Monday, January 10, 2011

Pests and diseases: Spider Mites

Spider Mites | Photo courtesy of

Perhaps you've begun to notice some of those plants you've brought inside for the winter have started to show slight to moderate signs of declining vigor. On top of the declining vigor, leaves that were once healthy and shiny are now slightly curled. Dotting the curling leaves are pale yellowing flecks, increasing in amount by day.

Then, you spot it … the minute, dense webbing on the underside of leaves. Upon final inspection with a hand lens, miniscule straw-colored creatures with a dark blotch or spot on either side of their bodies are found. A conclusion has been reached: You've got an infestation of Two-spotted Spider Mites or Tetranychus urticae on your hands.

Life Cycle: This temperature-regulated species has its greatest activity during periods of warm weather; but can also be found making itself comfortable inside your home where the air is very dry and temperatures are warm at this time of year. The life cycle starts with eggs that hatch into tiny six-legged larvae. This stage is then followed by two, eight-legged immature stages, followed by the final transformation into the adult stage. These tiny pests have a "resting" stage, called the chrysalis, that precedes each molt. Males and females both occur in populations. Produced at only certain periods, males are smaller than the females. In the absence of a male population, reproduction is asexual.
These voracious pests feed with a piercing mouthpart, enabling them to suck out the sap of the phloem (inner cells of plant). Once the sap is depleted from each cell, it collapses.  It's this you see as yellow stippling on leaves, where feeding has taken place in clusters of cells.  Due to the manner in which they feed and the design of their mouthparts, two-spotted spider mites are very capable of transmitting various plant diseases. They are seldom held responsible as they are generally homebodies, and rarely leave a host plant of their own volition. 

However, dispersal of populations can occur through human and/or animal contact. Simply brushing up against an already infested plant will leave you walking away with four or five clinging to a shirt sleeve. Coming in contact with house or garden plant(s) shortly after gives the stealthy little mites an easy way into your home. Outside, as mites move their way up to tips of foliage, wind can blow them into air currents and subsequently onto new plants. Semi-dormant adult females overwinter under sheltering debris near previously infested host plants. The adult females appear reddish orange during semi-dormancy.  In spring, weeds and early greens crops provide food before later-season plantings are done.

Plants Affected: Roses, pears, raspberries, peppers, asters, marigolds, impatiens, geraniums, sage, and thousands of other agricultural crops and house plants.

Symptoms: Pale, yellowing stippling at feeding sites on leaves due to loss of cell contents. Plant vigor is seriously reduced as a population grows in size. Premature leaf drop begins as leaves shrivel and dry out. Dense silk webbing is visible when populations are high.

Controls: Although many populations of two-spotted spider mites show a high resistance to pesticides, there are some effective control methods out there. There are many biological pest control agents available on the market today. Green Methods, located in Nottingham, NH, offers several predatory insect species geared specifically towards the many different kinds of spider mites. Neoseiulus fallacis resemble tiny spiders with their eight legs, but are predatory mites that feed on the very young and egg stages. Shipped to you as adults and immatures, they work very well as both a preventative and control measure. Weather conditions, such as temperature and humidity, need to be taken into consideration before release of these beneficial insects. Make sure to follow enclosed instructions on storage and release to ensure optimal results.

 For those of you who would prefer to fight these pests chemically, there are some pyrethrins on the market that can be effective as well. Liquid Rotenone-Pyrethrin Concentrate is a product that we offer here at Johnny's. Another product we have available that can be helpful in controlling a population, that is safe, easy to use, and certified organic, is Safer Soap.

 With a base of Potassium Salts derived from fatty acids, this is a product that can be applied to infested house plants with very minimal risk to the rest of your household. Dish soap diluted at a high rate can also prove effective when applied with a clean spray bottle. As always, read label instructions carefully and follow all safe handling procedures and disposal methods as outlined in pesticide label instructions.

Resources and "The Ultimate Guide to Backyard Bugs Garden Insects of North America" written by Whitney Cranshaw
Article by Sonya Reynolds, Greenhouse Coordinator, Johnny’s Selected Seeds


Kathleen Stoltzfus said...

Would the chemical methods be appropriate for use in a greenhouse? What about salt buildup in the soil of a greenhouse? Would the Safer Soap be a problem with that?

Susan in MA said...

If you only have small plants, it is safer and cheaper to control them by simply washing the entire plant (especially the underside) with a strong spray of water every other day and keeping the humidity high with a humidifier and a tray of water.

Johnny's Seeds News said...

Hi Kathleen. In one word, yes it is appropriate to use the Safer Soap in a greenhouse situation. The safer soap is not a long residual pesticide, so traces should not build up enough to make any drastic changes in salt levels in the soil of the greenhouse. Fertilizers are a more common culprit in salt buildup of greenhouse soils. The Potassium Salts, which are the active ingredient in the Safer Soap, are derived from fatty acids. Basically, the potassium salts weaken the insects’ protective outer shell and is designed to be reapplied every 7-10 days. Another benefit is it is safe for those beneficial insects that we all want, including bees. If choosing to use this product, although it being very safe to use, follow label use and directions when handling it.