Adding Insecticide to Foliar Fungicide for Dryland Soybean
Managing input costs is especially important for dryland soybean production in the Midsouth because yield will be limited due to periods of drought during the growing season, and profit potential will be lower.
In 2011 and 2012, research was conducted in Missouri to determine if adding an insecticide to a fungicide applied to dryland soybean is of value. Reports from this research were published in an articled entitled “Value of an Insecticide Added to a Fungicide for Soybean During Drought” on Mar. 4, 2016 in Crop, Forage, and Turfgrass Management
The premise for conducting the research was that tank mixing and applying an insecticide with a preventative fungicide application will reduce cost vs. that of a separate insecticide application. However, species of myriad insect pests occur at different levels during the growing season and thus may not be at treatment thresholds when a prophylactic fungicide application is made at the recommended time.
The specific objective of the research was to evaluate the impact of an insecticide plus fungicide application at the R3 and R5 growth stages of soybean vs. insecticide applied alone at treatment threshold levels separate from fungicide applications. Insect populations were documented with sweep net sampling prior to insecticide applications.
The treatments included pyraclostrobin (Headline) fungicide applied alone, lambda-cyhalothrin (Karate Z) insecticide applied alone, fungicide + insecticide applied together regardless of insect numbers, fungicide + insecticide applied together whenever insect treatment thresholds were reached, and an untreated control. Plots of the insect threshold treatment were monitored weekly from R3 to R7. The research period of both years had below-normal rainfall, and therefore was classified as abnormally dry.
The pertinent results from this research follow.
● A total of 21 different species/types of insect pests occurred during the R3 to R7 monitoring period. Beneficial insects totaled 13 different species/types during this period.
● The insecticide application at R3 resulted in a decrease in both pest and beneficial insect populations compared to the untreated control, while the insecticide application at R5 decreased the number of insect pests but did not decrease the population of beneficial insects. Thus, delaying the insecticide treatment until R5 had less of an impact on beneficial insect populations.
● Soybean seed yields averaged 38.5 to about 40 bu/acre, and there were no yield differences among any of the pesticide treatments. Thus, there was no economic benefit to adding an insecticide to the fungicide application in these studies conducted in dry conditions.
● None of the plots in the insect threshold treatment reached treatment levels, which means that adding an insecticide to the fungicide at either or both the R3 and R5 growth stages was unwarranted in this study. Thus, scouting for insect pests was especially important in these dryland studies for preventing unnecessary expense associated with insecticide applications that were piggy-backed on the fungicide application.
These results support the IPM approach for insect management in dryland soybean, which includes scouting to monitor insect pest populations in order to prevent unnecessary expense as well as to prevent a reduction in beneficial insect numbers.
It should not be assumed that the above results are applicable to all dryland soybean environments. However, these results do confirm/reaffirm that the IPM concept is especially important for insect management in dryland soybean, and that it is the only management strategy that will result in profitable insect management, preserve beneficial insect populations, and preserve present-day insecticide chemistry.
These results are supported by past research [Crop Sci. 31:1625-1629 (1991) and Crop Sci. 35:1657-1660 (1995)] that showed that 1) soybean looper larval weight is decreased and their developmental period is increased by plants growing on drier soils, and 2) yield reduction resulting from looper infestation is greater for irrigated than for nonirrigated soybean.
Composed by Larry G. Heatherly, July 2016, larryheatherly@bellsouth.net