Pest complex of soybean: Thirty nine species of insect pests belonging to seven orders and 22 families were found to infest at the different growth stages of soybean crop in Noakhali region, Bangladesh during rabi seasons of two consecutive years 2009-10 and 2010-11. Of these, only six species namely, hairy caterpillar, Spilarctia obliqua (Walker); leaf roller, Lamprosema indicata F.; common cutworm, Spodoptera litura F.; pod borer, Helicoverpa armigera (Hubner); stem fly, Ophiomyia phaseoli (Tryon) and white fly, Bemisia tabaci Genn. were considered as the major pests while the rests were of minor importance on the basis of population densities per plant, nature and extent of damages and yield reductions. The population density per plant of major insects namely, S.obliqua, L. indicata, .S. litura, H. armigera, O. phaseoli and B .tabaci ranged from 0.50-0.60, 2.00-2.50, 1.50-1.80, 0.35-0.40, 0.40-0.50 and 4.00-5.00, respectively in 2010 while it ranged from 0.45-0.55, 1.50-2.00,1.20-1.50 ,0.30-3.50, 0.30-0.40 and 3.00-3.50, respectively in 2011. Similarly, percent plant infestation by the major insects namely, S.obliqua, L. indicata, .S. litura, H. armigera, O. phaseoli and B .tabaci ranged from 70-80, 95-100, 90-100, 35-40, 40-45, 96-100, respectively in 2010 while it ranged from 60-70, 95-100, 80-90, 30-35, 35-40 and 95-100, respectively in 2011. On the basis of feeding behaviour, 17 species were grouped as sap sucker, 15 as leaf eater, 2 as leaf roller and eater, and the remaining five such as, Helicoverpa armigear, as pod borer and eater, Stomopteryx sp. as leaf miner, Agrotis sp. as stem cutter, Ophiomyia phaseoli and Melanagromyza obtusa as stem or shoot borers.
Nature of damage of the important pests: The larvae of S. obliqua damaged the soybean leaves and shoots and gregariously and fed on the older leaves, stems, shoots, flowers and pods causing serious damage to the plant. The yellowish green larvae of leaf roller rolled the leaves of soybean plants and several may be webbed together and feed inside. About 2-3 larvae remained in a single folded leaf. The pale green larvae of S. litura damaged the leaves and shoots by feeding voraciously. Infested leaves bore irregular holes, growth of the plant was arrested, flower and pod formation was hindered. Very small larvae of the pest bore into the stem through the petiole. They feed on cortex and pith of the stem causing tunneling of the stems and died. The green larvae of H. armigera feed on leaves and tender shoots firstly; later on they bore pods and feed inside. The green stink bugs suck sap from the tender shoots, leaves, and pods resulting distorted leaves and pods.
Succession of the pests: Soybean crop was first attacked by leaf beetle, Monolepta signata, black beetle, Cyrtozemia cognata, epilachna beetle, Epilachna 12 Punctata, pumkin beetle, Aulacophorai sp., black cutworm, Agrotis ipsilon, leaf hopper, Aphannus sordidus, jassids, Empoasca spp. at the seedling stage and their infestation continued up to pod formation stage of the crop during January 2010 and 2011. After 2-3 weeks, leaf roller, Lamprosema indicata F. common cutworm, Spodoptera litura F. hairy caterpillar, S. obliqua , pod borer, Helicoverpa armigera Hubner, green stink bug, Nezara viridula L., rice bug and other pentatomid bugs were frequently observed from flowering to maturity of the crop and recorded on February to April 2010 and 2011. The white fly and aphids were noticed from vegetative to pre-maturity of the crop and act as vectors of yellow mosaic virus (YMV) and leaf curled viruses that were seriously affected plant growth and yield of soybean crop in both the years. Stem fly was observed from seedling to pod formation stage of the crop while pod borer damage was recorded at the flowering and pod pre-maturity stage in both the years. Most of the major and minor pests appeared in the crop during vegetative to flowering stages (30-50 Days after sowing) and the maximum infestation occurred during flowering and pod formation stages of the crop in both the years. Although most of the insects recorded from soybean crop during the study period have been considered as minor, it is not unlikely that any one of the minor pests may attain the status of a major pest depending upon the environmental conditions and changing cropping pattern.