Comparative study of the effects of steam and solar heat treated cowpea seed on the development and control of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)
Africa produces more cowpea (Vigna unguiculata) than any other continent but utilization in many countries is reduced due to seed destruction by the larvae of bruchids. The dried edible seeds of legumes are frequently attacked by beetles of the family Bruchidae. There are several genera of stored-product bruchids associated with a range of host plants. Callosobruchus species are the major bruchid pests of cowpea in Africa. Callosobruchus maculatus is the major pest of stored cowpea in Africa. Damage to cowpea seeds by C. maculatus during storage is widespread in Africa and constitutes a major constraint to food availability. Cowpeas damaged by C. maculatus have reduced weight, poor germinating ability and are unfit for human consumption, due to loss of vital nutrients such as vitamins and thiamine. It is during storage that cowpeas suffer heavy quantitative and qualitative losses from the attack by C. maculatus. Even though there are various methods of control of C. maculatus, some of the effective methods such as chemical insecticides pose environmental, social, financial and safety considerations in the tropics. There is need for alternative and less hazardous methods of control. Solar heating of cowpeas to control C. maculatus is one of the safe methods. However, steaming is thought to cause some physical modifications such as starch gelatinization and protein denaturation leading to a case hardening "effect" on the surface cell layers of the cotyledons and could therefore be used as alternative method of control. In the present study, the effects of steam treated, solar heat treated and untreated cowpea seeds on the development and control of C. maculatus were studied under ambient laboratory conditions (temperature range 28.0 - 30°C and 62 - 74% RH). There was no significant difference (P> 0.05) in the number of eggs laid by C. maculatus under conditions 1 male: 1 female, 5 males: 5 females, 12 males: 12 females on treated and untreated cowpea seeds. C. maculatus developed successfully in untreated and solar heat treated cowpea seeds, but could not develop in steam treated cowpea seeds. Thus, the novel method of steam treatment of cowpea seeds is a useful pest management strategy that can be used to prevent C. maculatus infestation of cowpea seeds meant for long-term storage and consumption since the cooking properties and processing qualities of the cowpea were not affected.