Augmentative Biological Control of Spider Mites on Hops

Final report for GNC16-230

Project Type: Graduate Student
Funds awarded in 2016: $11,432.00
Projected End Date: 06/01/2018
Grant Recipient: Ohio State University
Region: North Central
State: Ohio
Graduate Student:
Faculty Advisor:
Celeste Wetly
Ohio State University
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Project Information


The twospotted spider mite, Tetranychus urticae (Acari: Tetranychidae), is one of the biggest challenges to hop production in Ohio.  As the production of hops is a new industry in the Midwest, research has not yet been done on the effectiveness of the pest management tactic known as augmentative biological control, where predators are released into a system to help control pest populations.  In our project, “Augmentative Biological Control of Spider Mites in Hops”, we will work with four hop growers in central and southwest Ohio to evaluate the ability of two predatory mite species, Galendromus occidentalis (Acari: Phytoseiidae) and Neoseiulus fallacis (Acari: Phytoseiidae), to successfully manage twospotted spider mite populations when released in a hop yard.  We will release these two predators at both a low and an intermediate prey density to determine at which density the predatory mites are better at managing spider mite populations.  As hop growers in the Ohio and throughout the Midwest need a cost effective pest management strategy to manage twospotted spider mites, we will compare the cost of augmentative biological control to the use of both organic and chemical pesticides currently being used.  Another key part of spider mite management is proper scouting techniques.  We will work with the same four growers to establish effective scouting programs in their hop yards.  These growers will also help us to produce extension videos that explain proper scouting techniques as well as the proper predator release techniques used in augmentative biological control.  These resources will be made available through the Ohio State Extension as well as the Ohio Hop Growers Guild.   The hop industry in the Midwest continues to grow and proper scouting techniques and the use of augmentative biological control will allow growers to improve production to keep up with the growing demands for locally produced hops.  The adoption of these tactics will be evaluated by a series of online surveys.

Project Objectives:

Expected Outcomes




Evaluation/Monitoring Plan, Measurement Methods


100 growers will learn about proper scouting techniques.


100 growers will learn about the use of augmentative biological control to control spider mites.



20 growers will develop and use proper scouting protocols that help them to better manage their pest populations.


20 growers will incorporate augmentative biological control tactics into their pest management strategy


Supply of locally produced hops will begin to meet demand



Profits increase


Use of augmentative biological control leads to a decrease in the use of chemical pesticides protecting the health of growers as well as increasing the presence of natural predators


Higher quality of life for growers, their families and their communities

Researched-based recommendation on the use of predatory mites G. occidentalis and N. fallacis in augmentative biological control to manage twospotted spider mites in hops


Economic analysis on augmentative biological control versus organic pesticides versus conventional pesticides


2 videos – proper scouting techniques, augmentative biological control


2 Printed Fact Sheets on

proper scouting techniques, augmentative biological control


2 extension bulletins (available on OSU website and Ohio Hop Grower Guild website)


1 research article (Journal of Economic Entomology)


1 on-farm trial on four farms


2 presentations at “First Friday Hop Tours”


1 presentation at  “Ohio Hop and Malting Barley Conference and Trade Show”



SARE funding


Time - Grad Student, Advisor, and Student Research Assistant


Cooperating growers for on-farm trials




Pre and post project survey of hop grower’s pest management tactics.


Pre and post project survey of hop grower’s scouting methods.




Materials and methods:



  • Measure the impact of natural enemies on T. urticae population growth
  • Determine if either a predator prey ratio of 1:10 or 1:5 is adequate for suppressing T. urticae populations


  • Location: 4 hop yards in Ohio
  • 100 replicates
  • Pair of leaves chosen on a hop plant
  • All arthropods and eggs removed from leaves
  • Ten female spider mites added to each leaf
  • Zero, one, or two predatory mites (Neoseiulus fallacis) added to each leaf
  • One leaf in each pair covered with a fine mesh bag (Fig. 2)
  • Leaves collected and spider mite eggs and motiles counted after one or two weeks

Enclosure Study Design



  • Document T. urticae seasonal population trends on hops
  • Evaluate augmentative biological control of T. urticae by two predatory mite species (Galendromus occidentalis and Neoseiulus fallacis )
  • Determine if a high or a low predator release rate is adequate for suppressing T. urticae populations

Methods 2016


  • Location: 4 hop yard in Ohio
  • Plot Size: 3 plants
  • Leaf samples collected weekly to monitor spider

  mite motiles and eggs

  • 2 leaves 1 m from ground
  • 2 leaves 1 m from top of plant

Predator Release

  • Randomized complete block
  • Treatments (8 replicates)
    • Control - no predator release
    • Low rate G. occidentalis - 10 per plant
    • High rate G. occidentalis - 20 per plant
    • Low rate N. fallacis - 10 per plant
    • High rate N. fallacis - 20 per plant
  • Predators released at a threshold of 1 spider mite / 10 leaves
  • If populations continued to increase, predators were re-released 2 weeks later (maximum two releases)

Yield Comparison

  • Hop cones harvested, yields measured
  • Treatments were compared using a one way t-test

Augmentation Study Plot Map – 2016

Methods 2017

  • Location: 4 hop yard in Ohio
  • two new sites, two same as 2016
  • More intense sampling than 2016
    • 5 leaves 1 m from ground
    • 2 leaves 1 m from top of plant
  • Treatments (17 replicates)
    • Control - no predator release
    • Low rate N. fallacis - 10 per plant
    • High rate N. fallacis - 50 per plant
  • All other methods remained the same as in 2016
Research results and discussion:



Spider Mite Motiles after Two Weeks


  • Arthropod predators present in Ohio hop yards can provide substantial spider mite control.
  • In conjunction with natural enemies present in the hop yard, N. fallacis was able provide adequate control of spider mites when released at a rate of two predators per ten spider mites.



2016 Hop Yields

Spider Mite Population Trends 2017


  • T. urticae start to appear on hop plants in early May.
  • Once present, populations quickly increase.
  • Populations vary greatly from leaf to leaf and plant to plant.
  • Populations crash in late-July / early-August before hop harvest in mid - August.
  • Several species of predatory mites, including N. fallacis, are present in Ohio hop yards.
  • The quality of commercially available G. occidentalis is not adequate for augmentative biological control.
  • When released at either a high or a low rate, N. fallacis was unable to suppress spider mite populations on hops.
  • Future studies might concentrate on identifying conservation biological control tactics.


Participation Summary
4 Farmers participating in research

Educational & Outreach Activities

3 Curricula, factsheets or educational tools
3 Webinars / talks / presentations

Participation Summary:

100 Farmers participated
Education/outreach description:

In July of 2017, I was invited to speak at the Ohio Hop Growers Guild Summer Symposium.  This was attended by about 50 growers.  I spoke about insect pest management in Ohio hop yards, this included proper scouting techniques as well as pest management techniques.

In August of 2017, we put together two extension videos, one on 'General Pest Management' and one on 'Managing Spider Mites'.  These were posted to youtube.

I will be speaking at the 'Ohio State University Hop Conference' on March 24th.  I will be giving a talk for the advanced growers entitled 'Controlling & Managing Spider Mites and Insects in Your Hop Yard' and a talk for beginning growers entitled 'Hop Pest Management.'

Project Outcomes

100 Farmers reporting change in knowledge, attitudes, skills and/or awareness
Project outcomes:

Understanding the role natural enemies play in controlling spider mite populations will help growers decide what steps to take in spider mite management.  This may encourage them to implement conservation biological control tactics, which can be very inexpensive and beneficial to tboth their crop and the environment.  This understanding may also reduce growers' inclination to spray miticides, knowing that miticides also affect predatory mites, an important natural enemy of spider mites.  A reduction in miticide use will save growers money and benefit the environment.

Understanding the cost of augmentative biological control and its lack of proven efficacy will help growers decide if and when to release predatory mites in their hop yard.  Knowing the economic impact certain pest management strategies have will help growers make more economical decisions.

Knowledge Gained:

Our project looked at management of twospotted spider mite populations on hops in Ohio.  Growers were interested in finding a way to control mite populations without spraying costly miticides that were potential harmful to predatory mites.

In the first year of our study, we looked at augmentative biological control of spider mites.  We released predatory mites to see if they could reduce spider mite populations.  Our results were inconclusive as to whether augmentative biological control of spider mites was an economically viable option for hop growers.  During our study we found that commercially available predatory mites vary greatly in quality and it is important to evaluate each shipment of predatory mites to ensure quality.

In the second year our study, we again looked at augmentative biological control of spider mites, but this time we looked at it in a more controlled environment.  We found that predatory mites are capable of reducing spider mite populations on an individual leaf, but when the same technique was applied on a larger scale results varied.  This is consistent with other studies that show augmentative biological control in a field setting is often not an effective approach. 

In our second year, we also looked at biological control of spider mites by natural enemies already present in the hop yard and found that these natural enemies were able to significantly reduce spider mite populations.  Conservation biological control can easily be overlooked as it a much more passive way of managing pest populations even though it can be an effective management strategy.


In the future, research should be done to determine what natural enemies are helping to control spider mite population in hop yards.  Once this is determined, growers can use conservation biological control to target these natural enemies using techniques such as providing alternative food sources and providing appropriate overwintering sites.

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.