Progress report for GNE24-316
Project Information
In agriculture, the mutualisms that ants and other predatory insects share with plants that produce extrafloral nectar (EFN) is understudied1. Extrafloral nectaries (EFNs) are structures that some plant groups produce that secrete nectar for the purpose of attracting omnivorous insects in exchange for protection from herbivores2. Ants are the most important insect group involved in this interaction, but other predatory insects visit EFNs and provide protection2–4. Conservation biocontrol is the concept of promoting habitat on farms that is attractive and hospitable to native beneficial insects, which provide essential services like pollination and pest removal. I aim to contribute to the knowledge of using EFN-producing plants as a means for conservation biocontrol in organic New England agroecosystems. Here, I outline a project designed around cucurbits using management practices common among organic farmers in Massachusetts. I will use partridge pea, an EFN-producing wildflower, as a companion plant cucumber plantings. I will collect insect community, plant damage, and harvest data throughout the growing season, and compare these data across blocks containing combinations of the above treatments. For outreach, I plan to collaborate with the Xerces Society and run on-site insect conservation workshops tailored toward local farmers. Information gained from studies like mine will build on our understanding of the plant-protecting activities that abundant omnivorous insects like ants provide, which may be a useful tool for farmers dedicated to sustainably growing food.
- Examine the effects of EFN-producing companion plants on insect communities associated with organic cucumber plantings in New England.
- Determine how EFN-producing companion plants influence the presence of pests, crop damage, and yield of cucumber plantings.
- Determine whether companion plants influence crops with and without their own EFNs differently.
The purpose of this project is to expand our current understanding on the utilization of plants that produce extrafloral nectaries (EFNs) as a tool for conservation biocontrol in organic vegetable systems. Extrafloral nectar evolved to reward omnivores like ants with sugars in exchange for defense against herbivory2,7. While ants are the primary mutualist, other beneficial predators known to feed on EFNs and reduce herbivory include parasitoids, beetles, lacewings, and even spiders2–4. Past studies have indicated the use of EFNs as a promising method of recruiting beneficial predatory insect abundance in agroecosystems, but their efficacy is highly dependent on a farm’s management practices, crops, and the surrounding ecosystem1. Therefore, I intend to explore methods of using native EFN-bearing plants to harness the behavior of local ants and other predaceous omnivores to advance pest control for organic growers in New England.
Partridge pea (Chamaecrista fasciculata - hereafter PP) is an EFN-producing legume native to the Eastern US that is highly attractive to predatory and pollinating insects. The use of this plant in agroecosystems is understudied, but they have great potential for enhancing the beneficial arthropod community in an area8. Here, I will focus on cucumber (a non-EFN cucurbit), which is popular among organic farmers in New England. I plan to use PP as a companion plant treatment alongside cucumber and assess its effect on insect community dynamics and cucumber yield. . The combination of plants in this system are good study candidates for this project. Cucumber is host to two major pests of economic concern to New England farmers:striped cucumber beetles (Acalymma vittatum) and spotted cucumber beetles (Diabrotica undecimpunctata howardi), which vector bacterial wilt that can cause up to 20% yield losses if left unmanaged by farmers9. Partridge pea intercrops may attract parasitoids and generalist predator insects that could assist in controlling the populations of these beetles and their subsequent damage - which is what we aim to assess.
There is an increasing societal demand for organic produce and less reliance on chemicals in food production10,11. In addition to consumer concerns, the use of broad-spectrum insecticides also harms non-target beneficial organisms like pollinators and predaceous arthropods12. Utilizing the prey-regulating ability of the predatory insects could help us mitigate crop damage, while reducing our reliance on these chemicals. Releasing lab-reared biocontrol organisms is common but is expensive and releasing species outside their native ranges can negatively non-target arthropods, especially endangered local species13,14. I would like to expand on this concept and learn more about the possibilities of enhancing local beneficial insect populations via the attraction of EFN-producing plants native to New England.
This proposal is part of my dissertation at Clark University, where I study the roles of ants in agroecosystems. The experiment outlined here is part of my third chapter, where I aim to run field experiments using cultivation methods common among organic farmers in Massachusetts. For the summer of 2024, I conducted my first set of trials for this chapter by utilizing PP as a companion plant alongside an EFN-producing vegetable: zucchini. The purpose of this study was to determine if the inclusion of an EFN companion plant will attract and bolster the local ant/beneficial arthropod community and cause a spillover effect onto the EFNs of the crops themselves. Due to limitations in budget and labor, this study took place in a single plot at one site. This first field season has concluded and analyses are ongoing as I prepare for 2025. The experiments outlined in this proposal will take place during the summer of 2025, and will not simply be an extension of my 2024 dataset, rather they will focus on more than one a new crop, include a weedy fallow treatment, and take place in larger plots at two separate sites.
Our findings would provide cucurbit growers with new information on this understudied and potentially effective chemical-free alternative to pest control and enhance our current understanding of the mutualism between plants that produce EFNs and the organisms they reward in exchange for defense. To date, there has not been an extensive study on the employment of ants as biological control in cucurbits via intercropped EFN recruitment1. As our population expands, food security issues will become more common, especially in developing regions of the world where factors like climate change and poor soil quality are likely to hit the hardest15,16. Developing new conservation biocontrol practices will reduce the need for costly pesticides and horticultural practices that create high barriers of entry that may dissuade people from underrepresented groups from starting their own farms. Any work that supports local farmers improves the surrounding community, helps eliminate food deserts in poor areas, and is a step toward phasing out our reliance on large food distribution corporations that we currently rely heavily on for produce.
Research
Study area
This field experiment will take place in Massachusetts on two sites using land rented or borrowed from farmers. I will utilize 0.20 acres at both sites for this study, which will allow for ample space between treatment blocks and a mowed perimeter. For this summer 2024 season, I am working with a hay field owner in Spencer, MA and I plan to continue to utilize this space in 2025, which they are allowing me to use for free. I will need to rent land that fits the parameters of my study (e.g., certified organic or no history of spraying) in order to include a second site. Land is primarily leased by the acre in MA, meaning that I will need to rent a full acre even though I will use a fraction. Farms in Central MA share common pests and beneficial species, yet each farm has slight variations in their insect community composition and management strategy (personal observation 2023).
Cucumber is the focal crop of this study. I will construct 24 equally-sized blocks at each site to act as individual replicates (Figure 1). Each block will be 3 x 5m, allowing for multiple rows of crops and companion plants with space for 3m mowed buffer areas between blocks to reduce the interactions among blocks of different treatments. One block will consist of one 5m long central crop row and an equal-length treatment zone on one side (Figure 2). In a randomized block design, these blocks will receive one of three possible planting treatments, with four replicates of each treatment x crop combination: 1) control treatment only surrounded by weedy fallow, 2) bare mulched ground, 3) cucumber with PP. For all analyses, the sample size equals the number of experimental units being sampled (48). I will treat all rows with the same watering regimes. The experiment will end three weeks after peak cucumber harvest..
Treatment justifications
Cucumber was chosen as the focal crop because it is an economically important vegetable for organic farmers in our area.. The weedy fallow control treatment is necessary in order to use as a reference point for unmanaged habitat that acts as refuge for local arthropods, and compare to a planting intentionally selected for its properties as an insectary crop17,18. The bare ground treatment will allow me to compare our treatments to conventional cucurbit production (e.g., weedless). Partridge pea is included as a companion plant treatment due to its great potential as an understudied attractor for beneficial arthropods8. Additionally, PP is a legume which can be utilized for soil nitrogen fixation, and its flowers are highly attractive to native pollinators (personal observations 2022-2024) which may prove an additional benefit of increased fruit set for cucumber.
Objective 1
I aim to determine the effectiveness of EFNs as attractors of ants, other predators, as well as pollinators. I will address this objective by both observing and collecting herbivore and ant/beneficial arthropods to determine their density/diversity among treatments. First, I will visually inspect insects within three subsets of plants from each block (both treatments and crops). These observations are standardized in two ways: 1) observers will look for insects for a 1.5 min at each sampling area and 2) the subset of plants being observed will take place within three equidistant 0.5 m squares around a section of both crops and companion plants within each treatment block. Observers will tally and identify insects to the lowest taxonomic unit.
I will collect insects throughout the growing season using various methods to obtain a representative sample of the insect community from each block. These methods include pitfall traps, sticky cards, vacuum/aspirator sampling, and baited traps. Collecting and preserving insect specimens this way is beneficial as I can identify individuals much further down taxonomically than with visual observations. I will also record environmental variables known to influence insect communities during each sampling date, including: soil temperature, soil moisture, soil pH, soil compaction, air temperature, air humidity, overhead canopy cover, precipitation, and plant volume. These environmental factors are necessary to record because each may vary by block and site, and accounting for them in analyses is crucial.
I will then use data generated to calculate insect diversity, richness, evenness, and abundance. I will use linear mixed models to examine how these measures of diversity are impacted by treatment groups and environmental variables. I will use Non-metric MultiDimensional Scaling (NMDS) analysis to compare insect community composition between treatments, sites, and based on environmental differences.
I also predict that PP will increase abundances of generalist omnivores like ants and lady beetles on focal crops, as well as parasitoid wasps/flies whose adults feed on EFNs, and that crop damage will be lowest and yield highest in this treatment8. I also predict that weedy fallow treatments will provide plant structure and serve as a refuge for predatory insects, but will not be as attractive as the PP blocks - and therefore this treatment will have lower beneficial insect diversity than the PP treatment.
Predictions regarding aphids
Ants are known to tend aphids in exchange for honeydew, which increases aphid numbers on plants. However, I do not predict that enhancing ant populations via EFN companion plants will be an issue regarding aphids in my study system. Aphids, while present on cucumber and zucchini, are not the main cucurbit pest of concern in New England. Acalymma vittatum is the most damaging pest of cucurbits growing in our region19, and I have witnessed the effects of this pest firsthand when conducting my first two seasons of samples across organic farms in Massachusetts. Net economic impacts of ant-aphid mutualisms are highly context dependent, but studies in other systems have shown positive effects of honeydew-farming ants that drive away other arthropod pest groups from plants harboring aphid colonies20. Lastly, EFNs and flowers of my companion plants are highly attractive to local wasps that parasitize aphids, as well as voracious aphid-eaters like hoverflies, ladybeetles, and lacewings5,8.
Objective 2
I plan to determine if EFN-bearing companion plant presence results in less crop herbivory and higher yield compared to controls. Here I will compare crop yield and fruit/leaf damage among the treatments described above. I will quantify leaf damage by scoring a subsection of each plants’ leaves by level of damage within a category23. I will also categorize and score fruit damage and take weight measurements as fruit is harvested. The inclusion of these categories is important, as different types of damage indicate different pests/disease. For example: damaged edges on leaves from chewing caterpillars, or spotting/necrosis are caused by fungal infections23. Schifani et al., 2020 also suggested that the presence of ants on experimental pear trees lowered necrotic damage caused by a fungus, via antimicrobial secretions from visiting ants. Therefore, recording the percentage of plants infected with disease will provide useful information. Lastly, I will use yield data to conduct a cost-benefit analysis to determine if the labor and materials required to install/maintain these companion plantings are worth it to a farmer.
I will calculate yield by obtaining total crop mass at the end of the season and will consist of two measurements: 1) total crop harvest biomass, and 2) marketable yield (total biomass minus unmarketable fruit). I will obtain biomass by weighing produce as it is harvested. I will then use linear mixed models with damage scores and yield as response variables, block treatment as a fixed effect, and include sample date, environmental factors and site as random effects to analyze the data. For the cost-benefit analysis, I will subtract the labor ($/hour of work setting up and maintaining treatment zones) and materials (e.g., seeds, mulch) utilized to maintain each treatment from the total yield at the end of the season. I will then use a single-factor Anova to compare differences in yield (response variable) across treatments.
I hypothesize that the recruitment of predators to EFN companion plants will influence the marketable crop yield and tissue damage in a system. I predict that when compared to control blocks, there will be an increase in marketable crop yield and lower host plant tissue damage from herbivores when PP are present. This is because PP will likely attract higher numbers of generalist predators (like ants) that I hypothesize are important in removing the most damaging pests in cucurbits (e.g., A. vittatum, D. howardi)21,22.For the cost-benefit analysis, I predict that the yield gains from PP will outweigh the overall cost of materials (e.g., seeds) and labor to include these plantings when compared to the management that goes into maintaining control blocks.
Objective 3
For my last objective I plan to determine if the presence of EFNs on the crops themselves has an impact on the insect communities that are associated with each and whether there is a synergistic effect of EFN-bearing companion plants on these two crop types. I will accomplish this by comparing the insect communities across crop types sampled between the 2024 (zucchini) and 2025 (cucumber) seasons and their respective treatments and sites. The collection methods and data used to address this objective are the same as the above Objective 1.
Using linear mixed models, I will examine how insect diversity, abundance, richness, and evenness are influenced by environmental conditions and how they vary across pairwise interactions of crop type and treatment. We will use Non-metric MultiDimensional Scaling (NMDS) analysis to compare insect community composition between companion plantings, crops, sites, while accounting for environmental factors.
I hypothesize that insect communities will vary between zucchini and cucumber, and among combinations of their companion plant treatments. I predict that carbohydrate-seeking predatory arthropods that utilize EFNs (e.g., ants, beetles) will be more apt to spill over to and forage on zucchini versus cucumber. The presence of EFN on zucchini will cause those arthropods to spend more time on this crop versus its non-EFN counterpart (i.e., cucumber). For zucchini specifically, I predict that more ants will forage on this crop’s EFNs (and therefore more will be collected from these plants) in “PP only” versus other treatments, as this treatment will attract and host the most ants.
Sharing of data and results
I will publish the results from this study in relevant scientific journals as part of my dissertation, as well as present to (e.g., local NOFA symposiums) and share with (e.g., data reports) farmers, landowners, and groups involved in the outreach presented in this proposal.
No data is yet available, 2024 data analysis is in progress and 2025 data collection will begin in May.
Education & Outreach Activities and Participation Summary
Participation Summary:
I plan to teach local organic farmers about insect conservation and beneficial insects beyond pollinators. I will achieve this by running an insect conservation/citizen science workshop in cooperation with a biologist from the Xerces Society. The Xerces Society is an arthropod conservation organization that provides traveling training modules across the country. Specifically, I would like to hire one of their conservation biologists to come and educate newcomer organic farmers about: 1) getting involved in citizen science projects, 2) identifying native beneficial insects, and 3) managing habitat on their farms to promote beneficial insects.
These workshops will take place at one of our research sites which will allow us to utilize our companion plantings as an example of common beneficial insects that are attracted to local agroecosystems. Additionally, I will share yield and insect diversity results with the participating farmers as I gather and analyze our data at the end of the field season. I will present the summaries of our results at one of the local symposia held by the Northeast Organic Farming Association (NOFA), as well as publish findings in one of their newsletters free to the public. I am currently in contact with a member of the Xerces Society’s Northeast Chapter and we are working on plans for this event during the 2025 season. I have been in contact with Laura Davis - an organic farmer and member of the board of directors for NOFA/Mass, as well as Julie Rawson who is the head of Many Hands Farm in Barre, MA, and a retired member of NOFA/Mass after working with them for three decades. By designing a workshop in coordination with the Xerces society, Laura Davis, and Julie Rawson, we will create an outreach event tailored toward the specific needs and learning goals of farmers new to the organic scene in Massachusetts.
As an additional form of public outreach, I am planning to hold display booths at one or more local in-person events during the summer of 2025 (e.g., NOFA winter/summer conferences, the MA Commercial Urban Farming Symposium, EMass craft training events, MA Farm to School). My lab’s undergraduate students and I will run these booths. At these events, we will educate visitors on what we do at Clark, talk about the importance of insect diversity in agriculture, and describe the science behind the project that I am outlining in this proposal. I also will design and hand out pamphlets describing how hobbyists and professional farmers can manage the landscape around their plantings by choosing companion plants that complement their specific crops. The in-person activities I have described here will also benefit the undergraduate students working for us during the summer, as they will encourage them to participate and build on their skills in public outreach as young scientists.
Project Outcomes
Field work has yet to be conducted.
Field work has yet to be conducted.
Field work has yet to be conducted.