Local West African strains of Sitophilus oryzae (L.) and Tribolium castaneum (Herbst) were placed in 200 g of wheat samples, one species after the other, during 6 weeks. The rate of infestation for S. oryzae was 6, 12, 24 or 48 pairs of adults, while it was fixed to 20 for T. castaneum. The generation of S. oryzae was killed by fumigation before T. castaneum infestation of the grain samples. The loss in dry weight and the frass production were recorded at the end of each rearing period of time for each species.

The dry matter loss caused by S. oryzae was highly correlated with the rate of adult infestation and measured as high as 600 mg per infesting pair of weevils. The frass production for both S. oryzae and T. castaneum was highly correlated with the rate of infestation by weevils only. For S. oryzae, the average frass production was about 60 mg per adult pair, while for T. castaneum it was variable from 27 to 44 mg per pair of the primary infestation level by the weevils.

The importance of these results is discussed in relation to standard loss assessment methods when there are several infesting species and particularly when there is a preponderance of secondary species over primary species.