Management Approaches:
Host Plants

Wild Hosts

Most African cereal stem borers are generally polyphagous and have several graminaceous and other wild hosts, in addition to more than one cultivated crop.  Several non-cultivated wild host plants of these stem borers have been recorded and documented by various workers.  As might be expected, the spectrum of wild, non-crop hosts attacked by stem borers is vast.  No doubt, the number of actual wild hosts is even greater, given that stem-borer damage to wild grasses is unlikely to be investigated.  Hosts are found in three main families - Cyperaceae, Gramineae (Poaceae) and Typhaceae.  An extensive survey carried out by the International Centre of Insect Physiology and Ecology (ICIPE) scientists during 1994-1997, were every possible kind of host plant was examined at different times of the year in various agroecologies throughout Kenya, resulted in the compilation of Table 2.  No plant was given host-plant status until any larva found in it had been reared through to the adult stage.

In the past, many host-range records did not take into account how the plant-insect association was determined.  In nature, an insect locates a host plant through a sequence of behavioural responses.  In the laboratory, feeding a larva in a closed environment ignores the natural feeding process and only measures the insect's acceptance of the plant as food.  Also, increasing evidence indicates that the host ranges and economic importance of most insects are dynamic, and often location- and time-specific.  For example, B. fusca, C. orichalcocilieus, E. saccharina and S. calamistis, the major African stem borers, originally would have attacked only wild graminaceous plants in tropical and subtropical parts of the continent.  As long as there were only wild plants, these insects were of no special consequence.  After the introduction and cultivation of maize and extensive planting of sorghum in vast areas of Africa where they did not occur originally, the insects followed the cultivated forms of their host plants and became more widely distributed and of economic importance.  In Africa, where the bulk of maize and sorghum is grown only on small plots surrounded by land occupied by wild graminaceous plants, these stem borers, which are normally associated with grasses, infest the cultivated crops in high populations, due to the presence of wild grass hosts in the immediate neighbourhood.  On the other hand, C. partellus, a species indigenous to Asia which has become established in East and Southern Africa, is also polyphagous and attacks several wild host plants.

Some authors have argued that the presence of alternative borer hosts in cereal-growing areas is detrimental in serving as a stem-borer reservoir, whereas others have pointed out that natural enemies can persist and increase their populations during the non-growing season, thanks to these alternative borer hosts.  For example, Napier grass, Pennisetum purpureum, which is an important pasture grass, is often encountered as a host of stem borers, and may be important in some areas as a reservoir host of stem-borer species.  Several wild sedges (Cyperus spp.) are known to be the natural hosts of E. saccharina.  Table 2 summarizes recent surveys, by the second author, of infestation of wild graminaceous hosts by various borers species in different agroecological zones in Kenya.  These data are far from complete at this time of writing, and, in particular, the presence of non-economic species of Busseola and Sesamia, provisionally identified as B. fusca and S. calamistis, need to be investigated.  In addition, several Chilo species, apart from C. partellus, are no doubt present, but have not yet been identified to species level.  However, the occurrence of borers, Phargmataecia boisduvalli (Herrich-Shaeffer) (Cossidae), Bactra stagnicolana Zellet (Tortricidae) and Poenoma serrata (Hampson) (Noctuidae) on some of these wild hosts, the first two never found on cereals, is of interest.  Of particular interest is the fact that some of these borers can and do act as reservoir hosts for natural enemies of economically important borers; for example, the braconid wasp Stenobracon rufus attacks P. biosduvalii.

There are plenty of examples illustrating the importance of uncultivated land as a source of pests for adjacent crops.  However, there is not now, nor does it seem there could be, a general theory that will predict the role of uncultivated land in insect invasions and outbreaks in cultivated crops.  The overall value of uncultivated land will have to be judged on a case-by-case basis, depending on the ecological mechanisms underlying the various disadvantages and advantages.  In cultivated crops, most of which are short-lived in comparison with natural communities, damage is typically caused by herbivores that migrate into fields from the outside.  These wild habitats often harbour food sources for many pest-insect species, and they may encourage insect invasion and outbreaks in neighbouring agroecosystems.  It is also possible that the presence of wild crop relatives in nearby habitats could result in the maintenance of genetic diversity in local, relatively non-vagile insect populations, leading to the emergence of new strains or biotypes that could overcome host-plant resistance.

On the other hand, wild hosts adjacent to cultivated crops can provide extremely important refugia for natural enemies, as well as sources of nectar, pollen and host/alternative prey.  There are also several examples of adjacent wild habitats being used to suppress insect outbreaks by keeping the pest on the so-called wild trap plants.  Furthermore, non-host plants intercropped with cereals have also been shown to have a significant effect on stem-borer damage.  In trials at ICIPE, molasses grass (Melinis minutiflora) intercropped with maize or sorghum significantly decreased levels of infestation by stem borers and also increased larval parasitism of stem borers by the braconid, Cotesia sesamiae (Khan et al., 1997)

During the off-season, when there are no cultivated crops in the field, in addition to the hibernating or diapausing populations in crop residues, stem borers remain present on wild host plants and infest the cultivated hosts after they are planted.  Although some basic information on the wild habitats as hosts of cereal stem borers in Africa is available, from the applied perpective, a systematic study of the dynamic relationship between the populations of insects in wild host plants and those on cultivated crops is important for the development of a sustainable integrated pest management (IPM) approach for cereal stem borers.  A more complete understanding of the role of such wild hosts in insect outbreaks will generate suitable management strategies for stem borers.  Appropriate methods of detecting migrants from wild habitats to cultivated crops and the information on the precise role of wild habits in stem-borer invasion of cereal crops are needed.  We also need to update and advance our knowledge on the phenomenon of cereal stem-borer outbreaks in Africa.  By discovering what accoujnts for differencesbetween natural systems and agroecosystems, we can learn much about the underlying ecological processes that create the observed patterns of distribution and abundance of stem borers in nature, and about those chemical features that protect wild plants from herbivores.  Plant breeders and entomologists can use this information in developing resistant/tolerant crop cultivars.

In addition, the information on the roles of wild habitats in providing refugia for natural enemies and the possible use of wild hosts in strip cropping and as trap crops will be helpfull in the management of stem borers in a cropping system.  Such knowledge can be used to strengthen and sustain the impact of IPM strategies.

Further studies on wild stem-borer hosts, taking into account local conditions, are currently underway in West Africa (International Institute of Tropical Agriculture) (IITA, Benin) and East Africa (ICIPE, Kenya).

Major references: Acland (1971); Harrison (1981); Rouanet (1984); Ibekwe (1986).

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