Managing Mite Problems

Managing Mite Problems

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

It might be mites, it might not.  Spider mites are often blamed for plants turning brown evenwhen they=re nowhere to be found on a failing plant.  How can you tell if your tree, shrub or garden plant has mites? 

What are Mites?

Spider mites are not insects.  They=re tiny arachnids related to the other arachnids that we know and Alove@, includingspiders, ticks, and harvestmen (daddy longlegs).  Like their relatives, spider mites have eight legs and lack wings, antennae and compound eyes.  Members of the insect family typically have six legs, antennae, compound eyes and wings.

Spider mites are visible, but extremely small (1/60th of an inch in length or smaller) and almost microscopic. One way to tell if a plant has mites is to tap a branch or some leaves suspected of having mites over a piece of white paper.  Invariably, little dust specks will fall onto the paper.  If after a second or two the little specks start crawling, it=s a good bet that they=re mites.

The other way to detect a mite problem is to look for their damage and other signs of their presence.  Spider mites don=t have chewing or piercing-sucking mouthparts. To access plant sap they pierce and rupture leaf cells with needle-like stylets.  They then suck up sap that seeps out of the punctured cells. This type of damage leads to the death of individual cells, giving the leaf a finely stippled appearance. As the damage increases a leaf appears discolored, then it turns yellow to brown and dies.  Some types of mites even inject a toxin when they feed which can lead to leaf distortion.

Another sign of a mite infestation is webbing.  The webbing is very fine and, depending on the type of mite, is often found on the undersides of leaves.  It shouldn=t be confused with more visible webbing made by spiders or the cottony masses from cottonwood and poplar trees.  However, it=s important to note that not all spider mites produce webbing... so you can have a mite infestation without webbing.

Mite Life Cycle

Knowing the life cycle and habits of a pest enables us to better manage that pest when it becomes a problem.  Because the two-spotted mite is the most common spider mite that causes problems for trees, shrubs, garden flowers, and vegetables let=s take a look at its way of life. 

The two-spotted spider mite, thus named because of the two dark spots on the back of the adult mite, start out as an egg.  The egg hatches into a larva that has only six legs.  This larva molts into a nymph with eight legs. The nymph molts into a larger nymph and then becomes an adult.  Both the larva and the nymphs resemble the adult.  This entire process takes about five to 20 days depending on the weather.  When winter comes, many types of mites overwinter as eggs. However, the two-spotted mite overwinters as an adult in the soil or in bark crevices on trees and shrubs.  They become active as soon as plant growth begins in the spring.

Mite Build-Up

During hot weather, two-spotted mite populations can build up quite quickly causing plant damage.  A female two-spotted spider mite lives approximately 30 days and lays about 100 eggs. One reason that hot weather leads to spider mite explosions is because of the low humidity.  With low humidity the excess water they excrete evaporates more rapidly.  This allows them to feed more heavily, favoring reproduction.  Another factor is that many of the natural mite predators are stressed by the hot, dry conditions and aren=t able to multiply quickly enough to keep up with the burgeoning mite population.

Hot weather doesn’t=t favor all types of mites. One very notable exception is the spruce spider mite that feeds on conifers, especially spruce and juniper.  This mite is most active during cooler spring and fall weather and actually goes dormant (in an egg stage) during hot summer weather.

Natural Mite Control

Spider mites do have some natural enemies.  They=re Adinner@ for predatory insects, such as dark- colored lady beetles, lacewings, predatory thrips, minute pirate bugs, and big-eyed bugs.  There are also a number of predatory mites that feed on spider mites and keep their populations under control especially when the weather isn=t extremely hot and dry.  One reason for spider mite outbreaks when the weather is not hot is that these predatory mites are killed off when insecticides are applied to plants.  They=re particularly sensitive to carbaryl (Sevin), but may also be harmed by other insecticides.   To keep spider mites in check, avoid using pesticides that kill off predators.

Controlling Mite Infestations

One cultural technique that helps keep mite populations in check is periodically syringing mite-prone plants with a forceful spray of water.  This forceful spray not only knocks off and kills the spider mites, but also dislodges the webbing that collects dusts and deters the natural predators.  It=s also important to keep plants from becoming drought stressed in hot weather.  Plants under stress are more vulnerable to spider mite attack.

If cultural controls don=t work, then pesticide sprays may be needed if significant damage is showing up on your plants.  Since mites aren’t=t insects, most garden insecticides are not very effective.  There are specific miticide chemicals for mite control, but they=re not readily available to home gardeners.  Two options that are available are insecticidal soaps and horticultural oils.

Insecticidal soaps are only moderately effective against mites.  For best results good coverage to both the upper and lower leaf surfaces is essential.  (Remember most mites are on the bottom of the leaves.)  The soap is only effective on the mites that are contacted with the soap spray.  Eggs are not harmed, so repeat applications are usually needed within seven to ten days in hot weather. Additional retreatments may be needed. 

During the growing season horticultural oils may be applied at the summer rate... but be sure to check the label for any hot weather precautions.  Horticultural oils may also be used on dormant plants in the spring at a dormant application rate for controlling overwintering adult mites and mite eggs. 

Spider mite infestations are not as common as some gardeners think.  I sometimes see infestations of spider mites on willows, marigolds, roses, and impatiens.  Spruce spider mites are occasionally a problem on area junipers and spruce, especially dwarf Alberta spruce.  I very seldom see spider mites on arborvitae or other landscape and garden plants.  So if your plants start to turn brown, check for stippling, crawling specks of dust, and very fine webbing on the undersides of leaves.  If in doubt, take a sample to your local Master Gardener Plant Clinic