Identifying Spider Mite Infestation February 25, 2007

Today I’d like to talk a bit about spider mites.  Spider mites are very tiny mites that live on the underside of leaves and eat them, causing damage and necrosis to the leaves.  In the photos I have posted here you can see the damage that these little guys can do.  First off, some gardeners mistake the damage to the leaves as a sort of fungus or mildew as it can look a bit like leaf spot or powdery mildew.  If you have a spider mite infestation you will notice that the top side of the leaf appears browinsh to silverish in color with slight mottling or “little dots”.  The easiest way to check if this is truly a spider mite infestation opposed to fungus is to take a piece of white paper and place it under the leaf, shake the leaf a bit, and then check the piece of paper.  Spider mites are compelled to be on the underside of a leaf so they will immediately set off running toward the edge of the paper to get underneath it.  Another thing to check is whether or not you notice tiny spider webs on the plant.  Keep in mind that they are very small, not quite microscopic but you may need a magnifying lens to verify this.  The two most common spider mites that do damage are the red mite and two spotted mite.  The red will appear reddish in color and the spotted have two distinct dots on their backs.  Spider mites do tend to stay in place, they do not move or migrate much from where they are born unless they are “vectored”.  Vectoring means that the mites have come in contact with something that moves (you, your clothes, your pets, birds, etc. that they can hitch a ride on to get to another area) to infect another plant or area.  This means that if you have handled a plant or tree that has been infected it is best to wash your hands, change clothes, etc. before handling an uninfected plant.  Most infestations begin in late spring and continue on until the end of summer as they prefer warm to hot conditions.  In winter thay will usually “overwinter” in grassy areas where they can stay warm.  They also like indoor plants and plants in greenhouses during winter because those conditions are usually warmer.  That seems to be the case with the few plants in and around the greenhouse that have been infected (see photos above).  If the infestation is slight, a good blast of water on the underside of the leaves may work.  Heavier infestations can be controlled with Safer Soap, a non-toxic soap treatment.  If you are going to use Safer Soap to control your infestation I would suggest that you try it on a few leaves of one plant to make sure it is not “phytotoxic” (i.e. won’t burn or kill the plant that you are treating).  Application of Safer Soap must be done to the underside of the leaves as it kills on contact.  Treatment also must be applied more than once, preferably 3 treatments 5 days apart as the soap kills the mites but not their eggs (their life cycle is only about 10 days).  If this does not work and you absolutely have exhausted all other methods, applications of pesticides such as Orthene or Avid can be used.  Keep in mind that pesticides are only a short-term solution because if you continually use them the spider mites will become tolerant, and further applications will no longer kill the spider mites.  Another solution is to use predatory mites, which can be purchased at a garden supply shop or farm supply shop.  The predatory mites will eat the spider mites without infesting your plants and is a much better alternative to pesticide use.  Also, if you are going to use predatory mites it is best to use them at the first onset of spider mites before they have done much damage.  A final method to limit the spread of the spider mites is to “quarantine” infected plants while you are treating them, which is what I’ve done.  I noticed a few oaks and redwoods with a minimal infestation in my greenhouse and immediately moved them outside and away from the greenhouse while I’m treating them; now they are much less likely to infect the rest of my plants and trees.  After I’m sure the infestation is gone I will move the previously infected plants back into the greenhouse.  Questions are more than welcome.  Happy Gardening!

Plantman