Growers who are making the decision whether to apply an insecticide or an insecticide-fungicide tank mix on soybeans have some university research findings and a simplified economic analysis to help them.
The following information is provided by a team of entomologists and plant disease specialists at Iowa State University. Rebekah Ritson is a graduate student in entomology and Matt O'Neal is a professor of entomology. Alison Robertson and Daren Mueller are ISU Extension plant pathologists, and Nate Bestor is a graduate student in plant pathology at ISU.
Trials were conducted at three Iowa locations (Sutherland, Ames and Nashua) over three years (2008 to 2010) to determine the effects of applications of insecticides (Asana, Leverage 2.7SE) and fungicides (Stratego YLD) applied alone or combined (i.e. a tank mix) at soybean growth stages R1 (beginning flowering) and R3 (beginning pod set) on soybean aphid populations and soybean yield in Iowa.
Because these pesticides were applied based on plant growth stage, regardless of the level of fungal disease or insect pressure, these treatments are referred to as prophylactic treatments. The ISU researchers compared the prophylactic approaches to an integrated pest management (IPM) approach, in which an insecticide was applied when soybean aphids reached an economic threshold of 250 aphids per plant.
What does the research show?
• Insecticide applications reduced aphid populations (Table 1).
• Using the IPM guidelines the researchers applied insecticide only 57% of the time. During 2010 the research team did not apply insecticide as aphid populations did not reach 250 aphids per plant.
• During this study, applying insecticides at R3 growth stage of soybeans or according to IPM guidelines reduced aphid populations more effectively than application at R1.
• Fungicides did not effect soybean aphid populations.
• Use of an insecticide-fungicide tank mix resulted in higher yields than use of an insecticide alone, but these differences were not statistically significant.
There are economic considerations you also need to consider
The ISU research team conducted a simplified economic analysis based on a break-even yield gain analysis to determine the economic viability of each of the management plans used in the field trials. Costs of pesticides, costs of application and scouting services, expected crop price, and expected yield were used to calculate a gain threshold (GT). A gain threshold is the increase in number of bushels of yield required to cover the costs of the applications. The team used prices that were typical for 2010 to estimate the cost of scouting and application service cost.
Application of an insecticide based on IPM guidelines resulted in approximately 80% probability (on average) of a yield increase great enough to surpass the gain threshold. In other words, 80% of the time, the use of an insecticide resulted in yields being high enough to pay for the cost of the insecticide and its application. Due to differences in overall costs (i.e. increase cost for scouting) and yields, prophylactic application of an insecticide or an insecticide-fungicide tank mix at soybean growth stage R3 provided slightly higher average probability (between 86% to 89%) of recouping treatment costs.
Study concludes scouting, and spraying only if needed, is the best bet
However, due to risks associated with prophylactic pesticide application, such as yield loss due to ground application (Hanna et al. 2007) and development of resistance to pesticides (Bradley 2010), the ISU specialists recommend that scouting still be used to ensure a pest problem is present before applying either insecticides or fungicides.
Sources: Bradley, C.A. 2010. Frogeye leaf spot pathogen with reduced sensitivity to fungicides found in Tennessee soybean field. University of Illinois Extension Bulletin. 24:172.
Hanna, S., S. Conley, and J. Santini. 2007. Managing fungicide applications in soybean. Purdue Extension: Soybean Productions Systems. SPS-103-W.