Intercropping for Pest Control in Organic Kale in Northern Florida

Progress report for GS20-223

Project Type: Graduate Student
Funds awarded in 2020: $16,279.00
Projected End Date: 08/31/2023
Grant Recipient: Florida State University
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Nora Underwood
Florida State University
Expand All

Project Information


Intercropping has been a successful tool for farmers for thousands of years. Modern agricultural practices have diverted attention away from intercropping, but this has come at the price of sustainability. Reliance on monocultures supported by pesticides has led to reductions in pesticide efficacy, contributed to environmental pollution, and harmed human health. Intercropping can increase economic return for farmers through yield increases, and as a component of integrated pest management (IPM). However, successful implementation of intercropping can be highly context-dependent, depending on the specific crop, timing, and spacing arrangements (scale of inter-mixing of crops) suited to a particular region. This project seeks to evaluate intercropping as a tool for sustainable vegetable production on small organic farms in the Southeast. This project will address the question of how the efficacy of intercropping varies with the spatial scale of mixtures, and evaluate the use of a novel combination of kale (Brassica oleracea) and elephant garlic (Allium ampeloprasum). Previous work suggests that Allium species (onions, garlic, leeks) can be beneficial companion plants for many crops. Elephant garlic is well suited for intercropping with kale in the North Florida region, but its use has not yet been evaluated. Working with a small farm and an agricultural research station we will use organic practices to grow kale monocultures and dicultures with elephant garlic at different spatial scales of mixture. Insect pests, damage to plants and natural enemies will be surveyed; kale and garlic yield will be recorded, and net costs and benefits to farmers determined.

Project Objectives:

The objectives of this project are to evaluate intercropping as an economically viable and environmentally sustainable alternative to pesticide use and monocropping for pest suppression. The project will also examine differences in the benefits (pest suppression, yield) to farmers in using different scales of intermixing of kale and elephant garlic. We expect pest suppression to result from volatiles that repel herbivores or disrupt herbivore-searching behavior, as well as potentially increasing the diversity of natural enemies. We are testing different spatial scales of intermixing because, due to the short-range action of plant volatiles, it is possible that intercropping with elephant garlic will only be beneficial when the plants grow in close proximity.

This project is intended to engage local farmers practicing sustainable agriculture in the Tallahassee area. This project was developed in consultation with two local farms (Turkey Hill Farm, Full Earth Farm) that are both leaders in local sustainable agriculture and in the Red Hills Small Farm Alliance, a coalition of 60 farmers in north Florida and south Georgia that share an on-line farmers market delivering to the Tallahassee area. Both of these farms grow kale and elephant garlic, but they do not currently practice intercropping. In addition to working at a regional agricultural research station, we propose to leverage the planned expansion of a local small farm (Longview Farms) from beef and poultry production into growing vegetables for market, using some of their new vegetable beds for experiments at a second site. 

Our primary goals are to:

1) Evaluate whether plots of kale (Brassica oleracea, cultivar: Lacinto) intercropped with elephant garlic (Allium ampeloprasum) enhance biological control, insect diversity, and increase yield compared to kale monocultures. We will test three different spatial scales of intercropping.

2) Disseminate research to farmers and growers, and conduct science outreach to elementary school students.


Materials and methods:

The experiment will consist of three treatments of kale-elephant garlic dicultures; elephant garlic intercropped within a row of kale, elephant garlic intercropped in alternating rows, and elephant intercropped with kale in side-by-side blocks (Figure 1). Because small farms harvest kale by hand and as individual leaves to be sold in bundles, alternating kale and elephant garlic plants within rows will not interfere with the kale harvest.

The research will be conducted at Longview Farms, a local farm in Havana, Florida, and at the University of Florida, Institute of Food and Agricultural Science (UF/IFAS) Extension research field, located in Quincy, Florida. Kale and elephant garlic will be grown at both sites. Longview Farms will have 12, 10 x 19.5 foot (3.33 x 6.5 m) plots, each surrounded with a 6-foot (2 m) border of bare soil. The boundary of bare soil is intended to reduce random insect movement between plots.  The 12 plots will contain three replicates each of the three  intercropping treatments plus the kale monocultures. The UF/IFAS Extension research field will consist of 24, 10 x 19.5 foot (3.33 x 6.5 m) plots, surrounded with a 6-foot (2 m) border of bare soil, allowing for twice as much replication. Each plot will have six rows of crops. At each location, elephant garlic and kale will be grown from October to May. Planting in the fall will be dependent on when the weather begins to cool down. Kale plants will be started in the greenhouse and transplanted in the field once conditions are suitable. Compost or manure will be used to fertilize all plots within treatments.

Schematic of proposed treatments for intercropping mixtures of kale (brown circles) with elephant garlic (green circles). The left panel illustrates kale intercropped with elephant garlic within rows, the center panel illustrates kale intercropped with elephant garlic in blocks, and the right panel illustrates kale intercropped with elephant garlic in alternating rows. Figure not to scale.


Objective 1 — Evaluate whether plots of kale intercropped with elephant garlic enhance biological control, insect diversity, and increase yield compared to kale monocultures.

 Treatment 1: kale intercropped with elephant garlic within rows

Treatment 2: kale intercropped with elephant garlic side-by-side in blocks

Treatment 3: kale intercropped with elephant garlic in alternating rows

Treatment 4: kale monoculture

The abundance of herbivores and natural enemies will be sampled biweekly on kale and elephant garlic crops. Observations and sampling will begin once elephant garlic has emerged from the soil and kale plants are established, approximately four weeks following planting. Multiple sampling methods will be used to assess the abundance and diversity of insects within different trophic guilds and to examine the composition of arthropod communities between plot treatments. A subset of individual plants within each plot will be sampled for visual observations of insect herbivores and predators. Yellow sticky traps will be used to sample aphids, leafminer adults, and whiteflies. Pitfall traps will be used to sample soil surface insects. A Berlese funnel will be used to assess arthropod diversity within soils. All insects will be identified to families, and when possible to species or genus.

Damage to a subset of kale leaves within each treatment will be recorded along with the  insect observations, on a scale of 1-4 as follows, 1= 0-5% damage, 2= 5-20% damage, 3= 20-50% damage, and 4= >50% damage. Location of damage on the plant and characteristic types of damage (chewing, leaf curling, sucking, etc.) will also be recorded. Kale harvests generally occur multiple times over the season, with farmers removing individual leaves as needed. We will calculate yield as the total number of leaves and weight harvested at each time interval, as well as cumulatively over the season. Elephant garlic will be harvested in late May or in June, 2021, depending on weather, and yield will be recorded.


Statistical analysis

Kale and elephant garlic yield and damage, and insect abundance and diversity will be compared among treatments. Data will be analyzed separately for the two locations, as the UF/IFAS site and the farm site differ in several respects. Insect abundance within trophic guilds between the treatment plots will be estimated using a generalized linear mixed-effects model (GLMM) with a Negative Binomial likelihood, if the data meet model assumptions. If necessary, appropriate transformations of the data will be done to meet model assumptions. A Negative Binomial likelihood will be used to account for overdispersion, since insect abundances are generally clumped. Replicate plots will be modeled as a random effect to account for repeated sampling. The effect of treatment on kale yield and damage within plots will be compared using ANOVA. Soil arthropod diversity will also be compared between plot treatments using ANOVA. Once samples are sorted into taxonomic groups, the proportion of herbivores to predators will be evaluated in each plot using ANOVA. Data will be transcribed from fieldbooks and backed up regularly to a cloud-based server and made public upon publication, or within a year of student graduation, whichever comes first.

Objective 2 - Disseminate research to farmers and growers, and conduct science outreach to elementary school students.

Presentations will be made at farmer outreach events such as the annual UF/IFAS Field Day in Quincy, FL to inform vegetable growers about the potential benefits of intercropping in general, as well as results from growing kale intercropped with elephant garlic. Many local farms around Tallahassee grow both kale and elephant garlic. After the first year of data collection, preliminary results will be presented at the Southern Sustainable Agriculture Working Group (SAWG) Conference, Georgia Organics Conference and Expo, and the Entomological Society of America Annual Meeting. A manuscript will be prepared during this time to submit to a peer-reviewed scientific journal.

Longview Farms has an established partnership with Cornerstone Learning Community, an independent preschool – 8thgrade school in Tallahassee, for activities related to their science curriculum. This project will include activities related to entomology, insect food webs, and when possible will engage students in the scientific method during their field trips to visit the farm.


Research results and discussion:

In 2022, I have presented preliminary results from the intercropping experiment conducted in 2021 at the Ecological Society of America 2022 annual meeting. Preliminary results suggest there was no significant effect of intercropping elephant garlic with kale on arthropod abundances compared to kale monocultures. However, there were significant differences in arthropod abundances between the small farm field site and agricultural research station, suggesting the surrounding habitat matrix may contribute to mediating effects of intercropping. I am still entering additional data and processing samples but anticipate presenting and distributing pamphlets with results to local farmers once all the data have been analyzed. I will also conduct a follow-up lab experiment this year to aid in the interpretation of results from the large field experiment. 

Participation Summary

Educational & Outreach Activities

1 Tours

Participation Summary:

Education/outreach description:

The pandemic this past year has made conducting outreach activities for this project challenging. I had the opportunity to record a brief presentation on a local farm (Longview Farm) for 3-5th grade students discussing this project and its relatedness to sustainable agriculture. I anticipate participating in more outreach as COVID restrictions for activities are lifted for students and other agricultural professionals. I anticipate presenting results from this project at the annual Entomological Society of America conference this fall, in addition to creating educational factsheets related to intercropping to distribute to farmers, and additional outreach with students during on-farm tours. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.