Controlling Spider Mites

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The Spider Mite

The spider mite is truly a relative of the spider and therefore not classified as an insect.  They are most prevalent in your garden during hot, dry weather.  In greenhouses, however, the spider mite can be a continuous pest.

It is important to recognize the early warning signs of spider mites before they get out of control.  On roses and other woody ornamentals, the first signs will be a lightening or purplish yellow color change to the upper side of the leaves.  This can even be misdiagnosed as downy mildew damage, but a quick look at the underside of the leaf will make all the difference.  Spider mites also attack many annuals and perennials.

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Crystalline structures note the presence of mites

These mites are so tiny that it may be hard for you to see them without magnification.  But if you have good eyesight or good bifocals you will see them on the underside of the leaves.   It’s here that they puncture the plant cells and suck the moisture from them.  It’s likely you may also see tiny crystalline structures as well as some webbing in extreme cases.  They are very active creatures and can quickly defoliate a plant.  At the least, these can setback outdoor plants by six weeks.  In greenhouse plants, whole crops can be lost.

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Spider mite webbing

Spider mite eggs can hatch in as little as 3 days, and new mites can begin reproducing in only 5 days. One female can lay up to 20 eggs per day and can live for up to 4 weeks.   Think about that.  She is laying hundreds of eggs meaning one female mite could start a population of a million mites in a month or less!   This incredible rate of reproduction is one reason mite populations quickly build resistance to pesticides, so chemical control methods can become somewhat ineffectual when the same miticide is used over a prolonged period.

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Visible mite damage on rose leaves.

Since insecticides will not kill mites, let’s discuss how to rid your plants of this pest.  If you only have a few outdoor bushes, you may want to remove the mites mechanically with a water blaster.  Most water wands have a removable nozzle that you can replace with a blasting nozzle.  Set the nozzle to a wide pattern but don’t be too conservative.  You will need a good deal of pressure.  Simply point the nozzle upward and blast those mites off the underside of the bush.  In most cases the mites will find an easier target.

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Mites on leaf underside.

If the mechanical method is not for you, or if you are growing in a greenhouse, you will need to use a miticide.  Miticides have come a long way over the past 15 years and thank goodness there are many to choose from currently.  We already discussed how rapid reproduction leads to pesticide resistance.   (This is very important to acknowledge.)  Almost every miticide label clearly states you should not use the product more than three times a year.  For that reason, greenhouse growers and gardeners with many plants will need to rotate two or more miticides in their spray program.

When choosing a miticide, be sure to read the label for products that are approved for your particular crop or cultivar.  Some miticides only kill adult mites and this means you would need to spray them twice (initial spray and then three days later) to kill both existing adults and newly hatched mites from eggs.   Most modern miticides have incorporated what is called an “ovicide” into their formulations.  This means they will kill all stages of mites, including the eggs.  These are preferable and most of these newer products continue to work for up to 21 days!  Examples are Floramite, Forbid and Shuttle but there are others that are as good or better.  There are even a few miticides that are “Translaminar”.  This means that the miticide, while sprayed on the top of the leaves, will penetrate through the leaf tissue and to the underside of the leaves.  This can be a big help where it is hard to reach the underside of leaves.  In almost every case I recommend the use of a good spreader-sticker (surfactant) with your miticide application.

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Mite life cycle

Let’s discuss how your plants get mites in the first place.  For outside gardens they are simply part of the environment and in hot dry climates you will almost always deal with mites.  It’s important to remember that there are many natural predators of spider mites and if you do your best to encourage these (ladybugs, praying mantis, green lacewings etc.) you may be able to eliminate the need for expensive miticides.   One way to encourage the good insects is to simply refrain from spraying insecticides.   Most insecticides are not selective and kill all insects, even the ones we want in our garden so use these sparingly.  If you are growing in a greenhouse environment, you will most likely deal with mites on a continuous basis and a good miticide rotation will be a must!

Finally, be sure to put newly purchased plants or cuttings in quarantine to be observed at least one week for disease, insect, and mite problems.  I get many calls each week from growers who discover their favorite miticide no longer works.  What I usually find is they have brought new plants or cuttings into their garden or greenhouse without quarantine.   The strain of spider mite on the newly purchased plants were already immune to some miticides due to the previous growers’ spray program.   Now the new grower has inherited a generation of mites that is resistant to their miticide of choice.