Agroecological methods to manage brassica pests on organic farms

Final report for GS16-160

Project Type: Graduate Student
Funds awarded in 2016: $11,000.00
Projected End Date: 02/28/2018
Grant Recipient: University of Texas-Rio Grande Valley
Region: Southern
State: Texas
Graduate Student:
Major Professor:
Dr. Alexis Racelis
University of Texas - Rio Grande Valley
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Project Information

Summary:

For farmers interested in transitioning to organic agriculture, or those that look for more sustainable options as they choose to avoid both the economic or ecological costs associated with conventional pesticide use, conservation biological control (CBC) may be a viable and attractive option.  CBC encourages the presence of native, beneficial predatory insects to regulate pest prevalence. This project builds on preliminary work that has been done exploring strategic intercropping as a method of pest regulation. This farming method, commonly known as push/pull agriculture, often involves the use of three different plant varieties 1) a cash or main crop, 2) “push” or repellent plant(s), and 3) “pull” or attractant plant(s) which function to attract both pests and their natural enemies.

This project tested the efficacy of various plant species as potential push-pull components on an organic vegetable farm in south Texas. An on-farm planting strategy incorporating push/pull components was employed to assess damage caused by green peach aphids (Myzus persicae (Sulzer)) and total plant biomass relative to a particular companion plant.

Project Objectives:
  • Develop a planting strategy that allows the growth of kale alongside a push/pull companion plant.
  • Assess kale cash crop for total biomass and green peach aphid impact relative to companion plant identity.

Research

Materials and methods:

In order to assess the efficacy of a companion plant for pest control, a planting strategy involving the growth of the cash crop kale (Brassica oleracea) next to a particular companion plant was employed. Data was collected over two growing seasons; February – May 2017 and October 2017 – February 2018. The planting strategies involved a grid-like planting pattern wherein each grid square measured 2ft by 2ft. Kale was planted at a corner of each experimental grid square and the companion plant in the center resulting in a pattern with four kale plants per companion plant. The companion plants dill (Anthenum gravolens), fennel (Foeniculum vulgare), cilantro (Coriandrum sativum), and hyacinth bean (Lablab purpureus) were utilized for the first growing season and bunching onion (Allium fistulosum), fennel, dill, and cilantro were used for the second. Data collection from the first growing season involved a full-destructive harvest wherein each kale plant was assessed for green peach aphid (Myzus persicae (Sulzer)) damage and overall biomass. Aphid damage was assessed by analyzing all individual plant leaves and classifying them as no damage, light damage, or heavy damage; no damage denoting no presence of aphids or aphid damage, light damage being 1-10 aphids present on the leaf and/or leaf damage that did not completely perforate the leaf, and heavy damage denoting the presence of 11+ aphids and/or leaf perforation damage. Biomass values were obtained by drying kale leaves at 40°C in an incubator for 72 hours and taking resulting total dry weight in grams. Data for the second growing season was collected via the harvest of six kale leaves per plant; two from the crop base, two from the center and two from the top. Analysis of biomass and pest impact was identical to the first growing season analyses.

Research results and discussion:

First growing season

The results from the first growing season show dill to be the companion plant that yielded both the highest kale biomass (178.9g) (Table 1) and lowest rate of heavy aphid damage (6.6%) (Table 2) while fennel and cilantro yielded similarly high rates of aphid damage with 25.3% and 14.8% of the leaves having heavy aphid damage respectively (Table 2) and low biomass yields of 95.2g and 92.5g respectively (Table 1). The control group and hyacinth bean kale plants yielded median biomass and aphid damage (Tables 1 & 2).

Table 1. Kale biomass comparisons from the first growing season.

Companion Plant

n

Average Biomass (g) ± SE

Control (Kale)

13

114.2 ± 14.4

Hyacinth Bean

8

132.1 ± 10.9

Fennel

4

  95.2 ± 17.3

Cilantro

8

  92.5 ± 14.0

Dill

2

178.9 ±   4.1   

 

 

 

Table 2.  Percent of kale leaves with no, light, and heavy aphid damage on leaves harvested from the first growing season.

Companion Plant

n

% No Damage

% Light Damage

% Heavy Damage

Control (Kale)

583

26.6

64.7

8.7

Hyacinth Bean

435

45.3

46.0

8.7

Fennel

178

41.6

33.1

25.3

Cilantro

393

36.9

48.3

14.8

Dill

227

27.8

65.6

6.6

 

 

Second Growing Season

Biomass yields from the second growing season ranked kale companion planted with fennel to be the most successful (10.5g) followed by onion and dill (9.5g), and lowest yields to be associated with cilantro and the control group (8.3g) (Table 3). The highest occurrence of no aphid damage was on the cilantro (56.7%) and dill (47.2%) experimental groups and heavy aphid damage was highest on the control (31.3%) and fennel (29.2%) groups (Table 4).

Table 3. Kale biomass comparisons from the second growing season.

Companion Plant

n

Average Biomass (g) ± SE

Control (Kale)

8

8.3 ± 0.8

Fennel

8

10.5 ± 0.6

Onion

9

9.5 ± 0.6

Cilantro

10

8.3 ± 0.8

Dill

6

9.5 ± 0.6

 

Table 4. Percent of kale leaves with no, light, and heavy aphid damage on leaves harvested from the second growing season.

Companion Plant

n

% No Damage

% Light Damage

% Heavy Damage

Control (Kale)

48

33.3

35.4

31.3

Fennel

48

22.9

47.9

29.2

Onion

54

37.0

37.0

26.0

Cilantro

60

56.7

35

8.3

Dill

36

47.2

30.6

22.2

 

In both growing seasons, kale companion planted with cilantro was found to have the lowest biomass yield and those planted with fennel had the highest aphid impact. However, the effects of cilantro and fennel as companion plants varied greatly between the two growing seasons with regard to the other variables. Kale companion planted with fennel produced the lowest biomass levels in the first growing season but the highest in the second, and kale planted with cilantro produced the highest aphid impact in the first growing season and lowest in the second.

Companion planting with dill produced the largest biomass and lowest occurrence of heavy aphid damage in the first growing season and produced a median biomass and low level of aphid damage in the second. These results are fairly consistent with one another and could be attributed to one of many possible reasons, including the attraction of beneficial insects that predated the aphids or repelling of the aphid pests by the companion plants. As such, dill has been identified as a “push” species for green peach aphids. No “pull” species was identified during this study. Further exploration into the mode of action responsible for the aphid impact and biomass yields can be investigated in future projects through the field collection of insects relative to companion plant with sticky traps and sink traps and laboratory olfactometry testing to track insect movement in response to plant semichemical volatiles present in companion plants.  

Ultimately, the results from the first growing season are thought to be more reliable for two reasons; 1) the complete destructive harvest gives a full analysis of the state of the kale cash crop versus a sampling and 2) difficulties with weather patterns that delayed harvesting resulted in kale growing for a longer period of time for the second season than a farmer would normally allow. Ideally, harvest of the second growing season would have taken place in December, but was pushed back to February due to weather complications. This may explain some contradictory results from the two growing seasons, though a third season harvested within the parameters of a normal growing season and assessed under the same biomass and aphid impact parameters would assist in determining whether or not this assumption is accurate.

Participation Summary

Educational & Outreach Activities

2 Webinars / talks / presentations

Participation Summary

Education/outreach description:

This research was presented in poster format at the Subtropical Agriculture and Environments Society Annual Conference in 2016 and at the University of Texas – Rio Grande Valley College of Sciences Annual Graduate Conference in 2017.

Project Outcomes

Project outcomes:

This project corroborated the results of preliminary work that reported dill as a repellent (push) plant for green peach aphids. Use of dill to manage aphid damage can reduce the need for insecticides leading to lower production cost which benefits both the farmers and consumers as well as increases the sustainability of a farm by limiting chemical insecticide use.

Recommendations:

This work can act as further preliminary research for future graduate student study. Future studies can incorporate insect capture and identification to determine the companion plant effects on localized pest and predator densities. Insect densities relative to companion plants were not examined in this study but would be beneficial in determining mode of action off each companion plant and should be prioritized in the event of future study.

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.