Research on Lepidopterous stem borers of Nigerian cereals began in 1951. Preliminary surveys finished in 1954 and from 1955 to 1961 the author studied these pests in Northern and Western Nigeria.
Guineacorn (Sorghum vulgare) and millet (Pennisetum typhoides) are the most important cereal crops in Northern Nigeria, and maize (Zea mays) is the only cereal of any importance in Western Nigeria. The crops are sown early in the wet season, which alternates with a dry season of two to seven months, and mature in three to five months. They are cultivated by hand and few farmers grow more than one acre of any one cereal.
The host ranges of all the known species of Nigerian cereal stem borers (viz., Busseola fusca (Fuller), Sesamia calamistis Hmps., S. penniseti Tams & Bowden, S. poephaga Tarns & Bowden, S. nonagrioides botanephaga Tams & Bowden, Manga basilinea Bowden, Coniesta ignefusalis (Hmps.), Chilotraea sp. ? argyrolepia (Hmps.), Chilo sp. ? phaeosema, Martin, Maliarpha separatella Rag. and Eldana saccharina Wlk.) are summarised and their distribution in Nigeria and relative importance there are discussed.
B. fusca and Coniesta ignefusalis are the most important stem borers of guineacorn and millet, respectively, and detailed accounts of their life-cycles and seasonal cycles are given, together with observations on adult activity, light-trap catches, larval migration, larval morphology and the effects of larval feeding patterns on plants of guineacorn and millet. Differences between the pupae of the three Noctuid genera (Busseola, Sesamia, Manga) and all stages of C. ignefusalis are illustrated. Some information on the biology of S. calamistis, S. poephaga and M. basilinea is also given.
At Samaru, the life-cycles of B. fusca and G. ignefusalis are completed in about two months under field conditions and in the absence of diapause. Three generations develop during the growing season, larvae of the third entering diapause until the next season. Many diapause larvae survive the dry season in crop residues, and the local practice of stacking stems for later use in building, etc., ensures reinfestation of subsequent crops, particularly near villages. The diapause ends between late April and early June, so that adults of the diapause generation emerge about three weeks after millet and guineacorn have germinated. Experiments indicate that diapause is terminated when larvae are provided with moist food or drink water. The relative abundance of B. fusca and C. ignefusalis in guineacorn, millet and maize largely reflects host selection by ovipositing females. C. ignefusalis lays freely only on millet and is therefore mainly confined to this crop, whereas B. fusca, though favouring guineacorn, also lays on the other two crops. Experiments indicate that more eggs are laid on larger plants.
Early millet and maize are harvested before borer populations reach their highest level. Guineacorn tolerates the considerable damage done by the high populations present at the end of the season but late millet and second-crop maize do not. Any factor hindering normal crop development tends to increase the damage done.
A provisional list of 14 primary parasites, three hyperparasites, one predator and four diseases is given. Tetrastichus atriclavus Wtstn., Apanteles sesamiae Cam., and Pediobius furvus (Gah.) were always present on B. fusca at the end of the season. Syzeuctus sp. was the most important parasite of C. ignefusalis and was regularly reared from diapause larvae, and other common parasites of this species later in the season were Sturmiopsis parasitica (Curran), T. atriclavus and Hyperchalcidia soudanensis Steffan. The over-all rate of parasitism seldom exceeded 10 per cent, of all larvae and pupae collected. Bacillus thuringiensis killed many diapause larvae of B. fusca and was the most important disease recorded.
The results of experiments relating yields to stem-borer attack in maize, early millet, late millet and guineacorn are presented and discussed. Early-sown maize suffers little damage but second-crop maize at Ibadan suffers losses of stand which, in local farms, generally exceed 20 per cent. In experiments, loss of stand did not always result in loss of yield, and evidence is presented that, under certain conditions, loss of stand is compensated for by the production of heavier cobs. At Samaru, the development of single larvae of B. fusca in otherwise healthy stems reduced their yielding capacity by 28 per cent, of the mean cob weight of healthy stems (P <0·001).
In six experiments on early millet, little reduction in yield occurred, even when infestations were much higher than those normally experienced, but late millet suffered almost complete loss of yield in two experiments and more than 90 per cent, of the stems were bored in. two other experiments, in which yields were low. These results and general observations indicate that stem borers, particularly C. ignefusalis, play a major part in limiting yields of late millet.
In six experiments on guineacorn, significantly so in three of them, the yield per bored stem was higher than that per unbored stem, and it appears that borers concentrated on or were concentrated on the larger, higher-yielding stems, possibly as a result of selective oviposition. In one other experiment, stems bored by first-generation larvae yielded significantly less than unbored stems but neither in this nor in any other experiment was over-all borer attack related to a reduction in yield and it is generally true that excellent yields of guineacorn can be obtained in the presence of high populations of borers.
Reduction of the carry over of diapause larvae, by destroying or composting crop residues or removing them from the field, has given good control on the experimental farms at Samaru but it is difficult to extend these measures to local farms. Farmers should clean up their fields before the beginning of the rains and stems that are to be used for building should be exposed in the open during the dry season; to reduce populations of diapause larvae. Insecticidal control has as yet shown little promise and plant breeding for resistance has not begun.
The necessity for further work to confirm the general conclusions drawn here is stressed and possible lines of investigation are indicated.