The Relationship of the Population Density of Larvae Spodoptera litura with the Leaf Damage and Decrease of Seed Yield for Soybean, Indonesia

This study aimed to illuminate the relationship of population density of larvae on the level of leaf damage and declining seed yields in some varieties of soybeans that have been implemented in Maros Experimental Garden from April to August, 2016. The experiment consisted of four population densities, i.e., 0, 2, 4 and 6 larvae instar-3 per plant and three soybean varieties, i.e., Anjasmoro, Argomulyo and Grobogan. The experiment was conducted in split plot design with varieties as main plot and population densities of larvae as subplot treatment with five replications. The results showed that the damage attack highest for Anjasmoro from 20.19% to 28.61%, Argomulyo from 14.68% to 21.18% and Grobogan from 13.28% to 18.00%. So the seed yields, the highest for Argomulyo 14.50 g/plant and lowest was for Grobogan 12.55 g/plant. The relation between population density of larvae and leaf damage intensity of Spodoptera litura in three days after investation (DAI) follows an equation, Anjasmoro, Y = 8.587x and r = 0.9583, Argomulyo, Y = 6.251x and r = 0.9558, and Grobogan, Y = 5.45x and r = 0.9459. The relationship between larval density population and the level of leaf damage in 6 DAI follows a regression equation, Anjasmoro, Y = 8.174x and r = 0.96107, Argomulyo, Y = 6.392x and r = 0.9609, and Grobogan, Y = 5.977x and r = 0.9626. The relationship between larvae population density and the level of leaf damage in 9 DAI follows a regression equation, Anjasmoro, Y = 5.8879x and r = 0.9358, Argomulyo, Y = 4.3671x and r = 0.954685, and Grobogan, Y = 3.917x and r = 0.9467. The declining seed yield caused by addition one larvae of S. litura, the highest on Anjasmoro variety (0.80 g/plant or 6.04%) and the lowest on Argomulyo (0.65 g/plant or 4.48%).