 Top: Two-spotted spider mite damage on marigold. Right: Two-spotted spider mite adult.It is time to be looking for the presence and damage caused by the two-spotted spider mite, Tetranychus urticae. This is a warm-season mite that is a problem during summer, with reduced development time and increased reproduction. The life cycle (from egg to adult) takes just seven days at 84ºF, and females can lay up to 200 eggs during their 30-day lifespan. Below are a series of questions and answers regarding damage, scouting, and management of two-spotted spider mite populations.
1. Why are two-spotted spider mites so difficult to manage or suppress?
There are a number of reasons why these populations are hard to manage including:
- They have a wide host-plant range.
- There is high female reproductive potential.
- The mites have a short life cycle.
- Multiple generations occur simultaneously.
- They have a rapid population growth rate.
- The potential for resistance to miticides.
2. What are the typical damage symptoms of plants infested with two-spotted spider mites?
These mites feed primarily on leaf undersides. They use their stylet-like mouthparts to pierce and feed on individual plant cells. Damaged leaves will appear bleached and stippled with small silvery-gray to yellowish speckles. In addition, fine-mottling may appear on the upper leaf surface. Leaves heavily-infested with mite populations may appear bronzed, turn brown, and fall off. These symptoms may vary depending on the host plant.
3. What is the best way to scout for two-spotted spider mites?
Since these mites do not fly, they won’t be captured on yellow or blue sticky cards. It is important to visually inspect plants regularly, using a 10x hand lens, to look closely at leaf undersides for the presence of mite eggs, larvae, nymphs and adults.
Focus scouting efforts on those plants that are susceptible to mite infestations. Also, be sure to check hanging baskets routinely. These plants are located in areas where the air temperature is warmer than the rest of the greenhouse, which is conducive for mite population development. Furthermore, as mite populations increase and plant quality declines, mites will fall onto any plants located below hanging baskets.
 Mottling of upper leaves caused by two-spotted spider mites.4. What cultural practices should be implemented to avoid mite outbreaks?
Avoid over-fertilizing plants, especially with nitrogen-based fertilizers, which results in the production of soft, succulent tissue that is easier for mites to penetrate with their mouthparts. In addition, well-fertilized plants tend to have higher mite populations.
Remove old plant material, which may serve as a source of mite populations when the next crop is started. Avoid water stressing plants because this increases susceptibility to mites. Remove heavily-infested plants and weeds from within and around greenhouses because weeds, including the nightshades and creeping wood sorrel, are secondary hosts of these mites. Irrigation practices, such as overhead watering, may reduce mite populations by washing them off of the leaves.
Quick Control Tips:
The key to managing/suppressing two-spotted spider mite populations include:
- Obtaining thorough coverage of all plant parts with miticide spray solutions, especially leaf undersides.
- Spray applications need to be performed at intervals between five and seven days, especially when using miticides with contact activity, to kill those life stages that were not inhibited from previous applications.
- Conduct spray applications when the susceptible life stages (e.g., larva, nymph and adult) are present.
- Always use the appropriate label rate or rates.
- Rotate miticides with different modes of action to reduce the potential of mite populations developing resistance.
- Use potassium salts of fatty acids (insecticidal soap), petroleum-based oils, and/or clarified hydrophobic extract of neem oil in rotation with other miticides to avoid resistance developing in mite populations. These active ingredients have activity on most life stages of the mites including eggs, and are less prone to resistance development.
Additional Tips
There are a number of commercially available predatory mites including Phytoseiulus persimilis, Galendromus occidentalis, Neoseiulus californicus and Neoseiulus fallacis. If used properly, any of these natural enemies can provide effective suppression and regulation of two-spotted spider mite populations. Biological control can be very effective. However, in some cases (when mite populations reach damaging levels), natural enemies won’t be able to suppress mite populations at sufficient levels to prevent aesthetic damage.
Some predatory mites are sensitive to certain miticides. Consult with your state extension entomologist or biological control supplier regarding information on release rates, timing of releases and other ways to maximize the effectiveness of the predatory mites.
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