The temporal evolution of the populations of Megalurothrips (=Taeniothrips) sjostedti (Trybom) (Thysanoptera, Thripidae) (larvae and adults) and of Maruca testulalis (Geyer) (Lepidoptera, Pyralidae) (larvae) as well as the rate of flower infestation by these insects was carried out under Held conditions using 10 cowpea varieties (IT 82D-889, IT 82D-287, IT 82D-716, IT 82D-789, IT 82D-703, IT 82D-713, IT 82D-890, IT 82D-709, TVu 57 and “Kpodjiguegue”).
The investigation was carried out during the 1984 early cropping season (from April to July) at Abomey-Calavi. In the research station the abundance of M. sjostedti populations was observed 47 days after planting (DAP) for one variety and 68 DAP for the nine other varieties (384–634 thrips/20 flowers according to the varieties). In addition, the decline of the insect population was recorded 40 DAP for four varieties and 75 DAP for the six other varieties (88–375 thrips per 20 flowers according to the varieties). But 90–100% of the sampled flowers were infested by M. sjostedti towards 40 DAP whereas 95–100% of flowers were injured 68 DAP.
Larval populations of M. testulalis reached their maximum value 40 DAP for four varieties and 47 DAP for the six other varieties (4–17 larvae/20 flowers according to the varieties). The population declined 61 DAP for two varieties, 68 DAP for seven varieties and 75 DAP for one variety (0–3 larvae/20 flowers according to the varieties). Regarding the rate of flower infestation by M. testulalis, the highest percentages (20–70% according to the varieties) were obtained among all varieties from the same sampling dates where the larval population peak was noted for the same varieties. In the same way, lowest percentages (0–15% according to the varieties) were recorded among all varieties on the same dates where the minimum value of larval populations of the pest was obtained.
M. sjostedti and M. testulalis fluctuate in opposed phase. The peak value of M. sjostedti generally coincides with the minimum value of M. testulalis and inversely. But the peak activity period of both insects occurs between 40 and 70 DAP. The average number of insects recorded all along the sampling period has shown significant differences (P=0.05) between the various cowpea varieties for M. sjostedti and M. testulalis. The biological and nutritional factors that contribute to such population fluctuations are discussed in the light of results obtained.
