Trap Cropping to Improve Tarnished Plant Bug Management in North Central Strawberry

Progress report for LNC20-436

Project Type: Research and Education
Funds awarded in 2020: $144,096.00
Projected End Date: 11/01/2022
Grant Recipient: University of Wisconsin - Madison
Region: North Central
State: Wisconsin
Project Coordinator:
Dr. Christelle Guédot
University of Wisconsin - Madison
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Project Information

Summary:

In 2017 over 3100 farms across the North Central Region (NCR) dedicated 4,928 acres to strawberry production, with an economic value of $17.5 million for growers in Michigan, Ohio, and Wisconsin, alone. Growers in Wisconsin and the Midwest have identified Lygus lineolaris, the tarnished plant bug, as their primary insect pest of concern. Although frequent chemical intervention remains the strategy of choice for managing TPB, it is costly and has led to widespread insecticide resistance in populations across the Southern United States.  Trap cropping may provide an effective, sustainable alternative management strategy. Trap cropping has proven highly effective against other Lygus species in California and Europe, and we expect that it can be employed to improve TPB management in the NCR. Behavioral laboratory assays conducted in our lab support this hypothesis, suggesting that TPB prefers alfalfa over strawberry. However, additional work is necessary to determine whether this preference can be exploited in the field. This project aims to assess the impact of incorporating perimeter plantings of alfalfa to manage TPB in strawberry and provide growers with an optimized cultural control strategy for this devastating pest. 

Trap crops concentrate insect pests away from a cash crop, reducing damage to the main crop and the cost of chemical treatments, thereby increasing farm profitability. In California, intercropping strawberries with alfalfa to manage Lygus hesperus reduces management costs by 78%, increasing profitability by more than $725/acre. Moreover, incorporating alfalfa strips has been shown to promote natural enemy communities in California strawberry, and the reduction in pesticide applications will in turn reduce the environmental and social costs associated with TPB management. By working with stakeholders to develop an easily-implemented, low-maintenance, and proactive strategy for managing the primary strawberry pest in the NCR, we will improve the profitability and sustainability of strawberry production in our region.

We will determine the impact of alfalfa strips on 1) TPB abundance and 2) beneficial arthropod communities in the trap and cash crops, determine the extent of TPB movement between trap and cash crops, and assess the potential of plant kairomones to improve attraction and retention of TPB in the trap crop. We will then disseminate our findings to stakeholders in Wisconsin and throughout the NCR via articles, blogs, and presentations at grower conferences and field days. 

Project Objectives:

1)    Determine the efficacy of alfalfa trap cropping as a cultural control strategy for managing TPB.

2)    Assess the impact of alfalfa trap cropping on beneficial and pest arthropod communities.

3)    Determine the extent of movement between the trap crop and the cash crop.

4)    Assess the potential of improving trap crop attractiveness with kairomones.

5)    Disseminate findings to stakeholders throughout the NCR.

Results from this research will provide growers with a new sustainable cultural control strategy, which will help reduce the need for pesticide applications to manage TPB, the most devastating pest of strawberry in the NCR. 

Introduction:

In 2017 over 3100 farms across the North Central Region (NCR) dedicated 4,928 acres to strawberry production, with an economic value of $17.5 million for growers in Michigan, Ohio, and Wisconsin, alone. Growers in Wisconsin and the Midwest have identified Lygus lineolaris, the tarnished plant bug, as their primary insect pest of concern. Although frequent chemical intervention remains the strategy of choice for managing TPB, it is costly and has led to widespread insecticide resistance in populations across the Southern United States.  Trap cropping may provide an effective, sustainable alternative management strategy. Trap cropping has shown promise against other Lygus species in California and Europe, and we expect that it can be employed to improve TPB management in the NCR.

Trap crops concentrate insect pests away from a cash crop, reducing damage to the main crop and the cost of chemical treatments, thereby increasing farm profitability.In California, intercropping strawberries with alfalfa to manage Lygus hesperus reduces management costs by 78%, increasing profitability by more than $725/acre. Moreover, incorporating alfalfa strips has been shown to promote natural enemy communities in California strawberry, and the reduction in pesticide applications will in turn reduce the environmental and social costs associated with TPB management. By working with stakeholders to develop an easily-implemented, low-maintenance, and proactive strategy for managing the primary strawberry pest in the NCR, we will improve the profitability and sustainability of strawberry production in our region.

Research

Hypothesis:

Objective 1: Determining the efficacy of alfalfa trap cropping as a cultural control strategy for TPB.

We hypothesize that alfalfa will attract and arrest TPB moving into strawberry fields and thus reduce the population of TPB in strawberry fields.

Objective 2: Assessing the impact of incorporating alfalfa trap strips on beneficial arthropod communities.

We hypothesize that the addition of alfalfa trap strips will alter the beneficial arthropod communities by 1) increasing the abundance and/or diversity of beneficial biocontrol agents.

Objective 3: Determine the extent of movement from the trap crop into strawberry 

We hypothesize that as the TPB population grows in alfalfa strips, density-dependent factors, such as intraspecific competition, will lead some TPB to move into the strawberry, thus increasing TPB abundance in the strawberry later in the season.

Objective 4: Assessing the potential of augmenting the attractiveness of alfalfa with kairomones.

We hypothesize that adding plant-derived semiochemical attractants will enhance the attractiveness of the trap crop, drawing more TPB to alfalfa.

Materials and methods:

We will establish a grower advisory panel to ensure that this project is implemented in a way that fully addresses stakeholder needs and concerns.  

Objective 1: Determining the efficacy of alfalfa trap cropping as a cultural control strategy for TPB.

We hypothesize that alfalfa will attract and arrest TPB moving into strawberry fields and thus reduce the population of TPB in strawberry fields.

To assess the efficacy of alfalfa strips as a trap crop for TPB, we worked with growers in early spring of year 1 to establish a 1 m wide alfalfa strip along the whole length of a quarter-acre strawberry patch (alfalfa treatment) at three berry farms across Southern Wisconsin with five replicates in year 1 and six replicates in year 2. Each treatment plot was paired at each farm with a quarter-acre control plot with the standard turf border (control: no alfalfa strip). 

We will assessed the seasonal phenology, abundance, and spatial distribution of TPB populations at each farm in both years by sampling along a transect weekly. TPB will be sampled by taking 20 sweeps from the centermost 10 meters of 1) the three edge-most rows of each strawberry field, 2) the tenth row of each strawberry field, and 3) each field perimeter (either alfalfa strip treatment or control strip). Sampling will occur weekly from May through July in 2020 and 2021. Insects collected in sweep nets will be stored in plastic bags. Upon return to the laboratory, insects will be transferred to vials containing 70% ethanol and identified as TPB or non-TPB. Each TPB will be identified by life stage (nymphs vs adults) and sexed. The impact of incorporating perimeter plantings of alfalfa on TPB infestation will then be assessed by comparing TPB abundance across paired treatment and control rows (i.e. edge-most row to edge-most row).  Within plot comparisons (i.e. center rows to edge rows to perimeter strips) of TPB abundance will also be made to determine whether incorporating alfalfa perimeter strips impacts the distribution of this pest in strawberry fields. A linear mixed effects model will be fit to assess the impact of Treatment, Row, and Week on TPB abundance. Pitfall: TPB populations may be too low at the predetermined grower sites and we will work with other growers to find more suitable sites with higher populations.

Objective 2: Assessing the impact of incorporating alfalfa trap strips on pest and beneficial arthropod communities.

We hypothesize that the addition of alfalfa trap strips will alter the beneficial and pest arthropod communities by 1) increasing the abundance and/or diversity of beneficial biocontrol agents, and 2) by increasing the richness and/or diversity of generalist pest species, such as thrips.

The incorporation of novel resources into strawberry fields may have unexpected non-target impacts.  We plan to assess the effect of alfalfa trap strips on pest and beneficial arthropods so that growers may be fully informed of the potential risks and benefits of adopting this new management strategy. 

To assess the impact of alfalfa strips on pest and beneficial arthropods, we will sample the arthropod communities at each strawberry patch with sweep nets, clear sticky cards, and pitfall traps between May and August of 2021 and 2022. We will place one 464 cm2 sticky cards and one pitfall trap at the center of  each location (perimeter, edge-most row, center-most row), for each treatment (alfalfa and control), at each farm, generating 50 samples every week.  These data will be evaluated alongside sweep net samples collected as part of Objective 1.   

Sticky cards placed at canopy level will allow us to monitor the movement of canopy-dwelling and flying insects, such as lacewings, spittlebug, and adult TPB, while pitfall traps will collect TPB nymphs and ground-dwelling arthropods, such as spiders, carabid beetles, and strawberry root weevil. Arthropod herbivores and natural enemies will be identified to family or superfamily. Simpson’s diversity index will be calculated and compared both between paired treatment and control rows and within each plot, as described in objective 1. Taxonomic and functional community composition data will also be examined via principal component analysis. These analyses ensure that we understand the impact of incorporating a trap crop for TPB on other strawberry pests and the natural enemy community. Pitfall: The majority of data will be derived from sweep net samples, we may miss rare ground-dwelling arthropods.

This approach will allow us to assess the efficacy of incorporating alfalfa perimeter strips into strawberry fields through the lenses of TPB management and conservation biological control, both overall and throughout the season.

Objective 3: Determine the extent of movement from the trap crop into strawberry 

We hypothesize that as the TPB population grows in alfalfa strips, density-dependent factors, such as intraspecific competition, will lead some TPB to move into the strawberry, thus increasing TPB abundance in the strawberry later in the season.

To determine the efficacy of the trap crop at retaining TPB, we will assess the extent of TPB movement between the trap crop and strawberry field, so that the timing for implementing supplemental management strategies, if necessary, can be optimized.

We will utilize an enzyme-linked immunosorbent assay (ELISA) to examine the relationship between population density, emigration, and immigration in the alfalfa trap crop. This data will allow us to develop better recommendations for managing TPB densities in the trap crop to prevent spillover. 

This work will be conducted over four weeks covering strawberry bloom and fruit set in year 2, when strawberry plants are most susceptible to TPB damage.  Perimeter plantings will be sprayed with a 12% egg white solution at a rate of 1 L/ 6 m from a gas-powered backpack sprayer, while the three edge-most strawberry rows will be sprayed with a 20% solution of fat-free milk powder at the same rate. Insects in each area will be marked with ovalbumin, protein from egg white, or casein, protein from milk powder, directly via spraying or by acquiring the protein marker through contact with marked plant material. The protein markers will be reapplied weekly after sampling, and as needed due to weather. TPB collected from the alfalfa, strawberry field edge, and interior of each strawberry field will be examined for the presence of casein, ovalbumin, or both, respectively. The protein markers will allow us to determine: 

  1. The extent of immigration from the strawberry field into the trap crop
  2. The extent of emigration from the trap crop into the strawberry field
  3. The extent of overall movement into the strawberry field from outside sources.

We will analyze the direction and magnitude of the TPB movement by comparing the mean number of casein-marked TPB collected from alfalfa, ovalbumin-marked TPB collected from strawberry edge, the number of casein-marked TPB collected from the strawberry interior, and the number of ovalbumin-marked TPB collected from the strawberry interior with ANOVA followed by Tukey’s HSD test. Pitfall: Poor weather may complicate the collection of this data, if this occurs we will continue collecting later into the summer.

This information, along with population density data collected in Objective 1, will allow growers to intervene as necessary to ensure that the trap crop remains a ‘dead-end’ rather than a source of TPB infestation. 

Objective 4: Assessing the potential of augmenting the attractiveness of alfalfa with kairomones.

Trap crop efficacy may be improved via augmentation with attractive chemicals.  We have identified five host-derived compounds from alfalfa and strawberry that elicit antennal responses in TPB antennae. 

We hypothesize that adding plant-derived semiochemical attractants will enhance the attractiveness of the trap crop, drawing more TPB to alfalfa.

Objective 4.1. Determine the optimal blend of chemicals for attracting TPB into traps. 

Electrophysiological experiments conducted in our lab revealed five chemicals that elicit antennal responses in adult TPB. Two field experiments to assess the attractiveness of these compounds to TPB will be conducted at the West Madison Agricultural Research Station in July and August of 2021.

In experiment 1, each chemical (Z-3-hexenol, α-pinene, sulcatone, E-β-ocimene, and linalool) and a blank control (6 treatments total) will be used to bait clear sticky cards. This will determine whether TPB are attracted to individual compounds, rather than a more complex bouquet.  

Experiment 2 will complement experiment 1. Attractive compounds identified in the previous experiment will be combined to produce potentially attractive blends. Treatments will consist of a blank control and all combinations of attractive compounds. If all individual compounds are more attractive than the control, treatments will consist of the 10 unique pairs of chemicals, the complete five-compound blend, and the blank control.   

Each experiment will be conducted over 4 weeks, with sticky traps and lures being replaced weekly. Each treatment will be replicated 10 times, and the number of TPB captured for each treatment will be compared using a repeated measures ANOVA followed by Tukey’s HSD test. Pitfall: Experiment 1 assumes that individual compounds will be attractive. If this is untrue (i.e. no individual compound is more attractive than the blank), we will continue experiment 2 with the complete five-component blend and a control. 

Objective 4.2. Assess the potential of augmenting the trap crop with semiochemicals.  

We will work with superintendents at two agricultural research stations in Madison and Arlington, WI to determine whether the blend optimized in Objective 4.1 enhances TPB attraction to alfalfa plantings. We will set up six paired plots of alfalfa with and without semiochemical augmentation. Sweep net samples will be collected from each plot weekly for four weeks and TPB abundance will be compared between augmented and control plots using a paired t-test.

This objective will identify plant chemicals that are attractive to TPB, providing chemicals suitable for improving monitoring of TPB, and improving the efficacy of the trap crop to attract and retain TPB, thus further decreasing TPB infestation in strawberry. 

Objective 5: Disseminate our findings to stakeholders throughout the NCR 

While our results may be interesting on their own, the true value of this project is only realized if stakeholders engage with the results and implement changes. We will present our research results at local and regional grower conferences (e.g., Wisconsin Fresh Fruit and Vegetable Conference) and write articles in extension newsletters, in the Wisconsin Fruit News, and on the Wisconsin Fruit website (https://fruit.wisc.edu). We will also work with the Wisconsin Berry Growers’ Association to organize field days that will showcase the ease of implementing this cultural control strategy and highlight the research results and benefits as well as participating grower experiences.

Christelle Guedot is an Associate Professor and Fruit Extension Specialist and she works closely with berry growers in Wisconsin to assess their needs. She has a great relationship with the board members of the Wisconsin Berry Grower Association (WBGA) as she serves as ex-officio member and collaborates with many berry growers on addressing insect issues such as the spotted-wing drosophila in the last eight years. She organizes the berry track to the Wisconsin Fresh Fruit and Vegetable Conference and is a co-editor and regular contributor to the Wisconsin Fruit News. 

Research results and discussion:

Objective 1: Determining the efficacy of alfalfa trap cropping as a cultural control strategy for TPB.

We hypothesize that alfalfa will attract and arrest TPB moving into strawberry fields and thus reduce the population of TPB in strawberry fields.

Treatment (F1,220 = 21.83, P < 0.0001) and Week (F5,220 = 42.54, P < 0.0001) were found to be significant predictors of total TPB abundance within the strawberry plots, while a marginal effect of Row (F3,15 = 2.79, P = 0.077) was observed. Overall, 51% fewer TPB were collected from strawberry fields with alfalfa perimeter plantings than the control plots. The treatment effect was most pronounced in strawberry rows 2 and 3 (Fig. 1).  

Within the treatment plots, Row and Week were significant predictors of TPB abundance (Row: F4,20 = 4.34, P = 0.011; Week: F5,125 = 33.40, P < 0.0001) and a marginally significant Row:Week interaction was detected (F4,20 = 1.63, P = 0.056). Examination of estimated marginal means revealed that significantly more TPB were collected from the alfalfa strip and row 1 than rows 2, 3, and 10 of the treatment strawberry plot (Fig. 2). This difference was only observed for Weeks 3-6, as TPB populations were low early in the season. Within the control plots, Week was the only significant predictor of TPB abundance (F5,125 = 29.45, P< 0.0001). One interpretation of this effect is that the trap crop effectively concentrated TPB in the alfalfa strip and first strawberry row. However, if the trap crop is effectively attracting and retaining TPB, we should see inflated TPB counts in the alfalfa compared to row 1. Objective 3 will clarify whether TPB are dispersing from alfalfa into the surrounding strawberry. 

This figure illustrates the effect of incorporating alfalfa perimeter strips into strawberry field design. TPB density was significantly reduced in rows 2 and 3 of trap cropped strawberry.
Figure 1: TPB Density by Treatment and Row. Astrisks denote significant differences between paired Treatment and Control plots. Row 0 is the field border/alfalfa strip.
This figure shows the within plot distribution of TPB. Significantly higher densities were found in the alfalfa strip and strawberry row 1 than strawberry rows 1, 2, and 10.
Figure 2: TPB density within the treatment plot. Letters denote significant differences.

Objective 2: Assessing the impact of incorporating alfalfa trap strips on beneficial arthropod communities.

We hypothesize that the addition of alfalfa trap strips will alter the beneficial arthropod communities by 1) increasing the abundance and/or diversity of beneficial biocontrol agents.

Forty beneficial taxa were identified from sweep net data and this community matrix was subjected to statistical analysis. A linear mixed model was fit to examine the effect of Treatment, Row, Week and interactions on species richness and Gini-Simpson diversity index within strawberry fields. Treatment was neither a significant predictor of species richness nor Gini-Simpson diversity within strawberry fields. However, richness displayed an edge-biased distribution and a significant relationship with Row (F3,20 = 4.75, P = 0.015) was detected (Fig. 3), while Week was a significant predictor both richness and the Gini-Simpson index (Richness: F5,192 = 21.93, P < 0.0001; Gini-Simpson: F5,205 = 13.41, P < 0.0001).

This figure depicts the effect of Row and Treatment on species richness and Gini-Simpson Diversity within strawberry fields with and without alfalfa trap crops.
Figure 3: Species Richness and Gini-Simpson Diversity index between strawberry fields with and without alfalfa trap crops. The presence of alfalfa did not significantly impact either biodiversity metric within the strawberry. Species richness was found to be somewhat edge-biased, letters denote significant differences.

Permutational multivariate analysis of variance (PerMANOVA) was applied to determine whether Treatment, Row, and Week were associated with shifts in the strawberry beneficial insect community revealing Row (F3,209 = 1.56, P = 0.032) and Week (F5,209 = 11.71, P = 0.001), but not Treatment, to be significant predictors of the beneficial insect community. Subsequent tests of multivariate dispersion indicated homogeneity of dispersion within Row and Week, we can therefore infer that the PerMANOVA is detecting a shift in the beneficial insect community with Row and Week rather than effects of dispersion. Importantly, this test is unable to account for the paired design of our study and may therefore fail to detect a true shift in community composition associated with Treatment. Additional analysis with multivariate mixed models will be necessary to resolve this issue.

Within the treatment plots, Row and Week were significant predictors of species richness (Row: F4,20 = 16.11, P < 0.0001; Week: F5,110 = 22.43, P < 0.0001) and Gini-Simpson diversity (Row: F4,126 = 4.39, P 0.002; Week: F5,126 = 7.78, P < 0.0001). A significant interaction driven by low diversity in the first two weeks of sampling was observed for richness (F20,110 = 2.141, P = 0.007), and a marginally significant interaction was observe for diversity (F20,126 = 1.5953, P = 0.064). Comparison of estimated marginal means revealed that the row effect is driven by increased richness and diversity in the alfalfa strip relative to all strawberry rows (Fig. 4). This was expected, as alfalfa is a high quality resource associated with a distinct beneficial insect community. Importantly, there is little evidence to suggest dispersal of beneficial insects from the alfalfa into the strawberry or surrounding landscape, so we should be cautious about overstating potential benefits when discussing our findings with growers. 

This figure depicts the effect of Row on species richness and Gini-Simpson Diversity within the treatment plot.
Figure 4: Species Richness and Gini-Simpson Diversity index within Treatment plots. These biodiversity metrics were found to be significantly higher in the alfalfa strip than associated strawberry. Letters denote significant differences.

Row and Week were also significant predictors of species richness in control plots (Row: F4,20 = 4.57, P = 0.008; Week: F5,110 = 19.42, P < 0.0001), but the Gini-Simpson diversity index was only associated with Week (F5,110 = 12.70, P < 0.0001). Examination of the estimated marginal means showed that species richness was significantly higher in the field border than in strawberry rows 2, 3, and 10 (Fig. 5), but that it was similar between the field border and row 1. 

This figure depicts species richness and Gini-Simpson diversity by row within the control plot.
Figure 5: Species richness and Gini-Simpson diversity within the control plot. No change in Gini-Simpson diversity was observed over space, while Richness was found to be somewhat edge-biased. Letters denote significant differences.

Objective 3: Determine the extent of movement from the trap crop into strawberry 

We hypothesize that as the TPB population grows in alfalfa strips, density-dependent factors, such as intraspecific competition, will lead some TPB to move into the strawberry, thus increasing TPB abundance in the strawberry later in the season.

This will be addressed in 2022

Objective 4: Assessing the potential of augmenting the attractiveness of alfalfa with kairomones.

We hypothesize that adding plant-derived semiochemical attractants will enhance the attractiveness of the trap crop, drawing more TPB to alfalfa.

A trapping experiment was conducted in 2021 to identify optimal parameters for trapping TPB monitoring. Ten arrays of red, blue, yellow, white, and clear sticky cards were placed alongside an alfalfa field and monitored weekly for two weeks. Mean TPB capture per card was compared via repeated measures ANOVA. Significantly more TPB were captured with red sticky cards than all other colors (Fig. 6), between which no differences were detected. Importantly red sticky cards captured 10x as many TPB as white sticky cards, the current standard for TPB monitoring. Based on these data we have an easily implemented change that growers can immediately use to improve TPB monitoring on their farms.

This figure depicts mean TPB capture for five colored traps.
Figure 6: Mean TPB capture for five trap colors. Red was significantly more efficient than all other colors for capturing TPB in alfalfa fields. Letters denote significant differences.

Kairomone-based monitoring will be assessed in 2022.

Objective 5: Disseminate our findings to stakeholders throughout the NCR 

This project has led to four grower-focused presentations at the Wisconsin Fresh Fruit and Vegetable Conference (2021 and 2022), Great Lakes EXPO (2020), and Midwest Organic and Sustainability Education Service Organic Research Forum (2021). 

 

Participation Summary

Project Activities

Alfalfa perimeter strips reduce tarnished plant bug populations in June-bearing strawberry fields.
Alfalfa trap cropping to manage Lygus lineolaris in North Central strawberry production.
A trap cropping strategy to manage Lygus Lineolaris in Wisconsin strawberry production
A trap cropping strategy to manage Lygus lineolaris in Wisconsin strawberry production
A Trap Crop for Lygus lineolaris in North Central Strawberry Production.
Trap cropping to improve tarnished plant bug management in north central strawberry production.

Educational & Outreach Activities

6 Webinars / talks / presentations

Participation Summary:

60 Farmers
100 Ag professionals participated
Education/outreach description:

Presentations of our findings were made to growers and agricultural professionals at the Wisconsin Fresh Fruit and Vegetable Conference in 2021 and 2022.

Our findings were presented as a virtual poster at the Great Lakes EXPO in 2020.

Our findings were presented as a virtual poster at the Midwest Organic and Sustainable Education Service Organic Research Forum in 2021.

Data from this study has also been presented to agricultural scientists and entomologists at the national Entomological Society of America meetings in 2020 and 2021.

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.