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.

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.

Effects of temperature on female longevity, fecundity and fertility of the sorghum shootfly, Atherigona seccata Rondani, were investigated. The longevity, preoviposition and oviposition periods, eggs per female per day, mean eggs per female and fertility (per cent eggs hatched), for the shootflies maintained at constant temperatures of 20, 25 and 30°C and alternating temperatures of 30°C (photophase) and 20°C (scotophase) referred to as 20–30°C were determined. At lower temperature (20°C), the longevity, preoviposition and oviposition periods were prolonged considerably, while daily egg production, mean fecundity and fertility were decreased. Higher temperature (30°C) had the reverse effect on preoviposition and oviposition periods as well as daily egg production. There was a significant positive correlation between the pupal weight and the number of eggs laid by the fly at 20°C. Egg mortality increased with advance in maternal age. The rate of increase in egg mortality was faster at 30°C than at lower temperatures.

The paper stresses the fact that an integrated pest management system is not a new concept leading to a more stable agroecosystem, but was practiced by the farmers for centuries; host-plant resistance is one of its major components. The system was destabilized by the introduction of new sorghum varieties and hybrids not resistant against the major sorghum insect pests like shootfly Atherigona soccata (Rondani); stem borer, Chilo partellus (Swinhoe); midge, Contarinia sorghicola (Coquillet); and headbugs, Calocoris angustatus (Lethiery). In order to balance the agroecosystem again, an integrated insect pest management system for sorghum has to be developed based on traditional pest management practices. These are host-plant resistance, cultural control and biological control. Insecticides should be used only if absolutely necessary. For a better understanding of such a control approach, a summary of the biology and population dynamics of the major insects is given, together with a brief account on the levels and mechanisms of host-plant resistance so far known. Based on this information, the rainfall pattern, plant duration to maturity, time of planting, natural enemies and insecticides, a sorghum based integrated pest management system is proposed for the monsoon and post-monsoon season in which host-plant resistance alone or in combination with the above mentioned control methods could be used.

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.

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.

This paper includes a description of the internal reproductive system of the female sorghum shootfly, Atherigona saccata, as well as studies on egg maturation and survival, and effects of mating and/or presence of the male on egg maturation, fecundity and fertility. The number of ovarioles per female ranged from 20 to 47. A. seccata exhibits autogeny, but carbohydrate is essential for egg production and survival. Nutritional deficiency led to a high incidence of oocyte resorption. A mixture of baker's yeast and sugar was a better adult food than brewer's yeast and glucose. Sorghum (cereal) aphid honeydew was a nutritious adult food. Shootflies provided with sorghum aphid honeydew showed an acceleration of vitellogenesis, a decrease in the frequency of oocyte resorption and a shortening of the preoviposition period. Egg maturation does not seem to be influenced directly by mating or by the presence of males. The continuous presence of the male also has no significant effect on fecundity. Shootfly females seem to mate only once and this single mating ensures fertilization of eggs laid even long after mating.

The sorghum shootfly, Atherigona seccata Rondani, is the main species of Atherigona attacking cultivated and wild species of Sorghum in Africa and Asia. The past 55 years' research on the pest has produced means for its management. The bionomics of A. soccata are well known and are being applied for its cultural control by adjusting planting dates. Natural enemies of the shootfly are few and their biology little known. The systemic insecticide carbofuran as a seed and seed furrow treatment provides effective fly control where the cost-benefit is favorable. Sources of host plant resistance to shootfly have been recognized for about 35 years. Incorporation of this resistance in high yielding varieties through plant breeding should be more rapid now that the mechanisms of resistance and their inheritance, through additive gene action, are better understood. These resistance factors include non-preference for oviposition, antibiosis and recovery through tillering.

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.

From field screening of two international nurseries of the sorghum shootfly (Atheriaona saccata), seven promising cultivars were selected for investigating the mechanism(s) of resistance. These cultivars were IS 1054, IS 1082, IS 2146, IS 2195, IS 2312, IS 3962 and IS 5613. CSH-1, a hybrid variety from India, was used as a susceptible control. Results from observations on larval development and mortality and the longevity and fecundity of resulting females when released on either CSH-1 or the parent cultivars are reported.

Mortality among the first-instar larvae was very high on IS 2146 followed by IS 2312 and IS 5613. IS 2146, IS 3962 and IS 2312 sustained the least growth of larvae. The larvae in these cultivars were often confined to the upper region of the central shoot.

The survival rate of flies from IS 2146 was very low, and the longevity was significantly reduced in the case of flies from IS 2146 and IS 2195. The preoviposition period was significantly longer when flies were released on IS 1082 and IS 2312. Fecundity of the flies did not differ significantly when released on CSH-1; however, it was significantly reduced when released on parent cultivars in all cases except IS 1054. It is concluded that IS 2146 possesses strong antibiosis for the sorghum shootfly followed by IS 2312 and IS 5613.

The use of fish meal and detergent water has been shown to be simple, economic and reliable for sampling of shootfly populations. Various mixtures of fish meal with other ingredients were evaluated. The most potent mixture was fish meal-yeast-ammonium sulphide, but generally fish meal alone was adequate and the most readily available attractant. The attractance of fish meal was found to be related to its degradation; it increased from the first to the third or fourth day, and then decreased. The chemicals responsible for the attractance of fish meal have yet to be identified, but in all probability they may be the amines resulting from biodegradation of the meal.