SPIDER MITES

What are they? Life Cycle
Spider mites are so small that they are sometimes difficult to see without a magnifying glass, and individual spider mites are almost microscopic. Spider mites are not insects; they are closely related to spiders, harvestmen (daddy longlegs), and believe it or not ticks. There are many species of mites, yet the one most likely to attack the indoor garden is the 2 spotted spider mite. The two spots on their back, which can be seen under a magnifying glass, can identify them.

 Mites are about 1/50" long, approximately the size of a grain of salt or slightly larger. They may be red, black or brown or pale green. In the summer, the adults and nymphs are white with two greenish spots.

Mites are triggered mainly by the cooler weather and shortened daylight hours in the fall, some or most of the mites turn red in color, stop feeding and egg laying, and then crawl off to protected nooks and crannies to hide through the winter. These eggs or adults overwinter in growing mediums, crevices, in ground debris, or on equipment that hasn't been properly cleaned, emerging when the climate conditions are again favorable to them and plant growth.

A female lays about 100 eggs during her lifetime. Depending upon climate conditions, the eggs hatch in as little as 2-3 days, and the adult stage is reached in 7-10 days. The female’s reproduction is greatly affected by climate and humidity with a significant difference in the days till their maturity dependent upon the temperature i.e.) egg to adult. For instance at 60 degrees Fahrenheit, it takes 30 days for the egg to become an adult, at 70 degrees 14.5 days and here is the astounding number - 90 degrees Fahrenheit they reach adulthood in just 3.5 days! So now imagine those populations increases over a month when the offspring start to reproduce less than a week after hatching - at 70 degrees she and her offspring number 13,000; at 80 degrees she and her offspring represent a staggering potential of 13,000,000 individuals over a single month - huge population increases!

Mites evaporate large quantities of water from their bodies, so they must suck juices from the plants. This is easier for them to do in a dry environment. Humid environments (above 60% RH) slow down their metabolism, life span and reproductive rate. Mites may be controlled somewhat by lowering the temperatures (at about 50 degrees Fahrenheit they start to hibernate), thus slowing down their life process considerable.

Symptoms and Damage

Feeding marks are usually the first sign of a mite infestation and many confuse these as nutrient deficiencies. Spider mites lack chewing or piercing-sucking mouthparts. Instead they have a pair of needle-like structures that they use to rupture leaf cells. A feeding spider mite pushes its mouth into the torn tissue and draws up cell sap. Small patches of cells are killed, resulting in a stippling or fine flecking on the upper surface of leaves giving the leaves a "sandblasted" look, it also appears that the foliage is losing its green coloration.

As the pests spread over a plant they spin a spider-like webbing over and between the leaves generally at the stems, enabling them to move about the plant (and from plant to plant) more quickly. Mites tend to congregate on the leaves rather than the flowers or fruits of most plants.

Spider Mites will walk down stems, across medium or equipment and across dry spaces in search of new host plants in which to set up house! They are also great hitchhikers who cling to your pets' fur, or your clothing, looking for a ride to more yummy plants.

Immediate Action

When an outbreak occurs you must act immediately:
  1. Carefully examine plants - separate the infested plants from the uninfested plants. Separation is very important !
  2. Install physical barriers such as sticky tape around the plant rims of the plant pots to prevent further migration of mites.
  3. Don't forget to thoroughly wash your hands and change your clothing. When tending and examining your plants, tend to your infested ones last - do not go back and forth between the uninfested and infested ones.
  4. Be sure to remove any fallen debris to a plastic bag and immediately dispose of to the outdoor trash. 5) Wash down your plants with a forceful jet of water (but not so forceful it will damage the foliage), be sure to spray the underside of the leaves - sometimes the water will knock them off and kill them.
  5. Raise the relative humidity above 60% (the wash will help do this).
YOU NOW HAVE TO MAKE THE DECISION ON HOW TO HANDLE YOUR OUTBREAK
  • Biologically by naturally using "good bugs"
  • Insecticidal Oils / Soaps or "Soft" Pesticides such as natural pyrethrum
  • Chemical produced miticides
Chemically

Resistance to chemicals has increased the difficulty of controlling of these pests. Spider mites can adapt to most chemical sprays within days if they are not directly hit with the initial spray. After that, they will just thumb their noses at you.

If using chemicals - use a miticide that has the least damaging effect on the environment, (natural pyrethrum and stronger synthetically produced permethrin (such as SMC) have been shown to have the best miticide abilities.

Because mites primarily occur on the undersides of leaves, applications of miticides need to be directed at both the lower and upper leaf surfaces.

Soaps / Oils / Soft Pesticides

These soaps kill the mite by penetrating its protective outer shell, disrupting their respiratory system and causing damage, which in turn causes them to dehydrate and die.

Insecticidal soaps must be sprayed directly on the pest in order to kill it, and once evaporated are no longer effective. When applied properly, insecticidal soaps have very little to no toxicity to humans, animals, or most beneficial insects. Most soaps such as Safer's can be used right up to the day of harvesting - just be sure to wash any fruits or foliage that you are going to consume. Residual is very short - once it has evaporated off the plant.

Insecticidal oils can also be used sparingly. These oils are a highly refined mineral oil that kills dormant adults and their eggs by suffocating them. Apply it early in the season to kill eggs, but not on very young plants - make sure that they are diluted according to package directions. Caution: continued use may clog up the plant's follicles causing further problems.

Neem Oil is a completely safe, non-toxic, biodegradable substance made by crushing the seeds of the Neem tree which has natural insecticidal properties. Neem oil makes the plant unpalatable to pests, it does not kill them but affects their behavior and physiology.

If you must quickly "decimate" a large mite population, it is practical and efficient to use an organic Pyrethrum in oil, sprays or insecticidal soaps.

"Pyrethrum" refers to the powder made with the dried flowers of the chrysanthemum, whereas the term "pyrethrins" refer to the six insecticide components occurring naturally in the powder. The effect of pyrethrum is immediate. Insects are paralyzed on contact. If a very low dose is used you need to take care that the mites are dead and not simply stunned to recover several hours later. One of the added benefits of Pyrethrum is that it has a much greater "flushing out" effect than any other insecticide; it disrupts pests from their hiding places forcing them to get out and to get exposed to the insecticide. Pyrethrum is effective against a large number of adult insects, but it is much less effective against larvae. It is not harmful to humans and animals, but is to fish and other aquatic organisms.

Biological Control using "good bugs."

One of the best controls for spider mites is a predatory mite-Phytoseiulus Persimilis. Persimilis is a red pear-shaped mite with long legs. An adult Persimilis will eat 5-20 prey (eggs or mites) per day. What makes them so effective against spider mites is that they reproduce more quickly at temperatures above 82F than the spider mite, and they feed on all stages of the pest spider mite. However persimilis must have high humidity temperatures-above 60%, which also affects the pest spider mite to reduce their egg laying. Persimilis are very voracious, and have one of the highest consumption rates. Because these mites are such efficient hunters, they can cause extinction of their spider mite prey, which is extremely desirable where little or no spider mite damage can be tolerated, such as in ornamental plants. Typically, persimilis will eventually exhaust their food supply and starve, and so it must be reintroduced when new spider mite infestations occur.

Neoseiulus californicus is a predator almost identical to persimilis but will tolerate warmer temperatures from 55? - 90? F., and greater humidity fluctuations. As well, they can starve for a longer time than persimilis, therefore they don't need to be reintroduced as often as persimilis would. This predator is often best for houseplants, and indoor gardening.

Another excellent predator for spider mite is the predatory midge, Feltiella Acarisuga. This midge larva is about 1/12th of an inch long and is a pinkish brown color. The female Feltiella will lay its eggs inside the spider mite colony and as soon as the young feltiella larvae hatch they begin their feast on spider mite eggs. The young larvae feed mainly on the eggs while the older larvae feed on all stages of spider mite. A larvae feltiella can feed on about 50 spider mites before pupating. The lifespan of the feltiella from egg to adult is 10-15 days and the female will lay about 12-14 eggs. Like Persimilis, when the food supply is exhausted they will also starve and thus must be reintroduced when new infestations occur. Feltiella will work more efficiently in much lower humidity and a broader temperature range.

Prevention

Last but not least, the importance of preventative measures

  1. Before planting or transplanting be sure disinfect pots with a 10 percent bleach solution, i.e., 1 part bleach to 9 parts water.
  2. Removal of debris and an early drench of insecticidal soap in areas where they were a particular problem last year can also help - but remember insecticidal soap will only kill on direct contact by breaking down the insects protective coating and causes them to dehydrate and die.
  3. If you have had continued problems in the past, it is very likely as soon as conditions are optimal that you will have problems again - then you may want to consider fumigation.
  4. Should you choose to fumigate before you start your indoor season, use products that are the least harmful to the environment. Be sure if you do this, that it is done prior to planting, as it is non-selective and will kill naturally occurring beneficial organisms.

You must remember that certain insecticides, especially broad-spectrum insecticides, whether natural or synthetic, kill beneficial organisms as well as spider mites. This can sometimes allow the spider mites or secondary pests to reappear/appear and become an even worse problem because their natural controls are gone.

Spider Mites - Back to Top
Last Updated: December, 2006
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