A study of pre-harvest infestation of maize by Sitophilus zeamais Motsch. (Coleoptera, Curculionidae) in the Kenya highlands

Abstract

The incidence of attack by Sitophilus zeamais Motsch in maize ears in the growing crop and in cribs during storage has been studied in the Kitale district of Kenya. There was no evidence that wild plants or cultivated wheat varieties supported high populations of this insect, the principal source of field infestation being stocks of stored maize. Ears from the previous crop ploughed in at time of cultivation and sowing rarely support S. zeamais long enough to allow cross-infestation of susceptible new crop ears. In a growing crop of the variety ‘Kitale synthetic’, 50–60 per cent of the ears were tightly enclosed by the sheath. The remainder had loose or open sheaths. In the same crop the numbers of infested grains and infested ears present immediately before harvest were estimated by placing ears in separate polythene bags just after harvest and recording daily the emergence of adults over a 5 week period. As a result it was estimated that 42 per cent of the tightly sheathed ears were infested, compared with 51 and 84 per cent respectively in the loosely and open sheathed ears. Assuming one insect per infested grain the number of infested grains for infested ears from all three categories combined was 10 per cent. The overall level for infested grains in all ears from the crop as a whole was 6 per cent. These data compare well with a field experiment in which two different pre-harvest infestation levels were measured, e.g. where the kernel damage shortly after harvest was less than 1 per cent it took 16 weeks to reach a level of 7 per cent, compared with an initial level of 3 per cent which reached 16 per cent after only 8 weeks. In a comparison of the susceptibility of six white dent maize varieties to pre-harvest infestation, Hybrid 622 proved far superior to the others in three important aspects. It had a low incidence of ears with open tips (27·5 per cent), a lower level of insect attack even when compared to another variety with a similar level of open tipped ears and a much lower moisture content at harvest. Field experiments indicated that S. zeamais would not develop in the kernel before harvest until the moisture content fell to below 60 per cent, which in Kitale district would be mid-September i.e. 3 months before harvest. As was appreciated many years ago, the most practical method of reducing pre-harvest attack is by preventing insect development in harvested grain. Lindane admixed with harvested ears from which the sheaths had been removed, killed emerging adults and achieved this. Unfortunately there are signs of resistance developing to this insecticide and alternative treatments are required. The admixture of an effective contact insecticide to prevent breeding after harvest, combined with careful cleaning and spraying of empty stores not later than pollination of the next field crop, and selection of maize varieties resistant to attack before and after harvest, appear to be the most effective way of reducing pre-harvest infestation.