Glass-vial bioassay: improvements towards soybean aphid (Aphis glycines) resistance monitoring

The soybean aphid is a yield-limiting pest of soybeans in North America, and the large-scale adoption of thiamethoxam seed-treatments for its management could increase the risk of selection for resistance to evolve. An effective bioassay technique is required in order to allow early detection of resistance to thiamethoxam in aphid field populations. A glass-vial bioassay has been used to detect and monitor resistance in various insect pests, including other aphid species. This method allows the use of small amount of insecticide solutions, large number of insects, and is rapid and reliable. However, for each biological organism, some adaptations in the pre-established protocol may be required to improve the effectiveness and establish a standard methodology. The objective of this study was to determine how different vial caps (cotton ball, or plastic caps), number of replications, aphids per vial,  and different storage temperatures could affect the sensitivity of the test method. The vials received five different thiamethoxam concentrations plus the control treatment (no insecticide). Each complete set of treatments was placed inside separate Styrofoam boxes in order to simulate different shipment times (24, 48, and 72 hours) at three different temperatures (-20^oC, 4^oC, and room temperature). Aphids were transferred into each vial, with mortality recorded at 6, 12, and 24 hours. This bioassay standardization will allow an optimized early detection of resistance to thiamethoxam in soybean aphids, improving the ability of consultants and extension personnel to make more accurate control recommendations, thereby benefiting soybean growers.

Key Words: soybean aphid, thiamethoxam, bioassay standardization