How do RoundUp® Ready soybeans affect the growth of fungi that may keep down the populations of some significant soybean pests? Experiments in the laboratory and soybean fields explored this question

Lewis, Leslie C.

Morjan, Wilmar E.

Pedigo, Larry P.

How do RoundUp® Ready soybeans affect the growth of fungi that may keep down the populations of some significant soybean pests? Experiments in the laboratory and soybean fields explored this question.</p

Pedigo, Larry

Lewis, Leslie

Morjan, Wilmar

Digital Repository @ Iowa State University (ISU)

00-41 COMPETITIVELeopold Center GRANT REPORT FOR SUSTAINABLE AGRICULTURE Principal Investigator: Larry P. Pedigo Co-investigators: Leslie C. Lewis Wilmar Morjan Entomology Iowa State University Budget: $32,000 for year one $32,000 for year two Question: Will RoundUp® applications on RoundUp® Ready soybeans destroy fungi that control insect pests, resulting in more insect outbreaks? Answer: Results of the research showed that ingredients in RoundUp® applications may cause larger spider mite populations in soybeans under dry conditions. Farmers who use the RoundUp® Ready system should be aware of this and monitor their soybeans for increased spider mite populations. Ecological impact of herbicides associatedwith transgenic soybeans on spider mitesAbstract: How do RoundUp® Ready soybeans affect the growth of fungi that may keep down the populations of some significant soybean pests? Experiments in the laboratory and soybean fields explored this question. Background A dramatic change in soybean production prac­tices has come with the adoption of transgenic crops resistant to herbicides. In 1997, 14 per­cent of U.S. soybean acreage was planted to RoundUp® Ready soybeans. By 2000, the total had increased to 59 percent. RoundUp® is a broad-spectrum herbicide that ranks among the top ten herbicides used in the United States. It has been determined that RoundUp® has fungicidal properties on some pathogenic and saprophytic fungi. Studying the interaction of glyphosate with fungi that attack insects (entomopathogenic fungi) is important because a wider use of this herbicide is predicted with glyphosate-tolerant crops. Under normal weather conditions, entomopathogenic fungi are considered the most important mortality factor of several ar­thropod pests (insects and spider mites) in Iowa, maintaining these pest populations be­low economic thresholds. The main objective of this research was to determine if outbreaks of spider mite and green cloverworm populations could be promoted with the use of transgenic herbicide resistant soybeans and their herbicides in commercial fields. The project objectives were to: 1. Determine the effect of glyphosate andglyphosate formulations on four entomopathogenic fungi important in Iowa crops, 2. Determine the suitability to greencloverworms of glyphosate-treated and untreated transgenic soybeans compared to traditional varieties, 3. Quantify twospotted spider mite and greencloverworm densities in commercial fields planted with transgenic glyphosate-resis-tant soybeans and fields with non-transgenic varieties, and 4. Determine the occurrence of Neozygites floridana on spider mites and Nomuraea rileyi on green cloverworm in commercial fields planted with transgenic glyphosate-resistant soybeans and non-transgenic va­rieties. Approach and methods Objective 1) Four species of entomopathogenic fungi were obtained from the USDA-ARS collection. They were placed in various growth media and received treatments (seven glyphosate formulations, five blank formula­tions, glyphosate and distilled water). In-vitro tests were conducted and growth inhibitors were measured. Objective 2) Green cloverworms were ex­posed via feeding experiments to transgenic soybeans treated with glyphosate and not treated with glyphosate. Developmental traits were calculated, including sex ratio and mor­phological measurements. Objectives 3) and 4) Observational data were obtained from commercial fields planted and managed by soybean producers so several Volume 11 (2002) 36 factors could not be controlled. Statistics were collected on spider mites and occurrence of N. floridana and green cloverworm and occur­rence of N. rileyi. Variables included leaf area, density, intensity, numbers of infected mites, cloverworm larvae mortality, and incidence of parasitism. Results and discussion Laboratory studies determined that some RoundUp® formulations, but not the active ingredient (glyphosate) had fungicidal proper­ties on four entomopathogenic fungi impor­tant in Iowa crops (Neozygites, Nomuraea rileyi, Beauveria bassiana, and Metarhizium anisopliae). Host suitability studies showed that green cloverworm does equally well de­velopmentally, and possibly reproductively, when fed traditional varieties and RoundUp® treated and non-treated RoundUp® Ready varieties. Field studies conducted in 1999 (Iowa, Minne­sota, and Ohio) and 2000 (Iowa) showed that in drought conditions, commercial fields planted with RoundUp® Ready soybeans had higher spider mite populations than fields planted with traditional varieties. Occurrence of N. floridana (entomopathogen of spider mites) was lower in fields planted with RoundUp® Ready soybeans than with tradi­tional varieties. No statistical differences were detected in green cloverworm populations or occurrence of its entomopathogenic fungus (N. rileyi) between traditional and RoundUp® Ready varieties. Conclusions Results from these studies suggest that out­breaks of twospotted spider mite are more likely to occur on RoundUp® Ready soybeans than on traditional varieties under environ­mental conditions suitable for spider mites. Results showed that glyphosate formulations had fungicidal properties against all fungal species. Impact of results Three studies were conducted to determine if outbreaks of twospotted spider mite and green cloverworm populations could be promoted with the use of transgenic glyphosate-resistant soybeans and glyphosate use in commercial fields. The first study evaluated the fungicidal effects, under laboratory conditions of glyphosate and seven glyphosate formulations on four entomopathogenic fungi. Results showed that glyphosate did not have fungi­cidal effects on any of the fungi tested. How­ever, glyphosate formulations had fungicidal properties against all fungal species. For ex­ample, N. floridana and M. anisopliae were susceptible to all glyphosate formulations. The strongest fungicidal properties were observed with the RoundUp® Ready-To-Use formula­tion. The results of this in-vitro study indicate that the four tested fungi are susceptible to various glyphosate formulations when exposed to field concentrations. Thus, application of these formulations in soybean fields could have an impact on the natural fungal epizoot­ics necessary for adequate pest regulation. Pest incidence in crops also may be affected by changes in plant suitability as a result of stress factors, such as herbicide metabolism in the plant. Therefore, the second study evaluated whether host suitability to green cloverworm differs between transgenic glyphosate-resis-tant soybeans compared to conventional vari­eties, as well as if plant stress, induced by glyphosate, affects plant suitability to green cloverworm. No statistical differences among treatments were detected on developmental time and survivorship, sex ratio, and morpho-Leopold Center Progress Reports 37 For more information contact Larry Pedigo, Entomology, Iowa State University, Ames, Iowa 50011; (515) 294-8694, e-mail lpedigo@iastate.edu logical performance. Although treatments did not affect size (calculated with principal com­ponent analyses), significant differences were observed between males and females. These results suggest that soybean genetic differ­ences (between conventional varieties and analogous transgenic varieties) or plant stress (induced by glyphosate metabolism) do not affect the plant suitability for green cloverworm. Thus, conventional varieties and varieties of RoundUp® Ready soybeans with or without glyphosate stress, contribute simi­larly to the insect’s development, survival, and reproductive potential. Field pest populations are strongly influenced by epizootics caused by entomopathogenic fungi. Thus, evaluations of field responses of these emtomopathogens to pesticides with fun­gicidal activity are important. In the third objective, commercial fields planted with transgenic glyphosate-resistant and non-transgenic soybeans were evaluated for twospotted spider mite and green cloverworm populations, as well incidence of their entomopathogenic fungi, N. floridana, and N. rileyi. Economic spider mite populations re­quired insecticide applications in six transgenic and three non-transgenic fields. Spider mite intensity was greater in transgenic soybeans than in non-transgenic soybeans, while per­centage of infection by N. floridana was smaller in transgenic (54 percent) than in non-transgenic (58 percent) fields. Populations of green cloverworm were similar in transgenic and non-transgenic fields and were character­ized as endemic. No statistical differences were detected between the treatments on the incidence of N. rileyi, which was less than 5 percent. Results from this observational study suggest that green cloverworm populations and incidence of its entomopathogenic fungus N. rileyi would be similar in fields planted with transgenic glyphosate-resistant soybeans and conventional varieties with endemic popula­tions. Results from this study all suggest that under favorable conditions for spider mites, fields planted with transgenic glyphosate-re-sistant soybeans may have greater predisposi­tion to outbreaks than fields planted with non-transgenic varieties. In conclusion, larger spider mite populations observed in RoundUp® Ready soybeans may be partially a result of differences in field characteristics. However, outbreaks may have been enhanced as a result of lower incidence of N. floridana in the RoundUp® Ready soybean fields due to fungicidal effects of inert ingredi­ents in glyphosate formulations. Education and outreach Three scholarly publications were written to summarize the results of this project and all have been submitted to journals. Volume 11 (2002) 38 

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Ecological impact of herbicides associated with transgenic soybeans on spider mites

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Ecological impact of herbicides associated with transgenic soybeans on spider mites

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