Insect pests are major yield reducers in sorghum under traditional farming practices in Africa. The use of resistant varieties in an integrated pest management scheme provides the most cost effective and practical means of sorghum insect control. Corneous grain sorghums are more weevil resistant than their softer endosperm counterparts. A wide range of sources of resistance to shootfiy, stem-borer and greenbug are known. Breeding for resistance to sorghum insects has been practised with varying degrees of success. Greenbug resistance breeding in sorghum has been successful in the U.S.A. Stem-borers are perhaps the most serious group of sorghum insects in Eastern Africa and further efforts are needed in breeding for resistance to these pests. In an effective pest resistance breeding work, the breeder and the entomologist must work as a team to make the fastest possible progress. Where several pests are of concern in an area, genotypes with multiple resistance in good agronomic backgrounds are needed.

Lepidopterous stem-borers are the most serious and widely distributed insect pests of sorghum. Among the several avenues of their control, the use of resistant varieties holds good prospects. In order to identify the sources of resistance to stem-borers (Chilo partellus, Busseola fusca, Eldana saccharina and Sesamia calamistis), a large number of sorghum lines from Kenya, India and Texas, U.S.A. have been screened under a very heavy natural infestation in Western Kenya. Several promising lines with resistance potential have been identified for the stem-borer complex which include: IS 10370, L-2 (TX 2780), IS 10364, IS 1044, S-178, IS 3962, IS 4213, IS 12447, IS 18479, IS 18326, L-l (A&B TX 2756), IS 4405, IS 10711, IS 18676, IS 5613, IS 18517, IS 18323, IS 4881, IS 1151 and IS 18427.

Greenhouse studies were conducted by artificially infesting potted rice plants with first instar larvae of Maliarpha separatella and Sesamia botanephaga Tarns and Bowden at two stages of plant growth, namely: early tillering stage and at the booting stage. Early infestation by M. separatella resulted in reduction of plant height and the number of filled grains. At the booting stage, the number of filled grains and their weight were higher in uninfested plants. Infestation by S. botanephaga at both stages of plant growth mainly affected the number of productive tillers. Grain weight was unaffected. Field studies on individual hills gave erratic results and there were instances when hills containing tillers bored by M. separatella produced higher yields than unattacked hills.

The importance of rice in West Africa and the problem posed by pests were highlighted. Stem-borers are regarded as major insect pests of rice and available information on yield losses attributable to insect pests in general and stem-borers in particular, was reviewed. Damage caused by stem-borers usually results in “dead hearts” and “white heads” though other effects including a synergistic interaction between stem-borers and rice blast have been recorded. Survey methods and sampling of rice stem-borers were discussed and brief descriptions on distribution, behaviour and biology of the major rice stem-borers namely, Chilo spp., Maliarpha separatella, Scirpophaga spp., Sesamia spp. and Diopsis spp. were given. Current control tactics under investigation in West Africa include chemical, cultural and biological methods.

Preliminary electrophysiological tests on certain tarsal and ovipositor sensilla of stem-borers, Chilo partellus and Eldana saccharina, show that the sensilla are innervated by mechano- and contact chemoreceptor cells. Tarsal receptors are sensitive to sucrose, whereas the ovipositor sensilla are not and responses of the ovipositor mechanoreceptor units can be related to the oviposition behaviour of the moths.

The paper which covers a two season study from 1981 through 1982, major and minor cropping seasons, describes an improved methodology of sampling for stem-/pod-borer pests within an intercropping farming system involving maize, cowpea and sorghum as target crops. The authors' approach to the intercropping study has been from a holistic angle in an attempt to gain an understanding of the processes which lead to intercropping advantages under the different cropping patterns. The target pests reported were categorized into three trophic levels: firstly, specialist feeders which were Maruca testulalis (Geyer) on cowpea and Atherigona seccata (Rondani) on sorghum; secondly, the relative specialist feeders as Chilo partelhts (Swinh.), Busseola fusca (Fuller), Eldana saccharina (Wlk.) and Sesamia calamistis (Hmps.) on maize and sorghum; and thirdly the generalist feeders represented by Heliothis armigera (Hbn.) which subsists on cowpea, maize and sorghum plants respectively. The entomological data assayed included levels of borer attack on plant spatial and temporal patterns. The population of C. partellus, the dominant stem-borer within the study area was significantly regulated within the intercropping systems. The borer incidence on the maize and sorghum monocrops and the maize/sorghum dicrop was earlier (>14 days after germination, DAG) and accentuated with time, whereas intercropping of the cereals with non cereals caused significant delay (>42 DAG) in the borer colonisation and establishment within the intercropping systems. Incidence of ‘dead hearts’ caused by the borers was significantly higher (P < 0.05) within sorghum than maize stands; a factor related to the significantly higher borer attacks on late tillers which suggest that plant ventrical heights within stands influenced borer attacks. Sorghum shootfly was restricted to within 42 DAG and very much regulated by intercropping. Maruca testulalis attack on cowpea plants was confined between 30 through 60 DAG of cowpeas. Presence or absence of non-preferred cereal hosts within the cropping patterns did not seem to interfere with the M. testulalis colonisation and establishment processes within the cropping patterns. Pod damages resulting from Maruca attack were however influenced by the cropping patterns with mixed maize–cowpea–sorghum intercrop, producing significantly higher healthier pods. The paper concludes that some intercropping patterns have many advantages over monocropping patterns with respect to pest colonisation and establishment.

This paper reviews progress made in the identification of the sex pheromones of lepidop-terous stem-borers and the use of these pheromones in crop protection. These uses include population monitoring with pheromone traps and control by mass trapping and mating disruption, and case histories for Ostrinia nubilalis and Chilo suppressalis are considered in detail to illustrate the problems encountered in applying these techniques. The pheromone blend, trap design, trap siting and pest biology are important factors in any monitoring system, and so far little progress has been made in correlating pheromone trap catches with subsequent larval infestations and crop damage by stem-borers. Control by mass trapping using pheromone traps has been little explored and is thought unlikely to provide a satisfactory control technique for many stem-borers. Control by mating disruption has been more extensively examined with encouraging results. The main potential of this technique would seem to lie in controlling pests of rice, maize and sugar-cane grown under plantation conditions.

The present article considers quantitative assessment of crop losses in relation to attack by three trophic levels of insect pests on maize, cowpea and sorghum intercropping systems. The trophic levels considered are (i) specialist feeders subsisting only on one crop within the intercropping systems, e.g. Maruca testulalis (Geyer) on cowpea, and Atherigona soccata (Rondani) on sorghum; (ii) the relative specialist feeders subsisting on two out of the three crops e.g. Busseola fusca (Fuller), Chilo partellus (Swinh.) Sesamia calamistis (Hmps.) and Eldana saccharina (Wlk.) feeding on maize and sorghum cereal plants; and (iii) the generalist feeders e.g. Heliothis armigera (Hbn.) which feeds on all three plant species. Tests for loss assessment were done under natural field conditions within maize, cowpea, sorghum monocropping patterns; maize–cowpea, maize–sorghum, sorghum–cowpea dicropping patterns; and the maize–cowpea–sorghum tricrop intercropping patterns. The authors discuss in detail their modified analytical methods based on the comparison of yield of sets of plants growing under identical conditions except that one set is unattacked by the stem-/pod-borers and the other set attacked, to generate economic losses. The comparative advantages of the planting systems on crop turnover is further discussed. The biomass and plant population fluctuations caused by pest attacks and ‘dead hearts’ at differential time scales in indices of yield have also been discussed. Yield per unit area of monocrop and intercropping patterns have been contrasted using the yield turnover of monocrop plant equivalence as the baseline for comparisons. Maize–cowpea–sorghum mixed intercrop of 1.45 land equivalent ratio (LER), and that for the sorghum–cowpea dicrop with 1.3 LER are considered the most advantageous cropping patterns. Yield losses significantly higher (P < 0.05) within maize-sorghum intercropping combinations with 0.89 LER further suggests that stem-borer colonisation which occurs at an earlier stage of crop establishment resulting in ‘dead hearts’ and significant reduction on plant population per unit area within the two cereal combinations contribute significantly to the overall reduction in crop turnover within such mixtures.

At harvest taller plants of both sorghum and maize produced a greater grain yield, but under intercropping conditions maize always out-yielded sorghum when planted at the same time, whether the sorghum was planted in the same hill as maize, or in alternate hills, or in alternate rows. There were generally more stem-borers in sorghum than in maize, with Chilo partellus predominating. Intercropping increased the level of stem-borers in maize, while infestation was very low in pure stands of maize, and the level of borer infestation in sorghum remained the same. Borers had a considerable effect on yield of maize at Mbita Point, on the shores of Lake Victoria: an infestation level of 38–52% of internodes bored resulted in significant yield loss. Implications of these results for predicting yield reduction due to stem-borers is discussed.

There are several stem-borer species recorded on important cereals such as rice, maize and sorghum as well as on grain legumes like cowpea. Those of considerable economic importance are: Chilo partellus, Sesamia infer ens, Sesamia eretica, Busseola fusca and Eldana saccharina on maize and sorghum; Sesamia infer ens. Chilo spp., Maliarpha separatella and Tryporyza incertulas on rice; and Maruca testu-lalis on cowpea.

Chemical control of insect pests will continue to be a major component of integrated pest management. However, control of tissue borers with insecticides is relatively more difficult than control of the insect pests feeding externally on foliage, shoots and stems. The foliar application of dust and spray formulation of most of the available insecticides has generally proved to be unsatisfactory as these cannot penetrate deep inside the tissues in sufficient quantities to kill the borers.

The recent advances in pest control technology, placing greater emphasis on use of various pest management components as well as insecticide selectively based on improved application methods has now made the control of stem-borers more effective and economical. Advantage can now be taken of the knowledge of the specific habits of the pests, period of peak activity of the most vulnerable stage and the economic thresholds for determining the appropriate time of application to control these pests. The development of new systemic insecticides and their granular formulation has further contributed towards the effective control of these borers. The recent research findings and recommendations for the control of cereal stem-borers are reviewed and discussed.

This paper reviews work on techniques in mass rearing of insect pests, especially stem- and pod-borers. The account covers advances in rearing of insects on native host plants and on artificial diets and possible ways of use of these insects in host plant resistance research.