Integration of Biological and Chemical Control of Twospotted Spider Mites in Containerized Nursery Production

Final Report for ANE95-028

Project Type: Research and Education
Funds awarded in 1995: $35,246.00
Projected End Date: 12/31/1996
Matching Non-Federal Funds: $18,836.00
ACE Funds: $35,246.00
Region: Northeast
State: Connecticut
Project Leader:
Timothy Abbey
Univ. of Connecticut Cooperative Extension System
Co-Leaders:
Richard Cowles
Connecticut Agricultural Experiment Station
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Project Information

Summary:

The predatory mite Neoseiulus fallacis, naturally occurring in vegetation surrounding container grown nurseries, was discovered to rapidly colonize burning bush (Euonymus alata ‘compacta’, EAC) and keep twospotted spider mites (Tetranychus urticae Koch) at non-damaging populations. EAC were artificially infested with sufficient spider mites to temporarily induce an outbreak, then were sprayed with miticides to determine their toxicity to the predatory mites. In this replicated experiment only two miticides, bifenthrin and chlorfenapyr, were highly toxic to predatory mites. Bifenthrin was selectively toxic to the beneficial predatory mites and caused resurgence of twospotted spider mite populations. Chlorfenapyr appeared to have short residual activity, and predatory mites increased within two weeks following its use. Of the remaining products abamectin was somewhat more toxic to predatory mites than clofentezine, hexythiazox, horticultural oil or spinosad, and all were selective against twospotted spider mites. With the exception of bifenthrin, these miticides in combination with predatory mites rapidly brought spider mite populations to very low levels. Of these products, abamectin and spinosad are natural products, while horticultural oil has very low mammalian toxicity. Abamectin is remarkably active: when predators are present its optimal use rate may be 0.015 lb active ingredient per acre.

Growers have been very willing to adopt selective miticides. Based on 1995 data, several nurseries participating with the Connecticut Nursery IPM Program switched to selective miticides. Growers previously spraying 4 – 9 times for mites only made two applications in 1996. Improved crop quality due to reduction in mite damage and phytotoxicity were concomitant with a change to selective miticides. We anticipate further reductions in pesticide use and increased profits as growers adopt findings developed from our research in 1996. Specific improvements expected are (1) reductions in the use rates of abamectin and horticultural oil to improve the selectivity of these products against twospotted spider mites, (2) diminished reliance on abamectin (beneficial with respect to resistance management), because we have found other selective miticides, and (3) reduced number or elimination of the need to apply miticides, because growers will avoid disruptive insecticides (carbamates and pyrethroids) and will use higher treatment thresholds for spider mites.

Project Objectives:

1. To determine which of three microbially-derived miticides provides the most selective mortality of twospotted spider mites relative to candidate phytoseiid predators.
2. To determine which species of phytoseiid mite predators can be used in a nursery.
3. To determine the most effective sampling method for mites in a nursery.
4. To determine the action threshold for mite populations that should trigger application of a selective miticide.
5. To determine whether a reduced-pesticide production system will release other pests from pesticidal suppression.
6. To substantially reduce the dosage equivalents of miticides applied in a nursery.
7. To extend the research and management results from a bellwether nursery to other nurseries.

Research

Materials and methods:

Container-grown nursery crops in the Northeast share many common features. A typical site has nearly level fields with overhead irrigation fed from adjacent farm ponds, from which run-off water is recycled. Plant material is protected from damaging winter temperatures by placement in plastic-covered hoophouses. The arrangement of plant material in blocks during the growing season is then dictated by spacing the pots within and between hoophouses once their plastic covers and metal supports have been removed. Nursery stock placed in hoophouses for winter storage fills in a nearly continuous canopy, with only a narrow walkway extending down the length of the house. Spraying insects, mites, or diseases when plants are under plastic involves running a hydraulic hose for half the length of the hoophouse (often 300 ft.) and walking backwards while spraying, then repeating the operation from the other end of the hoophouse. During the summer, blocks of plants are interspersed by a grid of driveways, from which tractor-driven spray machinery (hydraulic or mist-blower) may be operated. For small-scale spray operations, backpack sprayers are sometimes used.

The high density of plant material in nurseries and their required access to ponds has fortuitously dictated a high perimeter to area ratio, in which fields are surrounded by woodlands. Even in the midst of the largest nurseries, the edge of fields is not more distant than 1000 ft. This feature may be responsible for the ease with which predatory mites colonize nurseries from surrounding vegetation.

Research results and discussion:
Project Results

Objective 1. To determine which of three microbially-derived miticides provides the most selective mortality of twospotted spider mites (Tetranychus urticae Koch) relative to candidate phytoseiid predators.

Microbially-derived miticides of interest in this study were abamectin, spinosad, and chlorfenapyr, of which only chlorfenapyr has been chemically altered from a natural product. Other standard miticides (clofentezine, hexythiazox, and ultrafine oil) were included in the trial for comparison. Two methods were used to determine selectivity: a miticide trial using potted plants in a hoophouse, and laboratory dose-response assays. In the miticide trial, EAC in 2-gal. containers were artificially infested with sufficient spider mites to temporarily induce an outbreak, then were sprayed with miticides to determine their toxicity to predatory mites. In this replicated experiment only two miticides, bifenthrin and chlorfenapyr, were highly toxic to predatory mites (Fig. 1). Bifenthrin was selectively toxic to the beneficial predatory mites, so this product induced resurgence of twospotted spider mite populations. Chlorfenapyr appeared to have short residual activity, and predatory mites increased within two weeks following its use. Of the remaining products abamectin was somewhat more toxic to predatory mites than clofentezine, hexythiazox, horticultural oil or spinosad, and all were selective against twospotted spider mites. With the exception of bifenthrin, these miticides in combination with predatory mites rapidly brought spider mite populations to very low levels. Abamectin is remarkably active: when predators are present its optimal use rate may be 0.015 lb active ingredient per acre. Work is progressing on dose-response assays, and should be completed during the 1996-1997 winter.

Objective 2. To determine which species of phytoseiid mite predators can be used in a nursery.

Preliminary tests during the 1995 growing season were unsuccessful in establishing predatory mites on EAC within a cooperating nursery. Predators released included Phytoseiulus persimilis, Neoseiulus occidentalis, and Neoseiulus californicus. Lack of establishment was attributed to having insufficient populations of their food (spider mites) at the time of release in late August. Negligible twospotted spider mite populations were present throughout the season, as determined from intensive sampling.
In 1996, EAC brought to the Valley Laboratory to conduct the miticide trial (see above) were artificially infested with spider mite populations from commercial strawberry plantings and nursery stock. A naturally occurring population of predatory mites were detected in June and kept spider mite populations in check. These locally-occurring predatory mites were collected and submitted to a specialist (Dr. Harold Denmark) for identification. They were determined to be Neoseiulus fallacis (Garman), a species commonly found in Northeast region tree fruit production. A concerted effort to favor twospotted mite development, application of pyrethrin, and massive inoculations of the plants with spider mites was necessary to increase spider mites populations for the miticide trial.
Previous to this study, nursery growers were unaware of the potential for predatory mites to reduce spider mite populations, and researchers were unaware that naturally occuring predatory mites would commonly colonize commercial nurseries. Other predators encountered included the six spotted thrips (Scolothrips sexmaculata [Pergande]) and mite destroyer ladybeetles (Stethorus sp.). Based on repeated encounters with naturally occurring predatory mites in nursery stock, we hypothesize that N. fallacis disperses from woodlands surrounding the nurseries. We expect, because these are locally adapted predators, that they will perform as well or better than commercially reared mites. Also, because they colonize nursery crops by late June, they are present early enough to prevent mite outbreaks in nursery crops, obviating the need for releasing commercially-reared predators. Another line of evidence that points to the importance of these predators is the observation from the Connecticut Nursery IPM program that damaging populations of mites within other nursery crops was universally related to previous use of non-selective insecticides (especially cyfluthrin) inappropriately chosen to control aphids. Pyrethroids and carbamates would be expected to chemically exclude predatory mites, inducing secondary outbreaks of spider mites.

Objective 3. To determine the most effective sampling method for mites in a nursery. Objective 4. To determine the action threshold for mite populations that should trigger application of a selective miticide.

Intensive mite sampling took place in 1995 and 1996 at a cooperating nursery. By this time, the staff at the nursery had implemented changes in their spray practices (use of oil and avoidance of disruptive insecticides) that led to very low populations of spider mites in their EAC crop. The populations were so low that sampling methods never could be tested under conditions where mites could cause detectable damage. However, observations of mite outbreaks within the miticide trial (1996) provided a simple method for rapidly detecting mite populations in “hot spots.” In this crop, mite populations of 10 per leaf began to cause bronzing (within the centers of the shrub’s foliage) and became very dispersive, forming webbing at the top of the plant. When these populations were subsequently brought to low numbers by the combined action of predatory mites and miticides, the plants remained aesthetically acceptable. Therefore, when predatory mites are present, a threshold value of 10 mites (eggs and mobile stages) would make an appropriate economic threshold, and webbing can be used as an easily observed indication of where mites may be reaching such densities.

A threshold population of 10 mites per leaf is far higher than the populations currently tolerated by nursery growers (who often spray with populations below one mite per leaf). If growers can be convinced that higher action thresholds are acceptable, then any rapid sampling method, including beating samples, will be adequate for early detection of potentially troublesome areas within a nursery.

Objective 5. To determine whether a reduced-pesticide production system will release other pests from pesticidal suppression.

Observations of the EAC crops at nurseries participating in the Connecticut Nursery IPM Program in 1996 detected aphids as the only non-mite pest within the reduced miticide treatment program. Pests monitored in the crop included euonymus scale, aphids, Japanese beetle, oriental beetle and black vine weevil. The populations of aphids were not unusual, and did not require special treatments for suppression. In fact, a complex of ladybeetles (including Harmonia axyridis Pallis, the multicolored Asiatic ladybeetle) colonized these crops and did an excellent job at suppressing aphid populations.

Objective 6. To substantially reduce the dosage equivalents of miticides applied in a nursery.
Objective 7. To extend the research and management results from a bellwether nursery to other nurseries.

Growers have been very willing to adopt changes in their miticide use to include selective materials. Based on 1995 data, several nurseries participating with the Connecticut Nursery IPM Program switched to selective miticides. One grower previously spraying nine times for mites only made two applications in 1996. Improved crop quality due to reduction in mite damage and phytotoxicity were concomitant with this change to selective miticides. We anticipate further reductions in pesticide use and increased profits as growers adopt findings developed from our research in 1996.

Participation Summary

Education

Educational approach:
Dissemination of Findings

Preliminary results from this project were discussed at the Connecticut Nursery Grower’s Annual Meeting on January 17, 1996 with 43 attendees. A hands-on IPM workshop was conducted on August 5, 1996, at Summerhill Nursery in Madison, CT, with 33 growers attending.

Research findings and guidelines from this project (see “Operational Recommendations,” below) will be provided to growers, researchers and extension personnel by the following methods: (1) incorporation into the 1997 edition of the “Pesticide Guide toward Integrated Pest Management for Connecticut Nurseries,” published by the The Connecticut Agricultural Experiment Station; (2) an article for The Yankee Nursery Quarterly, a publication of University of Connecticut Cooperative Extension; (3) at The University of Connecticut Cooperative Extension 1997 Nursery IPM Field Day, (4) at the Connecticut Agricultural Experiment Station Valley Laboratory Field Day and Plant Science Day; (5) through personal communication between the principle investigators and nursery growers; and (6) presention as an invited talk, “Conservation of Natural Predatory Mite Populations on Nursery Crops Through the Use of Selective Miticides,” at the 68th Annual Meeting of the Eastern branch of the Entomological Society of America in Burlington, VT, on February 23-26, 1997.

Project Outcomes

Impacts of Results/Outcomes

A. Impact Assessment

This project has already demonstrated dramatic reductions in the quantities of pesticides used for managment of pests within nurseries. From a high value of 185 lb active ingredient (a.i./acre) in 9 dosage equivalents, the bellwether nursery (Nursery A) reduced its chemical use to 36 lb in 2 dosage equivalents, or an 80% reduction. Much of the weight of pesticide applied is horticultural oil. With the oil removed from the calculations, use of remaining products decreased from 5.25 to 0.02 lb per acre (a 99% reduction by weight, a 75% reduction in dosage equivalents). Abamectin can be responsible for dramatic changes in quantities of pesticide applied, since an application of 0.02 lb active ingredient is equivalent to 0.18 lb hexythiazox, 0.5 lb disulfoton, 0.75 lb oxythioquinox, 3.0 lb dicofol, or 36 lb of oil.

Reduced exposure of nursery workers to insecticides and miticides will result from having reduced spraying. Many growers rely upon hydraulic sprayers to apply pesticides, which place the applicators in contact with spray and treated plants. Applicators complain of skin and bronchial irritation associated with application of pyrethroids (bifenthrin or fluvalinate) and skin irritation to oxythioquinox. The materials suggested for an IPM program do not have these qualities, and with the exception of oil, could be applied with low-volume spray equipment that can lower worker exposure through engineering controls (such as using an enclosed spray cab).

B. Reductions of pesticide use resulting from this program

The following information on pesticide/cost reductions is from three nurseries for which we have compete records through 1996. Products applied prior to the introduction of IPM practices included pyrethroids: fluvalinate, cyfluthrin, and bifenthrin; the carbamate carbaryl; chlorinated compounds: endosulfan, dicofol, and dienochlor; organophosphates: acephate, disulfoton, and oxydemeton-methyl; and others: horticultural oil, oxythioquinox, and abamectin. Many of these products were used without determining need. Products used in 1996 under our guidance were limited to horticultural oil and abamectin. Specific impact on pesticide use patterns are given below.

1. Materials that have been eliminated from use on EAC and other mite-sensitive crops include the insecticides bifenthrin (Talstar), cyfluthrin (Tempo), fluvalinate (Mavrik), bendiocarb (Turcam), and carbaryl (Sevin).

2. Materials that were replaced through substitution include the conventional miticides oxythioquinox (Joust) because of its incompatibility with the use of oils, dicofol (Kelthane), endosulfan (Thiodan), and disulfoton (DiSyston).

3. Materials that will in the future be used at reduced rates include horticultural oil, with a 50-75% reduction in rate (from a 2% to a 0.5 – 1% solution) and abamectin (a 33 to 50% reduction, to a use rate of 4 fl. oz./ 100 gal.).

A product new to the nursery market, hexythiazox (Hexygon), may increase in use due to its compatibility with predatory mites, horticultural oil, and low volume spraying. Its use will involve a maximum of 0.18 lb active ingredient per acre each year. The addition of this tool is valuable because miticide resistance to abamectin could be delayed with product rotation.

C. New Hypotheses

The outstanding contribution of this work has been the discovery that naturally occurring and highly effective predators colonize nursery crops that have not been treated with disruptive insecticides. The critically important aspects that should be investigated further are the phenomenon of colonization and conservation methods for these predators.

We hypothesize, based on the theory of island geography, that predatory mites will more readily colonize crop plants at the perimeter of fields than at their center. One practical implication is possible earlier colonization by predatory mites of plants placed at the perimeter of a growing area, improving suppression of mite populations. It would also imply that growers should plan on placing mite-susceptible crops at the edges of fields. Another logical extension would be that growers may want to maintain hedgerows within the centers of large growing areas to support predatory mite populations and to reduce the maximum dispersal distance. Long, narrow fields would also have an advantageous geometry for mite dispersal.

With respect to conservation of naturally occurring predatory mites, we are confident that we have identified those classes of products (carbamates and pyrethroids) that are prone to induce mite outbreaks, due to chemical exclusion of predatory mites. We do not know whether fungicides or sprayable herbicides used in nurseries may also be toxic to N. fallacis. Laboratory tests of these other materials could prevent expensive disruptions to a working integrated mite management program.

Economic Analysis

Container-grown nurseries are highly intensive farming operations. Plant material may vary from 120,000 to 4,800 units (potted plants) per acre of production, depending on the size of the plant, the size of its canopy, and whether the plant material is spaced out or being held in a hoophouse. Wholesale value of plant material typically varies from $4 to $20 per unit (depending on size of container, rarity, and ease of growing), while retail value is 2-3 times the wholesale price. Areas dedicated to growing common varieties of plants may amount to 4-5 acres, but have up to 100,000 units and are worth several hundred thousand dollars.
Due to the expense of protection from winter temperatures and irrigation, plants grown in containers are usually kept heavily fertilized and at high plant densities. These factors contribute to make certain pests nearly impossible to manage with conventional pesticides. Twospotted mites are foremost among these pests: they are found on the undersides of leaves where coverage with contact miticides is difficult. Areas within dense canopies of leaves are poorly treated, from which mite populations may rebound following a spray.

Losses from mite infestations prior to the initiation of this project varied by site and crop. The following is an extreme example of the economic loss during 1994 at one nursery. Mite problems were so severe that there were nine applications of miticides applied biweekly during the growing season. To improve efficacy, material was applied with hydraulic sprayers in dilute sprays, requiring three spray crews working simultaneously to treat the entire crop. In spite of (or partly because of) these efforts, approximately $44,400 of EAC plant material (9% of the total crop) was killed due to mite injury, phytotoxicity, and a combination of the two. The material was lost from approximately 2.6 acres of production, so the loss in value of plant material alone amounted to $17,100 an acre. Table 2 breaks down the costs for conventional management and integrated management practices. Unlike the figures reported under section 5A, “Changes in Practice,” the figures given in Tables 2 and 3 include application costs and costs due to plant losses. These are actual costs from nurseries participating with the Connecticut Nursery IPM Program; the high values are from the case described above. The worst case scenario presented for the IPM Program gives the values expected from one low-rate application each of ultrafine oil and abamectin using a hydraulic sprayer (a labor intensive application method). A more likely scenario is a single seasonal application of miticide with a mist blower. The cost, including labor and equipment is $20, $86 or $100 per acre for horticultural oil, hexythiazox or abamectin, respectively. Growers in 1996 applied miticides even when we deemed the populations to be inconsequential. With increased tolerance of non-damaging mite populations and avoidance of disruptive insecticides, we anticipate that growers will be able to forego treating mites in many crops.

A change to integrated management practices is beneficial regarding material costs, labor costs, finished plant material quality, and risk. The use of naturally occurring predatory mites has several economic and risk benefits for this crop production system.

1. Unlike the use of commercially available predatory mites (costing $6 per 1000 mites) in California strawberry fields, in the Northeast nursery crop there are no material or application expenses associated with their use.

2. The predatory mites reproduce within the crop. Therefore, their population can track spider-mite populations, leading to a negative feedback control system which reduces risk.

3. Use of naturally occuring predators takes advantages of their having been locally adapted.

4. Halved rates of application of abamectin or oil should be used when predators are present, to increase selectivity and to allow predators to “mop up” remaining hot spots.

Farmer Adoption

A. Changes in Practice

As noted under “Findings and Accomplishments” a small number of influential nurseries have already adopted changes in the choice of miticides, dramatically reducing the number and quantity of miticides and insecticides now applied. Growers have adopted the following practices: (1) use of a more selective chemical product (abamectin) to control two-spotted spider mites (growers previously had been using general cover sprays with nonselective pesticides); (2) scouting for mites (they were taught proper identification of spider mites and Phytoseiid mites); and (3) improved spray timing based on information obtained through monitoring plant material.

When growers learned when and where to scout for twospotted spider mites early in the season (late May to early June), they discovered the importance of tracking pest populations over time and not waiting until pests have reached damaging levels. This translated into improved spray timing and thus better control. Growers will now also have to learn to accept higher treatment thresholds to make the most effective use of naturally occuring predators.

Details on the pesticide reductions at these nurseries is given below.

Nursery A – Pesticides applied for control of twospotted spider mites on Euonymus alata ‘Compacta’.

In 1994, this nursery applied 2,226 and 1,864 pounds of pesticide product and active ingredient, respectively. The totals dropped to 1,241 and 637 pounds for 1995. For 1996, pesticide usage fell to 355 and 259 pounds. This represents an 84% reduction in product and an 86% reduction in active ingredient from 1994 to 1996. Avid usage for these years was 0 ounces (the grower was reluctant to try the material), 222 ounces, and 144 ounces, respectively. Costs of chemicals used were $3,454 for 1994, $2,817 for 1995 and $2,386 for 1996.

Nursery B – Pesticides applied for control of two spotted spider mites on Euonymus alata ‘Compacta’ and other deciduous plant material

During 1994, there were 40 pounds of product and 19 pounds of active ingredient applied for mite control. These totals dropped to 0.96 and 0.26 pounds in 1995, and in l996, there were no products applied for mite control. This is a 100% reduction in pesticides applied for mite control. No Avid was used in 1994 (the grower was not familiar with the product), 5 ounces in 1995 and none in 1996. Pesticide costs for the three field seasons were $350, $50 and $0 respectively.

Nursery C – Pesticides applied for control of two spotted spider mites primarily on Ilex cornuta and Ilex crenata, also various Buddleia spp.

This nursery applied l06 and 76 pounds of product and active ingredient in 1995, respectively. In 1996, these amounts were 49 and 47 pounds. Avid was used in 1995 (27 ounces), but it was applied after predators had been killed by use of non-selective material and the pest population had increased. There was a 54% reduction in pounds of product and 38% reduction in active ingredient use for the two years. Use of Avid dropped to 19 ounces in 1996. Pesticide costs for the two years were $1,349 and $177.

B. Operational Recommendations

1. Eliminating pyrethroids (bifenthrin and fluvalinate) as suggested materials for the control of mites or insects, due to their high toxicity to predatory mites. A non-miticidal pyrethroid, cyfluthrin, has been removed from control guidelines for aphids, and precautions are noted for carbamates (carbaryl and bendiocarb) as products for controlling other pests because of their observed tendency to cause secondary mite outbreaks.

2. Hexythiazox or reduced rates of abamectin (4 fl oz per 100 gal.) or horticultural oil (0.5-1% concentration), are recommended to selectively suppress twospotted spider mites populations.

3. Scouting for mite webbing and bronzing are suggested as an indicators of “hot-spots” within fields and as easily detected management threshold indicators.

4. Scouting for predatory mites is suggested to confirm their presence in fields; this information is useful to determine appropriate rates of selective miticides (higher rates may be justified when predators are not detectable).

Areas needing additional study

Please note the section “New Hypotheses.” Testing island biogeography theory with this crop system could lead to a deeper understanding of practical ways to plant crops to minimize the damage from spider mites.

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.