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Abstract
This research examined 1) the effects of a thiamethoxam seed treatment on soybean aphid, Aphis glycines Matsumura, feeding behavior using electronic penetration graphs, 2) neonicotinoid translocation within specific soybean tissues and 3) the physiological effects of a thiamethoxam seed treatment on soybean using transcriptional profiling.
Soybean aphid stylet penetration behavior was not strongly affected by the thiamethoxam seed treatment during the initial 9 h feeding interval. However, after 9 h of exposure to thiamethoxam seed treatment aphids exhibited significant differences in feeding behavior compared to those feeding on untreated soybeans. Aphids on thiamethoxam-treated soybeans spent significantly less time feeding in the sieve element phase and exhibited a greater duration of non-probing events. These studies suggest soybean aphids may have difficulty ingesting phloem sap, which may be another important element in seed treatment protection.
Translocation studies investigated the uptake (total equivalents) of three seed-applied insecticides. The effect of moisture stress on insecticide uptake and translocation in VC, V1 and V2 stage soybeans was also examined. All insecticides displayed systemic and acropetal movement to the distal parts of the leaves with the majority of the insecticides remaining in the cotyledons. Moisture stress did not affect the total amount of imidacloprid and flupyradifurone in V1 and V2 soybeans. However, less clothianidin was recovered from V2 soybeans when plants were moisture stressed. Interestingly, flupyradifurone had significantly higher equivalents in the trifoliates than neonicotinoids under no stress and moisture stress conditions.
Next-generation sequencing examined the effects of a thiamethoxam seed treatment on gene expression in soybean. The interactive effects of seed treatment and moisture stress of 10 genes associated with plant defense pathways in soybean were also examined. Few genes were affected by the thiamethoxam seed treatment. However, large-scale changes in gene expression were present in thiamethoxam-treated and untreated soybeans across vegetative stages some of which may affect plant growth. Subsequently, genes associated with phytohormones in moisture-stressed soybeans were responsive to a thiamethoxam seed treatment.
