ife Brown than the uninfested (χ2 = 5, df = 1, p = 0.0254). The results further showed that there were non-significant differences between the infested and uninfested grains of Ife BPC (χ2 = 0.2, df = 1, p = 0.6547) and Oloyin (χ2 = 3.2, df = 1, p = 0.0736) varieties. Conclusions Adult D. basalis reduced emergence of C maculatus and reduced damage in cowpea seeds. Olfactory cues played a necessary role in host-searching efforts of D. basalis.