Maize (Zea mays) and sorghum (Sorghum bicolor) are the most important cereal crops for the people of Africa. Lepidopteran stem-borers are ubiquitous pests that attack these crops throughout their growth stages and the larvae cause damage ranging from 20 to 80% loss of yield. One approach to pest control in resource-poor regions is to develop management systems using the ‘push-pull’ or stimulodeterrent diversionary strategy1, whereby insects are repelled from a harvestable crop and simultaneously attracted to a ‘discard’ or ‘trap’ crop. For maximum efficacy, these systems should also exploit natural enemies, particularly hymenopteran parasitoids, which can be important in suppressing pest populations2. Indeed, reductions in such beneficial organisms frequently trigger pest outbreaks3.

To develop a diversionary strategy for small-scale African cereal production, we assessed a range of cultivated and wild plants in the Gramineae family (Poaceae) in field trials in Kenya for susceptibility to stem-borers, particularly the indigenous Busseola fusca (Lepidoptera, Noctuidae) and the introduced Chilo partellus (Lepidoptera, Pyralidae). In these trials, molasses grass showed no colonization by stem-borers. Further, volatiles extracted by hydrodistillation of the plant repelled gravid female stem-borers in a laboratory oviposition assay (for C. partellus, eggs laid per filter-paper disc: control 40.9, 100 μg M. minutiflora extract 2.2; Pn =8). In field trials at Mbita Point on Lake Victoria, M. minutiflora planted in alternate rows with maize significantly reduced stem-borer infestation of the main crop (damaged maize plants: single crop 39.2%, intercropped with M. minutiflora 4.6%; PC. sesamiae (Hymenoptera, Braconidae) (parasitized larvae in maize: single crop 5.4%, maize with M. minutiflora intercrop 20.7%; P

To identify the chemicals mediating this behaviour of stem-borers and parasitoids, we isolated volatiles from live M. minutiflora plants by entrainment into porous polymer4. Electrophysiologically active components in the solvent-eluted samples were located by coupled gas chromatography and electroantennography5. We tentatively identified active peaks by gas chromatography-mass spectrometry and confirmed their identity by co-injection with authentic compounds on two columns of different polarity, and using behavioural studies6. Characterized semiochemicals included α-terpinolene, the ocimene isomers, β-caryophyllene, humulene and (E)-4,8-dimethyl-1,3,7-nonatriene.

Production of some of the compounds released by intact M. minutiflora can also be induced in plants damaged by herbivorous insects7,8,9. The nonatriene in particular has been implicated as an ‘SOS’ signal recruiting predators and parasites10. The presence of such compounds in the M. minutiflora intercropping system could provide an explanation for the increased parasitism observed. In behavioural assays using a Y-tube olfactometer6, we showed that foraging female C. sesamiae were indeed attracted to live M. minutiflora plants and also responded in a dose-dependent manner to the hydrodistillation extract (Table 1), and to the nonatriene alone.

Table 1 Table 1 Response of C. sesamiae to plant or plant extract

The prospects for understanding and exploiting the interaction of hymenopteran parasitoids with their hosts have advanced rapidly, particularly with the discovery that semiochemicals released during herbivore damage can stimulate parasitoid foraging11,12,13. Our study suggests that intact plants with an inherent ability to release such stimuli could be used in new crop protection strategies.