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

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

Summary

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 productions 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.

Results
The predatory mite Neoseiulus fallacis, naturally occurring in vegetation surrounding container-grown nurseries, was discovered to rapidly colonize burning bush 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 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 Connecticutt Nursery IPM Program switched to selective miticides. Growers previously spraying four to nine times for mites only made two applications in 1996. Improved crop quality due to reduction of 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 of 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.

Impact
This project has already demonstrated dramatic reductions in the quantities of pesticides used for management of pests within nurseries. From a high volume of 185 lb active ingredient (a.i./acre) in nine dosage equivalents, the bellwether nursery (Nursery A) reduced its chemical use to 36 lbin two dosage equivalents, or an 80 percent reduction. Much of the weight of pesticides applied is horticultural oil. With the oil removed from the calculations, use of remaining products decreased from 5.25to 0.02 lb per acre (a 99 percent reduction by weight, a 75 percent 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.

Many growers rely upon hydraulic sprayers to apply pesticides, which place the applicators in contact with spray and treated plants. Applicators complain skin and bronchial irritation associated with application of pyrethroids (bifenthrin or fluvalinate) and skin irritation with 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).

The following information on pesticide/cost reductions is from three nurseries for which we have complete records through 1996. Products applied prior to the introduction of IPM practices include 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 percent reduction in rate (from a 2 percent to a 0.5-1percent solution) and abamectin (a 33 percent to 50 percent 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 oils, 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.

New Hypotheses
The outstanding contribution of this work has been the discovery that naturally occurring and highly effective predators colonize nursery crop s 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 of 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. fallicis. Laboratory tests of these other materials could prevent expensive disruptions to a working integrated mite management program.

Changes in Practice
As noted above, a small number of influential nurseries have already adopted changes in the choice of miticides, dramtically 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);
3. improved spray timing based on information obtained through monitoring plant material.

Reported December 1996.