Boundaries between cropland and natural plant communities provide food, water, and cover for wildlife. Similarly, field borders com­ posed of natural plant communities, especially those that include flowering plants, may have a marked effect on natural enemies of crop pests within crop fields. Monoculture crops typical of much of the Midwest do not provide adequate sources of food (nectar, pollen) or shelter and breeding sites for these natural enemies. Nectar and pollen are produced by a variety of native Midwestern plants; as these plants flower in succession throughout the growing season, they provide a continuous food source for the natural enemies (parasites) of pests attacking adjacent crops

Orr, David B.

Pleasants, John M.

Boundaries between cropland and natural plant communities provide food, water, and cover for wildlife. Similarly, field borders com­ posed of natural plant communities, especially those that include flowering plants, may have a marked effect on natural enemies of crop pests within crop fields. Monoculture crops typical of much of the Midwest do not provide adequate sources of food (nectar, pollen) or shelter and breeding sites for these natural enemies. Nectar and pollen are produced by a variety of native Midwestern plants; as these plants flower in succession throughout the growing season, they provide a continuous food source for the natural enemies (parasites) of pests attacking adjacent crops.</p

Orr, David

Pleasants, John

Digital Repository @ Iowa State University (ISU)

92-53 COMPETITIVE GRANTLeopold Center LEOPOLD CENTER FOR SUSTAINABLE AGRICULTURE Influence of native roadside plants on biological control of Iowa crop pests Background Boundaries between cropland and natural plant communities provide food, water, and cover for wildlife. Similarly, field borders com­posed of natural plant communities, especially those that include flowering plants, may have a marked effect on natural enemies of crop pests within crop fields. Monoculture crops typical of much of the Midwest do not provide adequate sources of food (nectar, pollen) or shelter and breeding sites for these natural enemies. Nectar and pollen are produced by a variety of native Midwestern plants; as these plants flower in succession throughout the growing season, they provide a continuous food source for the natural enemies (parasites) of pests attacking adjacent crops. The Iowa Office for Integrated Roadside Man­agement as well as the Iowa Department of Transportation are conducting and encourag­ing plantings of native vegetation, including a number of flowering native plant species, along Iowa roadsides. The goals of this program include reduced mowing and spraying costs, reduced soil erosion, more effective weed management, and enhanced aesthetic and natu­ral resource values along rural roadways. With Iowa's extensive road system, this effort rep­resents a potential total of approximately 600,000 acres of roadside rights of way. This area also represents a potential "nursery" for the natural enemies that attack insect pests of Iowa's agricultural crops. The goal of this project was to help determine whether prairie borders along crop fields could affect parasites of crop pests. Toward its main objective, this project studied these borders' effect on the European Corn Borer (ECB) in field corn. Second in importance in the Mid­west only to corn rootworm, the ECB is sus­ceptible to the parasitic wasp Macrocentrus grandii. In the Midwest, this wasp has two generations that are synchronized with the two generations of the ECB. The female wasp lays eggs in ECB larvae, and multiple adults emerge from the host cocoon. Immature parasites that overwinter in the overwintering (diapausing) larvae of the ECB produce the second genera­tion of M. grandii, which emerges in the spring. Phase one of this work involved identifying native prairie plant species that could provide significant food resources for M. grandii. Phase two measured wasp activity and parasitism levels on second generation ECB in portions of corn fields adjacent to prairie borders and portions with a more typical roadside border of brome grass. Approach and methods Potential of native prairie species to benefit M. grandii, 1992: To determine whether wasps were capable of feeding on and benefit­ing from the flowers of a particular plant species, investigators compared the longevity of individual wasps in small, environmentally controlled, walk-in chambers containing flow­ers of that species (plus water) with individu­als in chambers with no food source (water only) and individuals whose sole food source was honey. Newly emerged female M. grandii from a lab colony of ECB larvae were used. Testing was correlated with time periods when first and second generation ECB were present in the field. Because populations of M. grandii are very well synchronized with their hosts in the field, adult M. grandii were assumed to be active during these time periods—in 1992, from June 17 to July 6 (first generation ECB) and August 7 to September 6 (second genera­tion). REPORT Principal investigator David B. Orr (year one) Entomology John M. Pleasants (year two) Entomology and Zoology/Genetics Iowa State University Budget $22,474 for year one $23,170 for year two Leopold Center Progress Reports 73 For the first ECB generation, field-collected clusters of blooms were used from Purple Coneflower, Ox-Eye Daisy, Wild Rose, and Milkweed. During the second generation, the plant spe­cies tested were Blazing Star, Daisy Fleabane, Horse Mint, Goldenrod, Mountain Mint, Wa­ter Hemlock, and Ironweed. Because corolla (petal) tube depth can play an important role in determining accessibility of nectar for wasps, the depth of corolla tubes on live plants was measured and compared be­tween species that significantly increased M. grandii life span and those that did not. While the majority of field plots were under some form of minimum tillage, tillage type did not appear to be a critical factor because of the high mobility of both the wasp and the ECB. Investigators determined which plant species to use by compiling information on the flow­ering phenology (life stages) of insect-polli-nated prairie species. They also monitored two remnant prairie sites in Story County. Beginning in May, sites were visited every five days to record which species were in flower and, for common species, whether flow­ering was increasing or decreasing. The investigators constructed a database con­taining the following information: phenology, general pollinator types, presence of the plant species on Iowa Department of Transportation planting lists, longevity data for M. grandii, and information from scientific literature. Field studies of the effect of prairie borders on parasitism, 1993: Seven sites were chosen over a two-county area. Each had a corn field with a section of border at least 100 meters (m) long and 50 m wide of remnant prairie vegeta­tion (the experimental plot) and a section of border at least 100 m long and 25 m wide of roadside vegetation consisting primarily of bromegrass (the control plot). Each of the major study areas was 9 to 11 miles from the nearest other major area. Comparisons of M. grandii population levels in control and experimental plots were made using yellow cardboard sticky boards placed in the border and in the field from mid-June to mid-September. Boards were placed 6 m apart along a transect running perpendicular to the border strip and into the cornfield. Initially, boards were attached to wooden stakes; later in the season, when corn was taller, boards were attached to stalks at ear height. Boards were in place during seven periods of approxi­mately one week each (and replaced each week); the first two of these periods coincided with the flight of the first generations of ECB and M. grandii. The last four periods coin­cided with the second generations of the ECB and M. grandii. Boards were scored for the number of M. grandii present. Numbers of other beneficial insects were also counted. In October 1993, ECB infestation levels were compared in the control and experimental plots. Two locations per plot were examined, one at the edge of the field, 5 m in from the border (edge), and the other 50 m from the border (interior). Stalks were removed from the field plots, split, and ECB larvae counted. Larvae were removed so the M. grandii parasitism could be quantified. About 100 stalks were examined for each location. Stalks within the immediate vicinity of locations that had sticky boards were avoided. About 50 to 100 larvae were removed from each set of 100 stalks. Within two weeks, these larvae were removed from storage and transferred to a chamber where they were kept in diapause for at least three months. On April 4,1994, larvae were placed in a growth cham­ber and maintained; their fate and the dates on which they pupated, died, or parasites emerged were recorded. All larvae were accounted for by May 13. Some deaths were attributed to the fungus Beauveria bassiana, parasitism by M. grandii, other parasitism, and unknown causes. To determine whether ECB were using prairie or grass border strips as "action sites" for mating, an observer walked slowly through the strip, parallel to the adjacent corn field, for Volume 4 (1995) 74 35 m. A meter stick was gently moved back and forth through the vegetation and the num­ber of moths that flew up were counted. These surveys were done in the early morning hours from August 2 to August 13, 1993 during the main flight period of the second generation ECB. Data were analyzed separately by location. For example, to determine if ECB infestation levels were different for control and experi­mental plots, the number of ECB per stalk for the control plot edge location was compared with the number for the experimental edge location. Findings Potential value of native species, 1992: Re­sults indicated that M. grandii longevity was increased when it fed on plant species that had very short corollas or no corolla tube. The plant species that produced the greatest lon­gevity, indistinguishable in length from that produced by honey, was the Common Milk­weed, which produces copious nectar. Water hemlock, a member of the Apiciaceae family, was also a significant food source. Wild Rose, another species which increased longevity, does not produce nectar in the flower; instead it produces nectar on other plant parts, which often attracts ants (which in turn protect the plant); however, this study did not definitively show that parasitic wasps used this nectar. The data collectedon these and other prairie plants suggest that a corolla depth of 4.5 millimeters may be the threshold beyond which wasps cannot obtain the nectar in flowers. On this basis, plants can be evaluated for their poten­tial as nectar sources. The database indicates which species may be beneficial. Whether these species could be part of a restored road­side depends on how easily they can be estab­lished from seed. A fairly large set of plant species appear capable of providing nectar to wasps and thriving in roadsides. Field studies, 1993: Any effects of widespead flooding on ECB populations or other condi­tions were comparable across treatments. If wasps were using prairie plants for nectar, one might expect high numbers of wasps on sticky Leopold Center Progress Reports boards in the prairie border and higher num­bers in the fields next to prairie borders. For the first generation, there were actually more wasps in the grassy border than in the prairie border, but for neither generation were there any differences between experimental and control counts for the edge or interior sam­pling locations. Overall, there was no evi­dence from the sticky boards that wasps are more common in the prairie border or in the fields next to the prairie. It appears that wasps spend most of their time in fields, searching for hosts; differences in the number of wasps per board among the four field sampling locations (edge and interior/ prairie and grass) within a site are possibly attributable to M. grandii's attraction to areas within a field that have high ECB densities. However, there was no correlation between wasps per board and borers per stalk. Loca­tions with more wasps had more parasitized larvae per stalk. Thus the number of wasps trapped on sticky boards at a location within a site would appear to be an indicator of the relative level of parasitism. There were large differences among sites in the average number of wasps per board. Inves­tigators speculated that more heavily infested fields are more attractive to M. grandii fe­males, although results suggest that, if any­thing, more ECB means fewer wasps captured on sticky traps. It is possible that when corn borers are dense, more M. grandii females are walking on stalks searching for larvae than flying and being captured on the boards. Across sites, there was no relationship between the number of wasps and the number of parasit­ized larvae, but sites with more wasps did have a higher percentage of ECB larvae parasitized. Counts of ECB larvae per stalk differed sig­nificantly among locations. Field edges next to prairie borders had higher levels of ECB infestation than the edges of fields next to grass borders (see Fig. 1). Prairie and grass interiors did not appear to differ significantly. The prairie edge had higher numbers of larvae per stalk, probably because of the role of border strips as action sites. The ECB moths 75 use tall vegetation as mating sites from which the females fly into nearby fields to oviposit (lay eggs). The ECB moths prefer taller veg­etation but use bromegrass when other vegeta­tion is unavailable. In terms of parasitism of ECB larvae, all loca­tions were similar except for the prairie edge, which had a higher percent parasitism (Fig. 1). This was possibly due to the availability of nectar sources in the prairie border. Alterna­tively, there may have been more parasite activity on prairie edges simply because M. grandii is more attracted to the higher ECB densities at this location. Conclusions: A number of plant species were found to increase the longevity of M. grandii under laboratory conditions. Presumably these same species would benefit other parasitic wasps. An examination of the two extracted data sets on flowering phenology and corolla depth shows that for both first and second generations of M. grandii, there are a number of plant species which could potentially pro­vide nectar resources. (Flowers with short or no nectar tubes had the most positive effect.) To date, selection of species for roadside plantings have focused on those that would stabilize the soil and be aesthetically pleasing. Another criterion that could be used is whether the plant species would have a beneficial effect on natural enemies of crop pests. The field studies showed that prairie borders increased M. grandii parasitism by 32%. This effect was found only on the edge of the corn field. However, prairie borders also increased the level of corn borer infestation by 29%. Again, this effect was only found on the edge of the field. Thus, the benefits of prairie borders in increased parasitism are offset by higher corn borer infestation levels. Increased parasitism at prairie borders may be because wasps use the nectar resources provided by the flowers in the border or because wasps are attracted to the increased numbers of ECB there. This should not be considered as a deterrent to efforts to plant roadsides to prai­rie. The effect was localized to the edge of the fields in this study; thus the other benefits of such plantings outweigh the disadvantage. Implications The rationale for efforts to plant roadsides to native prairie vegetation has been based on stabilization of roadside soils, buffering to prevent field pesticides from running off into drainage ditches, weed control of ditches by limiting invasion of weeds from field edges, and aesthetic value. This study addressed the possible effects of roadside prairies on adja­cent fields. The results suggested that at least for systems involving corn, the European corn borer, and the parasitic wasp M. grandii, the benefits of native species in increasing parasit­ism of the crop pest are offset by increased pest infestation. Because this negative effect is localized to the edges of fields, its effect on crop yield will be small enough to be outweighed by the other benefits. However, this study did show that enhancement of crop-pest parasites cannot be included in the list of factors arguing for prairie restoration. This study considered only one parasite species, one crop pest, and one time period (second generation). Further studies are needed on the first generation in this system and on other crop pest-parasite systems in order to generalize these results. Volume 4 (1995) Fig. 1. Numbers of ECB larvae per stalk and percent of larvae parasitized for the different border types (grass/prairie) and different locations within each type (edge/interior). Values are averages for the seven study sites. For more information contact J. M. Pleasants, Zoology/Genetics, Iowa State University, Ames, Iowa, 50011, (515) 294-7204. 76 

English

Influence of native roadside plants on biological control of Iowa crop pests

http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1069&context=leopold_grantreports

Influence of native roadside plants on biological control of Iowa crop pests

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