Exploiting chemical ecology and species diversity: stem borer and striga control for maize and sorghum in Africa�� Zeyaur R Khan,1 John A Pickett,2* Johnnie van den Berg,3 Lester J Wadhams2 and Christine M Woodcock2 1 International Centre of Insect Physiology and Ecology, PO Box 30772, Nairobi, Kenya 2 IACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK 3 ARC-Grain Crops Institute, Private Bag X1251, Potchefstroom 2520, Republic of South Africa Abstract: Stem borers, comprising the larvae of a group of lepidopterous insects, and parasitic witchweeds, particularly Striga hermonthica and S asiatica, cause major yield losses in subsistence cereal production throughout sub-Saharan Africa. Studies are described that have led to the development of a ���push-pull��� strategy for minimising stem borer damage to maize and sorghum. This involved the selection of plant species that could be employed as trap crops to attract colonisation away from the cereal plants, or as intercrops to repel the pests. The two most successful trap crop plants were Napier grass, Pennisetum purpureum, and Sudan grass, Sorghum sudanensis. The intercrop giving maximum repellent effect was molasses grass, Melinis minutiflora, but two legume species, silverleaf, Desmodium uncinatum, and greenleaf, D intortum, gave good results and had the added advantage of suppressing development of S hermonthica. In terms of stem borer control, the plant chemistry responsible involves release of attractant semiochemicals from the trap plants and repellent semiochemicals from the intercrops. With M minutiflora, parasitism of stem borers was also increased by certain chemicals repellent to ovipositing adults. The mechanism of striga control has not been fully elucidated, but allelopathic effects from the Desmodium species have been shown to involve stimulation of germination and interference with haustorial development. Significant beneficial effects have been obtained with the individual components of these push-pull strategies. However, the most robust crop-protection package is obtained when these components are combined. The trap crop and intercrop plants also provide valuable forage for cattle, often reared in association with subsistence cereal production. There has been considerable take-up of the system within the communities where farmer-managed trials have been carried out, particularly in the Trans Nzoia and Suba districts of Kenya, and the programme is set to expand throughout and beyond Kenya. # 2000 Society of Chemical Industry Keywords: stem borers striga push-pull trap crop intercrop attractant repellent semiochemical 1 INTRODUCTION Stem borer and striga attack in subsistence cereal cultivation in sub-Saharan Africa can completely destroy the yield of crops such as maize and sorghum. The stem borers are larvae of lepidopterous insects (moths) comprising indigenous species such as the maize stalk borer, Busseola fusca (Full) (Noctuidae), and some non-indigenous species, for example the sorghum stem borer, Chilo partellus Swinh (Pyralidae). The adult moths locate suitable hosts and lay eggs, and the resulting larvae, in their early instars, shred the leaves before entering the stems of the plant, thereby causing yield losses and an increased susceptibility to wind lodging. Parasitic weeds, particularly the witch- weeds, or strigas, Striga hermonthica Benth and S asiatica Kuntze (Scrophulariaceae), produce seeds that can remain dormant within the soil for up to 20 years. Both germination and haustoria initiation are stimulated by cues from the host plant.1,2 This is followed by the development of haustoria and attach- ment to the host, which then provides nutrients for the parasite throughout its lifetime.3 Although pesticides and herbicides can be used to alleviate these problems, complete control is seldom (Received 2 June 2000 revised version received 17 June 2000 accepted 30 June 2000) * Correspondence to: John A Pickett, IACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK E-mail: john.pickett@bbsrc.ac.uk �� Based on a paper presented at the symposium ���The Economic and Commercial Impact of Integrated Crop Management���, organised by the SCI Crop Protection Group in collaboration with the Volcani Center, Israel and the Fresh Produce Consortium and held on 3���4 April 2000 at 14/15 Belgrave Square, London SW1X 8PS, UK Contract/grant sponsor: UK Biotechnology and Biological Sciences Research Council Contract/grant sponsor: Gatsby Charitable Foundation Contract/grant sponsor: Rockerfeller Foundation # 2000 Society of Chemical Industry. Pest Manag Sci 1526���498X/2000/$30.00 957 Pest Management Science Pest Manag Sci 56:957���962 (2000)

effected and, more importantly, they are too expensive for general use in these farming circumstances, which often involve small family farms of less than one hectare. Some direct treatments are available, for example use of fire ash against stem borers and hand- weeding against striga, but the favoured approach is to use an intercropping system.4���7 The mechanisms of these systems have generally been poorly researched, so that even when there is alleviation of pest or weed problems, maximisation of the effects has not been possible. The objective of the present work was to investigate the diversity of species already growing or having been introduced into resource-poor regions in Africa, seeking plants that were highly attractive to adult stem borers, to serve as trap crops, and for plants that were repellent to stem borers and/or antagonistic to striga development, to use as intercrops. Since intercropping and mixed cropping generally are readily practised in these regions of Africa, farmer take-up was expected to be rapid, provided that robust methods for stem borer and striga control could be developed and demon- strated in procedures acceptable to these particular farming practices. Full experimental details will be published elsewhere (Khan ZR, Chiliswa P, Gohole L, Hassanali A, Hooper AM, Khamis TM, LwandeW, Overholt WA, Kimani-Njogu SW, MwendiaC, Pick- ett JA, Smart LE, Wadhams LJ and Woodcock CM, in preparation). 2 EXPERIMENTAL 2.1 Selection of putative trap and intercrop plants Triplicate small plots of hundreds of plant species were grown at the field station of the International Centre of Insect Physiology and Ecology (ICIPE) at Mbita Point, on the banks of Lake Victoria, Suba district, Kenya. The species were principally in the family Poaceae (= Graminae), ie grasses, but plants from other monocotyledonous families, eg the Typhaceae and Cyperaceae, and some legumes (Fabaceae), were also grown. After establishment, samples of the different plants were examined for attack by stem borers, particularly the indigenous B fusca and the non-indigenous C partellus, by splitting open the stems and counting the different stages of larvae present. Stem borer colonisation, which results from an initial choice by ovipositing adults, was assessed and the most attractive (putative trap crops) and least attrac- tive (putative repellent intercrops) plant species were selected. Two poaceous plants attracted considerably more oviposition than maize. These were both forage crops, the Napier grass, Pennisetum purpureum Schu- mach, related to pearl millet, and the Sudan grass Sorghum sudanense Stapf, related to sorghum. Although Napier grass attracted more oviposition than maize, many of the larvae did not survive. However, Sudan grass allowed development of stem borers and also had a very high parasitisation rate, with 70���80% of the larvae being killed. Notably, the molasses grass, Melinis minutiflora Beauv, an indigenous poaceous plant which has been developed as a source of cattle forage throughout the world, particularly in South America, attracted no oviposition at all. M minutiflora is known locally to possess some anti-tick properties, causing ticks feeding on cattle to detach when the cattle are in contact with the grass. Legumes are not attacked by cereal stem borers, so assessment of their activity was made in association with maize as the host plant, but out of these studies, two plants in the Desmodium genus, silverleaf, D uncinatum DC and greenleaf, D intortum (Mill) Urb, were shown to repel ovipositing stem borers. 2.2 The ���push��� and ���pull��� against stem borers With putative ���push��� and ���pull��� plants having been selected in 1995, experimental trials began in 1996 at the Mbita Point field station in the Suba region and also in the Trans Nzoia region of Kenya, in collabora- tion with staff at the Kenyan Agricultural Research Institute (KARI) under the direction of Mr R Butaki. Initially, 30m 30m plots of maize were grown either as monocultures or surrounded by a 5-m border of Napier or Sudan grass to act as a trap crop. Again, assessment was made by splitting the stems and counting the larvae present. In plots with a border of Napier grass, although there was considerably more oviposition and early larval development in the trap crop compared with the maize, only 20% of the larvae survived on the Napier grass, whereas 80% survived through to adults on the maize. This additional control effect was caused by production of sticky sap by Napier grass tissues in response to penetration by first- and second-instar stem borer larvae. Most of the larvae were trapped in the sticky fluid and were killed. With the Sudan grass border, the trap crop contained eight times as many larvae as the maize, and stem borer numbers within the maize were reduced to one-third compared with the maize monoculture (measured at the 12-week plant growth stage). In the following year, scientist-managed trials on small private farms in the Trans Nzoia region showed that the Napier grass trap crop system reliably gave significant improvements in yield of approximately 1 to 1.5 tonnesha 1. In 1998���9, farmer-managed trials with Napier grass and Sudan grass on poorly-yielding farms in the Trans Nzoia and Suba regions showed similar successes, with yield increases again of the order of 1���1.2 tonnesha 1. Separate studies in both regions, initially at experi- mental sites, moving into scientist-managed and then farmer-managed trials, demonstrated the effectiveness of intercropping with M minutiflora. For example, numbers of C partellus larvae found in maize stems were reduced from 80 in the monoculture plots to c five in plots having a row of M minutiflora planted between each row of maize (six replicate samples). The traditional intercrop, cowpea, Vigna sinensis (L) Savi (Fabaceae), only reduced numbers of larvae down to 45. Similar reductions were found for the other stem borer species. Although not as effective as M minuti- 958 Pest Manag Sci 56:957���962 (2000) ZR Khan et al