Are pest regulation and erosion alleviation services conflicting or synergistic? Lessons from Sahel pearl millet

From 2010 to 2011 in Niger, the effects of a wind erosion-alleviating practice, i.e. mulching pearl millet fields with crop residue, on populations of and damage induced by millet stem borer (MSB), head miner (MHM), and head scarab (MHS) were studied. Significant differences in the survival of MSB diapausing larvae in millet stems at the end of the dry season were found at both Sadore and Doukoudoukou. At Sadore, mean MSB larval survival in the treatment where stems were exported at harvest and stored on platforms (treatment 1) was 1.2/stem, i.e. significantly higher than in all other treatments. Survival was between 0.1 and 0.2 larva in treatments 2 (with millet stems left standing from harvest throughout the dry season) and 3 (with millet stems flattened toward the end of the dry season, four months after harvest). It was less than 0.1 larva in treatments 4 (with millet stems flattened in the middle of the dry season, two months after harvest) and 5 (with millet stems flattened at the beginning of the dry season, at harvest). At Doukoudoukou, mean MSB larval survival in treatment 1 was 0.14/stem, i.e. significantly higher than in treatments 4 and 5 (0.02–0.03 larva), with intermediary findings obtained in treatments 2 and 3 (0.04–0.08 larva). The population and biomass of diapausing MHM pupae and of MHS larvae in the soil were low at both locations, and not affected by crop residue management. Thus, crop residue management whereby stems were left standing until the end of the dry season did not enhance control of millet head pests via increased predator activity, while it increased MSB survival in stems.