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Insect Pest Management for Flour Mills: Residual Efficacy of Aerosols
With the impending phase-out of the fumigant Methyl Bromide, the wheat industry is expanding the use of aerosol insecticides for insect pest management in milling facilities. Combinations of pyrethrin combined with the insect growth regulator (IGR) methoprene (Diacon II?) are often used to control the red flour beetle (RFB) and the confused flour beetle (CFB), two common stored-product pest insects in wheat mills throughout the USA. Insect growth regulators affect molting and development of immature insects, but generally do not kill adults, hence they are often combined with an insecticide that will kill adults.
Even though the RFB and CFB are similar in size and appearance they usually vary in susceptibility to insecticides. There is little information on how long the residues from an aerosol application will give control of either of these two important pest species. We placed wheat flour or one of seven packaging materials inside a flour mill, and then exposed these materials to commercial aerosol formulations containing either 1% or 3% pyrethrin with the same amount of Diacon II?. We removed the treated flour and packaging materials and held them for 16 weeks in the laboratory. Every two weeks for 16 weeks, we put larvae of each species on different sets of the exposed flour and the exposed packaging materials (with flour), and then determined if larvae could reach the adult stage.
Both formulations gave complete control of RFB on the flour and on the packaging materials for 16 weeks, but the formulations were less effective against the CFB because more CFB larvae were able to reach the adult stage compared to the RFB. Increasing the pyrethrin concentration from 1% to 3% did lead to a reduction in the number of CFB adults, indicating that there may have been an interaction between the pyrethrin and the Diacon II? components of the formulation. However, the residual efficacy is largely due to the presence of the IGR. In addition, results from other studies with Diacon II? used as a residual surface treatment, and results of studies with the IGRs hydroprene (Gentrol?) and pyriproxyfen (NyGuard?), indicate that the RFB is more susceptible than the CFB. Since both the RFB and CFB can infest flour mills, species identification is important when IGRs are being used for insect control.
Directional Flow of Aeration to Manage Insect Pests in Stored Wheat
Using low-volume ambient air to cool stored grain is a common management practice in the southern plains, but little research has been done recently to determine if the direction of airflow makes a difference regarding the cooling and insect pest populations. We conducted a 2-year study at the USDA ARS Center for Grain and Animal Health Research (CGAHR), using 1,000 bushel metal storage bins filled with wheat. We evaluated suction aeration, pulling air downward through the grain mass, as compared to pressure aeration, the standard strategy of pushing cool air upward through the grain mass. Results were consistent each of the two years and showed that temperatures on the upper surface of the grain mass were consistently cooler with suction aeration than with pressure aeration. The resulting insect pest populations were also generally lower in the bins with suction versus pressure aeration. Our results indicated that using suction aeration would cool the upper surface zone of the grain mass, which is vulnerable to insect infestation, and could reduce the need for additional pesticide inputs through this reduction in pest pressure.
The results of the previous study are promising and warrant further investigation in larger bins. A new study has been initiated to examine temperature gradients in the headspace zone of wheat storage bins with pressure versus suction aeration, and to determine the extent of the interactions between the headspace temperatures and those in the upper surface zone of the bulk grain mass. Studies are being conducted at the CGAHR, this time using 4,000 bushel bins. The first year has been completed, and results are similar to the earlier study in that more insects are found in the bins with pressure aeration compared to the bins with suction aeration. Headspace temperature patterns are different with the two aeration methods, and even with the larger bins the surface zone of the wheat appears to be cooler with suction aeration compared to pressure aeration.
Agricultural Research Service (ARS) News
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