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Update On Soybean Aphid Populations In South Dakota

Aug 11, 2016
By Adam J. Varenhorst
Assistant Professor & SDSU Extension Field Crop Entomologist
 
During late summer, soybean aphid populations can very quickly reach and exceed the economic threshold of 250 soybean aphids per plant. When scouting for soybean aphids be sure to also determine the average growth stage of the soybean plants within each field. Soybean aphid populations that exceed the 250 aphids per plant threshold can be managed up to the R5 growth stage (beans beginning to develop within pods at one of the four upmost nodes). However, insecticide applications on soybean that are at the R6 growth stage (full seed set in pods at one of the four uppermost nodes) or beyond are not associated with yield improvements and may cause issues with harvesting due to the pre-harvest intervals of insecticides.
 
Scouting Soybean Aphids
 
We are scouting soybean at the Southeast (Beresford), Volga, and Northeast (Watertown) research farms. For the last few weeks soybean aphid populations have been very low (less than 10 soybean aphids per scouted plant) and less than 50% of the scouted soybean plants were infested. However, these soybean aphid populations have been steadily increasing. Presently, 100% of the soybean plants scouted at the Volga and Northeast Research farms were infested with soybean aphids. Although most soybean plants at the Northeast Research Farm had soybean aphid population densities at approximately 5-10 soybean aphids per plant, there are a few plants within the field that have higher population densities (approximately 200 aphids per plant). At the Southeast farm, more than 50% of the plants scouted are infested, but again population densities are less than 10 soybean aphids per plant.
 
It is important to continue scouting for soybean aphids until soybeans reach the R6 growth stage to ensure that populations don’t exceed the 250 soybean aphids per plant economic threshold and potentially cause yield loss. While scouting, make sure to also monitor for the presence of other insects in the field such as defoliating insect pests and natural enemies of soybean aphid.
 
Natural Enemies
 
Natural enemies of the soybean aphid include multiple species of lady beetle adults (Figure 1) and larvae (Figure 2), syrphid or hover fly larvae (Figure 3), minute pirate bug adults (Figure 4) and nymphs, green lacewing larvae (Figure 5), damsel bugs adults (Figure 6) and nymphs, and also several small parasitoid wasps that are the source of aphid mummies (Figure 7). Although natural enemies may not be able to suppress large populations of soybean aphids, they are capable of slowing the population growth of soybean aphids and potentially reducing the need for insecticide applications.
 
   
Fig. 1. Convergent lady beetle feeding on soybean aphids.
                                     
Fig. 2. Convergent lady beetle larva feeding on soybean aphids.
 
 
Fig. 3. Syrphid larva feeding on soybean aphids. 
                                     
Fig. 4. Minute pirate bug adult feeding on soybean aphids.
 
 
Fig. 5. Green lacewing larva.  
                                     
Fig. 6. Damsel bug adult.
 
Fig. 7. Soybean aphid mummy.
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