The use of native perennial wildflowers and alfalfa trap crops to increase pollination and biological control in strawberries

Final Report for GNE12-036

Project Type: Graduate Student
Funds awarded in 2012: $14,534.00
Projected End Date: 12/31/2015
Grant Recipient: Cornell University
Region: Northeast
State: New York
Graduate Student:
Faculty Advisor:
Gregory Loeb
Cornell University
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Project Information

Summary:

The tarnished plant bug, Lygus lineolaris, is one of the top pests in strawberry and several other crops in the northeast. Due to the high value of strawberries, there is a low threshold for damage and growers often must apply insecticides to achieve control. However, this method of pest control is both costly and not sustainable, as resistance to as many as three classes of insecticides have developed in tarnished plant bug populations. In addition to the cost of pest control, many growers rely on rented honey bee colonies to ensure sufficient pollination. However, the many issues affecting the health of honey bee colonies has had negative impacts on their availability and cost. Increasing field diversity by means of a companion planting that augments the floral and nesting resources available to beneficial insects has proven to be a successful management tactic in many systems. In this study a carefully selected mixture of native, perennial plants and alfalfa was evaluated for its ability to enhance multiple ecosystem services in strawberry including pollination and biological control. Plot with alfalfa-wildflower plantings had higher pollinator visitation and greater pollination rates than control plots however these plots also had higher numbers of tarnished plant bugs and received greater pest damage. Ultimately fruit yield on plots with alfalfa wildflower strips was lower than yields from control plots.

Introduction:

The purpose of this project is to provide small fruit growers in the Northeast with a simple and effective management strategy for enhancing biodiversity and ecosystem services (biological control and pollination) using native perennials and alfalfa in companion plantings.

The tarnished plant bug (TPB) is a principle pest of strawberry, and many other fruit, vegetable and fiber crops, resulting in significant damage and control costs annually. TPB feeds on developing strawberry achenes and surrounding tissue causing arrested development of affected areas and a characteristic “catfacing”, reducing size and marketability of fruits. A low economic threshold for TPB in strawberry (fewer than 0.5 nymphs per inflorescence) necessitates frequent insecticide applications during the growing season for many growers. Such intense pesticide use is costly and can have negative environmental impacts.

In California and Ontario, alfalfa has been grown successfully as a trap crop to reduce plant bug damage in strawberry; however, preventing spillover of the pest from the trap crop into strawberry plantings has been problematic. On large production farms in California, tractor mounted vacuum systems have been used to remove plant bugs from alfalfa trap crops adjacent to strawberry. Alternatively, a method of biological control assisted trap cropping has been developed using the introduced parasitoids Peristenus digoneutis Loan and Peristenus relictus to control the western tarnished plant bug (Lygus hesperus) in the alfalfa trap crops and achieve parasitism rates as high as 60% (Pickett et al. 2009). This method offers a more economically feasible approach than vacuuming for the smaller farms in the Northeast; however, the method’s efficacy in limiting spillover and reducing TPB damage has not been established. Measuring a reduction in fruit damage for this trap cropping system is a critical step towards increasing adoption by growers.

Pickett et al. (2009) credit successful establishment and expansion of P. relictus in California to the occurrence of wildflowers in adjacent natural areas which likely provisioned alternative hosts and adult food resources, as well as refuge from mechanical and chemical disturbances that occur in crop habitats. Although Peristenus have successfully established in the Northeast, the presence of these resources in the agroecosystem may be important in influencing levels of pest control services achieved. Several studies have shown an increase in lifespan and fecundity of parasitoids provided with nectar resources (Heimpel and Jervis 2005). I hypothesize that providing nectar sources near strawberry fields will increase activity and fecundity of P. digoneutis resulting in higher parasitism rates and reduced damage. In addition, these habitats may be beneficial to other predatory insects such as syrphid flies, carabid beetles and spiders.

Similarly, I hypothesize that providing floral resources to pollinators will attract them to strawberry fields and support a large population thereby increasing pollination services. While pollination in strawberry can be accomplished by wind, yield and individual fruit size increases by as much as 25% with insect pollination (Zebrowska 1998). The European honey bee (Apis mellifera) has been used to increase pollination rates in strawberry; however, conditions in the Northeast early in the season during strawberry bloom are not ideal for honey bee activity. Additionally, declines in managed honey bee colonies have increased hive rental prices and threatened the stability of services provided by these pollinators (Winfree et al. 2007).

Native bees offer an alternative source of pollination services to strawberry. In New York, more than 60 species of bees have been observed visiting strawberry flowers (Connelly et al., 2015). These native bees are likely better adapted to local climates and in some cases, more efficient pollinators than honey bees. Chagnon et al. (1993) found honey bee and native bee pollination behaviors to be complementary resulting in greater overall pollination.

The type of floral resources added to a system may have a large impact on the success in increasing both parasitoid and pollinator abundance and ultimately the level of ecosystem services. Recently, some attention has been paid to the use of native plants in increasing beneficial insect populations (Frank et al. 2008; Isaacs et al. 2009, Mader et al. 2010). Prior to these studies, most research on enhancing ecosystem services using floral resources focused on a few non-native species such as buckwheat, phacelia, and coriander. In a survey of 43 Michigan native, perennial wildflower species Isaacs et al. (2009) identified several species which attract beneficials at similar, and in some cases greater, rates than traditionally studied non-natives. Additionally, native plants pose little risk of becoming invasive, require less care in establishment and maintenance, and are adapted to local insect communities.

            A carefully selected mixture of native perennial wildflowers established in conjunction with alfalfa adjacent to strawberry plantings will facilitate multiple ecosystem services including pollination and effective reduction in TPB populations through biological control assisted trap cropping resulting in less fruit damage, fewer insecticide applications and increased yield.

 

Chagnon M., J. Gingras and D, DeOliveira (1993) Complementary aspects of strawberry pollination by honey and indigenous bees (Hymenoptera). J. Econ. Entomol., 86(2): 416-420

Connelly, H., Poveda, K., & Loeb, G. (2015). Landscape simplification decreases wild bee pollination services to strawberry. Agriculture, Ecosystems & Environment, 211, 51-56.

Frank, S.D., P.M. Shrewsbury and O. Esiekpe (2008) Spatial and temporal variation in natural enemy assemblages on Maryland native plant species. Environ. Entomol., 37(2):478-486

Heimpel, G.E. and M.A. Jervis. (2005) Does floral nectar improve biological control by parasitoids? In Plant-Provided Food and Herbivore-Carnivore Interactions, ed. F. L. Wackers, P. C. J. van Rijn, and J. Bruin. Published by Cambridge University Press pp. 267-304

Isaacs,R., J. Tuell, A. Fiedler, M. Gardiner, and D Landis (2009) Maximizing arthropod-mediated ecosystem services in agricultural landscapes: the role of native plants. Front Ecol Environ. 7(4): 196–203

Mader, E., M. Spivak and E. Evans (2010) Managing alternative pollinators : a handbook for beekeepers, growers, and conservationists. SARE Handbook 11, NRAES-186 Co-published by SARE and NREAS.

Pickett C.H., S.L. Swezey, D.J. Nieto, J.A. Bryer, M. Erlandson, H. Goulet and M.D. Schwartz. (2009) Colonization and establishment of Peristenus relictus (Hymenoptera: Braconidae) for control of Lygus spp. (Hemiptera: Miridae) in strawberries on the California Central Coast. Biological Control 49:27–37

Winfree, R., N.M. Williams, J. Dushoff and C. Kremen (2007) Native bees provide insurance against ongoing honey bee losses. Ecology Letters, 10: 1105–1113

Zebrowska, J. (1998) Influence of pollination modes on yield components in strawberry {Fragaria x ananassa Duch.) Plant Breeding 117: 255-260

Project Objectives:

  1. Quantify the influence of wildflower-alfalfa strips on pollinator community composition and pollination services in strawberry.

                  A total of 5,684 bee visits to strawberry flowers were observed and 1,381 bees from more than 75 species were collected between May 2013 and May 2015. All bees were identified to species or morphospecies as appropriate during the fall and winter of 2013 and 2014.

 

  1. Investigate P. digoneutis biological control of tarnished plant bug to determine

a) if parasitoids near Wildflower-Alfalfa strips achieve a higher parasitism rate and,
b) if high parasitism correlates with reduced pest damage in the field.

                  More than 3,000 tarnished plant bug nymphs were sampled between May 2013 and June 2015. Preliminary analysis of parasitism rates began in October 2014 however thus far I have failed to detect and parasitoid larvae attacking the tarnished plant bug nymphs. This project is ongoing.

 

  1. Disentangle the relative benefits provided by either wildflowers or alfalfa through increased pollination and/or biological control on strawberry yield.

                  Yield estimates including pollination rates (Objective 1) and damage by tarnished plant bugs (Objective 2) were estimated from more than 2,500 individual fruits collected between June 2013 and June 2015.

Cooperators

Click linked name(s) to expand
  • Dr. Greg Loeb

Research

Materials and methods:

To address the proposed objectives, we collected data from 15 field sites from the spring of 2013 to the spring of 2015. Field sites were identified in both commercial strawberry fields and research plots established on Cornell research farms in Tompkins and neighboring counties in New York State in the spring of 2012. Sites are either managed organically or involve limited use of insecticides. Each field site is comprised of two plots of the same variety of June bearing strawberry, separated by a minimum of 200m in order to ensure independence. At each field site, a 15 x 3 foot Wildflower-Alfalfa strip was established directly adjacent to one of the strawberry plots in the fall of 2012. The second plot (control) did not have a Wildflower-Alfalfa companion planting but was otherwise managed the same as the first plot.

Wildflowers were introduced to the planting as one-year-old plugs to decrease establishment time and management. Species comprising the Wildflower-Alfalfa strip include: Alfalfa (Medicago sativa), Lanceleaf coreopsis (Coreopsis lanceolata), Smooth white beardtongue (Penstemon digitalis), Golden alexanders (Zizia aurea), Shrubby cinquefoil (Potentilla fructosa), Yellow coneflower (Ratibida pinnata), Meadowsweet (Spiraea alba), Yellow giant hyssop (Agastache neptoides), Cup plant (Silphim perfoliatum), Blue lobelia (Lobelia siphilitica), Late figwort (Scrophularia marlandica), Alum Root (Heuchera americana) and Late goldenrod (Salidago altissima). These flowers represent a spectrum of floral diversity in order to attract the greatest diversity of beneficial insects. They were specifically selected for their open access to nectar and attractiveness to the small pollinators and parasitoids that are important for the strawberry system. Additionally, bloom periods are overlapping so that flowers are present throughout the growing season. To produce the wildflowers for this project we collaborated with a local native plant nursery and every effort will be made to collect seed from local plant eco-types.

 

Pollinator community composition. To compare pollinator abundance and diversity, each plot was surveyed for bee activity by counting the number of strawberry flower visits and visitor identity along a 10 minute transect walk at each site. In order to get accurate species identifications pollinators were also collected using a sweep net along a 50 m transect immediately following the visitation survey. This protocol was repeated four times per season over the two-week bloom period with samples taken approx. every 4 days. This method was preferred over other collecting methods such as bee bowls because we felt there was a lower likelihood that our sampling would negatively impact pollination rates in the plots.

Pollinator performance. To compare the performance of pollinators between control and Wildflower-Alfalfa managed plots, we measured fruit weight on 25 hand cross-pollinated (positive control), and 25 open pollinated flowers in both plots at each site. Open pollinated flowers were marked just after opening but otherwise unaltered. Hand pollinated flowers received supplementary pollen applied with a paintbrush to the stigmas. These hand-pollinated plants allowed us to estimate the relative contribution of the pollinator community to yield at each site while controlling for environmental factors such as microclimate, which may have varied slightly across the study region. We used secondary flowers in all experiments as their later bloom time reduces their risk of being damaged by a late frost. Fruits were harvested daily when ripe and were weighed and scored for pollination rate and tarnished plant bug damage as a percentage of the total achenes either failing to develop due to lack of pollination (Figure 1a) or feed upon by tarnished plant bugs (Figure 1b).

 

Tarnished plant bug responses to field level diversification. After strawberry fruits have begun to form, we used the standard sampling protocol for estimating tarnished plant bug density in strawberry which involved tapping inflorescences over a white plate or bowl and aspirating fallen nymphs. Sampling proceeded until 25 nymphs were collected or all inflorescences in the plot had been sampled. The total number of inflorescences tapped was recorded and all nymphs were stored in 95% ethanol. This sampling protocol was repeated 3 times per season until fruit began to ripen after which the fruit are not longer vulnerable to TPB damage.

 

Relative benefits of Wildflowers and Alfalfa. In order to determine which components Alfalfa- Wildflower strip are driving beneficial or negative outcomes we performed a factorial experiment that will allow us to separate the effects of wildflowers and alfalfa on pollination and biological control.

Wildflowers vs. Alfalfa. Three of our field sites had four rather than two strawberry plots. Each plot was paired with either wildflowers alone, alfalfa alone, wildflowers and alfalfa or normally managed grass. The same pollinator and parasitoids responses that are measures in the paired plots were also recorded for each of these.

Research results and discussion:

Analyses of combined field data using structural equation modeling and generalized linear mixed effects models show that pollinator abundance was higher on plots with Alfalfa-Wildflower strips but only after their third year of establishment (Figure 2, F(6,79) = 87.91, p=<0.01). Similarly pollination rates were significantly higher on wildflower plots only in the third year (F(2,20) = 34.189, p=<0.01). These results match our predicted outcomes of increasing pollination by unmanaged bees in strawberry. The lag in effect of the strips on pollinator populations and consequently on pollination rates is not suprising considering that the flower species comprising the strips are perennial and do take a few years before they are fully established. Recently, similar research conducted on wildflower strips adjacent to Michigan blueberry also found a three year lag time before the pollinator community responded to the floral strip (Blaauw and Isaacs 2014).

 

                  The effects of the Wildflower-Alfalfa strip on tarnished plant bug densities and damage were more complex and depended on the characteristics of the landscape surrounding the farms. Plots on farms in complex landscapes with large proportions of natural area had overall greater densities of TPB nymphs (Figure 3., F(1,38) = 9.746, p=0.003) . In these landscapes wildflower-alfalfa plots tended to have fewer TPB nymphs. However on plots in simple landscapes with high proportions of agriculture in the landscape wildflower plots had significantly greater TPB densities (F(1,39) = 6.974, p=0.011).

 

                  Evidence based on fruit damage assessments suggested that tarnished plant bug damage is greater where pollination services are high and poor pollination damage is low and indeed our structural equation models did show a positive pathway between pollinator abundance and tarnished plant bug damage. Follow up studies showed that tarnished plant bug nymphs do show a preference for well-pollinated strawberry fruits and in simple landscapes wildflower strips tend to increase the abundance of tarnished plant bug nymphs in the strawberry plots. Interestingly tarnished plant bug abundance was significantly lower on wildflower plots in complex landscapes suggesting that biological control by P. digoneutis may be reducing their numbers. Overall yield was greatest on wildflower plots in complex landscapes and lowest on wildflower plots in simple landscapes (F(1,37) = 5.032, p=0.03).

 

                  Alfalfa and the wildflower planted in the strips appear to be equally good habitat for tarnished plant bug. There were no statistically significant differences in tarnished plant bug abundance between plot treatments (control, with alfalfa, with wildflowers, with wildflowers + alfalfa) however tarnished plant bug abundance and damage was always lowest on plots with alfalfa suggesting that it was successful as a trap crop. Tarnished plant bug abundance was highest on plots with wildflower strips only suggesting that they increase spillover of pests into the crop.

 

Blaauw, B. R., Isaacs, R. (2014), Flower plantings increase wild bee abundance and the pollination services provided to a pollination-dependent crop. Journal of Applied Ecology, 51: 890–898.

Research conclusions:

High levels of agricultural production are supported by ecosystem services provided freely by the biodiversity maintained in agroecosystems. Increasingly, biodiversity within agroecosystems and therefore, the supply of ecosystem services, is threatened by agricultural intensification. Several of the most critical ecosystem services in agricultural systems are provided by insects such as pollination and biological control. Many studies report a positive relationship between the presence of local floral resources , the level of ecosystem services received and fruit set. Maintaining a sufficient high level of floral resources in the agroecosystem can insure adequate crop pollination and biological control across time and space. This research advances of knowledge where and when deploying these measures can be expected to create a win-win for both pollination and biological control. Based on the results we have so far we have been able to secure additional funding ($6,000) from the Atkinson Center for a Sustainable Future to explore aspects related to the project including whether landscape characteristics may impact the phylogenetic diversity of strawberry pollinators.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary

Education/outreach description:

Following final data analysis a manuscript based on the finding of this project will be prepared for submission to PNAS. Over the course of the project I have given several outreach talks including the NYSAES Farm Days Open House in 2013, the Cornell Small Fruit Open House in 2014, and in the Berry Session at the NY State Fruit and Vegetable Expo in 2015. I have also given a number of talk to my colleagues and the broader scientific audience on preliminary results of the project including at the Cornell Entomology Symposium in 2013 and 2014 and the Plant-Insect Interactions Group Meeting in February 2015. In November I will present the final results at the Entomological Society Annual meeting during a symposium I am co-organizing.

Project Outcomes

Project outcomes:

Even with subsidies including those introduced by the President’s new Pollinator Initiative, the cost of establishing a wildflower strip can be substantial. These costs can be further exacerbated by increased pest damage due to spillover of pests from the strip into the crop. The knowledge gained from the current project will allow growers to make more informed decisions about whether or not it makes sense to invest in a floral resource planting.

 

Farmer Adoption

At this early stage in the research grower adoption of wildflower strips has been low, however one of the important finding of this research is that it is not appropriate for all growers to adopt this strategy. Although many of the growers who participated in this research have elected to preserve their wildflower planting or in some cases to expand it, I will advise all growers in agriculturally simple landscapes that without alternative pest management strategies for tarnished plant bug the strips on their farms should be removed. General grower interest in establishing these types of plantings has been increasing with the support of subsidies and with the increased attention given to pollinators by the President’s Pollinator Initiative. The objectives of my future outreach efforts will be to highlight the potential risks of establishing these plantings and to assist growers in establishing these plantings when it will create a win-win scenario for both pollination and biological control.

 

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

A major component of this research was to establish parasitism rates using a single-step multiplex PCR technique developed by Tilmon et al. (2000). This DNA-based technique allows for accurate determination of the presence and identity of parasitoids within the nymphs and does not require time consuming and less accurate dissection or rearing methods. Thus far we have not been successful in amplifying parasitoid DNA using this technique, however this component of the project will continue to be pursued although possibly by an alternative method such as dissection.

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.