The dry conditions and moderate temperatures over the weekend have been ideal for wheat maturation.  Unfortunately, they’ve also been terrific for aphid population growth!  Both cereal aphid and soybean aphid populations in the central & northern RRV continue to rise. 

Cereal Aphids - the later planted fields are at greatest risk of yield loss.  Fields that are in flag or early boot stage still have considerable development ahead of them and the standard recommendation continues to apply: spray as soon as a field reaches threshold (80% - 85% of stems with at least one aphid per stem).

The current conditions and the immediate forecast continue to favor aphid reproduction and aphid numbers may continue to rise. In Univ. of Minnesota trials, treating aphids at heading produced an economic return in only two trials – both occurred in years with heavy populations that had been steadily growing over a period of time. In both circumstances, greater returns were seen with earlier treatments. 

There are several fields in the central RRV that may fit these criteria.  For every 100 cumulative Aphid Days, the yield loss is estimated to be approximately 0.6 bu/ac. This rate of yield loss decreases as the plant matures for a number of reasons: physiological changes in the plant and maturation of the grain make it less susceptible to aphid damage, after heading the plant starts to become less suitable as a host, aphid populations start to decline, and natural mortality factors, such as predators, start to impact the population so that aphid populations generally start to decline within 2 weeks after heading.  From heading on, there usually isn’t enough time to accumulate sufficient aphid days to cause the amount of yield loss that would economically justify an insecticide application.  HOWEVER the very heavy populations in combination with the current conditions of dry weather and cooler temperatures may allow enough aphid days to accumulate to provide an economic.

The Bottom Line – If plants are pre-heading and at threshold, then immediate treatment is recommended, don’t wait until heading!  If plants are headed, the standard recommendation is not to treat, the exception is if very heavy populations exist a treatment might produce an economic return (100 Aphid Days = ~0.6 bu/ac loss).  Also remember to check the label for Pre-Harvest Intervals!!

Soybean Aphids - Yesterday I saw a 100 ac soybean field that requires treatment; over 90% of plants in this field were infested with soybean aphids exceeding 300/plant on average (and some with far more than 300/plant). This was apparently a field in which soybean aphids showed up very early in the season and it had a sub-threshold population evenly distributed across the field

It’s time to scout for soybean aphids!! Keep a close eye on fields with evenly distributed populations!!

The early appearance and development of soybean aphid populations and the high cost of fuel may make it tempting to tank-mix insecticides with upcoming herbicide applications, especially one of the synthetic pyrethroids which may provide extended aphid suppression. If the field is already at or near threshold (250/plant) and aphid populations are increasing, fine. BUT if you’re hoping to get season long aphid suppression, it’s a risky gamble and may have a low probability of paying off!

Be aware: in UMN trials only the high label rates provided extended soybean aphid suppression and the length of suppression most consistently encountered was only 10-14 days. After this, results were less consistent and may have been influenced by environmental conditions. In addition, the plants now are still relatively small but are rapidly growing - after the upcoming herbicide application, soybeans will continue to grow, adding a lot of new plant material, all of which will be unprotected by insecticide residue and open to immediate re-infestation by aphids.