Application of the Banker Trap Plant (BTraP) Concept of Trap Cropping for the Management of the Harlequin Bug, a Pest of Brassicaeae: A new paradigm in small farm IPM

Progress report for LS19-311

Project Type: Research and Education
Funds awarded in 2019: $257,987.00
Projected End Date: 09/30/2022
Grant Recipient: NC A&T University
Region: Southern
State: North Carolina
Principal Investigator:
Louis Jackai
N. Carolina Agricultural and Technical State University
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Project Information

Abstract:

The proposed project explores the use of an existing concept on ‘Banker Plants’ in greenhouse IPM, to expand our knowledge and ability to manage the harlequin bug, an important field pest that attacks collard, kale, cabbage, broccoli and other crucifers in the Southeastern U.S. causing severe reduction in market value or death of the plants. The economic impact of this pest can be far-reaching especially among small growers who produce most of the collard and kale found in many U.S. market outlets. Collard, kale and cabbage, like other Brassicaceae, are attacked by an insect pest complex from seedling throughout crop growth. This results in extensive damage that warrants frequent and predictable application of insecticides to keep the pest below economically damaging levels, sometimes resulting in over $1 billion in management costs and crop loss. Cases of resistance by some crucifer pests to all classes of insecticides have been reported in the U.S. and elsewhere. Heightened public concern regarding excess pesticides use on vegetables and the documented negative effects have provided the impetus for increased research to find viable alternatives to pesticide dependence. The proposed research will build on and improve the efficiency of current deployment of mustard trap crops for HB control in the field, as well as provide a footprint for their use in other cropping systems using the banker plant concept — a greenhouse pest management approach. This will be accomplished by manipulating either the fixed intra-row or movable inter-row establishment of a banker trap crop (pest sink) positioned at different distances within or between main row crops. We will examine the effect of banker trap plants on overall HB attraction to, and retention on, the trap crop by assessing egg number and parasitization, nymphal and adult distribution between trap plants and main crop plants, damage to main crop, and leaf biomass at harvest. The density ratio will be modeled to enable adjustment for use with other pests in this or another agroecosystem thus giving the findings wider pest management application. This approach will reduce the area occupied by the trap crop, especially important in the case of a sacrificial trap crop which has no other value, and also provide the grower the flexibility of having the trap plants always available at the most attractive stage (if the mobile BTraP is adopted). The long-term goal is to reduce insecticide use in vegetable production, increase farm profits while enhancing food and environmental safety as well as well being in farm communities. It will also conserve natural enemies and pollinators. This project models existing knowledge from one area of science (greenhouse biological control) to accomplish a different pest management goal. Its success would hopefully foster enough interest and research on trap crop use to attract increased funding. The project targets small vegetable growers working in different agroecosystems but can be scaled to medium-sized farms. On-farm demonstrations will be established in conjunction with collaborating growers in North Carolina and Virginia.

Project Objectives:
  • Using variable plant ratios to conduct laboratory bioassays and greenhouse cage studies to evaluate efficiency of “Banker trap Plants (BTraPs) established at variable fixed simulated intra-row densities using four different mustard varieties to obtain different trap crop:main crop ratios.
  • Carry out field evaluation to a) validate the optimal ratios of the lab experiment and then b) determine the effect of the optimal fixed-BTraP ratios from Objective 1 and 2a, compared with movable BTraPs variants on harlequin bug attraction and retention on two recommended mustard trap crops.
  • Conduct on-farm validation and the economic sustainability of the two best BTraP deployment systems determined from Objective 2.
  • Perform an economic assessment of the various trap crop models in Objective 2 and their deployment in Objective 3 to determine benefit/cost optimization, overall system profitability and which model choice will be the best.

Cooperators

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  • Dr. Leonard Githinji (Educator and Researcher)

Research

Materials and methods:

Project objectives from proposal:

  1. Using variable plant ratios to conduct laboratory bioassays and greenhouse cage studies to evaluate efficiency of “Banker trap Plants (BTraPs) established at variable fixed simulated intra-row densities using four different mustard varieties to obtain different trap crop:main crop ratios.
  2. Carry out field evaluation to a) validate the optimal ratios of the lab experiment and then b) determine the effect of the optimal fixed-BTraP ratios from Objective 1 and 2a, compared with movable BTraP models on harlequin bug attraction and retention on two recommended mustard trap crops.
  3. Conduct on-farm validation and the economic sustainability of the two best BTraP deployment systems determined from Objective 2.
  4. Perform an economic assessment of the various trap crop models in Objective 2 and their deployment in Objective 3 to determine benefit/cost optimization, overall system profitability and which model choice will be the best.

Completed Objectives

Objective 1. The original sequence of this objective was altered because of the difficulties highlighted in the Preamble. The insect population in the lab crashed unexpectedly making it impossible to carry out the experiments as planned. We therefore started with the field experiment in a modified design which included all four mustards and the originally prescribed TC:MC ratios of 0, 5, 10, 20 and 25%. This also experienced problems with extremely heavy rainfall which led to flooded fields; crucifers are intolerant to flooded soils. Despite the poor stands that resulted we were nonetheless able to obtain some results which are reported here.

Field experiment: The experiment had two 5m-row plots x 4 mustards x 5 TC:MC ratios (0, 5, 10, 20 and 25%) arranged in a CRB with 3 replications (4 in VA); it was conducted at NCA&T State University (NCA&T) Research Farm in Greensboro, NC, and in the Virginia Tech Experimental Farm in Kentland, Blacksburg, VA. A locally adapted collard variety was used as the main crop; in NC the variety Georgia Southern, and Vates  in Virginia.

Each 2-row plot had 20 plant stands, with some of them replaced by the different mustards according to the respective TC:MC treatments. These were randomly spaced out ensuring equal distance between them within the plot (Figure 1). Measurements were made by close direct visual inspection, on 5 previously tagged collard plants and up to 5 mustards/treatment plot) for estimates of harlequin bug (HB) adults and 3rd-5th stage nymphs, HB egg masses (oviposition) and flea beetle counts (Blacksburg only). At this stage collard yield was not the focus on yield since we let measurements continue through the season past the normal period for harvesting. No insecticide was applied except once (VA) and twice (NC) to reduce caterpillar damage, which tends to confound measurements taken for damage by HBs. Sampling was started two weeks after transplanting (DAT)—June 25th in Blacksburg and July 19 in Greensboro. The difference in the start of observations was due to heavy rains during the early part of the growing season which delayed transplanting in Greensboro.

Field design showing an example of mustard trap placement.among collards.
Figure 1. Field positioning of BTraP mustards interspersed between collards in each 2-row plot according to the specific treatment ratio in Greensboro, 2019.

Laboratory study: This was supposed to be conducted before the field study but we had to revise the order when our laboratory culture crashed due to unexpected and still unclear reasons. We were able to resuscitate the culture mid-summer and run the arena choice test (Figure 2) which simulated different TC:MC ratios (0, 5, 10, 15, 20, 25%) in which we added another ratio, 15%, which was not part of the field study. The second was a cage study with potted seedlings (Figure 2) in which different ratios were tested as in the choice arena, except with whole plants. It was replicated over time due to space limitations to run four replications of large cages simultaneously. It also required a large number of insects which was also a limitation. This second experiment was started late in the fall after the insect culture was fully re-established. At the time of this report, the experiment is still incomplete although initial results closely mirrored those obtained in the choice arena laboratory experiment.

Objective 2. Due to the COVID-19 pandemic, the activities (Objective 2 a & b) that were scheduled for year 2 were not carried out as planned in 2020. This was as a result of university restrictions on access to research fields and laboratories in response to state and federal guidelines. A modified objective 2 will be carried out during the 2021 season with one activity moved to the following and final field year (2022) when a no-cost extension request will be made, as part of Objective 3. 

In the 2021 field season, we carried out activities in objective 2a validating the results from the laboratory studies carried out in 2020. These involved determining the optimal trap crop ratio, thus effectively completing objective 2. We added a new component in which we evaluated the optimal placement of the trap crop (periphery, borders, or in the center) that had also been examined in the laboratory.

placement in the field.

Both trap ratio and position are critical for proper trap function. The rest of this objective 2 (comparing the trap crop efficiency of fixed versus movable traps crops deployed in the optimal ratios) will be carried out in 2022 as part of objective 3 following the COVID-19 related modifications of the objective  referred to earlier.  
Figures on the left show border (top), perimeter (middle) and center (bottom) placement of trap plants (white flags) using fixed ratios of trap : main crop 7WAT (see trap crop placement earlier in the season below). NCA&TSU Research Farm, 2021.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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These three photos (on the left) show the traps in position at the early growth stages of the collard crop.

Research results and discussion:

                        

Field experiment: The 2019 season had heavy rains throughout the region and in some areas such as the Greensboro where NCA&T research farm is located, there were sustained rains for several weeks causing a delay in the onset of transplanting as fields were water logged and/or soggy, and at Greensboro  made worse by the use of wheat straw mulch for weed management. We were nonetheless able to plant 3 replications of the experiment instead of four as originally planned as a result of widespread water-logging. Even then, the plant stands were poor and plant health was affected. That notwithstanding, we were able to salvage a good part of the study and collected enough data to enable us to draw reasonable conclusions that were supported by laboratory studies. The experiment in VT performed much better and provided useful information.

The harlequin bug (HB) population was high throughout the season. at both locations. In Greensboro, we computed the ratio of HBs as well as egg masses on the trap mustards to that on the collard (TC:MC). These data were used to plot trap crop retention potential (TCRP) that allows us to test the hypothesis that "if more insects are found on the mustard trap crop as compared to the main crop (collard), there will be concomitantly less damage on the collard". We can therefore make an indirect but reliable inference on actual damage since this was difficult to measure given high caterpillar damage at both locations. Based on this assumption, the highest TCRP values in Greensboro were obtained with the 25% ratio (where FBL > GR > OFM >> SGC), 20% (for FBL and OFM >> GR and SGC). There were clear differences between mustard varieties with the Florida Broad Leaf (FBL) and Old Fashion Mustard (OFM) giving the highest retention potential at 10 and 20% (Figure 3).  

Despite their lower TCRP values, the two other mustards (SGC and GR) also showed a similar trend. We conclude, based on these initial findings, that FBL and OFM were the two best mustard trap varieties in Greensboro, and the trap refuge of 10 and 20% TC:MC ratio the most effective mustard trap ratios (Figure 3). It is important to note that the peak TCRP values in this figure occurred at the optimal harvesting period of the collard, July 23rd and July 31st, for FBL and OFM, and were more spread out for the other two mustards. This overlaps broadly with the results from the lab study which is described below. The results from Blacksburg were similar but with different mustards (SGC > FBL > OFM) having the highest trap retention potential inferred in this case also using the direct HB counts (Figure 4). The 10% and 25% were most effective although there appeared to be an inexplicable peak in 5% with FBL. As earlier indicated, the 20% treatment was inadvertently omitted at this location.

No results are available at this time for Objective 2.

Laboratory study:

Results from the laboratory dual choice arena test (DCAT) using the most promising mustards (FBL and OFM) identified from previous studies show that the 15, 20, and 25% ratios were the most promising (see circle , Figure 5A). Feeding damage (or necrosis) on the TC was expressed against the collard MC as described for the field experiment to test the hypothesis that the longer the insect feeds on the trap, the greater the likelihood of increased retention of the HB. This enables us to determine which TC:MC ratio provides the best retention on the trap crop. The greatest differences in TCRP were obtained on FBL (Figure 5B). Using LSM analysis, significant overall differences were observed (F, 6.6; df, 4; p= 0.0012) between the five ratios, but not between the three top ratios (Figure 5A). Figure 6 shows the plot of the TCRP of all treatments at both locations.

Field Experiments in 2021:

The results from last season confirm lab findings regarding the optimal TC:MC ratio to be 10 and 20 using OFM and FBL. These will be used in the field experiments in 2022 for Objectives 3 and 4. The participating growers will be given the option to use either ratio depending on whether they want to grow a beneficial trap crop (to harvest some leaves for sale or consumption), or a sacrificial trap crop (no harvesting intended). So the intended use eventually determines how much land will be devoted to the trap crop. 

 

Findings from both the field (2 locations) and laboratory studies (Greensboro only) during the first reporting period allow us to draw the following overall inferences:

  1. In NC, the best ratios are 20 and 25% in the field, and 15, 20, 25 (not significantly different) from the lab experiments with the best mustards being FBL and OFM.
  2. In VA, the best ratios are 10 and 25% with SGC and to a lesser degree FBL mustard trap crop varieties. The 20% ratio was inadvertently omitted in this location.
  3. The common ratios between the lab and field studies in NC is 20%; however, there was no difference among the three top TC:MC ratios (15, 20, 25) in the lab study (Figure 5A). No grower wants to devote his/her field to a sacrificial trap crop (not intended for harvest) instead of the desired main crop. We will therefore advance 20 and 15% to the next level of testing unless further testing suggests otherwise. We will use  two varieties we will use FBL and OFM in subsequent testing (NC) (Objective 2b).
  4. The Southern Giant Curled (SGC) mustard has always performed well in VA. This mustard will be retained in that location. In addition, we will also add FBL so that there is one mustard trap variety common to both locations to provide a basis for making location comparisons. This is also important because a collaborating farmer in Virginia wishes to grow the SGC mustard variety.
  5. What these findings suggest is that it seems reasonable to grow an effective BTraP in only 10-20% of the land area instead of devoting large strips or a perimeter land area to a trap crop. This will be validated with further studies and demonstrations in the final year of the project that will include cost-benefit estimates to show farm profits using these trap crop ratios.

Recent results from the studies conducted in both laboratory and field experiments in 2021 confirm the following:

  1. The optimal trap crop ratio is 10 or 20% using either Old Fashion mustard or Florida Broadleaf mustard. These results were initially initiated in the laboratory in 2020 and completed in 2021. Field experiments performed in 2021 confirmed these findings. In general, the ratio of trap crop:main crop (TC:MC) used by a grower will depend on whether or not they intend to harvest some of the mustard for consumption of marketing (beneficial trap crop) or if they simply want to get rid of the bugs irrespective of the damage to the trap (sacrificial trap crop) as described earlier. 
  2. The other study that was initiated in the laboratory and validated in the field during 2021 was the spatial arrangement of the trap crop relative to the main crop. We evaluated three positions, periphery (the most commonly used trap arrangement), borders or in the center. In both the lab and field experiments, there was no significant difference between the placements. This is somewhat of a surprise given the conventional wisdom regarding the perimeter trap as the gold standard for trap crop placement.
  3. The findings in 2021 field season indicate that as the adult HB populations increased (3-5x more on the trap mustard than on the main collard crop) as the plants aged and  grew bigger (during the 5 sampling dates -- 5 - 9 weeks after transplanting -WAT); the adult population peaked at 6WAT, after which there was a decline on both the mustard trap crop (sink) and the collard crop. On the other hand the HB nymphal population increased slightly but steadily and peaked later in the season, at 9WAT, when the grower would have already done a lot of leaf-picking (Figure 7A&B)

    Figure 7B
    Figure 7A
    .
    Figure 7A & B. Populations of HB adults, nymphs and eggs on protected collard main crop (top) and mustard trap crop (bottom).

    This is a very significant finding. The results provide an insight as to when the grower would most likely become concerned (regrading the HB population or the damage inflicted on the collard) leading the grower to take remedial measures, either trap crop replacement -- the movable/replaceable trap crop (to be investigated during the 2022 season), or treat with insecticide (Figure 7B). They also tell us that at the time the adult bug population is at its peak on the trap crop (which coincides with the point of high trap degradation from damage) the grower should have made at least three leaf harvests.  So, the likelihood of more marketable leaf harvests beyond that point (6 weeks after transplanting) will depend on: if the population of the HB is reduced below the action threshold (score of 2-2.5) or b) insecticide is applied. It is critical for the grower to avoid having the trap  crop too heavily damaged so as to prevent the HB population from reaching the point of inflection (POI) when the bug population flips over to the main collard crop thus inflicting further damage on collard. (Figures 8A top).

    Figure 8AAction Threshold
    Figure 8A and B. Point of inflection (A-top) is attained when the bug population moves from the mustard trap to the main collard crop. At this point the action threshold has been exceeded, and the window of opportunity missed. (Figure 8B - bottom). Earlier action is required.

    During the 2022 field season, we will be conducting the activities in Objectives 3 and 4 which will be testing action needed at the identified juncture/window of opportunity a) to avoid the use of an insecticide to reduce the HB population, and b) evaluate the introduction a new (movable/replacement) mustard trap plant in place of  the damaged (and no longer attractive) plant. The cost-benefit of each will be determined.

    Grower visits the experiment at NCA&T Research Farm and discusses with researchers.visits NCA&T Farm.Grower discusses with co-PI

    Photographs of collaborating grower (in white short sleeves shirt) from Virginia during his visit to the  2021 experiment at NCA&T Research Farm; what he saw during his visit reinforced his belief in trap cropping.

Participation Summary

Education

Educational approach:

The graduate student on the project, Mr. Lawrence Owusu, graduated in the summer of 2021 after successfully successfully defending his MS thesis and completing other graduation requirements. His thesis title was: "Evaluating the impact of density and location of mustard trap crop on the management of harlequin bugs on collard ". We are currently developing two publications from his thesis.

Educational & Outreach Activities

4 Consultations
1 Curricula, factsheets or educational tools
1 On-farm demonstrations
1 Tours
1 Webinars / talks / presentations
2 Workshop field days
1 Other educational activities: 1

Participation Summary:

2 Farmers
5 Ag professionals participated
Education/outreach description:

 Collaborating farmers visited the field plots at our research farm in Greensboro. In addition, colleagues, graduate students and other interested parties were amazed on how effective the trap crops were in pulling away the harlequin bugs from collard. They have promised to try trap crops in their farms and gardens, and also plan on recommending this management strategy to their kinsfolk. At least two papers are in preparation.

Learning Outcomes

1 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation

Project Outcomes

2 Farmers changed or adopted a practice
1 Grant received that built upon this project
4 New working collaborations
Project outcomes:

Collaborating growers who visited the experimental site saw how effective the trap crop deployment was and promised to practice it ion their farms. They will have the opportunity to do so  this season (2022). My own assistants who had never observed the actual impact of trap cropping now want to try it out as well. Several students learned about the potential for reducing pesticide use in food production using trap crops.

Recommendations:

Will be made at the end of the project extension year.

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.