RESISTANCE STATUS OF SOME COWPEA SEED VARIETIES TO INFESTATION AND DAMAGE BY COWPEA SEED BRUCHID, CALLOSOBRUCHUS MACULATUS FABRICIUS (COLEOPTERE: BRUCHIDAE)

Abstract

Investigations were conducted on the resistance of twelve cowpea cultivars to infestation and damage by the cowpea seed bruchid, Callosobruchus maculatus ( F ). (Coleoptera: Bruchidae) in the laboratory at ambient tropical conditions using indices such as physical and nutrient characteristics of the seeds and biological activities of the bruchids on the various cowpea seed varieties. The cowpea cultivars used were IT98K-1092- 2. IT99K-49 1-7, IT99K-316.-2, Ife-brown, ITOOK-1217, IT98K-ll11-1, Sokoto local variety, IT97K-568-18, IT97K-I042-3 ITOOK-1263, IT93K- 452-1 and ITOOK-901-5. Infested cowpea seeds were obtained from Erekesan market in Akure, Nigeria and used to establish a laboratory culture of C. maculatus. The length and width of cowpea seeds were measured using micrometer screwguage while their weights were determined using the Mettler balance. Cowpea seed coat hardness was determined in terms of weight bearing ratio using California Bearing ratio machine. Oviposition and oviposition preference were determined in nochoice and choice' experiments respectively. No-choice experiment comprised two copulating pairs of adult C. maculatus (0-2 days old) introduced into three 500 ml Kilner jars containing 20 g of cowpea of different cultivars and left on the shelf for 7 days. After seven days, the beetles were removed from the jars, and the number of eggs laid were counted and recorded. Choice experiment was accomplished in an experimental protocol consisting of different cowpea seed varieties arranged in a circular manner at regular intervals on a Petri dish embedded with paraffin wax. Adult bruchids were introduced into the Petri dish and left undisturbed until death. The number of eggs laid on each cultivar were counted and recorded. The different varieties were also evaluated based on grain damage and beetle survival by introducing 2 male and 2 female bruchids into a Petri dish containing 20 g of each of the cultivar. The experimental set up was kept on the shelf for 36 days and the grain damage on the cowpea seeds were assessed and recorded. The number of adults that emerged were also recorded. Result obtained showed that IT98K-llll-l with susceptibility index (SI) of 8.45, Ife-brown with (SI) of 8.55 and IT97K-I042-3 with (SI) of 8.72 were the most resistant of all the varieties screened while Sokoto local variety was the most susceptible. The observed resistance may be due to seed coat hardness of 55.53, 67.43N and 134.87N in IT98K-llll-l, Ife-brown and IT97K-I042-3 respectively. Cowpea varieties with high fat content had low susceptibility indices. Thus depicting strong positive correlation between fat content and seed resistance to C. maculatus. The percentages of fat content in IT97K-I042-3, Ife-brown and IT98K-llll-lare the highest with mean value of 5.77, 4.79 and 5.55 respectively and had the least susceptibility indices 8.72, 8.55 and 8.45 respectively. These varieties had low carbohydrate content of 38.96 for IT97K-I042-3 and 40.41in IT98K-1111-1. Incorporation of planting of resistant cowpea varieties (notably IT98K-llll-l, Ife-brown and IT97- 1042-3) into farming systems will further enhance proactive, non-chemical management of C. maculatus, a notorious post-harvest pest of cowpea in the tropics.