Chilo partellus
(spotted stem borer)

C. partellus is native of Asia, but became established in East Africa in the 1950s. Since then it has spread to southern and central Africa and there has been an unconfirmed report from Benin (Inter-African Phytosanitary Council, 1985).

C. partellus is an important pest of cultivated cereals, especially maize, sorghum and pearl millet (Pennisetum glaucum). It has also been recorded from rice, foxtail millet (Setaria italica) and finger millet (Eleusine coracana) and from many grass hosts, including Sorghum halepense, Sorghum verticilliflorum, Panicum maximum, Pennisetum purpureum, Hyparrhenia rufa, Vossia cuspidata and Rottboellia compressa.

Many accounts of the biology and ecology of C. partellus have been published. Harris (1990) reviewed recent literature, noting that many research papers relate to work in India, especially at Indian Agricultural Research Institutes and universities, but that there had been a recent increase in the number of papers published from Africa, mostly resulting from work at ICIPE on this pest. Useful summaries of biological, ecological and other information include Nye (1960), Grist and Lever (1969), Siddig (1977), Appert and Deuse (1982), Hill (1983), Harris (1985, 1989), Rensburg (1987) and Polaszek (1998) which includes coloured photographs of live adults and larvae.

In Kenya, work at ICIPE has focused on larval movements (Berger, 1992); dispersal of early instars (Pats and Ekbom, 1992); flight periodicity (Pats and Wiktelius, 1992); and on pheromones (Lwande et al., 1993). In South Africa, work has been carried out at the Plant Protection Institute, Pretoria on duration of diapause (Kfir, 1991a); on seasonal abundance and parasitoids (Kfir, 1992); and on diapause termination (Kfir, 1993); studies have also been made at various locations in India.

Adults emerge from pupae in the late afternoon or early evening and are active at night. They rest on plants and plant debris during the day and are seldom seen, unless disturbed. Females release a pheromone to attract males (Nesbitt et al., 1979; Lwande et al., 1993) and mate soon after emergence. On two or three nights after emergence, each female lays up to a total of 200-600 scale-like overlapping eggs in 10-80 separate batches on the undersides of leaves, mostly near mid-ribs. Larvae hatch after 4-8 days, usually between 06.00 and 08.00 h, and initially feed in the leaf whorl. Subsequent larval instars tunnel into stems, eating out extensive galleries. Larval development is usually completed in 2-4 weeks. Larvae pupate in damaged stems and adults emerge 5-12 days later. The life cycle is completed in 25-50 days when climate and growing conditions are favourable. Five or more successive generations may develop in areas with suitable climates, such as southern India, but in less favourable areas larvae pass the winter or dry season in diapause in stems and stubble. They may remain inactive for up to six months, before pupating and completing their development early in the following growing season.

Biological and ecological studies that are of particular importance in understanding the factors that determine the pest status of C. partellus include Chapman et al. (1983) and Bernays et al. (1985) on survival and dispersal of first- and second-instar larvae on young sorghum plants; Woodhead and Taneja (1987) on larval behaviour and host-plant resistance in sorghum; Ampofo and Kidiavai (1987) on larval movements in maize; Alghali and Saxena (1988) on larval movement, feeding and development in sorghum; Rensburg et al. (1988) on larval infestations in maize; Alghali (1988) on oviposition in sorghum; Kumar (1988) on ovipositional responses in maize; and Sharma and Sharma (1987) on maize floral initiation and infestation.

Records of natural enemies of C. partellus are scattered in the stem borer literature and there have been few attempts to collate and authenticate them. Mohyuddin and Greathead (1970) published an annotated list of parasites of stem borers in East Africa, and Smith et al. (1993) published an illustrated account of the parasites of lepidopteran stem borers of tropical gramineous plants that includes C. partellus. The main parasitoids mentioned are Hymenoptera (Braconidae, Bethylidae, Chalcididae, Elasmidae, Eulophidae, Ichneumonidae, Scelionidae and Trichogrammatidae) and Diptera (Tachinidae). Smith et al. (1993) and Polaszek (1998) provide details of the biology of the main genera and species, together with identification keys.

The importance of predators has not been fully assessed. Mohyuddin and Greathead (1970) noted the importance of ants as predators on eggs, larvae and pupae.

Field infestations of C. partellus are detected by walking through young crops looking for characteristic holing of funnel leaves, the presence of dead hearts and holes in tunnelled stems. Samples of affected stems are then removed and dissected to retrieve larvae and pupae, from which adults are reared for identification. As similar symptoms may be caused by other species of lepidopterous stem borer, by some species of Diptera (especially Chloropidae and Atherigona shoot-flies) and by some diseases, retrieval of larvae or pupae and confirmation of their identity by rearing adults for identification by a taxonomic specialist is essential to ensure a correct diagnosis.

The presence of this species in older crops and in crop residues may be detected by taking random samples of stems or stools for dissection.

In external appearance C. partellus resembles many other species of Chilo (see Bleszynski, 1970, Plates I and II), but can be distinguished from them by diagnostic characters of the male and female genitalia. According to Bleszynzski (1970), it is most closely related to Chilo tamsi, a species described from southern India.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Introduction

The main international research organizations working on the development of effective control of C. partellus are ICRISAT, in Asia and Africa, mainly for sorghum and pearl millet, and ICIPE in Kenya, mainly for maize and sorghum. Active national research programmes are also well established in India and South Africa. There seems to be little current interest in this species as a pest of rice or of any crops other than maize, sorghum and pearl millet. A review of control in Africa is included in Polaszek (1998).

The relevance of other stem borers must be considered in situations where more than one species is of importance, as on maize in East Africa and southern Africa.

Cultural Control

Good crop hygiene, including the destruction of crop residues (stems and stubbles), and removal of volunteer crop plants and/or alternative hosts reduces carryover of diapausing larvae and active populations from one growing season to the next and will help to limit the most damaging attacks on young crops early in the growing season. Manipulation of sowing dates may also be used to avoid periods of peak adult activity.

Verma and Singh (1989) reviewed cultural control measures used against sorghum stem borers, which include time of sowing, crop rotation, tillage, plant spacing, water management, fertilizer management, removal of dead hearts, management of crop residues, field sanitation, mulching and intercropping.

Kfir (1990) assessed the prospects for cultural control of C. partellus (and Busseola fusca) in summer grain crops in South Africa.

Biological Control

Biological control by conservation, augmentation or introduction of natural enemies has potential, but has not been adequately researched. Greathead (1971) reviewed biological control attempts in tropical Africa, including the introduction of parasitoids for use against C. partellus and other species of Chilo. Ingram (1983) also reviewed the situation and noted the need for further critical ecological studies to determine more precisely which exotic parasitoid species are likely to be successfully introduced. Greathead (1990) reviewed the use of natural enemies against Chilo species in Africa and indicated possible lines of further development, including enhancement of the action of indigenous natural enemies by manipulation of the environment and the development of microbial biopesticides. Kfir (1991b, 1994) reported attempted classical biological control of C. partellus (and Busseola fusca) in South Africa by the introduction of egg, larval and pupal parasitoids and concluded that, because of adverse climatic and other factors, the prospect of satisfactory biological control by the introduction of exotic natural enemies is poor.

The efficacy of the entomopathogenic fungus Beauvaria bassiana as a control agent has been assessed on maize in Kenya (Maniania, 1993); formulations of the protozoan Nosema marucae have also been evaluated (Odindo and Mbai, 1993). A new species, Nosema partelli, has been described from C. partellus in South Africa (Walters and Kfir, 1993). Four commercial preparations of Bacillus thuringiensis have also been evaluated in laboratory trials against C. partellus (Brownbridge and Onyango, 1992 a,b).

Host-Plant Resistance

Plant breeding for resistance or tolerance is generally considered to be the best long-term method of control and much work has been done to identify and use sources of resistance, especially in maize and sorghum. ICRISAT has developed screening procedures for sorghum (Leuschner, 1989), and some progress has been made in breeding for resistance (Agrawal and Taneja, 1989) and in understanding mechanisms of resistance (Taneja and Woodhead, 1989). Gethi and Mihm (1997) reviewed progress in East Africa.

Chemical Control

Chemical control is effective in some circumstances but is inappropriate in much of Africa and Asia. Kishore (1989) reviewed recent research and recommendations for chemical control of stem borers on sorghum and maize in India and South Africa and concluded that control can be achieved by applications of granules or dusts to the leaf whorl early in crop growth to kill early larval instars. This method reduces pollution and limits harmful effects on non-target organisms.

Rensburg et al. (1991) reported on progress towards cost-effective control of C. partellus and other pests on maize in South Africa.

Integrated Pest Management

The development of IPM programmes against C. partellus is complicated by the fact that it is only one of a number of species of lepidopterous stem borers that attack its main crop hosts. Biological and ecological differences between these species affect their relative importance on different crops and at different localities. IPM programmes must therefore be developed at local levels to meet local needs. There seem to be few cases where this has actually been achieved.

Pheromonal Control

The components of the female pheromone have been identified (Nesbitt et al., 1979). The possible uses of pheromones to monitor and control stem borers were reviewed by Campion and Nesbitt (1983). They concluded that control by mass trapping is unlikely to be effective, but that control by mating disruption had shown encouraging results and might be used effectively on rice, maize and sugarcane grown under plantation conditions.

Phytosanitary Measures

No formal phytosanitary measures have been developed against C. partellus and its introduction to Africa from Asia, possibly by introduction of larvae or pupae in infested cereal or grass stems, pre-dates the establishment of phytosanitary services in Africa. Prevention of further spread within Africa might be attempted by controlling the movement of larvae and pupae in stems but, although the species is not known to be a migrant, it seems likely that adults may at times disperse over long distances, and it is impossible to prevent such movements.