This project examines the potential of increased crop genotypic diversity for sustainable pest control in field crops. Evidence from natural and agricultural systems suggest that increasing crop genotypic diversity with cultivar mixtures holds promise for controlling insect pests. This research is pursuing this strategy using soybean and soybean aphids as a model system and a combination of field and greenhouse experiments. Non-chemical methods of control are needed for soybean aphid, which has become the primary pest in soybeans. In conjunction to investigating the effects of increasing crop genotypic diversity, this project aims to help characterize the natural enemy community in soybeans that may help control the soybean aphid in Pennsylvania and the Northeast. Despite expectations of high soybean aphid populations for the field component of the project, aphid populations were very low. Under these low aphid populations, we saw no effect of genotypic diversity on pest pressure or natural enemy services. Effects on aphid populations and on natural enemies in small-scale greenhouse and field experiments were mixed. A number of natural enemies were identified from local growers’ fields using pan traps, pitfall traps, sweep netting (day and night), and direct counts.
The primary purpose of this project is to improve pest control by planting crop fields with genetically diverse cultivar mixtures. Natural plant communities are characterized by high species and genetic diversity. In contrast, modern agricultural fields are often planted with genetically uniform monocultures that facilitate crop production, processing, and marketing. However, this homogeneity maintains low arthropod species diversity and favors pest outbreaks, necessitating interventions with chemical pesticides. Work with polycultures indicates that these more diverse agroecosystems can sustain a more robust community of natural enemies that regulate populations of herbivores (Andow 1991), but polycultures often conflict with conventional production agriculture.
A promising pest management alternative that would be compatible with modern production is increasing plant genotypic (i.e., intraspecific) diversity by planting cultivar mixtures. This approach promises to provide many of the benefits of diversity while still allowing growers to maintain the species uniformity necessary for economies of scale. Increasing genotypic diversity in field crops is known to improve disease management and thousands of hectares of small grains and rice in Europe and Asia have been planted in mixtures of cultivars that vary in their resistance to key pathogen species. These cultivar mixtures better resist infection and slow the spread of disease, dampening disease pressure and reducing reliance on fungicides (Wolfe 1995; Zhu et al. 2000; Mundt 2002). Moreover, evidence from natural systems has recently demonstrated that genotypic diversity can also contribute to herbivore suppression and that genotypic diversity can have effects on arthropods rivaling those of plant species diversity. Recent studies with two native plant species, late goldenrod and evening primrose, have demonstrated that plant genotypic diversity increased insect diversity, including natural enemy diversity, which in turn reduced herbivore pressure, and increased plant productivity (Crutsinger et al. 2006; Johnson et al. 2006). In addition to these natural-enemy mediated or “top-down effects,” plant-mediated resistance, a “bottom-up effect,” also contributed to the success of genotypic diversity in limiting herbivores and improved plant productivity.
Synthesizing these distinct bodies of evidence strongly suggest that genotypic diversity has a role to play for managing insect pests in agroecosystems. However, little research in agroecosystems has examined the effects of genotypic diversity on arthropod communities. The few investigations that have been undertaken show promising results (e.g. Cantelo and Sanford 1984), but most of these have occurred in subsistence agriculture (Letourneau 1995; Abate et al. 2000).
My approach is to study soybean and soybean aphids as a model system to evaluate the potential utility of genotypically diverse cultivar mixtures for controlling aphids, which are among the most challenging insects to control in field crops and happen to possess several disease-like characteristics that make them promising for control via cultivar mixtures. First, similar to many pathogens, aphids are first wind-dispersed and then spread locally once colonies grow. Second, aphids and pathogens both display rapid population growth and host-plant specificity. Lastly, plants frequently perceive aphid attack in a manner similar to that by pathogens and induce defensive responses to aphid feeding comparable to those caused by pathogens (Walling 2000).
The soybean aphid currently drives insecticide use in soybeans (Smith and Pike 2002), providing a good model system for research on the sustainable pest management potential of cultivar mixtures. Soybean aphid was first detected in the U.S. in 2000 and quickly spread to all soybean-growing areas of the US. Severe infestations can reduce yield by up to 50% (Wang et al. 1994) and yield must often be protected using broad-spectrum organophosphate and pyrethroid insecticides, which are toxic to pesticide applicators and non-target organisms, pollute land and water resources and increase input costs for growers. Insecticide-use has recently shifted in soybeans, with increasing prevalence of soybean aphid accompanied by a rise in treated acreage. From 1996-2000, growers typically treated less than 2% of soybean acreage with insecticides. Insecticide use has since increased, jumping to 5.73% in 2002 and 16.59% in 2006 (USDA-ERS 2006).
This project aims to characterize the natural enemy community available in soybeans to suppress soybean aphids for several reasons. First, knowing the natural enemy community that is present is critical if we want to sustain or manipulate their populations as part of a sustainable pest management program. In addition, the natural enemy community in soybeans and the biological control services they provide for soybean aphid has been extensively characterized in the primary soybean-growing region of the Midwest, but this information is sparse for the Northeast. In addition, the temporal dynamics of the predator community has often been ignored, with foliar sampling occurring only during the day. This means that prior efforts may not properly account for predators moving into the foliage at night to forage for prey.
Abate et al. 2000. Pest management strategies in traditional agriculture: an African perspective. Annual Review of Entomology 45:631-659.
Andow. 1991. Vegetational diversity and arthropod population response. Annual Review of Entomology, 36(1):561–586.
Campbell and Hanula. 2007. Efficiency of Malaise traps and colored pan traps for collecting flower visiting insects from three forested ecosystems. Journal of Insect Conservation 11(4):399-408.
Cantelo and Sanford. 1984. Insect population response to mixed and uniform plantings of resistant and susceptible plant material. Environmental Entomology 13(5):1443-1445.
Crutsinger et al. 2006. Plant genotypic diversity predicts community structure and governs an ecosystem process. Science 313: 966-968.
Fox et al. 2004. Predators suppress Aphis glycines Matsumura population growth in soybean. Environmental Entomology. 33(3):608-618.
Johnson et al. 2006. Additive and interactive effects of plant genotypic diversity on arthropod communities and plant fitness. Ecology Letters 9(1):24-34.
Letourneau. 1995. Associational susceptibility: effects of cropping pattern and fertilizer on Malawian bean fly levels. Ecological Applications 5:823-829.
Mundt. 2002. Use of multiline cultivars and cultivar mixtures for disease management. Annual Review of Phytopathology 40:381-410.
Snyder. 2009. Coccinellids in diverse communities: Which niche fits? Biological Control 51:323–335.
USDA ERS. 2006 Agricultural Resource Management Survey: http://www.ers.usda.gov/Data/ARMS/CropOverview.htm
Walling. 2000. The myriad plant responses to herbivores. Journal of Plant Growth Regulation 19:195-216.
Wang et al. 1994. A study on the damage and economic threshold of the soybean aphid at the seedling stage. Plant Protection 20:12-13.
Zhu et al. 2000. Genetic diversity and disease control in rice. Nature 406:718-772.
Objective 1: Compare the effects of genotypically diverse mixtures vs. single lines on plant growth and yield and on aphid population growth.
Objective 2: Compare the effects of genotypically diverse mixtures vs. single line monocultures on natural enemies of the soybean aphid.
Objective 3: Compare predation services provided by genotypically diverse mixtures vs. those provided by single lines.
Objective 4: Characterize the natural enemy community of soybeans in Central Pennsylvania
Objective 5: Transfer knowledge gained from Objectives 1-4 to growers, extension agents and members of the public.
This project largely progressed as outlined in the original proposal and objectives and all objectives have been reached as originally outlined with few exceptions. For the large field experiment investigating the effects of genotypic diversity on yield, aphids and natural enemies (parts of Objectives 1, 2, 3), natural enemy identification and analysis for pitfall samples has been completed (Objective 2). A subset of the most important foliar-foraging natural enemies has been identified and the effects of genotypic diversity analyzed and further identification of the more minor natural enemies is ongoing (Objective 2). Given the lack of influence of genotypic diversity on aphid populations and the natural enemies examined, it is likely that there will be no effect of diversity on other natural enemies or, if so, the effect will be minimal. For Objective 3, the experiment comparing predation services in monocultures and mixtures using mesocosms (consisting of several plants in a pot with caged predators) was replaced with different experiments in both field and greenhouse settings that better examined the influence of diversity on predation services and that also added results for Objective 2. Difficulties with greenhouse facilities and reduced germination because of seed age also created barriers to completion of the proposed experiment. Objective 5 is ongoing, with results continuing to be disseminated and incorporated into fact sheets, educational materials, and presentations (see below). Given the low aphid populations and results from the large field experiment, I have not produced a fact sheet on the benefits of increasing crop genotypic diversity, although it is anticipated that future work will better support this effort. A picture-rich natural enemy fact sheet is near completion, but still needs to go through the proper channels at Penn State for formatting and publication. This will be linked to educational natural enemy videos.
To address Objective 1, a large field experiment was conducted at Penn State’s Rock Springs Research Farm. Sixty 30×30 foot plots of soybeans were planted in a randomized complete block design. Each plot contained 12 rows of soybeans planted on 30-inch rows. Low diversity treatments consisted of monocultures of six soybean varieties, while high diversity treatments consisted of all possible five-line mixtures formed from the pool of six varieties represented in monoculture. This design primarily tests for the effects of diversity per se. Each treatment was replicated five times. The plots were planted into a matrix of oats that separated the plots from each other and helped minimize movement of arthropods between plots. The planting area was sprayed with herbicide to burn down the oats prior to planting the soybeans. Over the course of the summer, soybean aphids were counted weekly on ten plants per plot to monitor aphid populations. Aphids were counted as soon as they began to appear (late June) until soybeans reached the R6 stage, at which point soybean aphid management is no longer relevant. Plot yield was assessed at the end of the season by harvesting 17.5 feet of two rows in each plot with a plot combine. These samples were dried in a drying oven and then massed.
A greenhouse experiment that complemented the large field experiment was conducted to explore the effect of genotypic diversity on pest populations under more controlled, predator-free conditions. The same treatments were used as in the field experiment with treatments consisting of monocultures and all possible five-line mixtures formed from a pool of six varieties. Each treatment was replicated seven times. Plants were seeded five to a pot. Five medium-sized soybean aphid nymphs were added to each plant. Aphid populations were then counted weekly for two weeks.
Natural enemies were sampled via several methods within the context of the large field experiment. Ground-dwelling arthropods were sampled with two pitfall traps per plot, placed within the center two rows. Pitfalls consisted of 16 oz. plastic deli containers sunken into the ground and flush with the soil surface. Samples were collected every two weeks. Potential natural enemies of soybean aphids were identified to the lowest taxonomic level possible (mainly species, genus or family). Foliar arthropods were sampled with a sweep net at the same time as the pitfalls were open. Two sets of two outer rows were sweep-netted during the day and night in each plot. Natural enemies were identified similar to those collected with pitfall traps. Predator counts were summed across sampling dates to provide enough individuals for analysis. Minute pirate bugs (Orius insidiosus) were assessed once mid-season near estimated peak population levels using the beat-pan method.
To better understand effects of genotypic diversity on attraction of natural enemies, we conducted a field and a lab experiment. The field experiment consisted of infesting pots of five plants, either monocultures or mixtures, with soybean aphids. These pots were placed within a soybean field, exposing them to natural enemies. Natural enemies were counted multiple times on the plants within the pots to assess attraction of natural enemies. At the end of the experiment, aphids were counted to measure the effect of natural enemies on the aphid populations on the plants within the pots (measuring predation services, Objective 3). Pots were then transferred into a field cage to allow cecidomyiid midge larvae to develop and they were then counted. These midges are predatory upon aphids as larvae.
We also conducted a lab experiment to compare predator attraction to monocultures and mixtures of soybeans. Convergent lady beetles (Hippodamia convergens) were given a choice between monocultures and mixtures of aphid-infested soybeans within a predator choice arena. The arena consisted of a plastic tub with a wooden base into which the pot was nested. Varieties and treatments were the same as in all prior experiments and pots were paired with the restriction that the variety in monoculture must be present in the mixture. The location of the ladybeetles were observed every half hour for six hours. Median observations for each diversity treatment were compared. The time spent by the lady beetles on each time of planting was also compared.
Predation services were assessed in the large field experiment by caging single plants with cages constructed from tomato cages and no-see-um mesh. Ten aphids were added to each plant at the beginning of the predation experiment and aphids were counted after 7 and 14 days. The proposed research used three treatments within each plot: predator exclusion, sham cage and no cage control. This was significantly expanded during the actual experiment to also include a day-predation cage and a night-predation cage. These cages were raised and lowered every morning and night for the duration of experiment, exposing the aphids to a subset of the natural enemy community. This addition, coupled with the day and night sweep net samples was done to help elucidate temporal dynamics of predation on the soybean aphid. This method has been used previously by our lab with success, but for unknown reasons the aphid populations were highly variable in this experiment. This could be due to problems with initial aphid infestation or from predators infiltrating the exclusion cages. Due to the uncertainty and variability with the data, results are not presented.
In addition, sentinel aphid cards were put in each plot to measure predation on two dates (July 6th and July 25th). On each date, two cards with three aphids apiece were attached to the bottom of soybean trifoliate in each plot during the morning and at night. Aphids were glued to a 1×2 cm piece of cardstock with non-toxic tacky glue. We revisited cards several times to witness predation and identify predators and to measure predation services. For data analysis purposes, the proportion of aphids predated after 12 hours was used.
The experiment utilizing potted plants placed in the field (outlined under Objective 2) also addressed Objective 3.
Natural enemy populations were sampled in three local growers’ soybean fields with sweep nets, pan traps (blue and yellow), pitfall traps, and direct observations. One field was managed organically (tillage for weed management), while the other two were conventionally managed with no-till (glyphosate application after planting). Natural enemy sampling occurred as soon as possible after weed management was completed. Natural enemies were identified to the lowest taxonomic level possible (mainly species, genus or family). Parasitism, as measured by direct counts, was very low due to low aphid populations. Parasitoid populations were therefore not assessed in the natural enemy samples since any parasitoids likely parasitize species other than the soybean aphid. Aphids were counted on 20 plants/field. Predators were counted on these plants, as well as on another 20 plant, yielding a total of 40 plants/field for direct counts. Sampling occurred until most plants had reached the R6 stage.
See Results and Discussion
There was no overall effect of crop diversity on either weekly aphid counts analyzed with a repeated measures analysis or cumulative aphid days. Cumulative aphid days (CAD) is a season long measure of aphid exposure that combines aphid data for multiple dates and is often used in research with soybean aphids (Fig 1). It did appear that genotypic diversity had a stabilizing effect on aphid populations, which could prove beneficial to growers due to dampening of aphid population spikes. This could maintain aphid populations below the economic threshold of 250 aphids/plant. Season-long, aphid populations were consistently and unexpectedly low and never began to approach the economic threshold. We were therefore unable to test in the field the potential of genotypic diversity as a management tool under conditions of high pest pressure, which is when we hypothesize that there is the greatest potential for an effect of diversity and is when any diversity effect would be relevant to growers. Similar to the effect on aphid numbers, there was no effect of diversity per se on plot yield (Fig. 2). There appeared to be variation among the different monoculture and mixtures, but these differences were not significant.
In the greenhouse experiment that compared aphid populations in high diversity pots to low diversity pots, substantial aphid populations were achieved after two weeks of population growth. However, similar to the results from the field, there was no overall effect of diversity per se on aphid populations. Significant variation in aphid populations at the individual plant level and at the pot level was observed and may drive these results.
Pitfall traps in the large field experiment caught a large variety of predatory arthropods that could consume soybean aphids. These included rove beetles, a number of species of ground and tiger beetles, predatory beetle larvae (not necessarily identified to family), a number of ant species, harvestman, wolf spiders, and members of other spider families. Total number of predators excluding ants was not affected by diversity level (Fig. 3), nor were either the total number of ants excluding aphid tending species (Lasius sp. and Prenolepis imparis) or the number of ants that may tend soybean aphids. In the foliar sampling, neither total lady beetles nor damsel bugs were affected by diversity. Orius populations, sampled with a beat pan, were also not influenced by diversity (Fig. 4).The lack of diversity effects on the major natural enemies captured within the plots is congruent with the lack of diversity effect seen for aphid populations.
Predators did not reduce aphid populations any differently in monoculture or mixture pots that were placed in the field as measured by percent reduction in aphid numbers (Fig. 5). Similarly, number of observations of predators were not different between treatments. However, more cecidomyiid larvae developed on the mixture pots than the monoculture pots after they were put into the field cage (Fig. 6). This suggests that adult cecidomyiids laid more eggs in mixtures pots, which could mean that a longer exposure to predators in the field may have resulted in differences in aphid numbers between treatments.
For median observations for each diversity treatment, there were no significant differences in the number of times beetles were observed on the low or high diversity pots, although there was a trend (P < 0.10) in the direction of more observations on the high diversity pots. In an analysis of the time spent on each type of planting, the lady beetles spent a larger percentage of their time on the mixture pots than on the monoculture pots (Fig. 7). These results suggest that diverse plantings may be more attractive to certain predators, independent of aphid population levels.
Results from the cage experiment are not presented due to the issues outlined in the Methods and Materials section. Despite these issues, it does appear that day-active predators provided a greater predation service than the night-active predators and suppressed aphid populations approximately as much as in the sham cages. The same was not true for the night predation treatments.
For both dates, there were no differences in aphid predation as measured by sentinel aphids glued to cards. Predators observed consuming aphid included lady beetles, damsel bugs, harvestman, earwigs, and mites.
The experiment utilizing potted plants placed in the field (outlined under Objective2) also addressed Objective 3.
Similar to the aphid populations at the Penn State Research Farm, aphid populations were extremely low in the growers ‘fields and never approached levels remotely close to the economic threshold (Fig. 8). This likely resulted in fairly low captures of some predators that feed primarily on aphids, since many of these predators aggregate to high aphid populations. Nevertheless, it is very clear that when aphids are protected from predators, by artificially caging them, they rapidly proliferate. Aphids were present in the field, which suggests that predators were actively suppressing their populations.
A wide variety of predators were captured via pan traps, pitfall traps, and sweep netting (Tab. 1, 2, 3, respectively). The diversity in sampling techniques ensures that the majority of predators that could help suppress aphid populations were captured and included in the results. . Some predators were included that are omnivorous, but that could feed on aphids. By contrasting the night and day sweep samples, it is clear that some predators are more effectively caught during the day (e.g. Orius insidiosus), while others are caught more at night (e.g. earwigs). The latter result suggests that earwigs move into the foliage to feed at night, potentially on aphids, and that day-only sampling might miss this component of the community. Direct counts primarily noted Orius insidiosus individuals, including both adults and nymphs. Adults were more common than nymphs. Populations peaked July 28th, with a combined total of 6 per 40/plants on the first farm, 11 at the second and 10 at the third.
Information from this project has been disseminated in a number of ways. Information from both the genotypic diversity and soybean aphid natural enemy components of this research have been incorporated into several fields days at Penn State’s Research Farm, which were well-attended by members of the Northeastern farming community, including both growers and industry representatives. We presented results from this project at multiple Entomological Society of America meetings, which has allowed us to share our results with members of the academic community and members of industry. Information was provided to cooperating growers during the project when available and a final report will be also be shared..
Field data and field observations have proved beneficial for the natural enemy factsheet, which is in the final stages of production. This fact sheet attempts to provide an overview of the taxonomically diverse natural enemies in soybeans (and other field crops) that suppress pest populations such as the soybean aphid. A major goal of the fact sheet is to be visually stimulating. Therefore, a large number of images have been incorporated into the fact sheet. A number of videos of natural enemies have also been produced as part of this project. These videos, produced with the help of Margaret Douglas, another NE SARE Graduate Student grant recipient, show natural enemies eating various pests and provide short descriptions of the organisms involved. The videos have been made available on a “Pests and Natural Enemies” Youtube channel (http://extension.psu.edu/plants/sustainable/news/2014/winter/3-natural-enemies) and will be linked to the online version of the natural enemy fact sheet once it has been uploaded. Several of the videos have already been used for a scout school program at Penn State. It is anticipated that the educational use of the fact sheet and accompanying videos will increase with time.
Increasing crop genotypic to manage insect pests is a promising tactic that holds potential for managing insect pests. Thus far, the effects of genotypic diversity on aphids and on natural enemies of aphids have been mixed. Unfortunately, the component of this experiment that incorporated the most potential for effects of diversity on bottom-up and top-down forces on aphid populations (the large experiment with 30×30 foot plots) was hindered by extremely low natural aphid infestations. Nevertheless, several parts of this project have produced promising results that show that genotypic diversity can influence natural enemies, which could result in reduced aphid populations. Importantly, in the absence of economically significant aphid populations, there was no cost to the grower for deploying cultivar mixtures in terms of yield. This, coupled with the lower amount of variability in the mixtures for aphid populations, means that planting cultivar mixtures may provide a level of insurance against aphid population spikes. By reducing population spikes, genotypically diverse could reduce insecticide applications and promote greater agricultural sustainability. A number of growers have shown interest in both results from this project and the rationale behind this project, which suggests there is the potential for grower adoption. Results from this project have and will continue to propel forward further research into the potential of crop genotypic diversity for sustainable pest management.
The results from the natural enemy characterization portion of this project add to a growing quantity of information about the natural enemies that are available in agricultural fields to suppress soybean aphids. While imperfect, our results from the cage study in which aphids were protected from predators clearly showed that aphids had the potential to reach economically damaging levels under environmental conditions in the field. We have demonstrated to local growers the benefits that natural enemies can provide. By recognizing the natural enemies that eat soybean aphids and appreciating the services they provide, it is more likely that growers will rely on natural control of soybean aphid. The fact sheet and natural enemy videos will likely be very useful to extension educators in teaching the benefits of natural enemies. These resources have also been written and produced so that they are both educational and entertaining. This means that they will likely serve as educational resources promoting agricultural sustainability for the general public.
Education & Outreach Activities and Participation Summary
Results from this project have been presented in a number of forms. This project has been the focus of numerous oral and poster presentations at both national and branch meetings of the Entomological Society of America. Poster presentations have been given three times to members of the Penn State community and to members of the general public at Penn State’s University Park campus. Both the genotypic diversity and natural enemy parts have been presented at two field days at Penn State’s Rock Springs Research Farm to approximately 75 attendees each time. The natural enemy component was incorporated into outreach activities aimed at the preschool to grade school level during summer camps and research farm visits by students.
So far, this projected has resulted in one fact sheet and another (natural enemy) will soon follow. Twenty-two natural enemy videos have been produced that cover a wide range of natural enemy taxa found in agricultural systems. Some of these videos have been incorporated into a presentation at the scout school at Penn State. Manuscripts that will incorporate results from Objectives 1-4 are in preparation.
Grettenberger, I.M, Tooker, J. F. Department of Entomology, Penn State Extension factSheet:http://ento.psu.edu/extension/factsheets/soybean-aphid-1
Videos: Available on the “Pests and Natural Enemies” Youtube channel (https://www.youtube.com/channel/UCP1bMuIKoBhQiGgXUP33b9Q)
See Impact of Results/Outcome
Final results will be provided to cooperating growers. A number of growers have expressed interest in crop genotypic diversity as a pest management tactic. Field days resulted in interactions with growers who have, independently and for reasons other than pest suppression, adopted cultivar mixtures. This project may lead to additional grower adoption for the potential of pest management. Cooperating growers were also interested in the natural enemy component of this research and in learning about the natural enemies that suppress pests in their fields.
Areas needing additional study
This study would ideally be repeated under high pest pressure to test the ability of crop genotypic diversity to influence economically damaging pest populations. Conducting large-plot research that compares pest populations in monocultures and mixtures across multiple sites and years would help elucidate the stabilizing benefits of diversity, which would better inform how increasing crop genotypic diversity could contribute to agricultural sustainability. Ideally, future research will also expand this research into other crop and pest combinations.