Effects of Weeding on the Population of Insect Pests and Yield of Soybean (Glycine max (L.) in Calabar, Nigeria

Abstract

Field experiments were conducted in the 1999 and 2000 late planting seasons to investigate the effects of weeds on the population of insect pests and yields of soybeans. Two soybean cultivars TGX 536-02D and TGX 1448-313D were planted as the main plot treatment while four weeding schedules (no weeding, 1, 2 or J weedings) were included as the sub-treatments. Some of the insects that were observed in the experimental plots included the defoliators such as: Medythia quaterna, Podagrica sp., Zonocerus variegatus The pod feeders among which were: Aspavia armigera, Nezara viridula, Acrosternum acuta, Cletus notatus, Mirperus sp, Riptortus dentipes and Chnootriba similes. The flower feeder such as Paederus sabeaus and the leaf roller (Hedylepta indica) were also observed. The weeded plots had significantly (P<0.05) lower insect pest population densities than the non-weeded plots Insect population density decreased significantly with increase in the number of weeding. Thus, insect population of 6.23/0.6 m-row were observed in the plots weeded thrice in 1999 and 11.34/0.6 m in the year 2000, 7.01 and 12.30/0.6 m row in the 2-weedings while the l-weeding gave 9.35 and 15.87/0.6 m row. The non-weeded plot had 11.8 and 17.02 insects/0.6 m row. The frequency of weeding was highly correlated with the insect population. Grain yields were significantly higher (1286.84 and 858.77 kg/ha) in the two years when weeding was done three times compared with when no weeding was done or with when weeding was done once and twice. In the two years, the percentage unfilled/damaged pods decreased as the number of weeding increased (r=0.986 and 0.935). There was also an indication of a high correlation (r=-0.961 and -0.938) between insect population and the grain yields in the two years.