Studies were conducted at the ICRISAT Sahelian Centre, Niger, to assess damage and yield loss by the millet stemborer, Coniesta ignefusalis (Lepidoptera: Pyralidae) on Pennisetum glaucum (L.) R. Brown. Although 78% of stems were infested, late attack by the millet stemborer on millet plants resulted in bored stems yielding more than unbored ones. Artificial infestations with 5 and 10 larvae per plant at 2 weeks after plant emergence (WAE) resulted in 50 to 70% plants with deadhearts and 24 to 100% avoidable yield loss. At 4 weeks after plant emergence, infestations with 5 larvae and 10 larvae per plant resulted in 7% increase in yield and 16% yield loss, respectively. Therefore, both the age of plant and density of borer population at the time of infestation can influence resulting stemborer damage and yield loss on pearl millet.

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.

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.

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.

Patterns of stemborer life history and the foraging patterns exhibited by a taxonomically diverse array of stemborer parasites are described. Foraging strategies provide functional descriptions of natural parasite groupings and include elements of the guild concept plus ecological, behavioural, and morphological adaptations that enable a parasite to locate and exploit a particular host effectively. Elements of the foraging strategy include: host stage attacked, microhabitat of the host, cues employed to locate and identify the host, and method of host attack. Possible constraints presented by the crop plants to the success of different foraging strategies exhibited by groups of parasites are explored.

Chilo partellus Swinhoe (Lepidoptera: Pyralidae) is an exotic stemborer of cereal crops in Africa which invaded the continent from Asia earlier this century. In addition to Ch. partellus, several indigenous stemborers are found in Africa. In 1991, Cotesia flavipes Cameron (Hymenoptera: Braconidae), a gregarious endoparasitoid of stemborers in Asia, was introduced into Kenya for biological control of Ch. partellus. Laboratory studies revealed that the parasitoid could successfully parasitise not only the target stemborer, but also two native stemborers that occur sympatrically with Ch. partellus in some locations. Interbreeding studies demonstrated the Co. flavipes would mate with a native congener, Cotesia sesamiae (Cameron), but no female offspring resulted from these matings. Investigations on competition between the two Cotesia spp. indicated that when Ch. partellus was the host, Co. flavipes appeared to be a superior parasitoid. Releases of Co. flavipes were made in 1993 in Kenya. Recoveries in 1994,1995 and 1996 demonstrated that the parasitoid was firmly established in two regions of Kenya and in northern Tanzania. Preliminary observations suggest that the parasitoid is causing greater mortality to stemborers in southwestern Kenya than in the coastal area.

The stemborers Busseola fusca (Fuller) (Lepidoptera: Noctuidae) and Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), are the most important pests of maize and grain sorghum in South Africa. Parasitoids, pathogens and predators curtailed populations of B. fusca and C. partellus on the crops but their activity was not enough to reduce pest populations to below-economic-damage levels. They also could not prevent the dispersal and subsequent widespread distribution of C. partellus after its introduction into South Africa. Nine parasitoids and hyperparasitoids have been recorded attacking both borer species. These parasitoids are probably more habitat-specific with regard to foraging for hosts than they are host-specific, as they appear to have moved from their indigenous hosts to use C. partellus as a new alternative host. Between 1977 and 1993 three braconids, one eulophid, three ichneumonids, two trichogrammatids and four tachinids were introduced into South Africa from 11 countries for biological control of stemborers. Recoveries of parasitoids were made only short periods after releases and in the vicinity of the release sites. Cold and dry winters, with night temperatures often falling to below freezing, prevail in the Highveld Region which is the major summer-grain-producing area of South Africa. These conditions, and the fact that borers enter diapause in the dry stalks for 7 months in winter, when the crops or other alternative host plants such as grasses do not grow, hamper the establishment of exotic natural enemies in the Highveld Region.

Progeny production and sex ratios of the parasitoids Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) and its exotic congener Cotesia flavipes Cameron (Hymenoptera: Braconidae) emerging from three species of field-collected stemborers were compared. There were no significant differences in numbers of progeny of the two parasitoids from the African stemborers, Chilo orichalcociliellus (Strand) (Lepidoptera: Pyralidae) and Sesamia calamistis Hampson (Lepidoptera: Noctuidae). Cotesia flavipes produced more progeny than C. sesamiae on large larvae of its coevolved Asian host, Chilo partellus (Swinhoe). No significant differences in the number of parasitoid progeny were observed for different-sized Ch. orichalcociliellus or S. calamistis larvae. These data suggest that under field conditions, Co. flavipes may be superior to Co. sesamiae as a parasitoid of its co-evolved host, Ch. partellus.

One tactic of importation of natural enemies for biological control of stemborers and other pests is the use of parasite species that have had no previous association with the pest. This ‘novel association’ approach offers the possibility of using parasites that have proven effective against a stemborer species other than the target stemborer pest. Nevertheless, creation of a novel host-parasite association requires that the novel host be ecologically and physiologically equivalent to the ancestral host. Selecting natural enemies based on ecological and physiological equivalence offers the best possibility of temporal, behavioural, nutritional and immunological compatibility in the host-parasite interaction, and reduces the chances of failure with the imported parasite species.

Twenty-seven species of Chilo which infest graminaceous crops are listed. The effects of various cultural practices such as time of planting, plant density, field sanitation, tillage, mulching, intercropping, proper fertilizer and water management on field populations of Chilo spp. are discussed. These practices have great potential as components of IPM for resource-limited farmers in the tropics.