Soybean Aphid Colonizing Soybean

Soybean aphids made their initial move into Minnesota soybean fields last week. Reports from around the state indicated low-level infestations in earlier-planted fields with VC to V1 stage soybeans. At Rochester, Fritz Breitenbach and Lisa Behnken reported up to 5% of the plants infested; 8-10 aphids infested the typical plant on their earliest planted soybean with none observed on later planting dates. Numbers were lower elsewhere. At Rosemount, Dave Ragsdale reported ca. 9% infestation frequency with 1-4 aphids per infested plant and winged aphids common. In a bulk field about a mile away that was planted on 4/29, my crew found a lower infestation rate, ca. 1% on Friday with 1-3 aphids per plant. At Lamberton, Bruce Potter did not find soybean aphids yet even though they searched over 1700 plants early this week. Near Fergus Falls, Doug Holen observed ca. 5-10% on field edge with 1-19 nymphs and adults per plant.

The situation should change markedly over the next two weeks. For example, the infestation frequency in one field near Rosemount went from < 0.5% on May 31 to 1.5% on June 3 to 4.1% on June 7 and should increase rapidly. Soybean aphids were still found on the new growth of buckthorn at Rosemount on Friday, indicating that colonization will continue for awhile, says Dave Ragsdale. Soybean colonization from buckthorn will be supplemented later by winged aphid (alate) production from heavily infested plants in the field. During initial colonization, alates deposit one or a few wingless young before moving on to other plants. Alates may also take up residence and establish larger colonies. For example, Dave Ragsdale observed 4 th stage nymphs last Friday and yesterday we found two heavily infested plants with 28 and 77 aphids.

As these nymphs mature and reproduce, aphid density increases to the point that the colony begins producing its own alates. These alates then leave for uninfested or lightly infested plants nearby within the same field or in adjacent fields and populations on the original plant subside.

The success of spring colonization sets the stage for subsequent infestations in soybean. Several factors influence the magnitude of soybean colonization:

• Egg density on buckthorn going into the winter,
• Overwintering survival of eggs,
• Spring buildup on buckthorn,
• Weather conditions during colonization and establishment, and
• Predator activity on both buckthorn and soybean.

It’s too early to tell whether or not we’re headed for a potential outbreak at this point in colonization. Soybean aphids are in the fields in higher numbers earlier than last year but we’ll have to see how the rainfall affects ongoing colonization and buildup of aphids. Soybean aphid infestations have done better under drought conditions (2001, 2003). Infestations fared poorly in wetter areas of Minnesota in 2002 and 2004.

The best strategy now is to sample a few earlier-planted fields near good overwintering sites. Smaller protected fields near buckthorn are likely to harbor the highest initial populations. Keep following progress in those fields and use them as a local index of colonization. We’d like to hear what you’re finding. Share your info on soybean aphid infestations on the soybean aphid website at http://www.soybeans.umn.edu/ . When infestation rates reach 50% of the plants, it’s time to start checking fields on a broader basis. Keep in mind that the threshold remains 250 aphids per plant on over 80% of the plants. We’re a long way from these levels.

Looking for a simpler sampling approach? Consider saving time by using the speed scouting approach developed by Erin Hodgson. This approach is based on one simple decision for each plant…Does the plant have 40 or more aphids? If so, it’s considered “infested.” The goal is to rapidly classify the field as either lightly infested (below threshold – resample in 7-10 days) or heavily infested (treat within 7 days). This scouting approach involves an initial sample of 11 plants; subsequent samples of 5 plants are taken only as needed to make a decision. A more complete discussion of this new scouting approach can be found at: http://www.soybeans.umn.edu/ . Just click on New Sampling Plan.

This is a new sampling plan with limited field testing, especially since the soybean aphid didn’t cooperate in 2004. The sampling that underlies it was conducted in 30” row spacings, so its performance in drilled soybean is unknown. If you would like to help evaluate this new sampling approach let me know at (612) 750-0993.

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