Black beetles (Heteronychus spp., Coleoptera: Dynastidae) are significant pests of upland rice in the Central Highlands of Madagascar, particularly in some conservation agriculture (CA) systems. On the other hand, certain species of cover crops used in CA systems may have a suppressive effect on black beetle populations or reduce the damage they cause to rice plants. Between 2009 and 2011, we conducted a laboratory study on the possible effects of adding dried ground plant parts (hereafter ‘residues’) to the soil on larvae (white grubs) and adults (black beetles) of Heteronychus bituberculatus. Ten cover crop species were tested: Cajanus cajan, Crotalaria grahamiana, Desmodium intortum and Vicia villosa (Fabaceae); Brachiaria ruziziensis and Eleusine coracana (Poaceae); Cosmos caudatus and Tagetes minuta (Asteraceae); Cleome hirta (Capparaceae); and Raphanus sativus (Brassicaceae). Adding residues of C. grahamiana, E. coracana and C. caudatus resulted in significantly higher larval mortality than in the control treatments, while residues of R. sativus had a marginally significant effect. Adding residues of C. grahamiana, C. caudatus, C. hirta, T. minuta and R. sativus significantly reduced damage caused by adult black beetles.
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