The effect of infestation of lepidopteran borers on yield of maize was assessed at three locations in the forest and forest/savanna transition zones of Cameroon, during the first and second season of 1992. Maize was treated once or twice with the systemic carbofuran at different developmental stages. The species encountered were the stem-boring noctuids Busseola fusca and Sesamia calamistis, which oviposit on and attack pre-tasseling stages of maize; the stem-boring pyralid Eldana saccharina, which attacks plants at and after tasseling; and two ear borers, namely the pyralid Mussidia nigrivenella and the tortricid Cryptophlebia leucotreta. Generally, carbofuran reduced infestations by B. fusca and S. calamistis but had little or no effect on species attacking the plant's later stages. Thus early treatments with granules into the whorl were more efficient than late treatments as a side dressing, and a second late treatment had little effect. During the first season, late-attacking species were predominant and grain yield losses of 17–44 % were found in one location only. During the second season, species attacking young plants (mainly B. fusca) became predominant in all three locations and yield losses of 30 to 41 % were found in two of the three locations. The percentage of ears with damage was especially high during the second season. As green maize is an important source of cash income in the area and damaged ears cannot be sold, the economic loss was likely considerable.

Studies were conducted in several ecological regions of Côte d'lvoire from 1986 to 1990 to monitor maize cob-borer populations and to investigate losses in maize grain yield due to this pest. The relationship between borer abundance and crop losses was studied using regression analyses, in which two regression models were developed. One model related yield loss to number of damaged grains and the other related yield loss to grain weight. Mussidia nigrivenella Ragonot (Lepidoptera: Pyralidae) was the dominant pest in most localities. Its population density was particularly high in the semi-deciduous rain forest and decreased westwards and northwards. The models explained 85.6% (yield loss vs damaged grains) and 72.5% (yield loss vs loss in grain weight) of the observed variation. Moreover, they showed that the weight loss due to cob-borers was much lower than that due to stem borers and also that the number of damaged grains increased rapidly with borer density.

Observation of the damage caused by Mussidia nigrivenella (Pyralidae) on corn cobs and cotton bolls begun in Togo in 1988 was continued in 1990 and 1991. The study was conducted at IRCT's Anié-Mono station at the Dalanda, Notsé and Kouvé test and on small holdings in the Notsé region. M. nigrivenella is the main pest on corn (over 90% of the caterpillars observed). Its larvae populations were higher in 1991 than in the previous year, owing to favourable climatic and cropping conditions.

On immature cotton bolls, C. leucotreta remains the dominant species. However, the extent of M. nigrivenella activity on mature bolls was revealed for the first time. In certain locations, over 90% of the caterpillars and chrysalids observed in attacked mature bolls belong to this species. Hence, cotton plants which are not destroyed after harvests can act as reservoirs for pyralid pests.

The larvae of Mussidia nigrivenella Ragonot, attack various fruits; it is the main maize cob borer in Ivory Coast. The development of attack during the cultivation, the behaviour of larvae in the field, the spatial distribution of eggs and larvae, the mortality of pre-imaginal instars and the annual fluctuations of populations are considered in this paper. This insect may represent an economical problem for seed producers in the south of the savannah area of Ivory Coast where its density exceeded 14 insects per cob, at harvest in 1984, whereas the risk of cob damaging by Mussidia nigrivenella appears to be very weak in the north and west of this region.

Mussidia nigrivenella Ragonot is the main cob borer of maize in Ivory Coast. An accurate description of male and female genitalia is given. Rearing in the laboratory has permitted determination of the number and duration of the pre-imaginal instars, fecundity of adults and fertility of eggs as also the sex-ratio. An artificial diet has been elaborated which provides conditions of similar development to those observed in the field: The insect has 5 to 6 larval instars and the total duration of development from the egg-laying to the adult emergence is 46 days for the male and 49 days for the female.

The corn earworm damage to seven maize cultivars TZB, TZPB, WYI, 096EP6, Farz 25, ‘Kewesoke’ and Local White, was assessed during the early and late cropping seasons of 1980 and 1981 at Ikenne and Ilora located in the rain-forest and derived savanna zones, respectively. The effectiveness of dimethoate and permethrin in controlling the earworms was evaluated. All the cultivars appeared susceptible to earworm infestation in the two ecological zones. Infestation was consistently and significantly higher in the early season and also in the rain-forest zone. It ranged from 71.5 to 86% in the early and from 28 to 36.8% in the late seasons, respectively, at Ikenne. At Ilora, infestation ranged from 53 to 82% and from 20 to 75% in the early and late seasons, respectively. Mussidia nigrivenella and Heliolhis armigera were the dominant species recorded damaging the grains.

Earworm infestation was significantly lower on plots treated wtih three applications of 0.05 and 0.1% a.i. permethrin at weekly intervals than on plots that received the same dosages and sprays of dimethoate and the control. Maize yields were significantly higher on permethrin-treated plots. The lower dosage of permethrin is recommended to reduce chemical residue on maize grains.