Dynamic of bruchid egg-laying in cowpea-based cropping systems: importance for determining an optimal release period of the oophagous parasitoid Uscana lariophaga Steffan (Hymenoptera: Trichogrammatidae)

Cowpea, Vigna unguiculata (L.) Walpers, is one of the most important leguminous crops in West Africa which is probably the largest area of cowpea production in the world[1]. Cowpea is a major staple food crop in Sub-Saharan Africa, especially in the dry savanna regions of West Africa. The seeds are a major source of plant proteins and vitamins for human [2], and also a significant source of income when the post-harvest constraints are overcome. In Burkina Faso, cowpea is often infested by two bruchid species, Callosobruchus maculatus Fab. and Bruchidius atrolineatus Pic. [3;4]. These pests oviposit on the ripening pods in the fields. Hatching larvae penetrate the pod and enter the seed, where they develop up to pupation. One of these bruchids, Callosobruchus maculatus (F.) (Coleoptera Bruchidae), is well adapted to storage. Emerging females from stored pods coming from the field continue to oviposit on pods or seeds during the storage in the granaries. C. maculatus is the only serious storage pest of cowpea, but it is generally very destructive and can result in complete loss of cowpea in individual granaries [5;6]. Many traditional methods, including the use of botanicals focus on prevention or control of C. maculates with a relative effectiveness [7;8;9]. Recently, a hermetic storage method using triple bagging proved to be effective for cowpea and several crop storage[10;11] and is currently being largely extended. However, in a view of developing a sustainable control strategy, several options should be considered. One promising option might be biological control since in West Africa, several parasitoid species are associated with C. maculatus populations both in the field and in storage systems[12; 13; 4; 14]. Previous investigations carried out in fields show that C. maculatus F. and B. atrolineatus eggs and larvae were parasitized respectively by Uscana lariophaga Stef. [15]and two larval parasitoid species, Dinarmus basalis Rond. and Eupelmus vuilleti Crw.[16; 4]. These naturally occurring parasitoids are responsible for substantial control of C. maculatus[16; 17; 18]. The indigenous egg parasitoid Uscana lariophaga Steffan (Hymenoptera:Trichogrammatidae) has been identified as the most important mortality factor for C. maculatus eggs in the field [15]and inside granaries where parasitism reached 6973%[18]. However, the ecology of this egg parasitoid species in cowpea fields in relation with bruchid egg-laying dynamic remains unclear. This study is a contribution to a better knowledge of temporal variations of bruchid egg-laying dynamic in cowpea fields. The results are expected to give insights for determining a suitable period to release U. lariophaga and enabled a significant reduction or suppression of initial infestation of cowpea pods at harvest. Moreover, intercropping is supposed to create particular microclimates favourable to pest natural enemies and infestation reduction [19]. Therefore we assessed the effects of intercropping cowpea with millet and sorghum on egg parasitism and initial infestation by bruchids.