The Effect of Edge-spraying a Broad-spectrum Organically-approved Insecticide to Control Hop Arthropod Pests while Retaining Beneficial Arthropods

Progress report for FNE21-977

Project Type: Farmer
Funds awarded in 2021: $12,502.00
Projected End Date: 12/31/2022
Grant Recipient: Aroostook Hops
Region: Northeast
State: Maine
Project Leader:
Krista Delahunty
Aroostook Hops
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Project Information

Project Objectives:

This project seeks to determine whether spraying PyGanic on hop field edges will reduce arthropod pest populations, while also minimizing negative impacts on beneficial predator species.   

Objective 1) To determine if some hops pests (PLH and DHA) establish at field edges first.

Objective 2) To determine if spraying a broad-spectrum lethal pesticide at hops field edges, in combination with less lethal insecticides throughout the whole field, can reduce populations of arthropods pests and prevent spread into the hop field interior.

Objective 3) To determine if this spatially adjusted insecticide application can minimize impact on beneficial predatory arthropods, and not lead to a spike in TSSM populations.    

Objective 4) To determine if hop yield is higher in subplots sprayed with PyGanic vs. control and edge vs. interior.      

To accomplish these objectives, we will measure the number of pest and beneficial arthropods in each experimental condition.  We hypothesize: 1) that spraying the field edge will reduce hops arthropod pests at the edge which will reduce their spread and establishment within the entire hopyard, and 2) that beneficial arthropods will be retained in the interior hopyard due to applying PyGanic at the edges, limiting mortality to beneficials here only.  

Introduction:

The most common and impactful arthropod pests on our hop (Humulus lupulus L.) farm are potato leafhopper (PLH), Empoasca fabae Harris, damson hop aphid (DHA), Phorodon humuli (Schrank), and two-spotted spider mite (TSSM), Tetranychus urticae Koch.  In high precipitation summers when downy mildew is our primary production challenge, these arthropods may have less impact.  However, in drier summers these insects can exacerbate negative impacts already experienced by low precipitation.  The spatial and temporal impact of these pests also varies within and across years.  For example, while we saw high numbers of TSSM in 2018, they were nearly undetected in 2020, even though the extreme drought environmental conditions appeared very favorable to them.  Secondly, we’ve also noticed that both PLH and DHA appear on the field edge first and that those plants show the telltale signs (e.g. hopper burn) first.  However, we’ve not surveyed edge vs. interior, although the literature does corroborate this anecdotal observation (Emmen et al., 2004; Lorenzana et al, 2017).  While TSSM do not appear to have the same ‘edge-first’ emergence pattern in a field, they can quickly increase population size following pyrethrin application, due to loss of beneficial TSSM predators.  PLH temporality also depends on the prevailing weather and atmospheric conditions, since they are a migratory insect that appears in northern Maine during late June or early July.  During 2020, PLH appeared to emerge earlier than prior years as an important pest, likely as impacted by the atmospheric conditions that brought significant southerly warm air to Maine from June 20 – June 30.  As the record-setting dry summer continued, we entered ‘severe’ and then ‘extreme’ drought during the middle and later part of our season.  Water stress, compounded by high populations of PLH, resulted in a 40% decline in hop yield compared to 2019. The impact of these arthropod pests is certainly exacerbated during drought years, but is also important in more average growing seasons.  These are also issues faced by hop farmers across the Northeast (in fact, in Northern Maine we are likely 2-3 weeks behind others in the arrival of PLH). 

As a certified-organic hops producer, our goal is to minimize the use of insecticides and maximize beneficial arthropod predator populations, which have been shown to be beneficial in controlling hop pests.  We have used two insecticides to attempt to control arthropod pests (AzaMax and Trilogy), although we do not have experimental data on their effectiveness, and these products mainly suppress feeding or prevent molting.  Another OMRI-certified product is a Chrysanthemum-derived insecticide (e.g. PyGanic) which is a broad-spectrum insecticide that has shown to be effective at a wide range of arthropods.  However, given its broad impact, it can also have a detrimental effect on arthropod predators.  The consequence of using PyGanic may in turn lead to a strong resurgence of arthropod pests that have a shorter generation interval than the predator species.  Thus, in our own hopyard we have only used PyGanic during two seasons when absolutely necessary and, even then, at the end of the season when we would harvest before pests could reestablish. 

If PLH and DHA are more likely to establish at field edges, then controlling them by spraying only in this zone could depress pest population growth while leaving a large area of refugia for beneficial insects and predators.  This could potentially increase yield, reduce pesticide use, and retain beneficial predators.  Thus, the secondary resurgence of pests that may occur post-PyGanic application may be minimized while controlling the pest establishment at its source.  By sampling both pest and predator populations, in both edge and interior areas of the field, and in PyGanic vs. non-PyGanic plots, we can assess the impacts of a spatially explicit integrated pest management (IPM) approach.  Further, by timing the first use of PyGanic to the appearance of PLH, we can delay its potentially negative effects until PLH are present (and also well before we typically see aphids in any concerning level).  If successful, this approach could result in minimized inputs and labor resources, maximized yield, and minimal impacts on beneficial arthropods.  This solution of only spraying edges would also affect the economic threshold for these hop pests, considering that PyGanic is a very expensive input, and its negative impact on beneficial arthropods could also result in a negative impact on yield - encouraging more farmers to adopt this solution.  

Cooperators

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  • Dr. James Dill - Technical Advisor

Research

Materials and methods:

We have a 1-acre and a 3-acre hopyard with 4 varieties in each hopyard, with 5 varieties overall (Cascade, Centennial, Nugget, Willamette, Mt. Hood).  As we considered experimental design, we weighed how to control for the experiment, while also accurately testing our hypothesis (with controls) versus spraying the entire edge.  If edge spraying acts like a fence, then any unsprayed area may serve as an establishment and entry point.  Thus, to test this possibility, for our 1-acre hopyard we sprayed the entire edge with pyrethrin, in order tocompare edge vs. interior samples for arthropods, plant health, and yield.  For our 3-acre hopyard, we established a randomized complete block design in order to test the effects of arthropod abundance in edge vs. interior and PyGanic-sprayed vs. control(s).  In order to accomplish this, we established spraying within 4 blocks of 250’ length across 5 rows of hops (60’ wide).  Within each block we will randomly assign half the block as PyGanic-sprayed and the other half will be a PyGanic-control.  Control treatments received water only, to control for the effect of the physical disruption of spraying that may cause PLH to relocate.  The entire area of both fields were also sprayed with alternate applications of Trilogy or AzaMax.  These are both neem-derived insecticides whose mode of action is anti-feedant or antivoltine (molt).  These actions may help suppress population growth and have been widely used in organic operations such as ours, but are not generally lethal.  We consider the PyGanic treatment as additive, which is why we will spray AzaMax/Trilolgy in all rows/treatments as a baseline.  Thus, we established five treatment combinations where we sampled arthropods, plant health, and yield. These include: PyGanic (edge), PyGanic (interior), PyGanic-control (edge PyGanic-control (interior), AzaMax/Trilogy (edge), and AzaMax/Trilogy (interior).  The PyGanic treated blocks are distributed north to south throughout our 500’ long field and span several varieties (Cascade, Centennial, Willamette, and Mt. Hood).  Two of these blocks have lengths of rows located along edges (north and south), while two blocks are in the interior where edge samples are on the east or west end of the block, but the rest of the rows are within 'interior'.  Thus, the design spanned our entire field and sampling at each cardinal direction.  While the edges along our fields are not uniform, they consist of fallow field, forest, lawn or cropland (broccoli in 2021).  Thus, there is minimized impact of adjacent crops that may either attract or trap PLH or other arthropods.

Insecticide applications were conducted on a roughly 10-day interval beginning in late June or after the first emergence of PLH.  We alternated AzaMax and Trilogy on one 10-day schedule, and applied PyGanic/water to treatment plots on a similar 10-day interval offset by 5 days.  Spraying occured in the evening to reduce impact on beneficial insects as well as to minimize the photodegradation of PyGanic (per the label).  We used moderate to high amounts of insecticides per the label, i.e. AzaMax (2 quarts/50 gallons), Trilogy (1 gallon/50 gallons), and PyGanic (0.5 oz./gallon) utilizing our blast sprayer.  The blast sprayer and tractor were calibrated and operated at a constant RPM to deliver consistent volume and range of spray.  Based on past experience, our sprayer may spray up to 50 feet, but most spray hits plant or ground within 30 feet.  However, given that potential drift, we planned our blocks to be 60 feet wide.  This way we can spray on each side toward the interior of the block to ensure relatively even coverage within the block, and to prevent drift to outside of the block within the AzaMax/Trilogy control subplots.   There is also non-treatment rows between each treatment block to act as a buffer.  

Arthropod sampling was conducted through field scouting using sticky traps and leaf observations.  Sticky traps were used to monitor beneficial and pest arthropods by placing them at edge (six feet from field edge) and interior (80 feet from field edge) locations six feet from the ground with the hop rows in between two bines.  One sticky trap per treatment/block in both fields resulted in 20 sticky traps collected.  Sticky traps were collected and replaced weekly.  Beneficial arthropod predators to be identified include Stethorus punctum, Anthocoridae, Geocoridae, Nabidae, Chrysopidae, Hemerobiidae, Coccinellidae, Syrphidae, Parasitica, and Araneae.   We are working to identify TSSM, PLH, total aphids, and DHA using a 10x-60x dissecting microscope.  We also conducted pre and post PyGanic/control spray leaf scouting within each experimental block by selecting 1 leaf per plant times ten plants per treatment; we counted PLH, aphids, TSSM and spider mite destroyer (Stethorus punctum).  We have used 3 leaves per plant in the past, but given within-plant consistency we think one leaf is sufficient to detect patterns, as the variation across plants may be higher than within plant.  We will conduct this scouting during the day prior to an evening spray application and again on the day after application.  Leaf sampling may also provide a better measure of TSSM than sticky traps. 

In addition to scouting arthropods, we will also assessed physical impacts of PLH on hops leaves, e.g. hopperburn. based on a modification of a scoring system used in alfalfa.  We scored hopperburn according to the following ordinal score, where 0:no sign of leaf yellowing or necrosis, 1:<10% leaf yellowing, mainly at margins, 2:10-25% leaf yellowing, but no necrosis, 3:>25% leaf yellowing and <10% necrosis, 4:>25% leaf yellowing and >10% necrosis, and 5:>50% of leaf area is yellow or necrotic. 

While we will focus on arthropod abundance and hop plant effects as dependent variables for data analysis, we also measured yield on a systematically selected set of 6 bines per treatment, surrounding the sticky trap sample location.  We were able to measure one bine at a time for both plant total biomass and hops cone yield for each treatment.  

This study will be completed over two seasons since the variation in weather and thus arthropod abundance is stochastic, and thus one year may be very different from another.  Once the second year of study is completed in 2022, we will collate all arthropod, hops leaf, and yield data to make comparisons across the treatment groups (including year as a factor) using ANOVA and repeated measures analysis.  We predict that PyGanic treatments will have lower pests and beneficial arthropods than the PyGanic control (water); we predict that interior plots will have fewer PLH and aphids than edge plots but may not differ in beneficial arthropods.  We also predict that interior AzaMax/Trilogy control plots will have more arthropod pests than PyGanic edge subplots.  We will also access NOAA weather data for Presque Isle, ME (about 8 miles north) to test correlations between weather data and arthropod pest abundance.  We will use daily maximum temperature, cooling degree days, and cumulative precipitation since May 1 (which we’ll calculate) in comparison to arthropod counts.  Data analyses will be summarized, graphed and written in our SARE report and outreach elements. 

Participation Summary
2 Farmers participating in research
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.