Bean seedling damage by root-feeding grubs (Schizonycha spp.) in Kenya as influenced by planting time, variety, and crop residue management

Use of legume cover crops to improve soil productivity has had unexpected effects on cropping system components in a maize (Zea mays L.) and bean (Phaseolus vulgaris L.) cropping system in Trans Nzoia district, Kenya. Smallholder farmers who recently adopted relay cropping of lablab (Lablab purpureus L.) observed a decline in bean yields where lablab residues had been incorporated prior to maize and bean planting. We investigated chafer grub (Schizonycha spp.) damage to beans and studied the impact of lablab and maize stover residue management strategy, planting time, and bean cultivar selection on seedling damage. Seedling assays (germination to flower initiation), involving four different bean varieties (GLP2, KK15, KK22, and KTL3), were carried out in three consecutive planting periods. The assays were superimposed on a long-term, factorial experiment where soils had been subjected to differential lablab and maize stover residue management strategies for 7 years. Factors were (1) lablab residues removed (L0) or retained (L1); (2) maize stover removed (S0) or retained (S1) and (3) residues left on the soil surface (I0) or incorporated (I1). Planting time significantly affected the degree of chafer grub damage, with later planted beans sustaining substantially more damage. Of the four bean varieties, KK15 and GLP2 were more grub tolerant than KK22 and KTL3. Grub damage was most severe in plots where lablab residues had been retained and maize stovers had been removed (L1S0). This factor combination had the highest seedling mortality, the poorest stand establishment, and the largest proportion of plants with severed roots systems. The results of this study show that the current farmer practice (lablab residue retention and maize stover removal) is conducive to increased grub-induced bean seedling damage and suggest that early planting, retention of maize stover, and selection of grub-tolerant varieties are important management strategies to reduce damage and subsequent productivity decline.