Does the Systemic Insecticide Imidacloprid (Gaucho) Have a Direct Effect on Yield of Grain Sorghum?

Gaucho¹ (imidacloprid; l-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine) is a seed treatment insecticide widely used on sorghum [Sorghum bicolor (L.) Moench] to control a number of pests, including greenbugs [Schizaphis graminum (Rondani)], chinch bugs [Blissus leucopterus leucopterus (Say)], and wireworms (Limonius sp.). A series of tests was conducted at five locations in Kansas during 1996 and 1997 and three locations during 1998 to determine whether a yield response results even in the absence of pests when Gaucho is used as a seed treatment on grain sorghum. Results varied. Gaucho improved yields at Hesston, especially in the June plantings but some hybrids benefited more than others. These differences may have been due to chinch bugs that were observed in the plots and were known to be troublesome in the area. Because their numbers were low, other factors may have played a role. At the other locations and in the absence of chinch bugs, Gaucho had less of an influence on yields. At Garden City, where some greenbugs developed, Gaucho provided early-season control, but infestations were too light to influence yields. At Hays, Gaucho treatments appeared to have some influence on the numbers of corn earworms [Helicoverpa zea (Boddie)] in sorghum heads in one planting, although yield differences were not significant. Overall, this study found no consistent yield increases in fields treated with Gaucho in the absence of observable pest activity. Research QuestionGaucho is the trade name for imidacloprid, a systemic insecticide commonly used as a seed treatment on sorghum for control of chinch bugs, greenbugs, and other insects. Its ability to control these insects and protect sorghum yield potential is well documented. However, the effects of Gaucho on sorghum growth, development, and yield in the absence of noticeable insect populations are less well understood. Growers and others using this product have suggested that a yield increase can occur in the absence of insect attack. We were interested in determining whether this is true and under what conditions it would occur. Literature SummaryThe insect controlling effects of imidacloprid are well documented. The insecticide is registered for control of such pests as greenbug, chinch bug, and wireworms. Other systemic insecticides such as disulfoton and carbofuran have been shown to have direct effects on plant growth and crop yields. For example, yields of peanut, tobacco, and sorghum were increased in soil treated with disulfoton and yields of tobacco, corn silage, corn grain, rice, and grain sorghum were increased with carbofuran soil treatments. However, other studies have shown no yield response to carbofuran by corn or to disulfoton or phorate by soybeans. Study DescriptionTests were conducted at five locations representing various soil types and agronomic practices in Kansas in 1996 and 1997 and three locations in 1998. Treated and untreated seed of five sorghum hybrids were obtained. Each treated lot was treated with Gaucho at a rate of 4oz. ai/cwt. Additional hybrids were evaluated at one location in 1996 and 1997. Plant stands were recorded at the 2-leaf stage of development. Head counts were made approximately 3 wk after the bloom stage. Plots were harvested with a modified two-row Gleaner combine, and yields and test weights were determined. Plots were examined at various intervals for the presence of insects. When infestations occurred, the numbers per plant were determined by examining several plants in each plot. Applied QuestionsWere insect populations present in these tests? Small numbers of chinch bugs were detected at Hesston and greenbugs were detected at Garden City but in general insect activity was not detected. When insects were present, did the insecticide treatments have any effect? Imidacloprid gave good control of greenbugs early in the season and caused about a 50% reduction in greenbug numbers late in the season at Garden City. At Hesston, chinch bugs were present but numbers were too low to quantify. Did the insecticide treated plots have a higher yield in the absence of insect attack? In general, no consistent yield increase occurred in Gaucho treated sorghum at five locations in Kansas. The exception was at Hesston, where significant increases were noted with some hybrids in both 1996 and 1997. RecommendationOur study suggests that yield improvement following use of Gaucho in fields with no noticeable pest activity is overrated. A consistent yield response was only observed at one location, which frequently experiences problems with chinch bugs. Low insect numbers were observed in the plots at this location. Some hybrids respond to the Gaucho treatment more than others.