by Doug Johnson
Kentucky Pest News
University of Kentucky

The agricultural industry is abuzz with the discovery of soybean rust in the US. Unquestionably, the development of sound management strategies for this disease will have a major impact on production practices. However, control of soybean rust can have implications well beyond the rust epidemic. One is the impact of a fungicide on fungi that attack arthropods that feed on the crop; the other is potential outcome from adding an insecticide to the spray tank as an “insurance” treatment. 

Impact of fungicides on beneficial

Specific fungal pathogens are an important part of population regulation of soybean feeders such as green cloverworms, soybean aphids, and two spotted spider mites. While we rarely see evidence of their impact, research studies have documented the importance of these fungal entomopathgens. Often, these organisms are susceptible to fungicides applied to crops to suppress plant pathogenic fungi. Consequently, the widespread use of fungicides to control soybean rust might also affect populations of some soybean insect and mite pests, which have the potential to damage the Kentucky soybean crop.

This is not a new concept.  As far back as 1908 scientists reported that Bordeaux mixture used in Florida citrus resulted in large increases in some insect pests because it suppressed fungi that kill insects.  Even as a graduate student studying entomology I worked on this same problem in soybean. A popular fungicide used in soybean at that time was shown to suppress the fungal pathogen which helped control the velvetbean caterpillar, a major insect pest.

A more current example of this almost nightmarish problem is illustrated by a situation concerning fungicidal control of a disease in potato. (See the link to Radcliffe, Ragsdale and Lagnaoui below).  In this case, the fungicide applied against a fungal disease of potato also suppressed a different fungus that controls aphids, allowing the aphid population to increase.  These aphids not only feed on potato but also transmit important virus pathogens.

This latter example may be particularly important to Kentucky grown soybeans. In the more northern soybean producing states the soybean aphid has become a significant insect pest.  In addition to the aphids feeding on the plants they may also move viral pathogens of soybean.  Thought this has not yet been a widespread problem, the soybean aphid has only been in the US for four years, and damaging populations have not been frequent over the Kentucky soybean production region. The blanket use of fungicides might remove an important natural control from these aphid populations, allowing them to both spread and increase in number.

Don’t add an insecticide if it is not needed.

An even more difficult problem could arise from a misuse of pesticides in an attempt to “cover the waterfront’.  If fungicide applications are necessary, there will be a temptation to put a little insecticide in the tank “just in case”.  While there are situations when insecticide / fungicide combinations are appropriate, the wholesale addition of insecticides to applications targeted at soybean rust could produce some very bad consequences. Insecticide applications, even when warranted, devastate natural biological control.  Applying insecticides when pests are present but not at treatment thresholds, could release them from natural control and allow them to increase at faster rates and to higher numbers.  The result could be more frequent pest outbreaks, or trouble from secondary pests that rarely threaten the crop.

Judicious use of pesticides

1) Follow the soybean rust management recommendations for our area.
2) Use insecticides when pest levels warrant an application. However, don’t put an insecticide on just because you are spraying for soybean rust and don’t put a fungicide in the tank just because you are spraying for soybean aphid! It is not likely to be cost-effective, and secondary pest outbreaks, resurgence, and resistance are much more likely to occur in a pesticide-intense system.  We have not seen the like of this possibility in Kentucky before.

Radcliffe, T., D. Ragsdale, and A. Lagnaoui.  1996. Fungicides Impact Aphid Control.  http://ipmworld.umn.edu/chapters/rrvpga.htm In: E. B. Radcliffe and W. D. Hutchison [eds.], Radcliffe's IPM World Textbook, URL: http://ipmworld.umn.edu, University of Minnesota, St. Paul, MN