DOI: 10.5061/dryad.hmgqnk9r5
title: Organic compost belowground and floral diversity aboveground interactively shape natural enemies in urban gardens

description: Plant diversity aboveground can exert top-down pressure on herbivores by
 attracting predatory insects, while organic soil amendments rich in
 beneficial microbes can limit herbivores from the bottom up by enhancing
 plant defensive chemistry. Aboveground and belowground forces always
 operate simultaneously to shape herbivore pressure, but understanding how
 they interact is a longstanding and persistent challenge. Here, we examine
 how organic composts mediate effects of plant diversity across trophic
 levels, using zucchini plants (Cucurbita pepo) as a study system. Over two
 field seasons, we manipulated vermicompost treatments in 18 experiments in
 school gardens that varied in surrounding plant and floral resource
 diversity, and measured responses of insect herbivores and their natural
 enemies.  Vermicompost strengthened a positive relationship
 between flower richness and foliar-feeding omnivores, suggesting that
 robust reservoirs of omnivores at flower-rich sites mounted stronger
 responses to compost-treated host plants. Predators increased with flower
 richness, but were not affected by vermicompost. Net outcomes of
 vermicompost and plant diversity were neutral for herbivores. Synthesis
 and Applications: Altogether, our results reveal that bottom-up
 factors protecting plants are modified by their environmental context, and
 may more effectively attract natural enemies in landscapes with diverse
 floral resources. Therefore, we recommend augmentation of biodiversity
 aboveground (i.e. floral resources) together with biodiversity belowground
 (organic soil amendments) to strengthen crop protection.
description: <strong>Experimental treatments</strong>
 In August 2021 and May 2022, experiments were established at nine
 school garden sites in Athens, GA, USA (Appendix Table S1). In 2022, three
 of the original school garden sites were dropped from the study due to
 teacher turnover and other factors beyond our control, and were replaced
 by three new sites. Therefore, experiments were repeated in six sites
 across both years, with a total of 12 unique sites across the two-year
 study. Within each year, all sites were separated by a minimum distance of
 0.8 km (Appendix Figure S1). Each site consisted of two
 adjacent raised garden beds (1.2 m × 2.4 m), with eight zucchini seedlings
 placed 0.6 m apart in two rows. One bed at each site was randomly selected
 for vermicompost treatments while the other served as a control. Four
 sites had pre-existing garden beds, while new beds were established at the
 remaining five sites. New beds were filled with organic raised bed
 planting mix (Kellogg Garden Organics, Carson, CA). In 2022, at sites
 where beds were already established, we homogenized the top 30 cm of soil
 in both beds by moving half the soil from one bed to the other with a tarp
 and mixing thoroughly. Dry organic poultry fertilizer (30 mL Symphony®
 5-4-23, Seven Springs Farm Supply, per label instructions) was mixed into
 planting holes for both control and vermicompost beds, while an additional
 120 mL vermicompost (castings produced by <em>Eisenia
 fetida</em> fed horse manure, juice pulp, and vegetable waste;
 Earthen Organics Worm Farm®, 1-0-0) was added to the holes in the beds
 assigned to receive vermicompost. Because vermicompost had a 1% nitrogen
 concentration, the amendment slightly augmented plant-available nitrogen
 relative to the control, but both control and vermicompost treatments
 included sufficient nitrogen for vigorous plant growth. Background
 nitrogen concentrations in soils are noted in supplemental data. The
 control and experimental beds received equal amounts of water every 24 h
 (each plant was watered for 10 seconds) until the plants established,
 after which they were watered every 48-72 h.
 <strong>Insect sampling</strong>
 Three weeks after plants established, weekly visual surveys of
 herbivores and natural enemies were initiated and continued for four weeks
 in 2021, and three weeks in 2022. Many plants in our 2022 study were
 killed by squash vine borers at four weeks of sampling (<em>Melittia
 cucurbitae</em>). Herbivores most common to our system included
 aphids, squash bugs (<em>Anasa tristis</em>), leaf/plant
 hoppers (Hemiptera: Cicadellidae & Flatidae), and fleahoppers
 (Hemiptera: Miridae). Omnivores preying on herbivores in our system
 included minute pirate bugs (<em>Orius insidiosus</em>) and
 big-eyed bugs (<em>Geocoris</em> spp.), and the most common
 predators were spiders (Araneae) and ladybeetles (Coleoptera:
 Coccinellidae). While spiders and ladybeetles occasionally feed on nectar
 and pollen, they were classified separately from ‘foliar-feeding
 omnivores’ because they do not feed on foliar tissue and are less
 responsive to changes in plant quality (Blubaugh 2023). On each plant, six
 leaves and petioles were carefully inspected on both sides (two new, two
 old, two intermediate), and all insects observed were counted, identified
 to family where possible on-site, and to order otherwise (for spiders).
 Insects in open flowers were recorded as well. Surveys terminated after
 three weeks in 2022 as nearly all plants from three sites were killed by
 squash vine borers. In 2021, four plants from each bed were randomly
 sampled on each sample date, and in 2022, all eight plants were sampled
 with the addition of more team members. Plants were excluded from surveys
 if they were severely damaged by deer browsing or by squash vine borers
 (damaged/dead plants from excluded from each site are noted in
 supplementary data). The timing of our two experiments
 ensured wide coverage of the communities of pests that attack cucurbit
 crops. In 2021, we captured late-season pest communities, which were
 dominated by squash bugs and aphids. In 2022, experiments were performed
 early in the growing season, where the most common pests were cucumber
 beetles, squash vine borers and aphids.
 <strong>Plant Diversity Survey</strong>
 To quantify plant diversity in the surrounding area, we gridded a
 450 m<sup>2</sup> area surrounding the raised beds, and
 recorded all crop and non-crop plant families observed. Our survey area
 was similar in scale to a similar study in urban agroecosystems (Philpott
 & Bichier 2017), and was sufficient to characterize most crop and
 non-crop plant diversity at all our sites. Schoolyard landscapes were
 primarily defined by turfgrass or pavement beyond the 12 m survey radius
 that typically included the entirety of all garden areas at each site. We
 estimated plant richness at the family level in both years of the
 experiment, and in 2022, we also counted inflorescences of all actively
 flowering plants, and estimated richness of flowering plants at the family
 level. Some garden sites included intentionally managed pollinator
 gardens, and others did not, creating a gradient in diversity of available
 flowers. We did not quantify floral traits or evaluate the quality of
 floral resources (i.e. nectar/pollen) offered by each plant.
 <strong>Crop performance and leaf
 quality</strong> To evaluate the effects of our
 treatments on the nutritional quality of foliar tissue, we collected the
 third unfurled leaf from the apical meristem from three randomly selected
 plants in each bed. Leaf tissue samples were processed at the Agricultural
 and Environmental Service Laboratory at the University of Georgia to
 determine percent total nitrogen concentrations with an Elementar Vario
 Max Total Combustion Analyzer (Langenselbold, Germany). We also recorded
 numbers of leaves, flowers and fruits (&lt;30 cm long), to estimate
 effects of our treatments on plant performance and crop yield. Ripe fruits
 were harvested after visual surveys completed.
description: # Organic compost belowground and floral diversity aboveground
 interactively shape natural enemies in urban gardens This dataset includes
 results from experimental manipulations of vermicompost treatments made to
 zucchini plants in 18 experiments performed over two years in urban school
 gardens. On each zucchini plant, we counted herbivorous pest insects,
 foliar-feeding omnivorous insects, and predatory arthropods. It also
 includes observational field surveys of floral resource richness
 (estimated at the family level) from the surrounding environment at each
 site. ## Description of the Data and file structure Missing data/Blank
 cells (when samples were lost or damaged) are noted as "n/a"
 throughout the data set. This spreadsheet contains four sheets: Sheet 1
 (annual.means.without.outliers) contains annual mean estimates of floral
 resources, insects, and soils from each site, pooled across all sample
 dates for each experiment. This sheet excludes outlier insect counts from
 three plants (out of 620) where insect counts were at least 3 SD higher
 than the next highest observation; Sheet 2 (annual.means.with.outliers)
 contains annual mean estimates of floral resources, insects, and soils
 from each site, pooled across all sample dates for each experiment. This
 sheet includes all observations. Sheet 3 (insect.survey.2022) contains raw
 counts of insects observed on zucchini plants sampled on each sample date
 in 2022; Sheet 4 (insect.survey.2021) contains raw counts of insects
 observed on zucchini plants sampled on each sample date in 2021; Sheet 5
 (plant.survey.2022) contains raw counts of plants and influorescences
 observed within 450 m^2 area surrounding experimental garden beds in 2022;
 Sheet 6 (plant.survey.2021) contains raw counts of plants observed
 observed within 450 m^2 area surrounding experimental garden beds in 2021;
 **Metadata:** **Descriptions of column headers:** treat: Experimental
 treatment (vermicompost and control) applied to each bed Year: Year field
 experiments were performed. Site: Name of each school where experimental
 gardens were established. herbivore: Anuual means of herbivorous insects
 counted on zucchini plants at each field site over all sample dates.
 omnivore: Anuual means of foliar-feeding omnivorous insects counted on
 zucchini plants at each field site over all sample dates. predator: Anuual
 means of predatory arthropods counted on zucchini plants at each field
 site over all sample dates. ant: Anuual means of ants counted on 8
 zucchini plants at each field site over all sample dates. flower.count:
 Counts of influoresences observed within a 450 m^2 area surrounding
 experimental garden beds. flower.richness: Richness of blooming plants at
 the family level within a 450 m^2 area surrounding experimental garden
 beds. plant.richnes: Richness of plants at the family level within a 450
 m^2 area surrounding experimental garden beds. non.bloom.rihcness:
 Richness of non-blooming ornamental plants at the family level within a
 450 m^2 area surrounding experimental garden beds. %soil.N: percent total
 Nitrogen (organic and inorganic)in soil samples collected from each
 experimental gardenn bed. %leaf.N: percent Nitrogen in foliar tissues
 samples collected from 3 zucchini plants in each experimental garden bed.
 fruits: Annual mean counts of ripe zucchini fruits harvested from each
 experimental garden bed. date: Month/day/year observations were made.
 insect: Common name identification of insects observed in visual surveys
 on zucchini plants in experimental garden beds. count: Sum of insects
 observed onn visual surveys on zucchini plants in experimental garden
 beds. trophic guild: Categorical identifier of insects' trophic guild
 (predator, omnivore, generalist herbivore, specialist herbivore) trophic
 level: categorical identifier of insects' trophic level (herbivore,
 natural enemy) Missing data/Blank cells (when samples were lost or
 damaged) are noted as "n/a" throughout the data set.