Plant mediated effects on parasitoid efficacy in a banker plant system

Final Report for GS11-099

Project Type: Graduate Student
Funds awarded in 2011: $9,930.00
Projected End Date: 12/31/2013
Region: Southern
State: North Carolina
Major Professor:
Dr. Steven Frank
North Carolina State University
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Project Information

Summary:

Aphids are an important topic of research because they feed on hundreds of ornamental and vegetable plant species reducing their aesthetic and monetary value. In a nationwide survey, growers placed aphids in the top three most important pests of ornamental plants in greenhouses (IR-4 2007). Sadly, current aphid biological control methods rely on mass releases of commercially sold parasitoids (augmentative biocontrol), leading to a greater financial cost in comparison to the pesticide standard (imidacloprid) (Vasquez et al. 2006). Therefore, in order to promote the use of biological control in commercial greenhouse pest management, biological control must be rendered more effective and affordable.

Introduction

Ornamental plants are the most valuable crop grown in North Carolina yielding $22,741 per acre (NCDA 2005). In the United States, ornamental plants are the second most valuable crop worth $14.7 billion (UDSA 2002). For this reason, effective and sustainable aphid management is a priority for ornamental growers (IR-4 2007). However, growers are hesitant to implement biological control because current implementation practices, in which growers have to repeatedly purchase and release natural enemies, make it unpredictable and expensive (Vasquez et al. 2006). The purpose of this project is to increase adoption of biological control by improving a banker plant system for aphid management. Banker plant systems consist of a non-crop grain plant that supports the non-pest bird cherry oat aphid (Rhopalosiphum padi). R. padi, in turn, supports the parasitoid wasp Aphidius colemani which also parasitizes pests such as green peach and cotton aphids. Therefore, A. colemani and reproduce and patrol greenhouses even when no pests are present(Frank 2010).
Unfortunately, there is little scientific data demonstrating how to use banker plants effectively in greenhouse aphid management. Existing research indicates that knowledge of A. colemani preference for different aphid species and banker plant placement in commercial greenhouses would help increase banker plant efficacy (Van Driesche et al. 2009). Plant architecture may also affect A. colemani’s efficiency at suppressing aphid pests. Currently, plant growers use plant growth regulators (PGR) to alter plant aesthetics and crop production timing. However, PGRs reduce intermodal distance and increase lateral branching which makes plants more architecturally complex. This increased complexity may make aphid host detection and parasitism more difficult for A. colemani, reducing parasitoid efficacy and population growth. Thus, more research is needed to investigate PGR effects on herbivore hosts and parasitoid attack rate. In order to optimize banker plant systems, more research is needed to understand what factors hinder or promote A. colemani’s attack rate.
We propose to improve the banker plant system for green peach aphid management by studying its effectiveness at suppressing aphids on pepper plants. This study system will render us capable of determining A. colemani‘s attack rate with different plant architecture, variation in preference between M. persicae and R. padi and the density of banker plants needed to efficiently control M. persicae infestations in a commercial greenhouse. By determining A. colemani‘s preference and efficiency at suppressing M. persicae, we will be able to increase efficacy and adoption of this sustainable pest management tactic.

Project Objectives:

The overall goal of this project is to develop a banker plant system using A. colemani as a sustainable, effective, and economical biological control agent for aphids in greenhouses. To achieve this, the specific objectives are to:
1) Determine the effect of plant architecture on parasitoid attack rate and aphid pest suppression
Rationale: Currently plant growers utilize PGRs to alter plant aesthetics and growth rate. PGRs alter plant architecture affecting herbivore placement on the plant and consequently parasitoid foraging efficiency. For instance, plants with more compact leaves are likely to provide refuges for the pest aphids. The drop in parasitoid foraging efficiency negatively affects parasitoid population growth as A. colemani cannot reproduce without its aphid host. Because so little is known regarding these topics, more research is needed to investigate PGR impacts on herbivore hosts and parasitoid attack rate.
1.1) Determine the effect of plant architecture on parasitoid attack rate and parasitoid and aphid development;
Rationale: This first experiment will be carried out in order to determine the variation in A. colemani’s attack rate as related to the different plant architectures. This experiment is conducted to stimulate an augmentative biocontrol setting where the parasitoids are released right after purchase.
1.2) Determine how crop-plant architecture affects banker plant efficacy
Rationale: This second experiment is to test whether or not the banker plant is providing sufficient parasitoids to decrease aphid numbers. When comparing the results from 1.1 to 1.2, we will be able to determine if parasitoids reared on the banker plant are capable of suppressing pest aphid population numbers at the same rate as the ‘simulated’ augmentative biocontrol methods are.
2) Determine parasitoid preference for and attack rate of pest aphid species compared to BCOA on banker plants.
Rationale: This experiment will be conducted in the lab to determine if, when given the choice, A. colemani attacks M. persicae more than R. padi. Past studies on A. colemani have demonstrated that the parasitoid exhibits preference for the host aphid on which it was reared (van Emden et al. 2002). Because past studies have shown that A. colemani reared on banker plants do indeed suppress pest aphid populations, this choice experiment will show what percentage of parastoids preferentially forage on the pest aphid versus their non-pest host aphid. The data from this study will also demonstrate the efficacy of a banker plant system in comparison to augmentative control.
3) Determine the efficiency of aphid banker plant systems in commercial greenhouse production
Rationale: To this date, the number of banker plants to place in a greenhouse has not been properly established. This experiment will help determine the correct density of banker plants needed within a commercial greenhouse. Banker plant placement and number is important because parasitoids emerging from banker plants must be able to locate and travel to pest aphids on crop plants throughout the greenhouse.

Cooperators

Click linked name(s) to expand
  • Travis McClure
  • Travis McClure
  • Sara Prado

Research

Materials and methods:

All experiments will be carried out using a banker plant system consisting of Barley (variety: Thoroughbred) with R. padi and A. colemani. The crop plant being tested is the ‘Black pearl’ pepper plant infested with M. persicae.
Objective 1: Determine the effect of plant architecture on parasitoid attack rate and aphid pest suppression
Hypothesis 1: Plant traits such as architecture that alter parasitoid attack rate or parasitoid abundance will influence the outcome of apparent competition between pest and non-pest aphids and the efficacy of banker plant systems.
1.1 Approach: I will use a PGR, Bonzi® (active ingredient: Paclobutrazol), to create ‘Black Pearl’ Pepper plants with more compact architecture and shorter internodal distance than untreated plants. This experiment will be conducted using a 2 x 2 factorial experimental design with 2 PGR treatments (treated or not) crossed with 2 parasitoid treatments (present or absent).
I will start by placing 20 PGR and 20 no-PGR plants in a 9-inch pot covered with a cylindrical cage. That same day, I will randomly place 3 aphids on each plant and allow the aphid population to grow. Nine days later, I will count the aphids and place 5 mated-female A. colemani in each cage. Five days later, I will begin recording the aphid abundance, location on the plant (buds, stem, low leaves, large leaves, other), andthe location and number of parasitized aphids (mummies). I collect the same data every week for 3 more weeks. In addition, to determine if PGR affects plant quality for parasitoid development I will collect 10 mummies from each cage each week. Adult parasitoids will be reared in the lab to measure size, fecundity, and gender.
I will use a two-way ANOVA to determine the effect of parasitoids and plant architecture on aphid population growth an t-tests to compare parasitoid mummy abundance and life history traits.
1.2. Approach: I will use 10 PGR and 10 no-PGR pepper plants. There will be 20 banker plants that will each be infested with 20 R. padi 4 days before 5 mated, female A. colemani are placed on the banker plants. That same day, I will infest 5 PGR and 5 no-PGR plants with 10 M. persicae. Four days later, I will place one pepper and one banker plant per 2-ft cage (total 20 cages). I will count the number of live aphids and parasitized aphids on pepper plants every 7 days for 3 weeks after whichI will dry and weigh the twenty pepper plants to compare their plant biomass. This will help me determine the difference in plant growth between the infested and uninfested plants. Aphid and parasitoid abundance and plant biomass will be compared using t-tests.

Objective 2: Determine parasitoid preference for and attack rate of pest aphid species compared to BCOA on banker plants.

Hypothesis 2: A. colemani has been shown to exhibit preference for the host aphid on which it was reared, however preference for Green peach aphid is known to be greater than for Bird cherry oat aphids. Store-bought A. colemani, which had no previous exposure these plant-aphid combinations will show equal preference for both Barley and Black pearl pepper plants as they have not previously been exposed to either host.
Approach: Using a Y-tube olfactometer, I’ll test whether or not parasitoids reared on a variety of banker plants exhibit preference for their host plant or host aphid over the pest aphid or crop plant. This will enable me to determine whether or not parasitoids would preferentially fly onto the pest aphids infesting the crop plant, or if they simply stay on the banker plant on which they were reared.
I will use A. colemani reared on barley, on the Black pearl pepper plant and A. colemani purchased from a commercial supplier (plant and aphid host is proprietary and unknown). With 18 separate experiments, I will compare these three parasitoids’ preference for green peach aphid on the pepper plant to bird cherry oat aphid on the barley plant, for the plants alone, and for the aphids alone.

Objective 3: Determine the efficiency of barley as a banker plant in commercial greenhouse production systems
Hypothesis 3: A previous study showed that A.colemani is able to evenly disperse in a circular area with 16m radius within one day. Furthermore, this parasitoid is able to locate and learn the position of infested plants within a mosaic of infested and uninfested plants, meaning that a greater distance with fewer banker plants may provide equal pest control to the shorter distance with more banker plants.
Approach: With the participation of a commercial grower, I will determine the distance at which banker plants must be placed from one another in order to achieve proper aphid pest control. I will divide a pepper plant greenhouse in four sections and place banker plants at varying radial distances from one another (2m, 4m, 6m, 8m). From the second week onward, aphids will be counted with the aid of a magnifier glass every 4 days for a period of 35 days after the initial banker plant placement. On each sampling day, I will collect 3 leaves from 10 randomly selected plants in each section of the greenhouse as well as 3 leaves from 5 randomly selected banker plants and bring them back to the lab for analysis. There I will count the number of parasitized and unparasitized aphids and thus determine the parasitism percent on both plants.
By monitoring aphid parasitism of the main crop plant, I will be able to determine the distance at which the parasitoids on the banker plants provide the greatest aphid control.

Research results and discussion:

Objective 1: Determine the effect of plant architecture on parasitoid attack rate and aphid pest suppression
We found that the plant growth regulator paclobutrazol makes plants more compact and bushy. On these paclobutrazol treated plants aphids prefer to hide amongst the many small leaves and branches along the stem. On untreated plants aphids are exposed on the bottom of leaves and on stems without cover. Thus parasitism is twice as high on untreated plants. The consequence of this is that on untreated plants parasitoids reduce aphid abundance by 90% but on treated plants parasitoids only reduce aphid abundance by 40%. Results of this experiment are published: Prado, S.G. and Frank, S.D. 2013. Compact plants reduce biological control of Myzus persicae by Aphidius colemani. Biological Control, 65, 184-189.
Having found that paclobutrazol can affect biological control by Aphidius colemani we conducted similar experiments with the 4 most common plant growth regulators to determine if PGRs also had direct negative effects on parasitoid growth or survival. We found that several PGRs negatively affected A. colemani developing on the treated plants. In some cases the PGRs reduced adult size or reduced the number of females emerging. On ancymidol treated plants no parasitoids survived to adult. This has clear implications for the efficacy of biological programs and is the first time the effects of PGRs has been investigated. The details of this work are published: Prado, S.G. and Frank, S.D. 2013. Tritrophic effects of plant growth regulators in an aphid-parasitoid system. Biological Control, 66, 72-76.

Objective 2: Determine parasitoid preference for and attack rate of pest aphid species compared to BCOA on banker plants.

Objective 3: Determine the efficiency of barley as a banker plant in commercial greenhouse production systems
We found that parasitoid source has important effects on preference for pest aphids and on parasitoid fitness and efficacy. Parasitoids reared on banker plants preferred pest aphids 50% of the time. This is good because it means that half of the parasitoids reared on banker plants move into the crop to find pests while half stay on the banker plant. Importantly, 90% of parasitoids emerging from the pest prefer to parasitize pests. Therefore, once pests are located they get parasitized over and over since they are preferred.
We found that parasitoid source also affect parasitoid fitness. For example parasitoids reared on banker plants had greater size, survival and female sex ratio than parasitoids purchased from companies. Many times parasitoids purchased from companies had less than 10% emergence. So when you purchase 500 parasitoids only 50 emerge as adults. Of these as few as 10% may be female. This means of the 500 wasps you purchase only 5 can parasitize aphids! Parasitoids from banker plants were over 50% female and had high emergence.
We compared biological control by banker plants to augmentative biological control in small greenhouses. With banker plants present pest aphid abundance grew very slowly during each 4 week experiment. After four weeks greenhouses receiving augmentative releases had 10 times as many pest aphids. Thus, differences in parasitoid preference, fitness, and abundance combined to improve biological control. Details of this work are published: Prado, S.G. and Frank, S.D. In Press. Optimal foraging by an aphid parasitoid affects the outcome of apparent competition. Ecological Entomology.
Working in commercial greenhouses we found that biological control kept parasitoid abundance stabile where as abundance in augmentative houses declined rapidly. Just 1 banker plant per 1000/ft2 prevented parasitoid abundance from declining.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Theses:
Prado, Sara G. 2012. Factors affecting biological control of Myzus persicae by Aphidius colemani. North Carolina State University, MS Entomology.

McClure, Travis. 2014. Optimizing aphid biological control with banker plant systems. North Carolina State University, MS Entomology.

Peer-reviewed publications:
Prado, S.G. and Frank, S.D. In Press. Optimal foraging by an aphid parasitoid affects the outcome of apparent competition. Ecological Entomology.

Prado, S.G. and Frank, S.D. 2013. Tritrophic effects of plant growth regulators in an aphid-parasitoid system. Biological Control, 66, 72-76. (see attachment)

Prado, S.G. and Frank, S.D. 2013. Compact plants reduce biological control of Myzus persicae by Aphidius colemani. Biological Control, 65, 184-189. (see attachment)

Edited Abstracts and Proceedings:
McClure, T. and Frank, S.D. 2013. The Effect of Banker Plant Species and Mixtures on Aphid and Parasitoid Abundance. Southern Nursery Association Research Conference Proceedings, 58.

Industry publications:
Frank, S.D. 2013. PGRs and Parasitoids: Do bushy plants have more pests? Grower Talks, November.

Frank, S.D. 2013. The enemy of your enemy: predators and parasitoids for biological control of aphids. Grower Talks, July.

Frank, S.D. 2011. Taking the Biocontrol Plunge. Greenhouse Product News 21:3.

Professional Presentations:
Frank, S.D. and Prado, S.G. 2012. Mechanisms affecting the efficacy of aphid banker plant systems. Entomological Society of America, National Meeting, Knoxville, TN.

Prado, S. and Frank, S.D. 2011. Effect of banker plant species on Aphidius colemani abundance and efficacy. Entomological Society of America, National Meeting, Reno, NV.

McClure, T. and Frank, S.D. 2013 scheduled. Diverse grain mixtures as banker plants for aphid biological control. Entomological Society of America, National Meeting, Austin, TX.

Prado, S.G. and Frank, S.D. 2012. Effect of parasitoid host choice on apparent competition between pest and non-pest aphids. Ecological Society of America, National Meeting, Portland, OR.

Prado, S. and Frank, S.D. 2011. Invasion of the body snatchers. North Carolina Entomological Society Meeting. Raleigh, NC.
Extension presentations:
September 24, 2013. Introduction to banker plants for biological control. Mountain IPM Workshop, NC Arboretum, Hendersonville, NC. Attendance 50.

June 1, 2011. Banker plants, biological control, and integrated management of greenhouse pests. California Association of Pest Control Advisors, San Diego Branch meeting, San Diego, CA. Attendance 100.

April 18, 2011. Banker plants, biological control, and new chemical approaches for managing greenhouse pests. 2011 Interstate Ornamental Plant Management Conference, Baltimore, MD. Attendance 155.

Webinars and multimedia:
Frank, S.D. 2013. Introduction to banker plant systems. Greenhouse Grower, FloriCAST Series. http://www.greenhousegrower.com/video/c:92/insect-control/1428/

Project Outcomes

Project outcomes:

Results of this work have been presented to growers in extension talks and industry publications. Although we cannot quantify the number of growers who have adopted banker plants during this project there is considerable interest. The reason biological control and banker plants have not been adopted more is that growers consider them to be unpredictable and ineffective. By showing the efficacy in university and commercial greenhouses this project could help convince growers to adopt biological control and reduce insecticide applications.
Ours was the first research to document the effects of PGRs on biological control. Growers and industry professionals have been astounded by our findings. Understanding these effects will improve biological control efficacy and thus adoption.

Economic Analysis

Summarize and analyze results regarding costs, returns, and risks of adopting approaches and techniques under study in your project. If your project does not produce information for this category, just leave it blank.

Farmer Adoption

Report here the extent to which farmers have adopted new technologies, production methods and systems addressed in the project. List the number of farmers reached by your project so far through publications, field days and workshops or other training/outreach activities. What specific recommendations would you make to farmers in terms of day-to day operations? What should a farmer do or stop doing? Attach any testimonials or letters from farmers in a mailed appendix.

We do not know the exact number of growers reached but Steve Frank speaks to approximately 1000 growers per year via extension presentations. Thousands more receive the industry magazines in which we have published. Our over all recommendation is for growers to monitor natural enemy and pest abundance when using PGRs and to select PGRs that are least toxic based on our results.

Recommendations:

Areas needing additional study

Suggest areas for future research, demonstration or training.
Much more work is needed to understand interaction that will improve greenhouse biological control. This is the best way to reduce pesticide use in ornamental and food production systems.

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.