Temperatures -

Temperature influences soybean aphid management decisions. Studies on the reproduction of soybean aphid indicate their reproduction decreases steadily as the temperature rises above 85F. This can impact management decisions. Remember, soybean aphids reproduce by producing live young, all of which are females and ready to reproduce their own daughters in 7 days. Consequently, aphid populations can increase very quickly.

Between 90-95F, soybean aphid reproduction stops, their mortality increases, and their feeding decreases (but does not stop). But this effect is reversed when the temperature drops. So if daytime temperatures are in the 90’s, there will be less aphid reproduction and feeding, but expect both to continue in the evening and mornings when the temperature is below 90-95F. It’s a matter of average temperatures; days in the 90s may result in less aphid population growth, but don’t expect them to disappear unless our daily average temperatures exceed 90-95F for several days.

High temperatures also impact the efficacy of some insecticides. Temperatures over 90F significantly decrease both the immediate toxicity and half-life (the period of time it takes a chemical to break down to half it’s original amount) of synthetic pyrethroid insecticides (SPs), (e.g. Warrior, Asana XL, Decis 1.5 EC, Proaxis, Taiga Z, etc). This means if SPs are applied when the temperature is over 90F, they won’t provide the control they do when temperatures are below 90F. If they’re applied when the temperature is in the 80’s and then the temperature rises above 90, they won’t provide the extended aphid suppression because their half-life is decreased. Temperatures of 90-95F have little or no effect on organophosphate insecticides (OPs), (e.g. Lorsban 4E, Penncap-M, Orthene, etc).

Extended Suppression –

The SPs have been shown to generally provide longer suppression of aphid populations than do most OPs. Although influenced by environmental conditions, UMN trials most consistently show SPs suppressing re-infestation of soybeans by aphids for 10-14 days, some conditions have resulted in even longer suppression. Our research indicates this is probably the result of direct residual toxicity but is more likely a repellency effect.

The top performing insecticides in both classes provide the same rates of initial mortality, but the OPs seem to work more quickly (i.e. as many aphids will die when treated with a good SP as with a good OP, but it will take longer for them to die). Likewise, we haven’t seen a difference in yields between the top performers in either class.

Typically in the RRV, we get our aphid populations in the early reproductive stages and are most likely to treat around R4. Currently, most local fields are at early R1, just starting to flower. This influences management decisions in two ways; 1) we have had impacting populations of soybean aphids develop as late as mid-July, if this happens this year, fields treated with SPs in late July may not have enough extended suppression to prevent treatable populations from developing again later, and 2) any new plant material added after application, and hopefully there’ll be a lot of it, will not be protected and will be vulnerable to aphid infestation and population growth.

Precipitation –

However, the earlier a field is treated, the more likely it is that field will require re-treatment later in the season.  As application date progresses through the season, there’s less time for re-infestation and population growth.  It should be noted that in most cases warranties cover only product and not the application costs, so it is not necessarily a good decision to spray early when aphids are still below threshold populations simply because of the warranty. 

Although soybean aphids are relatively well distributed in the area, populations still vary field to field, and spray decisions must be made accordingly. Some fields, with slower developing populations, may not exceed threshold until later in the season and may not have as high a risk of re-infestation as fields requiring treatment in early July.  Product warranties may provide an excellent opportunity for growers whose fields are currently over threshold, but it remains a field by field decision.