Manipulating beneficial insect habitat for control of thrips in greenhouses

Final Report for ONE07-071

Project Type: Partnership
Funds awarded in 2007: $4,998.00
Projected End Date: 12/31/2008
Matching Non-Federal Funds: $4,206.00
Region: Northeast
State: New York
Project Leader:
Carol Glenister
IPM Laboratories, Inc.
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Project Information


A strong Orius insidiosus population is necessary to exert pressure on thrips in greenhouses but greenhouse plants usually do not offer sufficient pollen and shelter to sustain such populations. As a result, the Orius does not establish in the greenhouse and growers are forced to resort to chemical interventions to protect market-bound plants from thrips damage.

This project sought to demonstrate that a habitat of continuously blooming plants could sustain establishment and reproduction of the thrips predator, Orius insidiosus and thereby maintain thrips populations below the action threshold. Bakers Acres of North Lansing participated in this trial by placing Habitat Plant Systems (HPS) in their herb greenhouse and offering a nearby bedding plant greenhouse to be used as a control.

For the HPS, we selected the same plants that successfully sustained Orius in our 2005 trial: two marigold plants (for pollen and harborage), 2 alyssum plants (for cool season pollen and nectar), 1 lantana (for hot season pollen and nectar) and a barley-cereal aphid banker plant (an aphid natural enemy support). We grouped these plants in hanging baskets called “Habitat Plant Systems” (HPS). We released commercially produced Orius and Aphidius in March, April, May, and June into both the HPS and the control greenhouses. We scouted from March through June, observing and recording the presence of Orius and thrips in and around the Habitat Plants Systems and compared these observations to the control greenhouse to determine whether offering habitat was an effective way to sustain Orius.

In 2007, we did not attain our objective of managing the thrips without pesticides. By improperly forcing early flowering of the marigolds, we induced weak root systems which did not compete well in the habitat pot assemblage of plants. This resulted in very few marigold flowers in the greenhouse throughout the season and rare incidence of Orius nymphs in the marigold flowers. In the absence of the marigolds, the sweet alyssum accounted for much more frequent occurrence of nymphs than did the marigolds. The presence of a nymph in a flower indicates that the Orius stopped in the flowers long enough to lay at least one egg.

However, we still obtained valuable information. For the third year, HPS were essential to the harborage for Orius. In two trials in 2007, we only detected Orius on the Habitat Pot Systems and we did not detect the Orius on the adjacent herbs. In the control greenhouse, we only detected 1 Orius adult once after a release and we did not detect one Orius nymph in the control house season-long. Later, in August we placed a flowering marigold plant in each of 10 herb crops, releasing more Orius and observing the interactions of thrips and Orius on the marigolds and crops. We tested whether the attractiveness of marigolds to thrips would cause thrips to increase on adjacent crop plants. The number of thrips declined on all 10 crops by an average of 70% over a 4 week period and the 10 marigolds by an average of 27% in the same time period, possibly due to the action of the Orius on the thrips. The Orius were only detectable on the marigolds, not in the adjacent herbs.

Thrips/predator ratios did not appear to be a useful measure of biological control without the additional knowledge of the presence or absence of Orius nymphs in the greenhouse.


Greenhouse crops are more susceptible to pest outbreaks than outdoor plantings due to the exclusion of natural enemies, either in time or in space. Even if a grower introduces beneficial insects into the greenhouse, many crops, including herbs, do not offer sufficient habitat to sustain the diverse natural enemy population necessary to control pests. As the movement and numbers of beneficial insects are directly linked to food and habitat needs, the lack of flowering plants in many greenhouses (a lack of food and refuge) means beneficials will move on, die, or fail to increase in number. In 2005 and 2006, we documented the effectiveness of habitat plants in attracting and retaining natural enemies in an herb greenhouse. Whitefly, aphid, and spider mite levels remained at acceptable levels throughout the season but we were unable to establish the thrips predator, Orius in sufficient populations to keep thrips numbers at acceptable levels. In 2007, we attempted to grow early marigold flowers and establish the predator Orius on them in March in the expectation that Orius would reproduce heavily and be in large numbers by the time that thrips populations normally rises in May.

Project Objectives:

Demonstrate successful thrips management by adding habitat for thrips natural enemies to a greenhouse growing herbs and ornamentals. Success will be demonstrated by

1). Establish increasing Orius populations in (Habitat Pot Systems) HPS before the thrips populations begin to increase. Observe and compare establishment of Orius in the control greenhouse without the HPS.

2). Suppress thrips population increase season-long. Based on our experience, we suspect maintenance of thrips/Orius ratios in marigold flowers numbers at or below 5 thrips per Orius may suffice to maintain season long control.

3). Demonstrate scouting methodology using marigolds as trap/indicator plants for beneficials as well as for thrips.

a). Observe the relationship between the number of thrips detected in marigold flowers in the HPS with the numbers of thrips found in the various crops.

b). Use HPS to monitor beneficial populations season-long.

c). Document thrips/Orius ratios season-long.

4). Develop further marketing/public awareness tools for the retail grower to demonstrate their sustainable pest control efforts.

5). Report on research efforts to grower groups via presentations and media.


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  • Maureen Sandsted


Materials and methods:

In 2007, two retail greenhouses were selected; Greenhouse 3 to house the Habitat Pot Systems (HPS) and Greenhouse 2 as a control under conventional pest management. Both Greenhouses 2 and 3 are approximately 94 ft. by 28 ft., or 2632 ft2; contain 24 benches, each about 55 ft2; and are temperature controlled. Greenhouse 3 contained about 250 varieties of herbs along with many hanging baskets of petunias and geraniums. Greenhouse 2 houses many different kinds of annuals; impatiens, petunias, lantana, ivy geraniums and marigolds being the most common.

Each Habitat Plant System (HPS) consisted of

2 marigolds, seeded on site January 9, 2007 and held under lights to induce early blooms
2 sweet alyssum, purchased as small plants
1 lantana, started from cuttings
a part of an Aphid Guard aphid banker plant purchased and divided so that there were 10 to 20 barley plants per HPS.

Scouting began on March 15, 2007 with an overall evaluation of pest populations in the selected greenhouses. Visual inspection of plants, 2 yellow sticky cards per house, and beating of specific plants were the primary methods of evaluation.

The 24 HPS were assembled and hung about 12 inches above each table in Greenhouse 3 during the third week in April. In depth assessments began in the 4th week. The aphids in the barley provided a constant food source for the beneficial insects throughout the summer.

Natural Enemy Population Monitoring
A Visual Encounter Survey was done upon arrival in both greenhouses, with no physical contact made with the HPS. The purpose of the “hands off” approach was to observe and more accurately record the natural enemies before they were disturbed and flew away. In addition to the Visual Encounter Survey, we randomly selected 10 HPS to be permanent sampling sites for in-depth observations by dividing the house into 6 sections per side, with 2 benches per section, and randomly selecting one bench per section by flipping a coin. When the marigold, lantana and alyssum were big enough, they were beaten 3-5 times onto a white piece of paper (this comprised one beat sample) and the pests and natural enemies that fell out were counted and placed back into the HPS. The number of fully and partially opened marigold flowers per beat sample and per HPS was also noted. Natural enemies were counted and recorded in every crop sample as well.

Pest Population Monitoring
Pest numbers were counted and noted during the in-depth monitoring of the HPS. This was done to examine the ratio between natural enemies and pests. Indicator plant species were selected in both greenhouses for inspection based on prior knowledge of thrips harborage. In the HPS herb house, we examined rosemary, passionflower vines, and jasmine (replaced by fennel when the jasmine were all sold) for thrips by randomly selecting 10 plants each and beating them onto white paper. In the control house we sampled ostospermum, petunia, and impatiens for thrips. Most plants were beaten 3-5 times but petunias were beaten 5-7 to compensate for the plants being very sticky.

Yellow sticky cards were placed in the eastern and western ends of both greenhouses. Insects were counted and the sticky cards were replaced every week.

We recorded the number of minutes for surveys of the HPS and the crops in each greenhouse in order to calculate the numbers of Orius detected per hour and thrips detected per hour.

After the predator establishment failed, we used 4 weeks in August to observe interactions of thrips and Orius on 10 herb crops including passion vine, each crop “guarded” by one plant of flowering marigolds in a 4” pot. The crops were lime balm, calamint, curry, scented geranium, lavender, oregano, rosemary, savory, woodsage, and passionflower. Each herb was on a separate table in the greenhouse. We made beat samples of each of the 10 crops per week (3 beat samples the first week, 10 beat samples the second and last weeks and 5 samples the third week) as well as beat samples of the marigold flowers guarding them, noting the number of thrips, number of Orius, and the number of marigold flowers. These observations gave us thrips/Orius ratios to observe as well as one more chance to attempt establishment of Orius before the short day length stopped Orius reproduction in early September.

Orius Introductions and Monitoring
Purchased Orius were weighed, mixed thoroughly and subsampled by weight (5 subsamples of approximately 1.5 grams per sample), counted, and the number of Orius extrapolated to the whole bottle. The bottle was then split evenly between the HPS and Control greenhouses.

Orius adults and nymphs were counted and recorded per beat sample along with the number of fully and partially open flowers per beat sample and per HPS.

Pest Control Measures
Orius were released on March 2, March 28, April 19, May 8, May 16, May 23, and June 7 in both greenhouses, with the exception that on May 8 the Orius for the Control greenhouse were accidentally applied to a third greenhouse (Table 1). In the second study all Orius were released in the herb greenhouse on August 2 (312 Orius adults), and August 9 (585 Orius adults).

The pesticide treatments are detailed in Table 1. Three treatments of Conserve were applied for thrips control to the HPS greenhouse in 2007: one in March to clean up the overwintering population before beginning the spring biological controls, on May 5 after the thrips numbers more than doubled from the previous week and the third in late June when there was an enormous flight of thrips into the greenhouse. The control greenhouse had 5 pesticide treatments in 2007.

Research results and discussion:

For the third consecutive year, we have observed the thrips natural enemy, Orius insidiosus, more easily in the HPS, and very rarely in the crops. No evidence of Orius reproduction was found in the conventional control greenhouse. Orius simply did not remain in the greenhouse, whereas Orius nymphs were detected most weeks in the HPS treatment (Figure 1). However, this year, the marigolds established very weakly in the HPS and offered far fewer marigold flowers in 2007 than in 2006 (Figure 2). When marigolds were not available, the Orius nymphs were detectable in the alyssum although at much lower numbers than marigolds commonly host (Figure 3). With sparse establishment of Orius in March through July, we could not demonstrate thrips suppression.

In developing predator/prey scouting methodology, we evaluated using a pest/predator ratio as a decision-making tool to assess the level of biological control. Figures 1 and 4 show a wide range of thrips to predator ratios, with the best ratios ranging between 17 to 1 and 6 to 1 in August. In all cases the actual presence of Orius nymphs, evidence of reproduction and establishment, seemed more meaningful than the ratios. In Figure 4, the Thrips/Orius ratio decreases from 17 to about 6 but then suddenly surges back up to 17 after most the adults and all the nymphs disappeared. It is likely that the nymphs simply grew up while the attractiveness of the flowers declined with the season and the adults flew away without laying any more eggs. If we had differentiated the size of the nymphs during the inspection of the best predator prey ratio of the year, 6:1, we could have observed that Orius reproduction had ceased and predicted an increase in thrips numbers. Given this information, an “action threshold” of 5 thrips to one Orius may not be a useful tool.

Based on sticky card counts, the thrips populations were slightly higher in 2007 than in 2006 and 2005, but followed a similar trend (Figure 5).

In the 4 week August study, Orius nymphs (evidence of Orius establishment) occurred in the first 3 of 4 weeks on the marigolds. Orius was only detected in the marigold samples, never on the adjacent crop. The number of thrips declined on all 10 crops by an average of 70% over a 4 week period and on 7 out of the 10 marigolds in the same time period (27% decline with the thrips increases averaged in)(Table 2). In those cases where the marigold thrips numbers did increase, they increased 7% on the marigold in the rosemary, 74% on the marigold in the Wood Sage, and 38% on the marigold in the passion vine. In the 4th week of the study, the number of Orius declined to 2 on 10 marigolds from 8 the previous week, and the average number of thrips nearly doubled on the marigolds, although there were still substantially fewer thrips than there were in the beginning of August. Lower Orius pressure probably allowed the thrips increase.

Figure 6 shows a graph of the relationship between the number of thrips detected in the HPS marigolds with the numbers of thrips found in the indicator crops. Except for the passion vine, the HPS marigold flowers showed much higher thrips populations than the crop.

For the third consecutive year, we have observed the thrips natural enemy, Orius insidiosus, more easily in the HPS, and rarely, if at all, in the crops.

In developing new scouting methodology to support biological control of thrips in greenhouses, we evaluated a pest/predator ratio as a measurement of the level of biological control. Although the ratio is an important piece of information, evidence of the presence of small Orius nymphs was more useful in predicting the future presence of Orius in the greenhouse.

We had set a preliminary “action threshold” ratio at 5 thrips per Orius, at which point either more Orius must be added to the system, or a corrective pesticide treatment is necessary. This is likely to be a very desirable ratio. However, we did observe thrips decline at a ratio of 17 thrips to 1 Orius, and as stated above, the more important information appeared to be the presence of young Orius nymphs in the greenhouse.

As a result of the 2007 observations, Bakers Acres used marigolds as trap plants in 2008 to pull thrips from the herb tables and simply discarded the marigolds whenever the thrips numbers exceeded 20 per beat sample.

Research conclusions:

1. Establish a thriving population of the thrips natural enemy, Orius insidiosus. Orius established in the Habitat Pot System greenhouse and did not establish in the control greenhouse. However, Orius reproduced and established minimally in 2007.

2. Suppress thrips increase in the HPS treatment. Orius did not suppress thrips in the spring and early summer, but showed evidence of thrips suppression in August.

3. Scouting Methodology

a.Except for the passion vine, the HPS marigold flowers were much better indicators of thrips presence than the crops. We used 4 weekly sets of observations in August to see if the extreme attractiveness of marigolds to thrips caused any increase in thrips on adjacent herbs. The numbers of thrips declined on all the herb crops in the presence of the marigolds.

b. We monitored the natural enemy populations season-long. The only place where the natural enemy, Orius, was detectable was in the HPS.
c.We documented thrips/Orius ratios season-long to develop an idea of acceptable thrips/Orius ratios. However, the August herb/marigold data suggest while predator/prey ratios may be important, it is just as important to see small nymphs and healthy flowers to support them.

4. Develop marketing/public awareness tools. We developed a web announcement for Bakers Acres to explain their use of habitat but have yet to integrate it into the website.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Outreach efforts in 2007 included presentations to:

Assn of Educational Research Greenhouse Curators Annual Meeting in Storrs, CT in July entitled Habitats for Long-Term Establishment of Beneficials

Association of Zoological Horticulturists in Tulsa, OK in October 2007 entitled Guardian Plants: Stories of Nurture, Sacrifice and Slaughter in the War on Pests

Northeast Greenhouse Seminar in Nanticoke, PA in November 2007 entitled Using Beneficial Insects to Control Pests

Cornell Cooperative Extension Program at the Penn Yann Produce Auction in NY in December 2007 entitled Biological Control of Flower Pests

USDA ARS Greenhouse Staff in Beltsville Maryland at their annual greenhouse training in February 2008 entitled IPM Indicators, Traps, and Habitat for Beneficial

January 2008 TriState IPM Workshop in Maine, New Hampshire and Vermont entitled Guardian Plants in IPM

There were 2 presentations at Bakers Acres in 2007. Michelle Ten Eyck, our scout, hosted a table top display on Mother’s Day in May where she engaged both adults and children in conversations about Bakers Acres use of beneficial insects and beneficial habitat. Michelle showed Bakers Acres customers the aphid banker plants, aphid parasites, Orius, live lady beetles, the insect cards developed for the SARE project in 2006 and pointed out the Point of Interest signs in the marigolds and lantana. About 20 customers took the Blooms for Beneficials fliers. In July, Carol Glenister introduced the habitat baskets to the Skaneateles Garden Club luncheon at Bakers Acres.

Bakers Acres marked the habitat plants that were for sale with the Blooms for Beneficials signs and reported on their participation in the study in their spring and fall newsletters.

Project Outcomes

Project outcomes:

Farmer Adoption

Bakers Acres adopted marigolds as thrips traps in their herb greenhouse in 2008. Boston Museum of Science installed marigolds and sweet alyssum to assist in Orius establishment in their butterfly greenhouse, Screech Owl Greenhouses in North Carolina used marigolds to trap thrips in 2008. University of Massachusetts grower grew alyssum to support Orius for thrips control on milkweed. Many grower throughout New England report using marigolds as thrips indicators, traps, and/or habitat plants.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

Living plant screens (trap crops) to deter thrips from entering the greenhouses and harbor natural enemies (guardian plants).

Bakers Acres used marigolds as a trap crop in 2008 in their herb house. Screech Owl Greenhouse in North Carolina reports doing the same. Boston Museum of Science installed marigolds and alyssum in their butterfly house to help with thrips control. Growers throughout New England report growing marigolds as indicators, traps, or habitat plants in their greenhouses.

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.