Several species of Chilo occur on sugar-cane, out of which five have major pest status. Infestation by these pests causes losses in cane yield and sugar recovery. There are constraints involved in the use of cultural, mechanical and chemical control measures on a large scale, but the nature of the crop facilitates adoption of biological control. Many species of natural enemies occur in the sugar-cane agro-ecosystem, which is relatively less disturbed. They play a significant role in maintaining the pest populations at reasonably low level.

In biological control, the egg parasite, Trichogramma spp., has been widely used with varying results. Among the larval parasites, Apanteles flavipes (Cameron) and Sturmiopsis inferens Townsend have been used on a limited scale and need further testing in other countries. The exotic tachinids introduced, failed to establish against Chilo spp. Not much work has been done on the predators, though the native predators play a significant role in reducing the pest densities. Among the entomopathogens, granulosis virus infecting Chilo infuscatellus Snellen has shown promise in the control of this pest and needs further evaluation in other agro-climatic regions. Future thrust for biological control of Chilo spp., in sugar-cane has also been indicated.

Knowledge on the natural enemies of graminaceous stem borers is outlined, with special reference to mortality of Chilo spp. as crop pests in Africa. The quantitative scanty information available suggests that over 90% mortality of eggs and larvae takes place before the larvae become established as borers in the crop and that mortality of the older larvae and pupae is much lower. Possible manipulations of natural enemies to increase mortality further are reviewed. In view of the largely negative results of past classical biological control introductions the prospects do not appear very promising. However, the results do indicate some possibilities which should be investigated, notably trials with Apanteles spp. (Braconidae) and intra-continental redistribution of important species with restricted distributions. For short-term biological control, development of microbial biopesticides seems a more promising strategy than augmentation of insect parasitoids by breeding and release, particularly on small farms. A third possibility, enhancement of the action of native natural enemies by environmental manipulation, merits more attention as part of an integrated crop production approach to the improvement of the quantity and reliability of yields for the small farmer.

Host preference studies were conducted on Trichogramma sp. nr. mwanzai at 25 ± 2°C, 40–60% r. h. In the laboratory, using eggs of C. partellus, B. fusca, E. saccharina, S. cerealella and B. mori in choice and no-choice tests. Adults emerged within 9–10 days; ca. 70% parasitoids of both sexes emerged and mated between 0800–1000 hr, with a peak at 0800 hr. There was no significant difference (P < 0.05) between the number of progeny per female from eggs of C. partellus (33.9 ± 9.2), B. fusca (30.1 ± 4.5) and S. cerealella (30.9 ± 9.9) in no-choice tests, but progeny production was significantly less from E. saccharina and no parasitoid emerged from B. mori. In host age selection tests on C. partellus, there was no significant difference (P < 0.05) between the number of offspring per female from eggs of age groups 0–2 days; offspring of day 3 were significantly less, and no parasitoid emerged from day 4 eggs. Up to 4 (mean, 1.9 ± 0.6) adults emerged from a single egg of C. partellus. Since mass rearing technology exists for C. partellus and is available at ICIPE, it was concluded that day 0–2 eggs of this borer are more suitable for mass production of T. sp. nr. mwanzai.

Studies were carried out at Mbita Point Field Station, South Nyanza, western Kenya from 1984 to 1987 to determine the role of Dentichasmias busseolae Heinrich in the population dynamics of Chilo partellus Swinhoe and its potential as a biological control agent against the stem borer. D. busseolae is a solitary endoparasitoid which parasitizes and completes its development in the pupa of C. partellus. The developmental period ranged from 64.5 ± 9.5 days at 15°C to 12.0 ± 1.5 days at 30°C for males, and from 66.7 ± 9.8 days to 13.1 ± 2.1 days for females, respectively.

The intrinsic rate of natural increase of the parasitoid was 0.1375, and the population multiplied 24.21 times in the mean generation time of 28 days. The functional response was curvilinear.

Field parasitism in maize and sorghum was evident 7 weeks after plant emergence (APE). Thereafter, parasitism was persistent until crop harvest. The range of parasitism was 0 to 45.5% (x = 23.3%) on maize and 9.7 to 90% ( X = 36.0%) on sorghum respectively. Using laboratory and field observations, a model for the host searching process of D. busseolae has been constructed. The potential of the parasitoid in biological control programmes of C. partellus is also discussed.

Laboratory and field assays carried out so far indicate that microsporidian pathogens, especially Nosema spp., may be exploited for the management of the spotted stalk borer Chilo partellus and possibly other Chilo spp. Several formulations have been tested for the application of Nosema spores in the field for borer management. Significantly high levels of control were achieved when the pathogen was sprayed on plants infested with borer larvae, compared with unsprayed plots. Sublethal dosages of Nosema spp. reduced the biological performance of C. partellus, as indicated by reduced fecundity in infected females, high mortality in pre-imagines as well as in the filial generations, and reduced lifespan. There was also a high level of deformity in the surviving adults. Such debilitating factors would subsequently result in a reduction of population levels.

In sugar-cane pests, granulosis viruses have been observed to cause high natural mortalities in C. infuscatellus and C. sacchariphagus. Field infestation has been reduced by the application of the virus. Further, the presence of polyhedrosis viruses in C. partellus, and Chilo iridiscent virus in C. suppressalis, points towards the possibility of using this group of pathogens in Chilo control.

Most field experiments in the microbial control of Chilo spp. have been performed using strains of Bacillus thuringiensis (B. t.). Generally, low levels of control were obtained, which is probably due to the relatively narrow host range of the B. t. strains used. B. t. formulations and different B. t. strains obtained from overseas culture collections or isolated from material collected in Kenya were screened for activity against C. partellus.

C. partellus was found to be susceptible to commercial preparations based on B. t. kurstaki only. The screening and bioassay work on identified B. t. strains showed differences in the toxicities of the various subspecies groups for C. partellus. There were also differences in the toxicity of isolates within the same subspecies. The most toxic exotic strain obtained was B. t. entomocidus. Excellent control of C. partellus larvae, on artificially infested plants, was obtained. Plant damage was significantly reduced and yield levels increased by 5 to 7 times over the untreated controls. Three strains, M-44–2, A-3, and A-C-2, gave the best levels of plant protection.

Different methods of applying the pathogen to the crop have also been investigated. Aqueous suspensions and a granule/powder formulation have given very good levels of borer control in screenhouse experiments.

Female sex pheromone components have been identified for seven species of Chilo, pests of cereals and sugar-cane. Synthetic pheromone blends for C. suppressalis, C. sacchariphagus, C. sacchariphagus indicus, C. auricilius and C. zacconius have shown satisfactory attractiveness to male moths in the field. Blends for C. partellus and C. infuscatellus seem to be incomplete. Pheromones and related compounds have been investigated for control of C. suppressalis by mating disruption in the Philippines, Japan, Spain and Korea. These studies have shown not only trap catch reduction, but also reduced viable egg production in field cages and effective control of damage by this pest on areas up to 60 ha. Initial studies have been carried out on control of C. sacchariphagus by mating disruption in Mauritius, and both the pheromone and an inhibitor have been shown to cause trap catch reduction of C. auricilius in India. Attempts have been made to control C. sacchariphagus indicus by mass trapping in India.

The flight phenology of the stem borer, Chilo partellus (Swinhoe) at the ICIPE's Mbita Point Field Station (MPFS) and on a nearby farmer's field was monitored using pheromone traps. The relation between pheromone trap catches of males and sorghum plant age, infestation, larval/pupal population density and sorghum cultivars used was examined. At MPFS C. partellus males were caught throughout the year in pheromone traps. Fluctuations of trap catches indicated that the pest completes two main generations on each of the long and short rain sorghum crops. On farmers' fields, males continued to be caught in large numbers after harvest, implying that the pest continues to breed on the sorghum stalks and tillers. Significantly lower observed catches of males occurred only on one (MB-3) of the eight different sorghum cultivars, during the period of 2–6 weeks after plant emergence. In general, positive correlations were observed between the number of males caught in pheromone traps and larval pupal population density, males and percentage plants infested, and males and plant age. However, the pheromone trap appeared to be less effective under high population density, particularly during the flight of the second generation moths, due to competition between pheromone trap and wild females. But the flight phenology as monitored by pheromone traps appeared to reflect major events in population development of C. partellus. Pheromone traps would be useful in detecting C. partellus and for monitoring its flight phenology.

A binary mixture of (Z)-ll-hexadecenal and (Z)-13-octadecenal was first identified as the sex pheromone components of female Chilo suppressalis. Later, an additional component was identified as (Z)-9-hexadecenal, which markedly enhanced the attractiveness to males when it was mixed to the binary mixture. Rubber septa containing ca. 0.3–1.0 mg of a mixture of the above three components in the natural ratio have been shown to keep up significantly higher attractiveness than virgin females for at least one month. It has also been shown that the pheromone trap is much more effective than a light trap, where both traps usually showed similar trap catch patterns. Several factors affecting trap catch, such as trap type, trap placement, etc., have been examined. Feasibility of the population monitoring with the synthetic pheromone was discussed.

Chilo species constitute one of the three major lepidopterous rice stem borers in Africa and their occurrence stretches across the different rice ecosystems; two major species, C. zacconius Bles. and C. diffusilineus (J. de Joannis) are found in West Africa, while C. partellus (Swinh.) and C. agamemnon Bles. appear to be the predominant species on rice in East and North Africa, respectively. A brief note is given on some aspects of the biology, especially the life history and behaviour of C. zacconius. Control measures are discussed as well as an integrated approach towards management based essentially on growing resistant varieties, conservation of natural enemies and adjusting planting dates.

Chilo spp. are among the most important stem borers of sugar-cane in the Old World. Chemical control is not very effective mainly because of the concealed habit of the borer larvae. Efforts have therefore mostly been directed to alternative methods of control such as cultural practices, varietal resistance, biological control and the use of sex pheromones. Varieties are not normally bred for borer resistance but very susceptible ones tend to eliminate themselves at the early stages of selection. Susceptibility is normally assessed on promising and released varieties. Classical introduction of exotic parasites has more or less reached a saturation point and biological control tends to be oriented towards the possible use of new or cross-bred strains. The importance of entomopathogens as a control component against Chilo spp. needs further studies. A great potential exists in the use of sex pheromones mainly as a means to disrupt mating, as shown by the promising results obtained through trials carried out in various countries. It is very important that the various methods of pest control available be used in combination with one another, taking into account the natural limiting factors of the environment. This would be an important contributing factor towards an objective of optimum yield at minimum cost in the management of Chilo spp. on sugarcane.

The stem borer Chilo spp. is a significant limiting factor of tropical cereal production. Consequently, international co-operation in research, education and training and resource sharing, including information exchange is essential for the Chilo problem to be minimized. Networking is likely to be the most beneficial mechanism for co-operation, for it has proved effective in bringing together faculties and establishing fora for exchange of experiences. The sustenance of co-operation and strategic devolution of appropriate responsibilities to national institutions are key considerations for enlisting national support and continued international co-operation.