Activity of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) on selected bambara groundnut (Vigna subterranea L. Verdc.) landraces and breeding lines

Abstract

Bambara groundnut, an indigenous African legume crop, is cultivated as a subsistence crop by resource poor farmers. In storage, yield losses are compounded through damage by insect pests, with Callosobruchus maculatus, being a leading one. The development of a variety with minimum susceptibility to this insect is thus desirable for the improvement of this nutritious crop. The study was carried out to determine the susceptibility of three bambara groundnut breeding lines (SSD5, SSD8, SSD9) and three landraces (Uniswa red, ASI7, OM1) to attack by the cowpea weevil, Callosobruchus maculatus and their germination success after damage. 20g of seeds of each variety replicated four times were used in the experiment. Physical characteristics of the seeds were noted and each replicate was infested with five pairs of C. maculatus and kept in a breeding chamber at 30°C. After an oviposition period of 7 days, the adult pairs were removed and number of eggs laid was counted. Subsequently, the developmental pattern of the insects, the amount of damage caused by the insects as well as susceptibility of the seeds to the insect was determined. The breeding lines were significantly larger and heavier than the landraces (p<0.05). The number of C. maculatus eggs laid were significantly different between the treatments (p=0.0012), with SSD8 and OM1 having significantly higher numbers laid. While % adult emergence was lower in SSD5, this was not significantly different between the varieties (p=0.l4l6). The susceptibility index was significantly different between varieties (p=0.0192) as well as between landraces and breeding lines (p=0.0255). On average, the landraces had higher SI (17.928 ± 2.4523) than the breeding lines (13.448 ± 5.9939). Germination success of damaged seeds was significantly higher in the landraces than the breeding lines (48.333± 18.007%). Results indicated that SSD5, SSD9 and AS17 were the most resistant to C. maculatus attack, while SSD8 and OM1 were the most susceptible. However, due to reduced germination success after damage, the breeding lines (SSD5 and SSD9) were not suitable for planting after storage while the landrace (AS17) was the most suitable due to its higher viability after C. maculatus damage. The results indicate that there is variability in resistance of the bambara groundnut varieties against the cowpea weevil. The use of resistant varieties could offer the simplest and cheapest way of improving bambara groundnuts production, especially if these maintain their viability after insect damage. The variability also emphasizes the need for the maintenance of genetic diversity when selecting for desired traits.

Keywords: C.maculatus, Vigna subterranea, bambara groundnut, susceptibility, landraces, breeding lines