Description of the Pest
The adult female twospotted spider mite is oval-shaped and measures approximately 1/50 inch (0.4 mm). The adult male is smaller than the female and has an elliptical shape; its body is tapered at the end. Twospotted spider mites are usually translucent or greenish-yellow in color, but can also be brown or orange-red. Just prior to diapause, adult females change to bright orange. Twospotted spider mites have dark spots on each side of their body, which is due to the accumulation of body waste products.
Adults overwinter in soil cracks beneath plant debris or on living plant tissue. Females lay eggs on the underside of leaves and depending on temperature, eggs hatch in 4 to 5 days. The complete life cycle takes approximately 14 to 16 days and there are several generations per season.
Feeding damage appears on the upper surface of leaves as silvery or dry spots. Heavy infestations cause leaves to turn bronze in color and drop. Economic damage can occur from a reduction in oil yield and quality, and a shorter duration of productivity of mint fields.
Infestations of twospotted spider mite can occur in all peppermint growing regions of California. Commonly, populations increase quickly during the hot and dry periods between June and August. Although most damage occurs during this period, economic populations have been observed during late April and early May. Control options include reducing plant stress through optimum irrigation and fertilization, biological control using predatory mite releases, and miticides.
The spider mite predator, Neoseiulus fallacis, can occur in all mint-growing areas of California. Adults have a pear-shaped body and measure approximately 1/50 inch long (0.4 mm). They are shiny with long legs and tan to light orange. Immatures are semi-transparent and cream colored.
Predatory mites can be distinguished from spider mites by their more active searching behavior. N. fallacis can also be purchased from commercial insectaries. Releases should take place when twospotted spider populations are well below the threshold of five mites per leaf. For optimum results, N. fallacis should be released at 2000 per acre when approximately 20% of sampled leaves have at least 1 spider mite. Preserve N. fallacis by using a compatible miticide.
Using biological control is an excellent tool to manage miticide resistance, because biological control organisms are indiscriminate in their feeding behavior.
Organically Acceptable Methods
Clarified extract of neem oil and predatory mites can effectively control twospotted spider mites. The efficacy of neem oil is slower than conventional miticides, because it is an insect growth regulator and does not kill by contact. When using neem oil, apply at a lower threshold of approximately three mites per leaf.
Minimize water stress and control dust along dirt roads, especially during hot weather.
Monitoring and Treatment Decisions
There are two different monitoring methods: enumerative and presence/absence sampling.
- Twospotted spider mites tend to be distributed unevenly throughout peppermint fields. Therefore sample several different locations when monitoring.
- Sample a minimum of 7 locations for every 30 acres. Sample more locations, depending on the field's history.
- The sampled location should measure approximately 15 feet by 15 feet.
- Sampling methods and treatments:
- Enumerative. Count all nymphs and adults on the underside of about 23 randomly selected leaves at every location. Treat if five mites per leaf or more are found.
- Presence/absence. If a leaf has two spider mites or more, note the leaf as infested, otherwise it is considered not infested. Apply a miticide if 15 of the 23 leaves sampled (~65%) are infested. This indicates a density of approximately five mites per leaf.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(conc.)||(dilute)||(hours)||(days)|
|The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Not all registered pesticides are listed. Always read the label of the product being used.|
|COMMENTS: Predatory mites can be released early in the season to establish or to augment resident populations. If an acaricide is needed (spider mite population is at economic level) and predators are present, be sure to use a selective material. Monitor to ensure that pest populations remain in balance with predator populations.|
|(Agri-Mek 0.15 EC, etc.)||10–20 fl oz||2.5–5 fl oz||12||21|
|MODE-OF-ACTION GROUP NUMBER1: 6|
|COMMENTS: Contact or ingestion toxin that paralyzes juveniles and adults. Direct contact toxicity to predators but not long lasting. Do not make more than 2 applications per growing season and allow at least 21 days between treatments. Do not exceed 20 fl oz/acre per application.|
|(Zeal)||2–3 oz||0.5–0.75 oz||12||28|
|MODE-OF-ACTION GROUP NUMBER1: 10B|
|COMMENTS: Acts as a contact toxin to eggs, inhibits molting of juveniles, and causes adult females of both pest and predator mites to produce sterile eggs. Do not apply more than once/season. Long residual activity that can be used early to midseason in the absence of concern for predator mites.|
|(Onager EC)||Label rates||12||28|
|MODE-OF-ACTION GROUP NUMBER1: 10A|
|COMMENTS: Apply after sampling indicates pest mites are increasing, but before significant damage or webbing is present. Mite growth regulator that is a contact toxin to eggs and young larval stages so it is best suited for an early season application if needed; causes adult females lay sterile eggs. Believed to have same effect on predator mite females as well. Do not make more than one application per year. Use allowed under a Special Local Needs registration.|
|(Fujimite 5EC)||1–4 pt||see label||12||14|
|MODE-OF-ACTION GROUP NUMBER1: 21|
|COMMENTS: Contact toxin to eggs, juveniles, and adults with long residual activity. Toxic to both pest and predator mites. A good choice under extreme mite pressure in the absence of beneficials.|
|(Omite 6EC)||1.5–2 qt||0.375–0.5 qt||21 days||28|
|(Omite 30WP)||7.5–10 lb||1.9–2.5 lb||21 days||28|
|MODE-OF-ACTION GROUP NUMBER1: 12C|
|COMMENTS: Contact toxin that targets juveniles and adults. Do not apply more than twice per season. Do not apply less than 40 days after or 30 days before an oil application. Toxic to predator mites at full label rates but becomes less toxic at rates that are 1/2 to 1/10 maximum label rate when western predatory mites are present. Below label rates are intended to balance predator and spider mite populations.|
|(Acramite 50WS)||0.75–1 lb||0.1875–0.25 lb||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 25|
|COMMENTS: Contact toxin that targets all stages. Relatively safe for beneficial and predaceous mites. Apply with ground equipment; requires complete coverage of both leaf surfaces for effective control. A good choice for post hullsplit control.|
|(Trilogy)||Label rates||4||See label|
|MODE-OF-ACTION GROUP NUMBER1: Botanical (26)|
|COMMENTS: Apply in a minimum of 25 gallons per acre. This product cannot be applied through any type of irrigation system. Toxic to bees, do not apply while bees are foraging.|
|**||For dilute applications, rate is per 100 gal water to be applied in 300–500 gal water/acre, depending on the label; for concentrate applications, use 80–100 gal water/acre, or lower if the label allows.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|#||Acceptable for use on organically grown produce.|
|1||Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. MODE-OF-ACTION GROUP NUMBERs are assigned by IRAC (Insecticide Resistance Action Committee).|