Thrips damage greenhouse plants by chewing on the foliage and flowers and deforming new growth, causing serious economic losses. Pesticides are giving diminishing control due to increased pesticide resistance in the thrips. Orius insidiosus, a thrips predator, can be used to suppress thrips, but requires certain habitat plants to remain in greenhouses. Marigolds are supportive of Orius, but they are also extremely attractive to thrips. This project was designed to determine if the thrips magnetism exerted by marigolds would harm greenhouse crops.
In collaboration with a New York grower and a Pennsylvania grower, we evaluated marigolds as a Guardian Plant for thrips both outside and inside the greenhouse. Guardian Plants act as traps for pests and habitat sites for pests’natural enemies. Marigolds in bloom are ideal trap plants for thrips because they pull thrips well, are easy to grow, fast to bloom, and inexpensive. At Peace Tree Farm in Pennsylvania, marigold plants directly inside the greenhouse intake vents served as an extreme filter for thrips, reducing the numbers of thrips detected inside the greenhouse to very low levels in comparison to the numbers of thrips coming in the vents. The marigolds also harbored numerous Orius. After 4 weeks, these marigolds were removed from the greenhouse along with the thrips.
At Bakers Acres, in New York, we observed clean herb crops adjacent to individual marigold plants with abundant thrips. When the marigolds became too infested, the individual marigolds along with the thrips were carefully discarded. Marigolds also competed with annual flowers for thrips, but to a lesser level than the non-flowering herbs.
Two cautionary notes: 1) Large numbers of thrips will abandon wilted marigolds. 2) Marigolds outside intake vents appear to act as way-stations to guide migrating thrips toward the air intakes.
Thrips are one of the five most significant pests in greenhouse crops and one of the most difficult to control. They cause damage by chewing on growing buds, foliage and flowers, which causes badly distorted new growth, as well as early senescence of flowers and foliage. In addition, thrips can be vectors for devastating viruses such as Impatiens Necrotic Spot Virus. Thrips outbreaks get especially severe when mass migrations occur from acres of drying flowers or crops and are taken in through the greenhouse air intake system, a frequent occurrence in late May and early June in the Northeastern United States.
Recently, thrips control options have caused great concern among growers all over the Northeast because the once excellent thrips pesticide, spinosyn, has become less effective against thrips. The level of resistance has gotten bad enough in Florida that Dow Agrosciences has banned the sale of spinosyn to two Florida counties(which had previously used spinosyn in multiple cropping systems) in an effort to slow down the development of resistance. Resistant thrips eggs can easily travel to northern states imbedded in the tissue of plant cuttings destined to be finished in greenhouses there. In 2008,thrips resistance spurred great interest in alternative thrips controls among greenhouse growers. Beneficial insects can be very effective where they suppress pest reproduction in the greenhouse. However, natural enemies are no match for the massive influxes of thrips that occur when they start to move in from acres of nearby drying plants.
For the Guardian Plant trap method to be successful, we must determine the appropriate threshold to know when to carry the marigolds and the thrips out of the greenhouse. We proposed to evaluate and document the use of marigolds as trap plants for the removal of thrips from a greenhouse crop in 4 ways:
1) Evaluate the level to which thrips will settle onto trap plants outside of the greenhouse.
This objective was accomplished. We concluded that trap plants should not be placed near greenhouse intake vents.
2) Evaluate the ability of marigolds to attract and retain thrips that have entered the greenhouse, and demonstrate the level of thrips removal from the greenhouse.
This objective was accomplished. The marigolds pulling power depended upon the plant species that it was competing against. More than 3000 thrips were removed from 3 greenhouses, the greatest number came from the herb greenhouse.
3) Compare numbers of thrips caught on marigold plants that are elevated 4 inches above the crop with marigolds that are at the same level as the surrounding plants.
This objective was not accomplished because the flowering marigold plants were significantly taller than the crop plants from the start. Accordingly, marigolds at the same level as the surrounding crop were not available for thrips comparisons.
4) Compare the number of thrips that are supplied by stressed or wilting marigolds with the number that are supplied by similarly infested plants with ample water in an empty greenhouse.
This objective was accomplished. Thrips abandoned wilted marigolds while they remained on healthy marigolds.
Peace Tree Farm set up five approximately 18” planters with 12 blooming “Bonanza” marigolds per planter approximately 10 feet from their intake vents and placed one 4”X6” yellow sticky card opposite each planter in the intake vents in order to sample the number of thrips entering the greenhouse. Approximately 200 trap marigolds in 4.5” pots were also placed on tables just inside the vents to document the level to which the thrips entered the greenhouse. These marigolds were about 10 inches tall and on tables about 30 inches below the vent, so they were not visible outside the vents. The trap plants were well watered and carefully maintained to be sure that wilting plants did not release large numbers of thrips towards the vents. Grower, Joe Volpe, sampled all planters weekly for 4 weeks from June 19 to July 10, counting all the thrips and all the flowers. Just inside the vent, he sampled 10 marigold plants, recording the number of thrips and the number of flowers. He also counted thrips from 10 beat samples of vegetable plants (no flowers) throughout the greenhouse.
At Bakers Acres, greenhouse 2 has an open-air sales area in front of its intake door with many tables full of annual plants, including large numbers of marigolds. We will refer to this area as the “gazebo filter”. Greenhouse 3 is the same size, shape, and orientation as greenhouse 2 but does not have this filter of annual plants in front of its intake door. The intake on each greenhouse is a large garage door. We believe that migrating thrips enter both greenhouses with incoming air through the large garage doors at the front of the greenhouses. Both greenhouses were scouted weekly for 12 weeks. One marigold was placed on each table and used to trap thrips. Observations included 6 crop beat samples on each of 6 crops, 6 marigold beat samples and 2 3” x 5” yellow sticky cards per greenhouse per week. Crops were constantly moving and changing so there is not complete data on all crops. Crops sampled most days in greenhouse 2 were impatiens, lobelia, portulaca, salvia, torenia, and verbena. Likewise, in greenhouse 3, the herbs were chervil, mint, rosemary, sage, scented geranium, and viola. The number of thrips on the marigolds and other plants in bloom was assessed as number per flower, and averaged over the number of flowers in a beat sample. For plants not in bloom, beat samples were made on six 6 inch terminals of each species. We compared counts on marigold trap plants, and counts on the crops in each greenhouse in order to document the relative thrips attractiveness of the marigolds in comparison to the other crops.
In addition, we compared the number of thrips entering the greenhouses directly vs via the gazebo filter by placing one yellow sticky card to either side of the doorways of Greenhouse 2 and 3 and 2 cards within the “gazebo filter” . These cards did not survive every time due to weather and people, but we did get 5 dates in which we could compare the results.
Marigold plants were carefully removed from the greenhouses whenever they had more than 20 thrips. We documented the number of thrips and the number of flowers on the plant. and replaced it with a fresh marigold plant grown outdoors that had almost zero thrips. All 24 marigolds from the 24 tables in each greenhouse were subject to this treatment.
We compared the number of thrips that can be supplied by stressed or wilting marigolds with the number that is supplied by similarly infested plants that have ample water in an empty greenhouse. We selected 8 thrips infested marigold plants each week for 3 weeks and placed each inside an individual , 2 ft. x 2 ft. x 2 ft., wooden frame box with greenhouse plastic covering all sides. These enclosures were set up in a vacant hoophouse at Bakers’ Acres.. One 3 by 5 inch yellow sticky card was placed in each corner of the top of each box. Half the plants had a continuous water supply and the other half had no water. On Day 1, beat samples of a recorded number of marigold flowers were taken from each plant. We multiplied the average number of thrips per flower times the number of flowers to estimate the number of thrips on each plant. A Student’s t-test concluded that the marigolds assigned to the wet and dry treatments were all from the same population. On Day 3, the plants with no water were wilted, while the plants with water were not. All thrips on the sticky cards and on the plants were counted. Each plant was beaten twice against a white sheet of paper and all thrips counted. The experiment was performed on July 26 – 28, August 6 – 8, and August 17 – 19.
This study was designed to test whether a thrips magnet like marigolds can be safely used as a Guardian Plant to trap thrips away from greenhouse crops, especially when the “trap” is notorious for harboring the pest. We tested marigolds 2 ways: inside the greenhouse to pull thrips off the crop, and outside the greenhouse to prevent the thrips from entering the greenhouse. At the Pennsylvania site, the marigolds just inside the vents filtered thrips coming into greenhouses through air vents. There were 2 to 7 times as many thrips per flower on the vent side marigolds in comparison to marigolds in planters on the ground outside the vents (Fig. 1). The marigolds also served as predation and reproduction sites for the thrips predator, Orius, which were numerous (Fig. 2). Vegetable plants inside the greenhouse harbored less than one twentieth of the thrips found on the marigolds over 4 observation dates (Fig. 3). Thrips counts on the crop and sticky cards in the Pennsylvania greenhouse were extremely low and no pesticide treatments for thrips were required. Thrips and marigolds were removed from the greenhouse after the 4th week of observations.
At the NY site, the marigolds concentrated thrips from all the nearby flowering crops, including its greatest competitor, verbena and another thrips-prone crop, impatiens (Figs 4 and 5). In greenhouse 2 annual flowers, the marigolds commonly had 3 to 4 times as many thrips as the sampled crops (Fig.6). In greenhouse 3, with mostly non-flowering herbs, the marigolds commonly harbored more than 15 times as many thrips as the crops sampled did (Fig. 7). Individual infested marigold plants were removed from each crop any time that the number of thrips exceeded 20 per sample and replaced with a clean marigold from outdoors. The number of thrips removed with the marigolds are shown in Table 1. The most thrips were collected and removed by marigolds in the herb greenhouse, where the marigolds pulled many more thrips from the non-flowering herbs.
The test to see if the annual flowers in the gazebo in front of greenhouse 2 could reduce the number of thrips entering greenhouse 2 in comparison to greenhouse 3 had a negative result. It appears that any thrips that are attracted to the gazebo dispersed out of the gazebo and into greenhouse 2 as evidenced by the sticky card counts (Fig 8). When combined with the Pennsylvania data, we conclude that any outdoor trap plants should be placed so that they draw the thrips away from the greenhouse intake vents.
The trial testing whether thrips will disperse from wilting marigolds showed them abandoning the wilted marigolds in 2 out of the 3 dates (Table 2). On the third date, so many Orius were in the marigolds that the data was more variable and thus there was an 8% chance that the 2 treatments were not significantly different.. One of the wet marigolds had 8 Orius and only 12 thrips left (causing the high standard deviation of 43.6).
These trials showed that a thrips magnet like marigolds can be used to pull pests out of the crop and produce the thrips predator, Orius. However growers must manage these plants carefully because wilting plants can release all their thrips into the crop. Additionally, trap plant placement outside vents can draw additional thrips into the greenhouse.
- Figure 4. Number of thrips on marigold vs verbena.
- Table 1. Number of Thrips Removed on Marigolds at New York Site
- Figure 2. No. of Orius on marigolds vs. vegetables.
- Figure 3. Number of thrips per indoor ventside marigold plant compared to number per vegetable plant.
- Figure 7. NY HOUSE 3: Average No. Thrips per Marigold Sample vs per Crop Sample (Herbs)
- Figure 8. Average yellow sticky card counts at NY Site Doorways
- Figure 1. Number of thrips per marigold flower indoors vs. outdoors.
- Figure 5. Number of thrips on marigold vs impatiens.
- Table 2. The Effect of Drought Stress on Dispersal of Thrips from Marigolds
- Figure 6. NY HOUSE 2: Average No. Thrips per Marigold Sample vs per Crop Sample (Annual Flowers)
- Table 2: The Effect of Drought Stress on Dispersal of Thrips from Marigolds
The Guardian Plant practice of maintaining pest –prone plants in a greenhouse opposes some IPM sanitation recommendations of excluding or removing pest-prone plants from the greenhouse. These trials showed that a thrips magnet like marigolds can be used to pull pests out of the crop and produce the thrips predator, Orius, as long as growers are prepared to remove them when they become too infested with thrips. Additionally, this work offers some explanation of the folk-remedy where marigolds are used to suppress pests in gardens.
These results have been shared with more than 500 people via powerpoint presentations to greenhouse grower groups both by Grower, Lloyd Traven of Peace Tree Farm and Carol Glenister of IPM Laboratories.
Education & Outreach Activities and Participation Summary
The 2009 work demonstrated that marigolds can serve as Guardian Plants by pulling thrips away from crops and supporting the reproduction of the thrips predator Orius. The results have been shared with the public in the following articles and presentations listed in Table 3. Approximately 450 people attended these presentations.
This study shows that keeping marigolds in the greenhouse as thrips traps and Orius habitat will not increase thrips infestation of the crop, if managed properly, and can maintain a population of Orius. Growers that purchase Orius for thrips control spend approximately $50 per 500 plus overnight freight (approximately $30). For greenhouses that have no plant habitat suitable to hold the Orius in the greenhouse, this investment can literally disappear within a few days, so weekly infusions of Orius are required. Some greenhouse growers do indeed make this weekly investment. For small growers, however, regular purchases of Orius are too expensive. By adding Guardian Plants such as marigolds, they can influence the Orius to stay and multiply. Marigold seed is inexpensive, the marigolds are quick to grow and flower, and they happen to also serve as trap plants for thrips. For greenhouse managers already in the business of growing large quantities of plants on schedule, their decision to invest time, materials, space and unique watering requirements in marigolds is balanced against the cost of the Orius. Some growers choose the marigolds or other suitable habitat, while others make regular purchases of Orius. Yet even those growers that make regular purchases are providing habitat to maintain and grow their investment.
Bakers’ Acres has continued to use the marigolds as trap plants. Peace Tree is not using them at this time. There is a general upsurge in grower efforts to create habitat for beneficials that has been supported by the dissemination of this project along with work by many other people. In addition, 2 very prominent growers that attended Carol Glenister’s Guardian Plant presentation in Tolland, CT have been planting mixed containers for support of natural enemies and showcasing them at grower meetings organized by Cooperative Extension at their greenhouses. One grower, Chris Champagne at Grower Direct in Somers CT, planted habitat baskets that included white alyssum, marigolds, dill and fennel, but has recently identified White Sensation cosmos as an extreme Orius attractant. The other, Roger McGaughey of Michaels Greenhouse in Cheshire CT, is including zinnias in his mixed planters for the same purpose and distributing these planters both in the greenhouse and outdoors.
Areas needing additional study
Retention of purchased Orius will assure their action against pests. Dispersal away from the crop may be driven by insufficient host plants or by the size and placement of host plant groupings. Orius release expense would likely be more efficient if the number released were correlated to the number of compatible flowers available to host them instead the current crop area calculations. A comparison of Orius retention after a release of 1 per flower, 1 per 5 flowers, and 1 per 20 flowers would give pest managers concrete information on ideal release rates.
At Peace Tree Farm and at other large greenhouses, it has appeared that the Orius were more prone to stay on large groupings of host plants than on plants placed individually among the crop. Data on optimum position and spacing of Guardian Plants are needed.