Exploiting plant genotypic diversity for sustainable insect pest management

Exploiting plant genotypic diversity for sustainable insect pest management

Summary

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. The second year of field data collection have been completed, as has a portion of the outreach. 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. To better understand how genotypic diversity may affect natural enemies, we conducted lab and field experiments that measured predator attraction to monocultures and mixtures. Further greenhouse experiments and outreach activities are still needed to complete this project.

Objectives/Performance Targets

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.

Accomplishments/Milestones

Objective 1:

To address objective one, a large field experiment was conducted at Penn State’s Research Farm. Sixty plots of soybeans were planted in a randomized complete block design. 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. Each treatment was replicated five times. Over the course of the summer, soybean aphids were counted weekly on ten plants per plot to monitor aphid populations. Plot yield was assessed at the end of the season by harvesting 17.5 feet of two rows in each plot. These samples were dried in a drying oven and then massed. Instead of simply harvesting machine-planted plants to measure plant biomass, fifteen seeds/plot were hand planted at the beginning of the season so that the line-identity of each plant would be known and were later harvested.

There was no overall effect of diversity on either weekly aphid counts analyzed with a repeated measures analysis or cumulative aphid days, which is a season long measure of aphid exposure that combines aphid data for multiple dates (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. Season-long, aphid populations were consistently and unexpectedly low and never even began to approach the economic threshold of 250 aphids/plant. 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 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.

One greenhouse experiment has been completed that compared aphid populations in high diversity pots of five plants of different lines (genotypes) to low diversity pots of five plants of the same line. 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. Further examination of the data will reveal whether the aphid populations on individual plants of a specific variety within a mixture can be predicted from that particular variety in monoculture.

Objective 2: Compare the effects of genotypically diverse mixtures vs. single line monocultures on natural enemies of the soybean aphid.

Natural enemy populations were sampled biweekly in the plots described above during the first year of this project. To sample foliar arthropods, we sweep netted two rows of the plot. This was repeated during both the day and the night to capture both day- and night-active foliar predators. Pitfall traps were used to sample ground predators biweekly. To better assess Orius (Hemiptera: Anthocoridae) populations within the plots, one beat pan sample was taken near estimated peak population levels.

Identification of arthropod samples has been partially completed. However, some of the key potential natural enemies captured, such as lady beetles (Coleoptera: Coccinellidae) and damsel bugs (Hemiptera: Nabidae), were not affected by the level of genotypic diversity within a plot. Orius also were not affected by diversity level as measured through the beat sheet sample. Both genotypic diversity levels had approximately 6 Orius per plot. Other arthropods that have been identified from the samples have shown no clear pattern with respect to diversity level, but the analysis of the completely identified samples may reveal different patterns.

To better understand effects of genotypic diversity on attraction of natural enemies, we conducted a field and 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. 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.

Predators did not reduce aphid populations any differently in monoculture or mixture pots as measured by percent reduction in aphid numbers (Fig. 3). Similarly, predator observations were not different between treatments. However, there were more cecidomyiid larvae on the mixture pots than the monoculture pots (Fig. 4), suggesting that a longer exposure to predators in the field may have resulted in differences in aphid numbers between treatments.

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. The location of the ladybeetles were observed every half hour for six hours.

Median observations for each diversity treatment were compared and 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. 5).

Objective 3: Compare predation services provided by genotypically diverse mixtures vs. those provided by single lines.

Predation services were assessed 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 will help elucidate temporal dynamics of predation on the soybean aphid.

In addition, sentinel aphids were put in each plot to measure predation on two dates. On each date, two cards with three aphids apiece were placed in each plot during the morning and at night. We revisited cards several times to witness predation and identify predators and to measure predation services.

Further data analysis is still needed to understand the results from the cage portion of the large field experiment. The sentinel aphids were successfully preyed upon by predators, including ladybeetles, mites, earwigs and ants. However, aphid removal and therefore predation services appeared to be consistent between mixture and monoculture plots.

Further greenhouse experiments are planned for the upcoming year that will compare predation services in soybean mesocosms.

Objective 4: Characterize the natural enemy community of soybeans in Central Pennsylvania
Natural enemy information from the above experiment will be used to help characterize the natural enemy community in soybeans. In addition, we sampled natural enemy populations in soybean fields of local growers with sweep nets, pan traps and pitfall traps biweekly. We also measured aphid populations, but aphid numbers were low, leading to similarly low numbers of natural enemies in growers’ soybean fields. Specimens collected from local fields have thus far been partially identified. Foliar dwelling arthropods caught that could be key predators of the soybean include several species of ladybeetles, damsel bugs, harvestman, and earwigs. Ground dwelling predators consist of a number of ground beetle species, ants and wolf spiders, especially Pardosa spp. A number of “ground dwelling” predators were also captured in the foliage.

Objective 5: Transfer knowledge gained from Objectives 1-4 to growers, extension agents and members of the public.

Work for this objective is still undergoing due to the continued work on this project. 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 was well-attended by members of the Northeastern farming community. We presented results from this project at multiple Entomological Society of America meetings. Information was provided to cooperating growers when available and final results will be shared once completed.

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 that suppress soybean aphid populations as well as other potential pests of soybeans.

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Impacts and Contributions/Outcomes

While our project is still incomplete, we believe that our project has contributed to agricultural sustainability. Outreach events and presentations have allowed us to share our results with the agricultural community. Thus far, the effects of genotypic diversity on aphids and on natural enemies of aphids have been mixed. Still, we have shown that genotypic diversity may provide benefits, at least in the form of dampening aphid population spikes. We were unable to test our hypothesis that genotypic diversity can help manage economically significant aphid populations, so we thus far been unable to strongly recommend the tactic as a sustainable pest management tactic based on the data we have produced obtained thus far. Importantly, in the absence of aphids, there was no reduction in yield and therefore no costs. We have also demonstrated to local growers the benefits that natural enemies can provide. 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. In our outreach activities, these results have illustrated how maintenance of natural enemy populations is critical for sustainable pest management.

Collaborators: