BIOCONTROL OF TWOSPOTTED SPIDER MITE

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Biological Control Guidelines

Mark A. Morris and Joyce Takeyasu, A. M. Todd Co.

Note: this information is considered unpublished work and should not be used as final or finished results. It has been included in IPMP 3.0 because it may not be available from other sources, and in some cases may include information that may not reach final publication.

One species of predator mite, Neoseiulus (Amblyseius) fallacis, is very common in peppermint fields. N. fallacis adults are somewhat shiny, tan to slightly orange in color, and are oblong-shaped. They will provide excellent biological control of twospotted spider mites, if populations are conserved. When populations are disrupted due to incompatible pesticides or heavy flaming, recolonization usually occurs from habitats surrounding mint in Western Oregon. In Eastern Oregon, N. fallacis has not been collected on surrounding vegetation, and is only likely ro recolonize from small refugia within the field. In addition, this species may also be re-introduced manually. It may be obtained from biological control supply houses and released in the field to initiate new populations. Released predators will spread throughout the field over several weeks to months, depending on the number released and weather conditions. They overwinter in dead leaves, on stems, and on green foliage in mint, and will provide long term biological control if not disrupted.

The following table lists pesticides tested to have no or low, moderate, and highly toxic effects against N. fallacis. Note that the Pacific Northwest Pest Control Handbooks should be consulted to check the registration status of all pesticides used in mint. More information on pesticide compatibility is placed in recent OSU Entomology Dept. Research Progress Reports (Morris 1993, Morris 1994).

No or low toxicity to predator mites (0 - 10% mortality):

Bacillis thuringiensis (BT)         Parasitic nematodes
Omite (at lower rates)              Comite (at lower rates)
Tilt                                            Authority
Basagran                                  Class Act
Command                                 Karmex
Rally                                         Sinbar
Solution 32 (1:100 dilution)       Thiolux
Prowl                  

Moderate toxicity to predator mites (11 - 30% mortality):

Orthene                                    Lorsban¹
Malathion                                 Sulfur
Metasystox-R                            Assure
Fusilade                                   Poast
Tough                                      Solution 32 (1:10 dilution)
Prowl (topical)

Highly toxic to predator mites (31 - >50% mortality):

Vydate²                                    Lannate
Kelthane                                 Carbofuran
Mocap¹                                    Gramoxone
Carbaryl                                  Asana
Brigade/Capture                     Goal (predator eggs did not hatch)
Malathion                                Buctril (even at 1:1,000 dilution)

¹ Less toxic if applied by chemigation with large amounts of water
² Less toxic if applied with concurrent irrigation

Where a disruptive pesticide is concerned, avoidance is the best approach. If there is a selective pesticide that is just as effective, the selective pesticide should be used. For example, Basagran is a more selective alternative than Buctril in warmer weather when mint is no longer dormant and predator mites are active. When a suitable alternative is not available, the strategy is to use disruptive pesticides in a slective manner. Application timing is one way of making a disruptive pesticide more selective. Applying a pesticide while predator mites are overwintering is one strategy to avoid direct contact with a disruptive pesticide. N. fallacis overwinters in soil debris , which provides a certain degree of shelter. Although it is moderately toxic, Goal can be used safely when it is applied when the mint is dormant and predator mites are overwintering. Similarly, applying Prowl at this time is the least toxic approach to both mint and predator mites. If there are not options but to apply a disruptive pesticide when predators are active, it is important to consider the population dynamics of both predator mites and spider mites at the time of application. Applying a disruptive pesticide before predator mites have substantially reduced the spider mite population may result in a subsequent spider mite outbreak.

Elimination of natural enemies by the misuse of pesticides may result in subsequent pesticide applications for other pests. Despite efforts to use disruptive pesticides in a selective manner, it may not always be possible, However, we can keep disruptive of biological control agents to a minimum and also slow the development of pesticide resistance by treating fields only when absolutely necessary.

Heavy (double) flaming of the field will reduce predator densities to near zero, while single flaming usually allows sufficient numbers to survive, so that re-introduction may not be needed. Single flaming may adversely affect predator/prey ratios, however, that can lead to spider mite population flare-ups. (see Morris et al., 2000).

Preliminary release recommendations:

Preliminary release recommendations are based on recent work with N. fallacis on peppermint and strawberry. The number to release depends upon whether short-term or long term control is desired. Short term control (2-4 weeks, longer for high spider mite infestations) can be expected with relatively high (innundative) release rates, about 7,000-10,000 predators per acre. The cost of these releases should be roughly equal to about 2 applications of conventional miticides. Long-term control (within 2-4 months) can be expected with very low (inoculative) release rates, about 500-1,000 predators per acre. This approach takes advantage of the predators very high natural dispersal ability. Predators may be taken from fields where spider mite populations have crashed and predators remain abundant, or when predator:prey ratios are 1:10 or better. Miticides including omite and comite (at lower rates) are compatible with predator mite releases and may be necessary to provide control while predator populations build up.

To inoculatively release N. fallacis, spread predators evenly throughout the field, especially towards the upwind side and wherever spider mite densities are highest. If the lower release rates are used, place at least 10-12 healthy female N. fallacis per 5 sq. ft. site, with ca. 30-35 ft. between sites. This rate is equivalent to 500 predators per acre. Apply the predators to the crop as soon as possible after recieving shipments for best results. Note that predator mites will not survive storage, except perhaps for a few days under refrigeration. Avoid making releases when humidities are low. Be sure to check for good, healthy females, which are easy to recognize from their robust, shiny appearance and very active search behavior. Small, dull-colored and sluggish females are not as likely to survive. Contact OSU Entomology Dept. for a list of recommended insectaries and distributors that handle N. fallacis.