No. 8 Article 4/May 13, 2005

Insecticides, Fungicides, and Insecticidal Seed Treatments for Soybeans--What Do Our Data Tell Us?

In 2004, concerns about the future occurrence of Asian soybean rust and soybean aphids in soybean fields at the same time prompted recommendations for the application of a fungicide-insecticide combination. If both pests are present, such an application makes sense. However, some people have suggested that applying this pesticide combination would provide yield benefits even if the pests are not present. Obviously, from a pest management standpoint, this type of application does not make sense, and we have addressed this issue before ("Insects Infesting Soybean? Or Not?" in issue no. 18, July 23, 2004, of the Bulletin).

The recent registration of Cruiser insecticide as part of soybean seed treatment also has generated considerable interest. We wrote an article about the use of Cruiser in a soybean seed treatment, too ("Cruiser Registered for Use on Soybeans" in issue no. 3, April 5, 2005, of the Bulletin). Fortunately, we were able to conduct some research trials in 2004 to determine the impact of fungicides, insecticides, and insecticidal seed treatments on insect densities and soybean yields. Unfortunately, the target insects (bean leaf beetles and soybean aphids) were virtually inconspicuous in our trials last year. Nonetheless, we generated some yield data that might shed some light on the issue. Following are descriptions of the trials and interpretations of the results.

Two research trials were established in Whiteside and Stephenson counties on May 4 and 5, 2004, respectively. The trials were established in areas where the presence of bean leaf beetles early in the growing season, and possibly throughout the growing season, was optimized (i.e., the plots were planted in fields bordering wooded areas and alfalfa fields). Unfortunately, densities of bean leaf beetles in the areas where the trials were established (and throughout most of Illinois) were quite low throughout 2004. No economically threatening densities of bean leaf beetles were observed in any of the treatment plots at any time. However, the trials were sampled throughout the growing season, and numbers of all insects (both pest and beneficial species) were recorded. In addition, the plots were harvested in October 2004. Yield data gathered from the two trials are presented in Table 1.

Although average yields ranged from 41.34 bushels per acre (untreated check) to 57.78 bushels per acre (Cruiser + Warrior) in the Stephenson County trial, yields were not significantly different among treatments. In Whiteside County, average yields ranged from 39.12 bushels per acre (Warrior applied once in mid-July) to 50.45 bushels per acre (Gaucho), and there was a significant difference in yield between these two treatments. The average yield in the untreated check (42.68 bushels per acre) was not significantly different from the average yield in the Gaucho-treated plots.

Although there were few significant differences (probably no biological differences) in numbers of bean leaf beetles among treatments in both studies, some differences in yields were noted. There were no significant differences in yields among treatments in Stephenson County (Table 1), but numerical differences were noticeable (e.g., 16.44 bushels-per-acre difference between the Cruiser + Warrior-treated plots and the untreated check plots). Unfortunately, the variability within our trial area did not allow us to identify statistical differences among treatments.

A third trial to determine the impact of 12 different treatments on soybean aphids and yield was established in Kendall County in 2004. The treatments included fungicides, insecticides, and insecticidal seed treatments, alone or in combination. The trial was planted late (in June) purposely to enhance the likelihood of development of economically threatening densities of soybean aphids. The trial was established in an area in which infestations of soybean aphids had been relatively common, and occasionally significant, since 2000. Unfortunately, no economically threatening densities of soybean aphids, or any other insect pest, developed throughout 2004. However, all plots were sampled for insects several times throughout the season, and the plots were harvested in October. Yield data are presented in Table 2.

Densities of all insects in the trial established in Kendall County were extremely low in 2004. Nonetheless, there was a significant difference between the average yield of Apron Maxx + Warrior + Quadris-treated plots (64.99 bushels per acre) and the Apron Maxx-treated plots (55.24 bushels per acre). However, the average yield in the untreated check plots (63.60 bushels per acre) was not significantly different from the average yield of the Apron Maxx + Warrior + Quadris-treated plots.

Some of the yield data gathered from the three trials in 2004 lend some credence to the suggestion that a fungicide + insecticide application would provide a yield benefit, even in the absence of pests. However, the data were not consistent among trials. And it's difficult to gain any perspective on the utility of an insecticidal seed treatment from the one trial in 2004. Previous experiments have revealed that insecticidal seed treatments are quite effective against early-season bean leaf beetles. Consequently, more data will be necessary to either support or refute the claims of yield benefits attributed to fungicides, insecticides, and insecticidal seed treatments. Ultimately, economics and environmental consequences of such treatments must be estimated/assessed.

Plant pathologists and entomologists at the University of Kentucky have published some interesting information regarding the use of insecticides and fungicides. For summaries of their efforts, we direct you to two articles in the Proceedings of the 2005 Illinois Crop Protection Technology Conference ( [Adobe PDF]):

We encourage our readers to view numerous sources of information before making a pest management decision. Interpretations from different perspectives are healthy and usually result in informed decisions.--Kevin Steffey, Mike Gray, and Ron Estes

Bean leaf beetle feeding on soybean seedling (photo courtesy of Kevin Black, Growmark).

Small colony of soybean aphids (photo courtesy of Marlin Rice, Iowa State University).

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