EFFECT OF MICROAC ON NUMBERS OF SOYBEAN APHID, 2005

Abstract

Infestations by SA can reduce the yield of soybeans and the efficacy of various biorational spray treatments need evaluation as alternatives to chemical insecticides for SA control. The efficacy of an entomopathogenic fungal formulation, MicroAC, against SA was compared to that of a distilled-water spray check and an untreated check. Two identical efficacy tests were conducted in growth chambers at the Northern Grain Insects Research Laboratory, Brookings, SD. Each test used a RCB design with 10 replications. In each test, 30 greenhouse-raised soybean plants, growing individually in separate pots, were infested in the late VE stage (unifoliate leaves fully expanded, first trifoliate leaves expanding) with five SA (adults or late third instars) on the underside of each unifoliate leaf (total of 10 SA per plant). The plants were placed in a growth chamber (16:8 L:D photoperiod; mean temperature and R.H. of 21.4 C and 45.9%, first test; 21.3 C and 73.9%, second test). Seven days after infestation, SA were counted on each plant. Sprays were then applied 20 cm from each plant using the wide-angle mist setting on the nozzle of a handpumped squirt bottle. Sprays were applied to run-off with eight squirts (about 9.2 ml) directed per plant, four with the nozzle horizontal and four with nozzle tip pointed downward at approximately 30-degrees. The number of spores contained in an 8-squirt volume of MicroAC was enumerated by direct observation of diluted product suspensions on a light microscope using phase contrast microscopy and a hemacytometer. Sporulation rate was measured as colonyforming units (CFU) in dilution-series cultures plated on Sabouraud dextrose agar with 2 % yeast extract under growth chamber conditions; CFU were counted after a 4-day incubation period when colonies were visible and separate. After spray application, clear 10-cm-diam × 40-cm-tall cylindrical acetate cages with 16-mesh screen tops were placed over each plant, and plants were returned to the growth chamber. At 10 days after treatment (DAT), (23 Jun, first test; 11 Jul, second test), plants were clipped at soil level, and SA were counted on each plant. For each test, SA counts were subjected to ANOVA and treatment means separated using Tukey’s HSD (P = 0.05). Post-spray SA counts were log (x + 1)-transformed before analysis.