Controlling Spider MitesAndy Crossland
Adams County Master Gardener According to Greek mythology there was once a girl named Arachne, who had great talent as a spinner and weaver. But she was arrogant about her talent and challenged the gods to a contest. During the contest she proved to be better at the loom than the goddess Athena, who became jealous and angry. She attacked Arachne and destroyed her work. In humiliation, Arachne hung herself. The goddess took pity on her and used a magic potion to change her into a small creature with many legs. The rope with which she hung herself was turned into a silken thread. It was with this thread that she was permitted to continue her spinning and weaving. Thus the spider was created.
It is from Arachnes name that we get the word arachnid. If you have six legs youre an insect, but if you have eight legs youre an arachnid. Spiders, scorpions and ticks all belong to the class Arachnid. So do mites.
There are many species of mites. The two-spotted spider mite Tetranychus urticate is the most pervasive of these little arachnids and has become a worldwide pest. It destroys crops and trees. It destroys houseplants, plants in greenhouses and ornamentals in the garden. Two-spotted spider mites are very small. You will need a magnifying glass to confirm their identity. Overwintering females are reddish orange but, typically, depending on the environment and what they are eating, males and females are dark to pale green or even a translucent yellow. The two spots are distinctive. There is one dark spot on either side of the abdomen. These arent really spots. You are actually looking into the creature and seeing its food being digested. The food is made of plant juices.
The first sign of plant damage caused by these mites is a stippling pattern on the leaves. Look for a series of small dots that are brown. This is where the mites have pierced the leaf to get at the juices. Leaves under severe attack will turn either a yellow or bronze color. They may also begin to curl as they dry out. Once the damage reaches a certain level, there will be no recovery. Photosynthesis is halted. Carbon dioxide can no longer be processed. The leaf will eventually drop off. If enough leaves are attacked, the plant will die. The two-spotted spider mite lays its eggs amid a mass of white webbing on the underside of the leaves. This webbing is made of silken threads that the mites also use to get about the plant. If left unchecked, they will eventually wrap the entire plant with these threads, making it look like a Halloween decoration.
The adult mites needs to get rid of excess moisture on a constant basis. This is done through evaporation. Mites are most active when temperatures are warm and humidity is low or moderate. Under these conditions mites eat more and reproduce more often. If it gets very humid and very hot, above 104 degrees, they suffer the lethargy that we all tend to feel under such conditions. They eat less and reproduce less because the rate of evaporation is diminished. A strong winter will bring their activities to a halt. In a mild winter they slow down but may still continue to eat and reproduce. And in the warm home or greenhouse, they wont slow down at all.
The success of the two-spotted spider mite is due to several factors. First, reproductive rates are astonishing. One female can lay up to 200 hundred eggs over a ten-day period. In as little as five days, an egg can develop into a mature adult. With seven or more generations being produced during the summer months, the population can obviously get out of control very quickly. As a group, the two-spot is made extremely durable by its genetic make up. Females develop from fertilized eggs and have two sets of chromosomes. Even if they dont mate, females still give rise to males, but only males. Females that do mate can produce both males and females. Males have only one set of chromosomes. This means they are highly susceptible to mutation. They adapt, and quickly, to changes in their environment. For instance, some of the chemicals that were once lethal to them are no longer effective. If we spray with these chemicals now, and they happen to kill enemies of the spider mite, then the mite population is free to expand. And expand it does. What is to be done?
Mechanical Control
If you find spider mites are damaging your plants, isolate those plants. This should be done because of the numerous means of travel available to the mite. The air is never still. Variations in temperature cause it to move. The two-spot is so light that it uses these variations as a means of transport. They drop down from the leaves on strands of silken thread. They are then carried by the moving air to neighboring plants, much like Tarzan moves from tree to tree on vines. Just walking by a plant we cause the air to move and thus provide the mites with express transport. Also, they have eight legs, so walking is not a problem. They may just drop to the ground and walk to a new home site.
So, first: isolate.
One means of control is to use a stream of cold water from the hose to knock the mites from the plant. This can be effective. But be careful: if the stream of water is too powerful, you may damage the plant. And be sure to spray the underside of the leaves. Also move the plant away from its usual resting place before setting the hose on it. The water from the hose may kill a few through drowning, but it is mostly a means of removal. The mites will want to climb right back up onto the plant once the hose is put away. Spraying the plant with ice water every day for a week, again being sure to spray the entire plant, especially the undersides of the leaves, has been reported to eliminate infestation as they are inhibited by cool, damp conditions. Cold water or ice water creates an unfriendly environment, especially when applied on a regular basis. Actually, before pesticides were available, the cold-water treatment was the main means of control.
Biological Control
If you have mice in your house and you get a cat to take care of the problem, then you have just practiced what is known as biological control. You have introduced a predator into the unwanted population. You have no mice anymore. But you do now have a cat. This is the danger of biological control. A new concern may arise from the species that has been introduced. It can exemplify the adage the cure is worse than the disease. With regard to the two-spot there is a means of biological control that has proven effective and, surprisingly, the introduced predator, takes care of itself. It is called Phytoseiulus persimilis.
This is truly a case of mite versus mite. Phytoseiulus is a mite, but it doesnt feed on plants. It eats mites. Once they have eaten all the two-spotted spider mites, they turn on themselves. They eat each other. Voila! The mice are gone. And so is the cat.
Chemical control
WiltPruf is a chemical available at most garden supply stores. It is designed to be used during times of drought. It slows evaporation of water through plant leaves by covering the leaves with a thin film. This is harmless to the plant when used as directed. Try spraying the mites with this product. The theory here is that since the two-spot is so dependent on evaporation for its survival, a coating of Wiltpruf prevents that evaporation and the mites die. It does not kill the eggs. Apply three times at five day intervals.
UltraFine is a highly refined petroleum oil spray that will kill both the mites and their eggs. Repeat the application in ten days or so in case you missed eggs or mites on the first application. Remember: get the underside of the leaves as well as the top sides and use caution on soft-leaved plants as foliage damage can occur.
Rose Defense Spray by Greenlight is a botanical oil from the seed of the neem tree that smothers the mites but is gentle on the plants.
Isopropyl alcohol and water at a 50% dilution with a few drops of dishwashing detergent to a quart is an effective mitecide. Its best to spray it in early morning or late in the day when evaporation is slower. If possible repeating the application 15-20 minutes later will ensure complete control.
Weve found no damage to foliage even with repeated treatment which can be done at a 4-5 day interval in case a few stray mites were missed. Avid is another well-known chemical that will kill the mites. Be careful. Follow the instructions on the label. If you cant find any of these chemicals and you decide to try something else, just remember that these critters have eight legs. They are not insects. You are looking for a miticide.
If you are having problems with these mites, or have questions about other aspects of plant care, please call you local Master Gardener Office.
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