Today’s article addresses the topic of tank mixing insecticide for soybean aphid with fungicides at R3 soybean growth stage, or a bit earlier this season the same conversation was circulating regarding insecticide with 2nd pass Roundup (glyphosate) herbicide application in soybeans.
Considerations for Spraying Insecticides for Soybean Aphid in Soybean
Eileen Cullen, Extension Entomologist, UW-Madison Entomology Department
Soybean aphid population densities continue to be low to moderate. I have received reports of only a few soybean fields treated for economic threshold populations (250 aphids/plant average throughout the field) in the last week. Other than that population densities remain low to moderate, or nonexistent, and below threshold.
Please refer to the July 9th WCM newsletter article for information on soybean aphid economic threshold and how you can track soybean aphid population densities online for Wisconsin
http://ipcm.wisc.edu/WCMNews/tabid/53/EntryId/764/Soybean-Aphid-Densities-are-Low-But-Start-Scouting-Now.aspx.
Today’s article addresses the topic of tank mixing insecticide for soybean aphid with fungicides at R3 soybean growth stage, or a bit earlier this season the same conversation was circulating regarding insecticide with 2nd pass Roundup (glyphosate) herbicide application in soybeans.
While convenient, insecticide tank mixing in the absence of soybean aphid, or at soybean aphid populations below economic threshold, is not recommended. Most phone calls and field visits over the last two weeks growers, Certified Crop Consultants, UW Extension county agents and others have mentioned promotions for tank mixing an insecticide with a fungicide to provide “long residual” insecticide effect (3 weeks or more) on soybean plants in the event soybean aphid populations build to economic threshold or colonize in greater numbers in the weeks ahead.
Optimal application times for fungicides, herbicides and insecticides rarely coincide with each other. Tank mixes would be justified if plant pathogen and insect densities simultaneously reach economic levels at optimal plant growth stages for economic yield return, but this is not typically the case. Please see Dr. Shawn Conley and Dr. Paul Esker’s companion article on the tank mix topic regarding foliar fungicides in soybeans in this issue of the WCM newsletter. The key to economic yield protection for soybean aphid management is timing treatment at economic threshold population to prevent aphids from reaching the economic injury level and conserve natural enemies in the field (predators and parasitoids) for as long as possible.
Moreover, university entomology field research at UW Madison and throughout the North Central region, has found no economic yield return to treating soybean aphid below the economic threshold of 250 aphids/plant.
For a full discussion of that topic, please visit my previous WCM article link:
Soybean Aphid Update for Late August 2008
http://ipcm.wisc.edu/WCMNews/tabid/53/EntryId/624/Soybean-Aphid-Update-for-Late-August-2008.aspx In the article link, under Frequently Asked Questions, please scroll down to Question 5. “The price of beans needs to be factored into the threshold. Do higher bean prices mean we lower the economic threshold below 250 aphids/plant?” The economic threshold has not changed, and is not lowered. Please check out the link above for a refresher on those details.
So, we know it is not economically profitable, or advisable, to tank mix an insecticide in the absence of soybean aphid pressure, or at population densities below economic threshold. What are other parts of the tank mix, insecticide “residual” equation?
The concept of preventative soybean aphid treatment without field scouting data, or at aphid numbers below economic threshold, in order to obtain residual control for “incoming” pests for a pre-defined period (for example, 21 days) is not a valid IPM approach.
UW Extension and grower and Certified Crop Consultant concern is justified as unnecessary insecticide tank mixes are likely to increase the number of insecticide applications needed over the season. When natural enemies are eliminated prematurely, this creates an “enemy free” space in which soybean aphids can colonize and quickly rebound to high numbers, requiring insecticide treatment. Repeated applications within each growing season increase selection pressure for insect resistance development. Treatment cost is another factor to consider.
In addition to pyrethroid and organophosphate insecticide products, premix foliar insecticides are available that combine a nicotinoid active ingredient such as imidacloprid or thiamethoxam with a pyrethroid. Keep in mind, while we categorize nicotinoids as systemic insecticides when applied as seed treatments and through the early vegetative soybean growth stages, when the soybean plant is actively growing and nictotinoids are applied foliar in season, they are translaminar and active in the foliage to which they are applied. As new growth and new leaves develop on the plant, foliar applied nicotinoids are not systemic to all plant parts to the degree that it would provide complete residual control to new growth and incoming aphids a month after application. The bottom line is that any of the registered insecticides will be effective against soybean aphid, as long as they are optimally timed to economic threshold populations, based on solid field scouting data.
I’ve been asked to review some insecticide label definitions in this article as well, especially as it pertains to the terms insecticide Preharvest Interval (PHI) and Residual.
REI: All pesticide products used in the production of agricultural plants have a prescribed-time re-entry interval (REI) for worker protection. REI is stated in hours on the label, usually 48-hour, 24-hour or 12-hour depending on the product signal word. For the most part, entry into treated areas during the prescribed time REI is not allowed.
PHI: The preharvest interval is the elapsed time between a pesticide application and harvest of the crop. The PHI is defined explicitly in days. PHI on an insecticide label is where we typically see 21, 30 or in some cases a fewer or greater number of days. PHI depends on the specific pesticide label you are following, and the crop to which it was applied.Residual: Foliar insecticide residual activity is essentially the period following treatment and initial mortality of the insect population present at application during which insect suppression continues to be effective to a relative degree. Residual activity is different from contact or systemic activity, rather this is the residual effect of insecticide on treated surfaces encountered by surviving or newly arrived insects after treatment. Typically, in insecticide efficacy trials a pre-treatment count is taken, experimental treatments applied, and post-treatment insect counts are compared to the untreated check plots 4, 7, and 14 days after treatment (DAT) and up to 21 or 28 DAT. Residual activity can vary and several inter-related factors are involved (e.g., insecticide active ingredient, formulation, temperature, UV light intensity, susceptibility of insect treated, pre-treatment population density, percent control achieved, to name a few). Insecticide labels do not typically post “residual” activity information.Residual activity should be distinguished from well defined, (and label law) PHI’s stated on labels in days. An insecticide treatment decision, application timing and post treatment monitoring of treatment efficacy against the pest and impact on predators and parasitoids are based on field scouting data in an economically profitable and environmentally sound IPM program.
Additional Resources:
Avoid a tank tri-mix on soybean
Why it’s not a good idea to tank mix fungicide, herbicide and insecticidehttp://www.planthealth.info/rust_mgmnt.htm