SW MN IPM Stuff 2016 Issue 15

August 9, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-15

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Soybean Aphid Watch 2016

Soybean aphids continue to show rapid population increases in some areas of Minnesota, spectacularly rapid in some areas, including fields at the University of Minnesota Southwest Research and Outreach Center. More areas are now seeing more general populations at economic threshold and being treated. However, populations in many fields and many areas remain low. This could change and yes, there is still plenty of time for it to happen.

It is a bad idea to assume:

1) It's too late in the year for soybean aphids to hurt yield. Aphid populations are late in developing this year. Because of planting delays, soybean development is also behind in some areas of MN, including SW. The filling of soybean pods is just getting started and there are still several more weeks in which yield can be lost to soybean aphids as yield can be lost until the R6.5 stage. If populations continue to increase and persist late, we could still loose 10-15 bushels in most plantings and 40% in late-planted soybeans.

Soybean aphids are a consistent threat to yield loss in MN. Particularly because of the tight farm economy, you should have budgeted for foliar soybean aphid control in 2016. If you don't need to treat, great, that goes on the black side of the balance sheet. If you do need treat to avoid economic loss (remember the difference between economic injury level and economic threshold), you are likely to have a good return on your investment. If you used your crop protection budget on invisible leaf diseases or insecticide seed treatments that provide minimal aphid control well...

The economic or action threshold for soybean aphid treatment is: 80% or more of the plants with aphids, an average of 250 aphids/plant and aphid populations are increasing. Use it until soybeans reach the R6 stage.

white dwarves

Figure 1. The small light-colored morph (white dwarves) of the soybean aphid that occurs mid-season. These will reproduce.

2) I looked last week and they were low so I don't have a problem now. Did you look effectively? On more mature soybeans, aphids are now distributed throughout the soybean canopy. It makes scouting more difficult.

If you are seeing spots of honeydew on lower canopy leaves, you are (or were) likely to be over 250 aphids/plant.

In some fields, aphid populations are now mostly the small "white dwarves" in the lower canopy (Figure 1). Count these aphids as they can reproduce rapidly and these populations will begin to produce "normal" aphids later in the season. These populations can build to many thousand aphids/plant late in the season.

winged aphids on soybean

Figure 2. The presence of winged aphids and/or nymphs with darker heads and squared shoulders and wing pads are a clue that the colony will disperse within the field or longer distance.

winged aphid

Figure 3. We see a dispersal of winged aphids about this time each year.

Things change and populations built rapidly over the past week. In many fields, a large number of winged aphids are being produced on crowded plants. Look for a large percentage of colonies of aphid nymphs with darker heads and wing pads as a clue that aphids may be preparing to leave the plant, and perhaps the field.

Conversely, plants (or leaves) that have few aphids are now being colonized by winged aphids. Fields with high populations can rapidly decline as winged aphids leave and fields with low aphid populations can rapidly increase with winged aphid immigrants.


soybean aphid colony infected by fungal pathogens

Figure 4. Soybean aphid colony infected by fungal pathogens.

Often they arrive in fields that previously had few aphids and few predators. Younger soybean plants present a particularly attractive landing strip. The comings and goings of the winged ones makes predicting population increases more difficult. This migration happens every year at about this time and soybean aphids can be transported long distances on weather systems.

We are just now starting to see a small amount of aphid mortality from fungal pathogens. Look for groups of dead, discolored aphids. High humidity and cool temperatures will favor an epidemic.

There is some significant rain in the forecast and that may help with aphid populations from a disease and sap quality standpoint. Population collapses from disease, predation or emigration are the reason why the soybean action threshold includes the caveat of "soybean aphid populations are increasing".

3) I sprayed earlier so I won't have a problem. Rapid population increases and the winged aphids that are now free to move about the country are two reasons that fields need to be watched until the R6 stage occurs.

Fields that were sprayed earlier may be re-infested because predators and parasites were removed and thus can experience explosive soybean aphid population increases. It is one reason we recommend waiting to spray until the 250 aphid/plant aphid threshold. Fields that were sprayed early should be watched closely for re-colonization by winged aphids.

Finally, in some areas of MN, pyrethroid insecticides are not performing as well as they have previously. As aphid treatment we are seeing additional cases of poor performance by pyrethroids. Some of these have occurred outside the area where there were issues last year. The problem fields have not yet been documented with an assay, but they do not seem to be due to application errors.

Check aphid control 3-4 days after an insecticide is applied. Alive, but dying aphids can be dislodged by shaking the plant. An indication of an insecticide resistant aphid population is dead predator insects, pockets of healthy aphids with dead aphids in the remainder of the field. A reminder: Please report any problems with insecticide control of aphids you observe at: Assessing and reporting potential cases of soybean aphid resistance to pyrethroids.

Happy trails,

Bruce Potter