The effect of N, P and K fertilisers, measured as percentage leaf dry matter, was assessed on larval populations of Maruca testulalis (Geyer) (Lepidoptera: Pyralidae) recorded on cowpea, Vigna unguiculata, var. IT 82E-32 under natural infestation. The study was carried out during the 1987 long rains (March-July), at Abomey-Calavi's experimental farm. A factorial experiment with 9 completely randomised treatments in three replicates of N-P-K fertilisers was used. Phosphorus was applied at three levels (P0, P1, and P2) corresponding to 0, 40 and 80 kg of P2O5/ha respectively. Potassium was also applied at three levels (K0, K1 and K2) representing 0, 40, and 80 kg K2O/ha respectively. In all plots nitrogen was applied as 20 kg /ha of urea. All the fertilisers were spread on the soil on the day of planting. Fourteen sampling dates for flushing shoots, flower buds, flowers and pods were subsequently made 29, 33, 36, 40, 43, 47, 50, 54, 57, 61, 64, 68, 71 and 75 days after sowing (DAS) to evaluate the infestation rate by M. testulalis larvae. Sampling for. N, P and K contents in cowpea leaves was conducted 29, 47 and 61 DAS. As for interaction between P and K and the pest, only the averages of larvae from interaction P0 K0, P2 K0 and P0 K1 on the one hand, and those from the interaction of P0 K0, P0K2 and P0K2 on the other hand, were statistically, (P < 0,05) different. Multiple regression analysis showed that the content of N, P and K in cowpea leaves throughout the sampling period influenced by 40% the total variation of larval population of M. testulalis in the plant. The remaining 60% were due to other factors of experimental agrosystem. Application of N, P and K fertilisers to the cowpea and their assimilation by the plant are primarily aimed at increasing the plant vigour and grain yield. In contrast, the plant vigour induced by these nutrients leads to an increase in the larval population of M. testulalis in the plant and also to a decrease in the highest yields that can be expected from the use of these fertilisers. Therefore, an agrobiological competition takes place as a result of two inductive opposite effects inherent from fertilisation. In connection with this situation, the control of M. testulalis using fertilisers could be one option to maintain a buffer effect between the pest and the plant, thus therefore help in increasing cowpea yield.