Update:
After correcting for differing plant size, our analysis determined that feeding by DSB larvae did not cause a significant reduction in yield of individual soybean plants. This result is contradictory to the previous year’s data, which showed a 10% reduction in yield when plants were infested with DSB. Differences in findings from the two years are likely due to major differences in planting dates and growing conditions between the two field seasons. We did, however measure that plants infested with DSB produce seeds that are on average 5% lighter than seeds of uninfested plants within the same field.

From sweep net monitoring, we found that adults are detectable in soybean fields at relatively low numbers (~5 adults per 100 sweeps) over an extended period of time with no apparent synchrony in emergence. In addition, monitoring infested stems from the previous season showed that adults appear to emerge from overwintering larvae and pupae over an extended period of time, which contributes to their prolonged presence in soybean fields. The earliest adult emergence from stubble was measured on June 27, and the final emergence on August 6. Field populations did begin to dwindle in early August, and very few adults were captured after the first week of August. We also collaborated with Dr. Jocelyn Millar at the University of California Riverside to determine whether chemicals produced by adult DSB function as a long-distance pheromone to attract mates, as they do in other species within this beetle family. We deployed Japanese beetle traps baited with either the putative pheromone or a solvent control, since we found in the previous year that DSB were rarely caught in traps that were not in contact with soybean plants. We found some support for the attractiveness of the pheromone compared to the control, however overall captures were low, with only 10 beetles captured. Other trap configurations may produce better results, as the Japanese beetle traps were prone to filling with water during repeated rain events.

Older studies of DSB found that fall tillage could inflict significant mortality on overwintering DSB larvae. However, this tillage may also disrupt overwintering parasitoids and other natural enemies that could attack different stages of DSB. Results of the field experiment show fewer parasitic wasps emerge from fields that were chisel plowed the previous fall when compared to no-till and planting cover crops. However, we found no evidence of any mortality inflicted by parasitoids in the DSB larvae reared in the lab. Parasitic wasps and flies are not likely to inflict high levels of mortality on DSB, and are not likely to provide adequate biological control.

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Research to Develop a Management Program for the Dectes Stem Borer by Finding and Targeting Its Weak Links found that larger diameter soybean plants are more likely to be infested with DSB than smaller diameter plants. However, after correcting for differing plant size, feeding by DSB larvae was not found to cause a significant reduction in yield of individual soybean plants. Findings from the previous year’s data showed a 10% reduction in yield when plants were infested with DSB. Differences in findings from the two years were likely due to major differences in planting dates and growing conditions between the two field seasons. Plants infested with DSB produced seeds that were 5% lighter on average than seeds of uninfested plants within the same field.

Adults DSB were detectable in soybean fields at relatively low numbers over an extended period of time. Monitoring of infested stems from the previous season showed that adults appear to emerge from overwintering larvae and pupae over an extended period of time, which contributes to their prolonged presence in soybean fields. The earliest adult emergence from stubble was measured on June 27, and the final emergence on August 6.

Fewer parasitic wasps emerged from fields that were chisel plowed the previous fall compared to no-till and planting cover crops. However, no evidence of any mortality inflicted by parasitoids was observed in lab-reared DSB larvae. Parasitic wasps and flies are not likely to inflict high levels of mortality on DSB, and are not likely to provide adequate biological control.