Sorghum is one of the most important cereal crops in the semi-arid tropics. Several insect pests damage it. The possibility of identifying genotypes with multiple resistance to these pests and transferring the relevant genes into high-yielding varieties and hybrids has been considered as an attractive approach to reducing yield losses. A set of 12 cytoplasmic male-sterile and maintainer lines, 12 restorer lines and their F1 hybrids were evaluated for resistance to sorghum shoot fly Atherigona soccata Rondani, spotted stemborer Chilo partellus (Swinhoe) and sugarcane aphid Melanaphis sacchari (Zehntner) under field conditions. There were significant differences among the genotypes tested. A total of 50% of male-sterile lines, 41.7% maintainers, 58.3% restorers and 35.4% of the hybrids tested showed moderate to high level of resistance to the three pests. The male-sterile and restorer lines showing resistance to different insects can be exploited for developing hybrids with multiple insect resistance for cultivation by the resource-poor farmers in the semi-arid tropics.

The population dynamics of busseola fusca (Fuller) was studied from 1997 to 1999 at Halhale and Halhal Begos, located at altitudes of 1960 and 1850 m above sea level respectively in the highlands of Eritrea. Sowing was done in early April in Halhale and late June in Halhal Begos throughout the study period. All agronomic practices recommended for sorghum production in the two areas were followed to ensure good crop stand, except for application of pesticides. A total of 15 generations, nine in Halhale and six in Halhal Begos were observed during the study periods. Busseola fusca had three and two generations per year in Halhale and Halhal Begos, respectively. Peak egg laying at Halhale was in early May, July and early September, while at Halhal Begos it was in July and early September. The study revealed that stage-specific mortalities of B. fusca were caused by infertility, disappearance (predation by ants and earwigs, lack of suitable host, abiotic factors), residuals and parasitoids. The key mortality factor was disappearance of small larvae (35–66 % mortality). The mortality of the other stages was low but had a composite effect on the overall generation mortality. The mortality of small larvae was density-dependent and density-independent at Halhale and Halhal Begos, respectively. The changes in population between generations were due to small larval mortality of B. fusca from one generation to the next. The life table study showed that mortality due to natural factors did not reduce B. fusca population below the damage level. Integrated management strategies are discussed to reduce the damage caused by B. fusca on sorghum.

In West and Central Africa, panicle-feeding mirid bugs, particularly Eurystylus oldi Poppius (Hemiptera: Miridae), are major pests of improved sorghum cultivars. Population dynamics of panicle-feeding bugs were studied on the sorghum cultivar ICSV 197, sown on eight dates each year from 1992 to 1994 at Samanko, Mali. Relationships between weather factors and abundance of paniclefeeding bugs were examined for the three most commonly found mirids, E. oldi, Campylomma spp. and Creontiades pallidus Rambur (representing on average respectively 58, 39 and 2% of all mirid species). Although only one generation of E. oldi could develop between flowering and grain maturity on a particular panicle, there were three successive generations each year during the extended grainmaturing period in the experiment with eight sowing dates, with a major peak in abundance in early October, followed by a gradual decrease until late November. Abundance of Cr. pallidus peaked in late October, while that of Campylomma spp. showed no relationship with date of sowing.

An increase in abundance of E. oldi nymphs (y1) was favoured by high precipitation, reduced maximum temperature and high minimum relative humidity during and 2–3 weeks before the grainfilling period [a strong negative relationship was found with the number consecutive days without precipitation from 78–88 days after sowing (das) (x1): y1 = −11.7x1 + 149.3 (r2 = 0.67)]. The abundance of Cr. pallidus (y2) was correlated with wide temperature ranges and low precipitation during the same periods (notably precipitation frequency from 75–91 das: x2): y2 = −2.28x2 + 24.6 (r2 = 0.65). The abundance of Campylomma spp. (y3) was negatively correlated with amount of rainfall between 19–32 das (x3): y3 = −3.82x3 + 705.9 (r2 = 0.49). Reasons for these trends are suggested and their possible application for management of panicle-feeding bugs discussed.

Thirty-five grain sorghum genotypes representing 6 variable groups (A/B-lines, R-lines, commercial varieties, germplasms, mutants and locals) were evaluated for the orientation, colonisation and oviposition responses of the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), governing the antixenosis component of resistance. In free-choice tests, the orientation and colonisation responses of the weevil were identical among all the genotypes at 24 h, varied at 48 h, and differed widely by 72 h after adult release. High degrees of antixenosis for colonisation by the adult weevils were observed among all the genotypes except 2077A and 2219B, which suggests a predominant reaction to gustatory rather than visual or olfactory stimuli. One to five egg plugs per seed were oviposited, frequently near the endosperm close to the seed base. Greater levels of antixenosis for oviposition were noticed in 2077B, DJ 6514 and IS 11758 in free-choice tests, and 2219B, M 148–138, P 721 and Nizamabad (M) in no-choice tests. To the contrary, M 35–1, Swati and Lakadi showed greater susceptibility for oviposition. Significantly less damage to seed was observed on 2219A/B, 116B, IS 9487, IS 11758, CSV 8R(M) and Local Yellow. Although the differences in seed weight loss were not significant, this parameter was relatively lower in AKMS 14B and 2219B than in the other genotypes. This indicates the need to increase the levels of resistance among the parental A/B-lines in the development of hybrids, in order to ensure better protection from rice weevil infestation in stored sorghum.

Surveys were conducted to determine the species composition of stemborers and their natural enemies in maize and sorghum in 1999 and 2000 in eastern, northern, southern and western Ethiopia. The six species of stemborers found attacking maize and sorghum were Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), Busseola fusca Fuller, Sesamia calamistis Hampson (Lepidoptera: Noctuidae), S. nonagrioides botanephaga Tarns & Bowden (Lepidoptera: Noctuidae), Rhynchaenus niger (Horn) (Coleoptera: Rhynchophoridae) and Pissodes dubious (Ström) (Coleoptera: Rhynchophoridae). Twenty species of primary parasitoids of eggs, larvae and pupae of Ch. partellus, B. fusca and S. calamistis were recorded. Other natural enemies, including predators, fungi and nematodes, were also found. Cotesia flavipes Cameron (Hymenoptera: Braconidae) was the dominant and the most widely distributed parasitoid of stemborers in Ethiopia, and was reared from field-collected Ch. partellus, B. fusca and S. calamistis.

Studies to determine the relative abundance and distribution of stemborer species and their associated parasitoids in maize and sorghum were carried out during the 1995–1996 and 1999–2000 cropping seasons on Unguja and Pemba, the two main islands of Zanzibar. Three stemborer species were found on both islands in maize and sorghum, with the exotic species Chilo partellus (Swinhoe) (Crambidae) being the most abundant and widely distributed species, accounting for 75.3 % of stemborer attack. It was followed by Sesamia calamistis Hampson (Noctuidae) and Ch. orichalcociliellus Strand (Crambidae). Stemborer abundance was significantly higher in Pemba (1.47 ± 0.20) than Unguja (0.85 ± 0.05). Overall stemborer density in Zanzibar, during the period of study, was 1 03 ± 0.08 stemborers per plant. Stemborer infestation was significantly higher during the short rainy season than long rainy season but it did not vary between maize and sorghum or coral rag and plantation areas. The indigenous parasitoids recorded included ten larval parasitoids: Bassus sp., Chelonus sp., Cotesia sp., Cotesia sesamiae (Cameron), Cotesia ruficrus (Haliday), Dolichogenidea sp., Dolichogenidea aethiopica Wilkinson, Dolichogenidea polaszeki Walker, Megaselia sp. and an unidentified Tachinidae. Among these, the braconid Co. sesamiae was the most common parasitoid, attacking all the stemborer species and recorded from 85.2 % of parasitised larvae. However, parasitism was low— 3.9 % on Unguja and 1.9 % on Pemba. In addition, the efficiency of Co. sesamiae was reduced by two hyperparasitoids—Aphagnomus fijiensis Ferrière and Elasmus sp. Seven pupal parasitoids were recorded, namely Dentichasmias busseolae Heinrich, Brachymeria sp. Westwood, Brachymeria olethria Waterston, Pediobius furvus Gahan, Psilochalcis soudanensis Steffan, Syzeuctus ruberrimus Benoit and an unidentified Chalcididae. The differences in species diversity between the two islands are discussed in the context of the equilibrium theory of biogeography and the geographical size. Classical biological control of the introduced stemborer Ch. partellus using the exotic parasitoid Co. flavipes to supplement the indigenous parasitoids in population regulation is proposed as a potentially useful option for Zanzibar.

The incidence and distribution of the sorghum head bug, Eurystylus oldi Poppius (Heteroptera: Miridae), and other panicle pests of sorghum in research stations and farmers' fields in West and Central Africa (WCA) were assessed from 1985 to 1994. Maximum head bug abundance was observed during the dough stage. Head bug abundance was greater on improved cultivars, while the local guineense sorghums with long glumes were less susceptible, both at the research stations and the farmers' fields. Grain damage was moderate to severe, although farmers were mostly unaware of this insect and its damage potential, because it remains hidden inside the panicle. Sorghum midge (Stenodiplosis sorghicola Coq.) damage was very high on farmer's fields in Burkina Faso, Niger, and Nigeria. The shootfly Atherigona soccata Rond., stemborer Busseola fusca Fuller, grasshoppers, spittle bug Locris rubens Erichson, Campylomma spp., Creontiades pallidus Ramb., Agonoscelis spp., head caterpillars Helicoverpa armigera Hub., Pyroderces simplex Wlsm. and Eublemma gayneri Roths., were other insect pests damaging sorghum in the areas surveyed. It is proposed that future efforts in sorghum improvement for WCA should focus on developing cultivars with resistance to sorghum head bugs.

Forty forage sorghum genotypes, including 30 hybrids, 5 inbred forage varieties and 3 resistant and 2 susceptible checks were screened for resistance to the sorghum shootfly, Atherigona soccata (Rondani) (Diptera: Muscidae) and the spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) under natural infestation in July-September 1996 and 1997. Six sorghum genotypes including two forage sorghum hybrids, HH 1 and NFSH 10659, three resistant check entries, IS 2205, IS 5469 and IS 18551 and inbred forage variety HC 171 showed multiple resistance to both the shootfly and the stemborer. The forage sorghum hybrid HH 1 also gave the highest green fodder yield. It is therefore suggested that this cultivar may be used in breeding programmes as a donor parent for developing sorghum cultivars resistant to A. soccata and C. partellus, or used directly for general cultivation.

Studies on the population dynamics and species composition of the sorghum stemborers Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), Busseola fusca (Fuller) (Lepidoptera: Noctuidae) and Sesamia sp. were conducted at Melkassa, Rift Valley of Ethiopia, during the 1990 and 1992 cropping seasons. Chilo partellus at 92% dominated the species composition. Busseola frisca made up 7.7% and Sesamia calamistis less than 1% of the borers. Occurrence of partellus declined with delay in planting until the third planting ( early June to early July) then increased during late planting in July. Busseola fusca alone, however, tended to increase in numbers with delay in planting. In Melkassa area, early planting in July could be considered as the most suitable, with low borer numbers, damage symptoms and resulting in fewer stalks with peduncle damage. Grain yield was also higher during early planting in July while the lowest yield was obtained during late planting in July. Weather factors showed no consistency but relative humidity seemed to have a significantly negative correlation with insect numbers in the last two plantings in 1992.

Field experiments were carried out at Coimbatore, Tamil Nadu State, India, for four consecutive years (1990–94), to monitor the seasonal activity of Cotesia flavipes Cameron (Hymenoptera: Braconidae), a larval parasitoid of gramineous borers, including the sugarcane shoot borer (Chilo infuscatellus Snellen), sugarcane internode borer (Chilo sacchariphagus indiens (Kapur) and sorghum stemborer (Chilo partellus Swinhoe) (all Lepidoptera: Crambidae). The highest levels of parasitism (0.0–17.9%) were recorded on Ch. partellus followed by Ch. sacchariphagus indiens (0.0–8.3%) and Ch. infuscatelliis (0.0–1.1%). The parasitoid was collected in Ch. partellus and Ch. sacchariphagus indiens in all the four study years, whereas it was not observed in Ch. infuscatelliis during 1993–94. Parasitism rates in Ch. partellus were positively correlated with minimum temperature and afternoon RH. In Ch. sacchariphagus indiens, parasitism levels showed significant negative correlation with minimum temperature. Parasitism rates in Ch. saccharipliagus indidus larvae collected during harvest from seven sugar factory areas of Tamil Nadu State were lower in 1992–93 (0.0–1.8%) and higher in 1993–94 (0.0–3.2%) than those recorded at Coimbatore in the corresponding period (December-March). Cotesia flavipes was the only larval parasitoid recorded from the borers both at Coimbatore and the seven sugar factory areas surveyed in Tamil Nadu.

Three field trials were conducted to determine the effect of various levels of resistance of South African sorghum hybrids on follow-up generations of Busseola fusca Fuller (Lepidoptera: Noctuidae) and Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae). In one trial, plants were artificially infested with B. fusca larvae while the other trials were subjected to natural infestation of C. partellus. The number of damaged internodes and moth exit holes per stem were used to calculate a relative survival index which estimates the survival of larvae to the adult stage. A moth production index was calculated to estimate the role of a hybrid in area-wide suppression of stemborer populations. In the B. fusca trial, resistant and susceptible experimental hybrids produced 40 000 and 113 000 moths per hectare respectively, compared to 80 000 and 83 000 moths per hectare produced by the commercial hybrids. Larval damage to whorl leaves, assessed on a 1 to 9 scale, indicated leaf-feeding resistance of commercial hybrids to B. fusca ranging from susceptible to intermediate. Yield losses suffered by the hybrids in the B. fusca trial were greater than that suffered by a susceptible experimental hybrid, indicating susceptibility of South African hybrids to B. fusca. The evaluation of sorghum varieties for their potential to reduce stemborer populations should indicate their usefulness in integrated pest management systems.

Tannin extracts of two sorghum, Sorghum bicolor (L.) Moench. cultivars, BKS5 and BTX 623 were assessed for their bioactivity against the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and the angoumois grain moth, Sitotroga cerealella (Olivier) (Lepidoptera: Gelechiidae), in whole wheat flour pellets, and against the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) in whole wheat flour at three treatment levels. The number of adult progeny was reduced and number of days to emergence increased by tannins extracted from both sorghum cultivars at all treatment levels. However, the effects were not significant in all cases. In a short-term (25 days) feeding test using S.oryzae adults, the presence of sorghum tannins from cv. BKS 5 in the pellets significantly reduced feeding by the insects.

Intraspecific competition in the rice weevil, Sitophilus oryzae (L.) and the angoumois grain moth, Sitotroga cerealella (Oliv.) in grain sorghum was studied in laboratory experiments. Progeny production per female of each of the two species when reared separately, decreased as the initial parent population density increased. The number of progeny produced by Sitophilus oryzae increased to a peak, and then declined sharply with a further increase in initial parent population density, characteristic of a scramble competition. In contrast, the progeny produced by Sitotroga cerealella increased with an increase in parental density to reach a fairly constant maximum (flat topped curve), characteristic of a contest competition. It is concluded that intraspecific competition will not limit the potential of the grain moth S. cerealella of becoming a major pest of stored sorghum.

Combining ability of resistance to the sorghum shootfly, Atherigona soccata and spotted stemborer, Chilo partellus in grain sorghum was analysed on a seven-parents diallel in a field trial at the ICIPE Mbita Point Field Station, western Kenya during the 1990 long rainy season. Lines with best performance for each of the four measured resistance parameters were also found to be the best general combiners. Resistance to leaf damage and number of holes/metre were controlled by additive gene effects whereas resistance to deadheart and stem tunnelling were controlled by additive and non-additive gene effects. Parents IS-1044, IS-12308 and IS-2269 with their good level of resistance, and desirable general combining ability (GCA) effects are proposed for utilisation as resistant combiners in breeding programmes for shootfly and stemborer resistance in grain sorghum.

Studies were conducted from 1993–1995 at Samanko, Mali, on rearing and pheromone trapping of Busseola fusca Fuller (Lepidoptera: Noctuidae), and on the insecticidal effect of extracts of Jatropha curcas (Euphorbiaceae) on B. fusca and Sesamia calamistis Hampson (Lepidoptera: Noctuidae). A technique for mass rearingof B.fusca larvae on a chickpea and sorghum leaf-based meridic diet was developed, requiring about 10 g of diet per larva. An average of 7.3 successive generations were obtained per year. Pupal and larval mortalities were less than 15%. Oviposition was obtained on artificial substrates, with a fecundity of 300–500 eggs per female. Laboratory tests were conducted with diets in which extracts of nuts of J. curcas had been added. A 100% larvicidal effect was obtained with B. fusca and S. calamistis at 0.01% and 1% crude oil concentration respectively, and with 0.025% of the phorbol ester fraction on S. cahmistis. Trapping experiments using traps made of aluminum trays baited with synthetic pheromone showed that B. fusca male moths flights could be satisfactorily monitored. An improvement of traps and/or pheromone effectiveness is, however, necessary before their use as a control method.

Two field experiments were conducted over two seasons to assess the effect of resistance levels of sorghum varieties on follow-up generations of Chilo partellus. Sorghum varieties, released by the Southern African Development Community/International Crops Research Institute for the Semi-Arid Tropics (SADC/ICRISAT) through the Sorghum and Millet Improvement Program (SMIP) in the southern African region, were evaluated for their ability to provide for larvae to complete development in stalks and emerge as moths. Plants were naturally infested by C. partellus. The number of moth exit holes per stem was used to calculate survival of larvae to the adult stage. A moth production index was calculated to estimate the role of each sorghum variety in area-wide suppression of stemborer abundance. Between 97,500 and 213,125 moths per hectare emerged from the different sorghum varieties in trial 1 while between 0 and 77,500 emerged per hectare in trail 2. Resistant sorghum varieties may help suppress pest population build-up and be of considerable value in development of IPM systems for resource-poor farmers.

The phenotypic stability of resistance to the stemborer, Chilo partellus and grain yield in 10 newly developed sorghum hybrids namely HYD-1, HYD-2, HYD-3, HYD-4, HYD-5, HYD-6, HYD-7, HYD-8, HYD-9 and HYD-10, and two local checks, Serena (tolerant) and IS-1044 (resistant), were evaluated for seven seasons at the International Centre of Insect Physiology and Ecology (ICIPE) Mbita Point Field Station and Ungoye Field Site in western Kenya. Two stability parameters were determined: (1) The linear regression (b1) of an entry mean on an environmental index, and (2) the deviation from regression (S2d). The significance found for the linear component of environment implies that the variation over the different environments is adequate to test for cultivar stability. Hybrids HYD-1, HYD-5, HYD-8 and HYD-10 were identified as the most suitable genotypes for future commercial production.

An investigation was carried out on the effects of three patterns of sorghum (Sorghum bicolor) and cowpea (Vigna unguiculata) intercrops on larval establishment of the spotted stemborer, Chilo partellus, on sorghum at different growth stages. The intercrop patterns were: sorghum sown simultaneously with cowpea, sorghum sown 2 weeks before cowpea and sorghum sown 2 weeks after cowpea. Sole sorghum and sole cowpea were controls. Sorghum plants were infested at 4, 6 and 8 weeks after emergence (WAE). Ten days after infestation, the infested plants and their immediate neighbours in the same row were uprooted and dissected. The number of larvae recovered and their stages of development were recorded. Some predatory arthropods were monitored by direct egg counts and use of pitfall traps. Grain yields were recorded at harvest to determine land productivity. Larval establishment was significantly (P < 0.05) higher on sorghum plants in sole crop than in intercrops; it was also higher on sorghum plants infested 6 WAE than on other dates. More C. partellus larvae were lost from sorghum sown after cowpea than in other cropping patterns, and more larvae were lost from sorghum plants infested at 4 WAE than on other dates. More predatory arthropods were observed in cowpea cover compared to those found in sole sorghum—the arthropods may be responsible for the observed loss in C. partellus larvae in the intercrop. Land productivity was higher in sorghum sown simultaneously with or after cowpea than in other cropping patterns. Thus, intercropping increased larval loss in C. partellus and improved productivity.

Survival and development of Chilo partellus were studied on five sorghum cultivars (IS-18363, IS-18520, ICS-1, ICS-2 and IS-4660) in the laboratory at Mbita Point Field Station, Kenya, at 26 ± 2°C and 65 ± 5% RH. Larval developmental period differed significantly on different cultivars. Number of instars was numerically greater for the females than for the males on all cultivars. Average pupal weight varied with sex; females were twice as heavy as males. The pupal period was longer for males than for females on all cultivars, with the exception of ICS-1, where the pupal period of males was equal to that of females. Ovipositional period was significantly longer on IS-18520 (5.18 ± 0.30 days) than on other cultivars. The shortest ovipositional period (3.70 ± 0.27 days) was recorded on IS-4660. There were no statistically significant differences in adult longevity. Sex ratio of the moths also did not affect the adult longevity of C. partellus reared on the five sorghum cultivars. The variations found in the development of C. partellus on different cultivars were attributed to the nutritional value of the food source.

Seedlings of 462 sorghum lines were screened to identify resistance to the sugarcane aphid, Melanaphis sacchari (Zehntner). About 7% of the lines exhibited resistance in the seedling stage in greenhouse trials. In an advanced screening experiment, 12 sorghum lines had resistance ratings of 1.0–3.0 (highly resistant or resistant). In a similar test, older and larger plants of the same lines were as resistant as in the seedling stage. Lines IS12664C, IS12609C, IS12158C, and IS12661C were highly resistant in preliminary and advanced screening trials. Antixenosis was shown to be a mechanism of resistance. Sorghum lines IS12664C, IS1144C, IS1598C, and IS12661C were less preferred than ‘Mtode’, a susceptible sorghum. Based on greenhouse and field trials, antibiosis adversely affected aphid longevity, days reproducing, and progeny production. On the least antibiotic sorghums, longevity, days reproducing, and number of nymphs produced were as high as 13.7, 6.0, and 26.6, respectively. On the most antibiotic sorghums, these values were 4.5, 0.0, and 0.0. Resistance appeared to be controlled by a dominant gene, but there were indications of modifiers linked to major genes.