Impact of Gliricidia sepium and Cassia spectabilis hedgerows on weeds and insect pests of upland rice

Abstract

Double hedgerows of two leguminous multipurpose trees Gliricidia sepium (Jacq.) Walp. and Cassia spectabilis DC. were established in highly eroded, low organic matter, acid uplands in northern Mindanao, Philippines. Alleyways were grown with upland rice Oryza sativa L. and fertilised with G. sepium green manure or C. spectabilis mulch (or in combination) and assessed for weed and insect pest suppression. Mulch reduced weed biomass which was mostly composed of grasses on infertile soil. Green manure favoured broadleaf weeds and increased weed biomass reducing the effect of mulching. Greater weed suppression may be achieved. Improvement in soil fertility increased the crops’ competitiveness against weeds by concentrating it to crop furrows. Greater crop nutrition from hedgerow biomass resulted in increases of rice seedling maggot Atherigona oryzae Malloch and stem borer (mainly Sesamia inferens (Walker)) damage and white grub (mainly Holotrichia mindanaoana Brenske and Leucopholis irrorata (Chevrolat)) larval densities over the control. Terrace formation reduced white grub densities the first year. The trend reversed in the second year as shade from the hedgerows and mulch probably favoured survival of white grub larvae by reducing soil temperature and increasing soil moisture. Despite higher insect pest densities, the moderate tillering UPLRi5 tolerated weeds and insect pest damage to produce higher yields in the biomass treatments than the control. But hedgerows may increase the incidence of rice blast Pyricularia oryzae Cav. by decreasing air movement. As purchased fertilisers provide limited crop response on depleted soils, alley cropping provides an economical means to overcome that constraint in small-scale tropical agriculture.