Effectiveness of a Dry Formulation of Spinosad Against Stored-Grain Insects

1. Effectiveness of a Dry Formulation of Spinosad Against Stored-Grain Insects Anna Iversen Getchell and Bhadriraju Subramanyam Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, U.S.A. (aiversen@ksu.edu) ABSTRACT 7-DAY ADULT SURVIVAL C B The efficacy and persistence of a liquid formulation of spinosad (SpinTor2SC), a commercial bacterial insecticide, against stored-grain insects has been documented through laboratory and field trials. The effectiveness of the dry formulation of spinosad (0.5% purity) against stored-grain insects is unknown. Adults (25) of the lesser grain borer, red flour beetle, rice weevil, and rusty grain beetle, were exposed to untreated wheat and corn kernels and kernels treated with 1 mg (AI)/kg of spinosad. Adult mortality was determined after 7 days of exposure and progeny production and kernel damage after 49 days. Spinosad effectively controlled lesser grain borers, and rusty grain beetles on both corn and wheat. The red flour beetles and rice weevils were less susceptible to spinosad based on adult mortality, progeny production, and kernel damage. A D * * * * * * * INTRODUCTION Fig 1. Adult and larva of rice weevil (A), rusty grain beetle (B), red flour beetle (C), and lesser grain borer (D). Pictures courtesy of USDA (http://www.usda.gov/gipsa/tech-servsup/visualref/insects.htm) Spinosad is a commercial bacterial insecticide. The activity of spinosad is attributed to the metabolites Spinosyns A and D, which are fermentation products of the actinomycete bacteria, Saccharopolyspora spinosa (Mertz &Yao). Spinosad has low mammalian toxicity and is environmentally benign. Previous laboratory and field tests with a liquid spinosad formulation (SpinTor 2SC) have shown it to be highly effective in killing several stored-grain insects at 1 mg(AI)/kg (Fang et al. 2002a,b; Huang and Subramanyam 2003, Flinn et al. 2004). Dry formulations of insecticides do not require water for dilution and power supply for the sprayer, and are relatively easy to use by producers. Fig 2. Number of adults (out of 25) of the four insect species that survived on corn and wheat after 7 days of exposure. An asterisk (*) indicates that survival was significantly different between 0 and 1 mg/kg treatments (P < 0.05; t-test). 49-DAY PROGENY PRODUCTION 49-DAY KERNEL DAMAGE * * * OBJECTIVES • Assess the effectiveness of a dry formulation of spinosad on survival of red flour beetles (RFB), Tribolium castaneum (Herbst); lesser grain borers (LGB), Rhyzopertha dominica (F.); rice weevils (RW), Sitophilus oryzae (L.); and rusty grain beetles (RGB), Cryptolestes ferrugineus (Stephens), on corn and wheat after a 7-d exposure (Fig. 1). • Determine progeny production of the four insect species on corn and wheat and assess kernel damage after 49 d. * * * * * * * * * * MATERIALS and METHODS Fig 3. Number of progeny of each species produced after 49 days of exposure to corn and wheat. An asterisk (*) indicates that the number of progeny produced was significantly different between 0 and 1 mg/kg treatments (P < 0.05; t-test). Fig 4. Kernel damage caused by insects after 49 days of exposure to corn and wheat. An asterisk (*) indicates that the number of kernels damaged was significantly different between 0 and 1 mg/kg treatments (P < 0.05; t-test). Grain Treatment: 100 g of hard red winter wheat or yellow dent corn were treated with dry spinosad containing 0.5% AI to provide a rate of 1 mg(AI)/kg. Untreated wheat and corn served as the control treatment. Grain and spinosad were admixed for 2 min in a 0.45-liter jar and then transferred to 150 ml plastic containers. Each 100 g lot was infested with 25 adults of a species. All containers were placed in a chamber at 28oC and 65% RH. A completely random design was used for the experiment with five replications per treatment. RESULTS The 7-d adult survival, 49-d progeny production, and 49-d kernel damage were all significant between the two grain types (F, range = 5.62-45.39; df = 1, 64; P, range = 0.021-<0.0001), among insect species (F = 110.77-165.50; df = 3, 64; P < 0.0001), and between the two rates (F = 117.30-865.85; df = 1, 64; P < 0.0001). Reduction in survival of adults of all four species on spinosad-treated grain relative to that on untreated grain ranged from 6.7 – 100% (Fig. 2). Except for red flour beetles and rice weevils, very few or no adults of the lesser grain borer and rusty grain beetle survived the 7-d exposure to spinosad. Adult survival, in general, was lower (P < 0.05) on spinosad-treated grain than on untreated grain. Suppression of progeny production among the four species on both grains treated with spinosad ranged from 79.8 – 100% (Fig. 3). Except for the rusty grain beetle on wheat, fewer progeny (P < 0.05) were produced on spinosad-treated grain than on untreated grain. Reduction in damage on spinosad-treated grain infested with lesser grain borer, red flour beetle, and rice weevil ranged from 40.0 – 89.2% (Fig. 4), and for each of these species, fewer kernels (P < 0.05) were damaged on spinosad-treated than untreated grain. Damage caused by the rusty grain beetles on both corn and wheat in 0 and 1 mg/kg treatments was essentially similar. Adult survival: After 7-d, five replicates of the control and spinosad treatments were examined to count live and dead adults. Adults that were mobile were considered alive. After counting, all adults and grain samples were discarded. CONCLUSIONS The dry formulation of spinosad at 1 mg/kg provided effective control of lesser grain borer, red flour beetle, and rusty grain beetle, although adults of the red flour beetle were less susceptible than the other species to spinosad. Effectiveness of spinosad against red flour beetle neonates is responsible for the complete progeny suppression observed. The rice weevil was able to produce more progeny and cause more kernel damage on spinosad-treated grains than the other species, possibly because of the slow action of spinosad on this species. Based on adult survival and kernel damage, spinosad performed slightly better on wheat than on corn. Progeny production of each species on spinosad-treated wheat and corn was essentially similar, although more progeny were produced on untreated wheat when compared with untreated corn. The results observed here are consistent with our earlier findings with a spinosad liquid formulation (Fang et al. 2002a, b). The dry formulation of spinosad is effective against the lesser grain borer, red flour beetle, and rusty grain beetle. Damage caused by the rice weevil exceeded the federal Defect Action Level of 32 insect-damaged kernels/100 g. Spinosad may need to be used in conjunction with other grain protectants (Storicide II) for effective control of rice weevil. The activity and persistence of the dry formulation of spinosad still needs to be verified under field conditions. Progeny Production: After 49-d, five additional replicates of control and spinosad treatments were examined to count the number of adult progeny produced in the case of lesser grain borers and rice weevils. For red flour beetles and rusty grain beetles, the number of adult progeny and any live old larvae in samples were counted. The original number of adults (25) added to the replicate was subtracted to determine the total progeny produced in each replicate. Damaged Kernel Assessment: All kernels in each replicate were examined for feeding damage or adult emergence holes (lesser grain borers and rice weevils). The number of kernels damaged in each replicate was enumerated. REFERENCES Fang, L., Bh. Subramanyam, and F. Arthur. 2002a. Effectiveness of spinosad on four classes of wheat against five stored-product insects. J. Econ. Entomol. 95: 640-650. Fang, L., B h. Subramanyam, and S. Dolder. 2002b. Persistance and efficacy of spinosad in farm-stored grain. J. Econ. Entomol. 95: 1102-1109. Flinn, P., Bh. Subramanyam, and F. Arthur. 2004. Comparison of aeration and spinosad for suppressing insects in stored wheat. J. Econ. Entomol. 97: 1465-1473. Huang, F., and Bh. Subramanyam. 2004. Responses of Corcyra cephalonica (Stainton) to pirimiphos-methyl, spinosad, and combinations of pirimiphos-methyl and synergized pyrethrins. Pest Manag. Sci. 60: 191-198. Data Analysis: Adult survival, progeny production, and kernel damage data (x) were transformed to log (x + 1) scale. Data were subjected to 3-way analysis of variance and two-sample t-tests. All treatment effects were considered significant at the a = 0.05 level. ACKNOWLEDGMENTS:We thankDaniel Hopper, Brandi Kaufman, Nicole Nafziger, and Carmelita Goosen for laboratory help. This work was supported by Dow AgroSciences, Gustafson and CSREES-USDA (RAMP) under Agreement No. 00-51101-9674.