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.

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.

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.

Larval parasitism of the sorghum shoot fly, Atherigona soccata Rondani (Diptera: Muscidae), by Neotrichoporoides nyemitawus Rohwer (Hymenoptera: Eulophidae) was studied in the laboratory. Ten shoot fly larvae of each instar (3) and two periods of exposure (24, 48 h) were used in a factorial design with four replicates. Significant differences of parasitism were observed with respect to instars, periods of exposure, and the interaction instar-period of exposure. The second larval instar was most parasitized (68.75 and 85% of parasitism after 24 and 48 h, respectively), followed by the first instar (46.25% of parasitism) exposed for 48 h to adult parasitoids. N. nyemitawus was an effective shoot fly endo-larval parasitoid. Observations on N. nyemitawus searching sorghum seedlings for shoot fly larvae are summarized.

Most germplasm lines and improved sorghum genotypes identified as resistant to shoot fly (Atherigona soccata Rond.), stem borer (Chilo partellus Swin.), midge (Contarinia sorghicola Coq.) and head bugs (Calocoris angustatus Leth.) are resistant to only one of these insects which usually will infest the same crop during the same season. At ICRISAT Center, India, a technique was developed in which entries were subjected to an array of combinations of pest infestations for identifying and separating genotypes with resistance to one or more insects.

Using this technique, 220 resistance sources and breeding lines were evaluated for multiple insect resistance. Less than 10% of shoot fly resistance sources had acceptable resistance (< 3 on a scale of 1–9) to shoot fly, but over 50% showed good resistance (scores of 1–3) to stem borer. IS 18551 and IS 2195 were the best entries with resistance to both shoot fly and stem borer. However, the majority (80–90%) of shoot fly and stem borer resistant sources were highly susceptible to midge with scores > 8. Similarly, all midge resistant sources were highly susceptible to shoot fly but less so to stem borer where seven midge lines had scores < 5. IS 22464 was the best midge line with a score of 3 for stem borer resistance. Advanced breeding lines showed a wider range of resistance to shoot fly, stem borer and midge with a higher frequency for resistance to stem borer, PS 28060−3 and PM 14388−1 were the most promising breeding lines.

In many parts of Africa, field sorghum is attacked by a number of insect pests, notable among them being the sorghum shootfly, a range of lepidopterous stem borers, midge and a complex of head bugs. Studies on yield losses caused by these insects are scanty, yet such information is essential in formulating an IPM strategy for the crop. This study, therefore, describes the various methods of assessing and quantifying the on-farm (pre-harvest) yield losses caused by the above insect pests.

This paper reports the results of extensive pest surveys conducted on farmers' fields and pest population studies at two national research stations between 1979 and 1981 in Burkina Faso. Major findings indicate that, under normal planting conditions, infestation of the sorghum shootfly, Atherigona seccata is very low in farmers' fields. This pest appears to be restricted to research stations. Stem borer infestation is caused by a complex of species, and species predominance varies with rainfall. South of latitude 12°N, Busseola fusca is the predominant species but further north it is replaced by Acigona ignefusalis, which is a primary pest of pearl millet. Panicle damage caused by the sorghum midge, Contarinia sorghicola, varies with season; is usually very low in the dry northern Sahelian zone but severe south of latitude 13°N. Maximum midge populations occurred between mid-September and early October.

Sorghum shootfly, Atherigona soccata, oviposition and infestation on eight resistant sorghum cultivars (IS nos: 2122,2123, 2205, 2291, 4660, 5092, 5480 and 18551) and two susceptible cultivars (Serena and CSH-1), were studied under field conditions (natural infestation). The eight resistant cultivars and CSH-1 were also tested for preference for oviposition in a two-choice situation in the screenhouse. The eight resistant cultivars received considerably less shootfly eggs and showed fewer “dead hearts” than the susceptible cultivars. The results indicate that non-preference for oviposition is an important mechanism for resistance to shootfly in eight test cultivars. There was also evidence for a certain degree of antibiosis in IS 2291.

Temperature-time and temperature–velocity equations are given for egg, larval and pupal stages of the sorghum shootfly, Atherigona soccata. Preimaginal development (from egg deposition to adult emergence) on sorghum CSH-1 seedlings lasted 19.5, 26.3, 33.8 and 76.8 days at 30, 25, 20 and 15°C, respectively. The sorghum shootfly showed optimal development and survival at 30°C. Lower threshold temperatures were 13.8, 10.2 and 11.8°C for the egg, larva and pupa, respectively. Host plant condition affected larval development and survival; pupal weight was higher and larval mortality lower in insects reared on fertilized and normally watered seedlings than on unfertilized and water stressed seedlings.

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.

The development and morphology of the immature stages were examined by means of dissections, linear measurements, light and scanning electron microscopy of the egg, the larval instars and the puparium.

The measurements were of body lengths and widths and the lengths of the cephalopharyngeal skeleton. Observations on the morphology of the immature stages concentrated on the surface structure of the egg, the head capsule, the anterior and posterior spiracles, the ventral plate and the cephalopharyngeal skeleton of the different larval instars.

Development and survival of larvae at different growth stages of the sorghum plant were investigated in the laboratory. Survival and rate of larval development were dependent on the size of the host plant. Survival was usually highest when plants about 2 weeks old were infected, low in very young plants and lowest in plants more than 50 days old. Within groups of insects hatched from eggs laid on the same day, the occurrence of various larval instars overlapped. The first instar was found from the 2nd to the 8th day after egg-laying, the second instar from the 4th to the 12th day, and the third instar from the 5th to the 18th day. Pupation started on the 14th day and adults started to emerge on the 21st day.

As an important measure for increasing grain production, hybrid sorghum has been cultivated in the whole area of the Peoples Republic of China since 1970. Over the past 10 years the sorghum shootfly has gradually become the worst pest of sorghum in several places in the southern parts of China. In 1973, some regions of Gung Dong, Gui Chou and Gung Xi Provinces were affected by the shootfly with a rate of 30% for the heavier damage, and a few even exceeding 90%.

Field and laboratory observations revealed that the placement of eggs of the sorghum shootfly, Atherigona soccata, among sorghum stems tended to be random or slightly aggregated rather than regular, which suggests that the site of oviposition by a female is little or not determined by the presence of other eggs already laid. The possible effects of this type of distribution on the mortality of the first-instar larvae are briefly discussed.

Sorghum (Sorghum bicolor) is an important food crop, especially of the semi-arid areas of Asia and Africa. One of the major constraints affecting the popularity of high-yielding hybrids and varieties is their extremely high susceptibility to insect pests.

Painter (1951) has reviewed in detail the work done on resistance in sorghum to the chinch bug (Blissus leucopterus), corn leaf aphid (Aphis maidis) and sorghum midge (Contarinia sorghicola). In the last decade and a half considerable work has been done on resistance to the shootfly (Atherigona seccata), stem borer (Chilo partellus) and sorghum midge (Contarinia sorghicola). By conducting multilo-cation screenings of world germplasm under natural and artificial infestation conditions, not only have promising sources of resistance been identified but a number of them have been utilised in crossing programmes to develop high-yielding resistant cultivars. Some sources with multiple resistance have also been identified.

The mechanisms of resistance in sorghum to the major pests have been investigated. It is seen that the mechanisms are generally complex and the observed field resistance is the total reaction of several components. There are several leads available which justify further intensive work, especially on identifying naturally occurring plant products associated with antibiosis mechanism.

The assumption was made that for a complete understanding of the feeding behaviour and nutrition of Atherigona soccata larvae on various cultivars of sorghum, it was necessary to investigate the relationship between plant chemicals occurring in resistant sorghum plants and the digestive enzymes of larvae. However, at least an introduction to the anatomical organization of the alimentary canal of larvae is needed to understand the digestion and absorption of food taken from resistant and susceptible sorghum lines.

The present preliminary investigation was undertaken to determine qualitatively some of the enzymes present in the gut and salivary glands of the third instar by locating enzymes histochemically on polyacrylamide gels.

Surveys were conducted in two different ecological zones (semi-arid and arid) of Kenya to monitor egg, larval and pupal populations of the sorghum shootfly, Atherigona seccata, on a wild host, Sorghum arundinaceum. Populations were found to be usually higher on wild sorghum than on local varieties of Sorghum bicolor, the cultivated host. During dry periods, shootfly eggs and larvae were still found on wild sorghum, especially in moist areas such as beds of temporary streams or river banks. Population levels seem to be related primarily with the availability of susceptible stems, which in itself is determined by rainfall, soil conditions, density of other vegetation and by the phenology and the distribution of the host plant, Sorghum arundinaceum, being a pioneer species which colonizes recently disturbed areas and is subsequently replaced by other grass species. Since no evidence of the existence of an aestivation diapause has been found, it is reasonable to assume that Sorghum arundinaceum is a major reservoir for A. seccata, especially during the dry season.

Movement, feeding behaviour and growth of larvae of the sorghum shootfly, Atheriyona saccata, were studied under controlled conditions on a susceptible sorghum variety, CSH-1. The time of hatching coincided with the presence of moisture on the leaf, a condition favourable to movement of the larvae to the base of the leaf. Cutting of the central shoot at its base by the larva caused the symptom referred to as ‘dead heart’. The three larval instars are described and changes in the cephalopharyngeal skeleton discussed.

Movement of the larva to the base is not due to positive geotaxis. Larvae in plants placed horizontally to the ground did not develop normally and a high mortality resulted. The larvae are strongly cannibalistic inside the host-plant but not outside of it. The first-instar larvae do not migrate from one plant to another, but the third instars may do so if the source of food is exhausted.