Efficacy of insecticides and fungicides for treatment of soybean seeds against adults of Myochrous armatus (Baly, 1865) (Coleoptera: Chrysomelidae)

The soybean mealworm Myochrous armatus (Baly, 1865) (Coleoptera: Chrysomelidae) feeds on leaves and stems in the initial stages of soybean growth, reducing grain yield and quality. The aim of this study was to evaluate the effect of soybean seed treatment with systemic insecticides and fungicides to protect against M. armatus adults. These adults were exposed to active ingredients and combinations of systemic insecticides and fungicides, and the effects were observed after direct planting. The study was conducted in an experimental field in the municipality of São Gabriel do Oeste, in state of MS, Brazil. The field was sown with the 8473RSF Desafios RR soybean cultivar, in randomized blocks with six treatments and four replications per treatment, such as the control (untreated); pyraclostrobin, thiophanate -methyl, fipronil; clothianidin + thiophanate -methyl, fluazinam; cyantraniliprole + thiophanate -methyl, fluazinam; fipronil + thiophanate -methyl, fluazinam; and thiamethoxam + fipronil + thiophanate -methyl, fluazinam, used at dosages of 100 kg of soybean seeds for the protect soybean plants against attacks by M. armatus and fungi. Five adult insects were released per cage for artificial infestation. The lowest number of attacked plants occurred in the treatments with pyraclostrobin; fipronil and thiamethoxam + fipronil, 21 days after infestation (DAI). The treatment with thiamethoxam + fipronil differed statistically from the control, with efficiency of 85%. The lowest percentage of dead plants was found in the treatment with thiamethoxam + fipronil at 0.96%. The treatment with thiamethoxam + fipronil in combination was most efficient for controlling M. armatus.