Supporting Natural Enemies of the Cabbage Aphid with Hedgerow Plantings

Progress report for OW18-007

Project Type: Professional + Producer
Funds awarded in 2018: $48,554.00
Projected End Date: 03/31/2021
Grant Recipient: Wild Bee Project
Region: Western
State: Utah
Principal Investigator:
Laura Horn
Wild Bee Project
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Project Information

Abstract:

Our project seeks to expand the adoption of Conservation Biological Control practices for aphid
management on vegetable farms in northern Utah and the Intermountain West. Most farmers in
the region are faced with significant hurdles in learning and upfront costs required to implement
such practices. Aphid control is a fruitful “introduction” to Conservation Biological Control as
aphids have many natural enemies, and are a persistent problem for vegetable growers. Cabbage
aphid problems in particular have increased in recent years, as more farmers in the region grow
kale and other brassica crops year-round to meet customer demand.
Our project will establish aphid-control hedgerows at six farms, three of which will be used as
demonstration sites. Hedgerows will be evaluated for their potential to attract
natural enemies of cabbage aphid and to reduce aphid damage to cole crops, using associated
control plots (at varying distances from hedgerows) for comparison.
Education and outreach components of the project focus on building knowledge and visibility of
Conservation Biological Control practices at the grassroots level, and on making connections
among urban vegetable growers and the ways they implement Conservation Biological Control strategies in our region. We will extend Conservation Biological Control education and the results of this study to growers throughout
the Intermountain region with USU Extension or other publications, with an emphasis on the cost-benefit analysis of incorporating long-term Conservation Biological Control strategies such as hedgerows for pest control.
Our project attempts to lower the barriers to adoption of Conservation Biological Control
practices by emphasizing 1) that Conservation Biological Control is just formalizing and expanding on practices many farmers are already implementing, 2) Conservation Biological Control plants can be chosen for their direct profit potential (culinary or medicinal herbs, seeds, or bouquets for sale), and 3) the multiple resource conservation benefits of hedgerows specifically (e.g., reducing soil erosion, filtering runoff, preventing weed encroachment).

Project Objectives:

Objective 1. Evaluate whether hedgerows attract natural enemies of cabbage aphid

Objective 2. Evaluate whether fewer cole crops reach economic loss thresholds with
hedgerows

Objective 3. Use established hedgerows and outcomes of this study (Objectives 1 and 2) to
support broader education goals with Conservation Biological Control through
presentations to urban and small-acreage vegetable farmers.

Objective 4. Encourage grower-to-grower information exchange regarding Conservation
Biological Control with workshops or farm tours at demonstration farm sites.

 

Cooperators

Click linked name(s) to expand

Research

Materials and methods:

Methods for establishing hedgerows (Objective 1a):

Site Analysis, planning, and design: SPRING 2018

  1. Site 1 (Green Team Farm). A 55 ft x 10 ft block was chosen for the hedgerow, along/directly adjacent to a 60-ft long greenhouse. The area had been used for storage and was compacted with low fertility soils, and few weeds. Topography was generally flat, but for the purposes of water catchment/conservation the area was sculpted to slope slightly downhill away from the greenhouse to capture runoff from the roof, and allow runoff to flow across the planting. A berm was constructed around three sides of the planting, and a pathway between the greenhouse edge and the planting for access. Plant choices: drought-tolerant mix of grasses, shrubs, and flowering perennials to provide continuous food and shelter for beneficial insects. Plants also selected to provide a yearly source of herbs and farmscaping perennials (most likely the grasses, most easily divided) to sell or add more beneficial insect habitat to the farm. 
  2. Site 2 (Hand Sown Homegrown Farm) A 80 ft x 4 ft block was chosen for the hedgerow, located between a fence and an open irrigation ditch that conveys water to the farm. The area was covered with turf grass and perennial weeds, having been mowed with a weed whacker several times per season. The hedgerow site is adjacent to the growing area, which is enclosed by the fence. A visual screen and windbreak is needed at the site to mitigate pests and weed seeds arriving by wind from the shared access road area and irrigation ditch along the south property line. Plant choices: flowering columnar shrubs and native bunchgrasses to fit in the narrow space and grow tall (8 ft) to provide a windbreak and visual screen in 3-5 years; also several flowering perennials planted in the gaps at each end of the hedgerow to attract and retain beneficial insects.
  3. Site 3 (Wheeler Historic Farm) A 100 ft x 5 ft block was chosen for the hedgerow, which follows the edge of a historic farmhouse yard, and an irrigation ditch for part of its length (60 ft). The area had been bulldozed in 2017 for renovation of the historic farmhouse and garden, and was populated by some annual and perennial weeds. A hedgerow is needed as a living fence to enclose the historic farmhouse garden and provide stabilizing vegetation along the irrigation ditch. Plant choices: modern cultivars of traditional shrubs will be used, to aesthetically match the historic farmhouse, provide a realistic representation of a historic hedgerow, and provide recognizable species to the public for the “demonstration” objective.

Site Preparation and planting: FALL 2018

  1. Site 1 (Green Team Farm). To prepare the site, the site was irrigated for 24 hours, soil was then loosened with a broadfork, and approximately 2″ of bark fines were spaded into the planting area. Plants were installed followed by installation of six Netafim irrigation lines (layout shown in photos). Species include feather reed grass, blue gramma grass, rabbitbrush, Russian sage, catmint, alyssum, candytuft, echinacea, black eyed susan, goldenrod, licorice mint hyssop, butterfly weed, and purple aster. 

Hedge adjacent to greenhouse, designed to capture runoff. Planted Sept 2018
Signage from June Bee Festival (Tour)

  1. Site 2 (Hand Sown Homegrown Farm) To prepare the site, turf grass and weeds were removed by hand and approximately 1″ of bark fines were spaded into the planting area. Plants were installed followed by installation of two Netafim irrigation lines (layout shown in photos). Species include black tower elderberry, golden tower elderberry, standing ovation serviceberry, little bluestem grass, blue gramma grass, echinacea, candytuft, black eyed susan, goldenrod, and autumn joy stonecrop.

  1. Site 3 (Wheeler Historic Farm) To prepare the site, weeds were removed by hand and approximately 2″ of bark fines were spaded into the planting area.  Plants were installed followed by installation of three Netafim irrigation lines (layout shown in photos). Species include knockout rose and Korean dwarf lilac for the shrub row and tufted hairgrass along the outside edge (the dark, uniform grass clumps shown in photos). Note: Existing perennial grasses (the lighter green, irregular clumps) immediately adjacent to the ditch were left in place to provide temporary stabilization while the tufted hairgrass becomes established. 

South end of hedge
South end of hedge – another view
North end of hedge

Site analysis, planning, design, preparation and planting 

SPRING 2019

Site 4 (Borski Farm)

A 80 ft x 4 ft block was chosen for the hedgerow, on the west side of a concrete wall / property barrier. The area had been weedy and unsuitable for growing vegetables and was a repository for field debris. Topography was flat although situated about 6 inches higher than the main field. The area was weeded by hand and mulched in fall 2018 in preparation for planting. A mix of hardy and drought-tolerant shrubs, grasses, and perennials were chosen. 

To prepare the site for planting in 2019, the site was loosened with a broadfork and weeded although few weeds were present. Two lines of Netafim were laid, followed by plants over the next two days. Species included feather reed grass, blue gramma grass, blanket flower, candytuft, evening primrose, sage, basket-of-gold, blue flax, blue and red penstemon, butterfly bush, cinquefoil, red hot poker, rabbitbrush, catmint, alyssum, and goldenrod. 

Hedge pictured in August 2019 (planting occurred in May).

 

Site 5 (HHR Farm)

A 100 ft x 4 ft block, wrapping around three sides of the vegetable plot area, was chosen for the hedgerow. Fencing encloses the garden on these three sides, so the hedgerow site is just inside the fencing. Due to the high weed pressure at the site, only hardy flowering shrubs were chosen for the hedgerow in order to facilitate weeding until establishment. The site was mulched once and weeded by hand three times in 2018 and early 2019 in preparation for planting.  In Spring 2019 two lines of Netafim were laid, followed by planting. Shrub species included golden currant, golden tower elderberry, standing ovation serviceberry, and two species of ninebark (a hardy ornamental). 

FALL 2019

Site 6 (Backyard Urban Garden or B.U.G. Farm)

A 50 ft x 4 ft block along the east side of an existing cedar hedge at the western border of the farm was chosen for the hedgerow. The block was located three feet from the cedar hedge to minimize contact with the (possibly) acidic soils. The site had low weed pressure but was partially shaded at one end. Shade-tolerant shrubs were chosen for the shaded end, and a diverse mix of flowering shrubs and perennials were chosen for the remainder. The site was hand weeded, mulched, and loosened with a broadfork prior to planting. Two lines of drip tape similar to that used elsewhere on the farm were installed by the farmer prior to planting in early September 2019. Species included dwarf peashrub, butterfly bush, false spirea, Russian sage, and cinquefoil. 

Hedge after planting, September 2019.

 

 

Measure abundance and diversity of natural enemies on cole crop (Objective 1b)

SPRING – FALL 2019

Abundance of beneficial insects (ladybugs, hoverflies, and parasitoid wasps) was assessed by visual count every other week at five sites on cole/brassica crops, with an emphasis on kale (curly, Russian, or lacinato). Xerces Society protocols for “Traveling Count” or “Stationary Count” were used, depending on the collector’s preference (count type remained consistent across collections for each collector). At some sites, data collection was interrupted when cole/brassica crops were covered or pulled out, or when weather was unsuitable (frequently in May and June, 2019). 

Diversity: Voucher specimens were collected using kill jars directly (ladybugs) or with an aerial net (hoverflies). Parasitoid wasps (approximately 200) were collected via aphid mummies placed in Eppendorf vials and reared to adulthood.  

 

Measure cabbage aphid infestation levels / crop losses due to aphids (Objective 2)

SPRING – FALL 2019

Aphid infestation levels on cole/brassica crops were assessed weekly at five sites. A subset of five plants per brassica variety were chosen for data collection at each weekly assessment. Per-plant or pear-leaf estimates were made depending on plant size. Counts were conducted whenever it was not raining heavily. Counts were suspended when crops were covered or pulled out.

To gauge perceptions, farmers were asked about their personal assessment of aphid infestation levels and kale crop losses (from cabbage aphid) in 2019 via text, email, or in person. 

Research results and discussion:

 

Natural enemy abundance: Data indicates that all three target natural enemies are present in low or moderate abundance across sites (less than one, or approximately one individual observed per minute). Isolated surges in (different) natural enemies occurred during two aphid infestations, at different sites. 

Natural enemy diversity: Identification of flies and parasitoid wasps is ongoing. Identification of ladybugs indicates that diversity is low across sites (predominantly two exotic species). 

Aphid infestation levels: Three seasonal cabbage aphid infestation peaks were identified, one in early spring (consisting of overwintering/first generation aphids), one in mid-summer, and one in late fall.

The mid-summer aphid peak was identified as having the greatest potential for natural pest control via Conservation Biological Control and will be the focus of this project going forward, as early and late aphid peaks occur when natural enemies are absent from most sites. I will be focusing on hedgerow plant bloom times and the natural enemy species with natural occurrence during June and July, to test natural pest control potential during the mid-summer aphid surge. 

Participation Summary

Educational & Outreach Activities

1 Published press articles, newsletters
2 Tours
2 Workshop field days

Participation Summary

40 Farmers
10 Ag professionals participated
Education/outreach description:
  1. Tour of farmscaping elements at Green Team Farm, Salt Lake City, UT. Event = “Bee Fest, A Celebration of Pollination” June 16, 2018. Tour of Green Team Farm midway through the festival led by James Loomis (GT Farm Director), with approximately 5 participants who were farmers, included stops emphasized the multitude of existing beneficial insect plantings on the farm including the OW18-007 planned hedgerow site where Western SARE signage was posted.
  2. Workshop and tour of farmscaping elements at Wheeler Farm, Murray, UT. Event = “Urban Farming and Farmscaping Conference” August 30, 2018. Approximately 10 participants at the workshop were farmers. Approximately 5 farmer participants attended the farmscaping elements tour led by Laura Horn that emphasized the existing beneficial insect plantings including two planned installations funded by OW18-007 where Western SARE signage was posted.
  3. “Conservation Biocontrol” Presentation at Integrated Pest Management Workshop (hosted by USU Extension), West Jordan, UT, January 14, 2020. Laura Horn presenting to approximately 25 vegetable farmers about Conservation Biocontrol strategies followed by Q&A. 
  4. Conservation Biocontrol article in Utah Pests Newsletter, Spring 2020 edition. Approximately 750 words written by Laura Horn, outlining Conservation Biocontrol strategies for Utah and progress on Western SARE project OW18-007.
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.