Spider Mites

Spider Mite Damage (left); Spider Mites - click to enlarge (right)

Description:
        Spider mites are not insects but are more closely related to spiders.  These arachnids have four pairs of legs, no antennae and a single, oval body region.  Most spider mites have the ability to produce a fine silk webbing.  Spider mites are very tiny, being less than 1/50 inch (0.4mm) long when adults.

        Spider mites have tiny mouthparts modified for piercing individual plant cells and removing the contents.  This results in tiny yellow or white speckles.  When many of these feeding spots occur near each other, the foliage takes on a yellow or bronzed cast.  Once the foliage of a plant becomes bronzed, it often drops prematurely.

        Heavily infested plants may be discolored, stunted or even killed.  Web producing spider mites may coat the foliage with the fine silk which collects dust and looks dirty.

        All spider mites go through the same stages of development.  Adult females usually lay eggs on their host plants.  The eggs hatch in days to weeks into the first stage, called a larva.  Larvae are round bodied and have only three pairs of legs.  The larvae feed for a few days, seek a sheltered spot to rest and then molt into the first nymphal stage.  The first nymph now has four pairs of legs.  The first nymphs feed a few days, rest and molt into the second nymph.  The second nymphs feed, rest and molt into the adult stage.  The males are usually the size of the second nymph and have pointed abdomens.  The females have rounded abdomens and are the largest mites present.

Prevention:
        Early detection of spider mites, before damage is noticed, is important.  The tiny spider mites can be detected by taking a piece of white paper or cardboard and striking some plant foliage on it.  The mites can be seen walking slowly on the paper.  If 10 or more mites per sample are common, controls may be needed.

        Hard jets of water can often knock these bugs off the plant.  This is good to reduce their number on the plant, but it can spread them to other plants.  Isolation of infected plants is recommended.

Defeating with Sprays:
        Most spider mites can be controlled with insecticidal oils and soaps.  These are not difficult to kill, but they do require a thorough coverage of the plant with the oil or soap.  Repeating this for several weeks is necessary.

Predators:
        There are numerous insects (lacewings and lady beetles) that prey on spider mites.  However, the most commonly sold predators are other types of mites.  Predatory mites (usually Phytoseiulus spp., Amblyseius spp. or Metaseiulus spp.) can be purchased and released onto infested plants.

Spider mite Sources:
http://www.ext.colostate.edu/pubs/insect/05507.html
http://ohioline.osu.edu/hyg-fact/2000/2012.html

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