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

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 23^rd and July 31^st, 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 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 an unexpected result given the conventional wisdom which regards 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 grew older and bigger (during the 5 sampling dates -- 5 - 9 weeks after transplanting -WAT); the adult population peaked at 6WAT, followed by 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 had several harvests (Figure 7A&B)

   
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   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 which provides 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 (this will be investigated during the 2022 season), or treat with insecticide (Figure 7B). They also tell us that at the time the adult bug population peaks 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: a) 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 heavy trap  crop damaged in order 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) and potentially diminishing the profits.

   
   Figure 8A and B. Point of inflection (Fig 8A, top) is attained when the bug population reverts from the mustard trap to the main collard crop. At this point the action threshold (damage level 2-2.5, Fig 8B, bottom) has been exceeded, and the window of opportunity missed (Figure 8B - bottom). Earlier remedial action would be recommended.

   During the 2022 field season, we will conduct activities in Objectives 3: to test action needed at the identified juncture/window of opportunity a) to avoid the use of an insecticide to reduce the HB population, on farm and in research plots, and b) evaluate the introduction a new (movable/replaceable) mustard trap plant in place of  the damaged TC (which would no longer attractive). Objective 4: determine the cost-benefit of each will be determined based on the two optimum TC ratios (10%, 20%) of the best two mustard varieties (OFM and FBL) on our field 

   
   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.

Results:

Our observations from the 2022 research studies on the final objectives further confirm predictions based on the laboratory and field experiments in 2021, albeit with some mixed results due to environmental and logistic events that were beyond the control of the researchers. The main focus was to verify the need for/value of fixed and replaceable TCs superimposed on the selected TC ratios and placement. These experiments were also to determine on-farm performance (ie grower participation) and the cost-benefit of the different treatments.  The latter would normally be conducted at the end of all field work but could not be carried out because of the early termination of funding (6 months before the end of the season as planned, having lost 12 months due to COVID-19). This meant that the co-PI who was tasked with this aspect of the project would not have a salary. We were able to carry out the field experiments at the research farm and at a growers\'s farm, Jeffries Farm, Reidsville, NC. The study at the Research farm has just concluded while  the on-farm demonstration is at an advanced stage. The HB population was inconsistent although it clearly indicated that there was no difference in damage between the fixed and replaceable TC treatments as shown in the figures (below), even though the HB population was higher on mustard than on collard at all sampling dates. 

The Jeffries Farm had low levels of HBs on the crop throughout the season unlike at the NCAT research farm because of  the late planting (Mid-August) at the former. The complete data are yet to be received and processed as harvesting is not over yet. However, based on the observations from both locations at this point (see charts and photographs below) we have made made additions to the model we have been developing for trap crop use on collard (see model figure below in the outcomes section). We believe the model is scalable for use on other crucifers and possibly other crop systems.

 Proposed model for increased TC function (for adoption by the grower community) 

1. Grow two trap crops varieties (Old Fashion Mustard, and Florida Broad Leaf Mustard)  in a staggered sequence along the borders. The mustards should comprise 10-20% of the entire field crop, according to the grower's wishes.
2. The interval between the planting/transplanting is 7-10 days with the second plating reversing the border placement of the TCs (see figure).
3. Staggered planting allows for a longer presence of flowers on the field; flowers have been established as the most attractive phenological stage, even though the vegetative stage is also attractive. This will account for higher attraction and longer retention and a delay of the point of inflection since the sink component (flowers) will remain active. 
4. This model has the potential of eliminating the need to replace the trap crop.
5. The model is inherently flexible to allow the TC to be placed almost anywhere in the main crop as there appears to be no real advantage of any location (we compared border, periphery and center locations and found no statistical difference among them). 
6. It is fair to say this model has not been tested on larger farms, but we believe the concept will work if the farm is considered a collective of smaller farms and treated accordingly.

These three photographs from the Jeffries Farm show a great crop of collard and a bonus crop of mustard, and a delighted grower, Mr. Wilbert Jeffries.

The trap worked very well especially so because the harlequin bug population was low where the farm is located. However, the grower realized the reduced damage to the collard leaves this season. That was most likely first because the low population of harlequin bugs was pulled and redirected away from the collard by the mustard trap which has been shown to work quite well even at high harlequin bug populations. The yield from this farm is however yet to be collated as harvesting is still in progress.