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.