This is the time of year when it has started to frost and many gardeners have moved tender plants indoors to overwinter them. Be it inside a greenhouse or your sunny south window, this is also the time of year that spider mites love to sneak up and overtake the unsuspecting indoor gardener. If you have ever grown susceptible houseplants, plants in a greenhouse, or even highly susceptible plants like Passiflora outside, then you are probably familiar with this really obnoxious little pest. Spider mites are one of the most common garden pests around.
If you haven’t encountered this incorrigible little pest, you should count yourself a lucky gardener, and should still learn to spot the signs of an infestation. As with many pests, getting rid of spider mites when they first show up is a lot easier than managing a large infestation. Spider mites can be extremely difficult to get rid of. In fact, the best course of action for a severely infected potted plant (particularly a houseplant) is to discard the plant.
What is a spider mite?
As their name implies, spider mites are not an insect, but instead an arachnid closely related to ticks and spiders. (This is an important detail to remember when we get down to how to control them.) Spider mites suck plant sap and can severely damage or kill a plant if left uncontrolled. If you do not take measures to control a spider mite infestation, you will at the very least end up with a very unsightly plant that will quickly spread its plague to other susceptible plants in the area. At worst it and all the neighboring plants will end up dead.
While biology and control is the same for all of them, there are actually many different types of spider mites. The most common one is the two spotted spider mite. You will not be able to see the spots without a magnifying glass as these mites are very tiny. But you can see the mites with the naked eye if you know where to look and look carefully.
How can I recognize spider mites on my plants?
You will need to look very carefully at the undersides of the leaves. Spider mites don’t often go on top of the leaves unless the infestation is advanced. If you look at the backs of infected leaves you can typically see tiny little orange-red or sometimes green dots moving about. Tiny dots the size of the dot on this ‘i’ or smaller.
Their damage is typically much, much easier to see than the mites themselves. Leaves of plants with spider mites feeding will have little yellow dots on them. Leaves will yellow and eventually fall off. If the infestation is severe the back of the leaf, or even the whole plant can be covered in ‘spider’ webs. These webs help to protect the spider mites. They make it difficult for you to effectively spray them with contact insecticides. It however can also can make identification of this tiny little pest easy. If you have webs you know you have spider mites not aphids or other common pests.
Spider mites thrives in warm, dry environments. Sounds kind of like your house…or a greenhouse. And it is true, these environments are the PERFECT place for them to thrive. These mites are tiny tiny, and like to hide out on the backs of leaves. This makes them very easy to miss. Often times they are so sneaky, you don’t notice them until there is a very large population present. Plus in these locations, there are no natural predators present to help keep them in check.
You should check your plants regularly
If you are growing a susceptible plant (Passiflora, beans, raspberries, and many houseplants are highly susceptible), it is worth your while to get into the habit of frequently inspecting your plants for signs of infection.
This is particularly important if you put plants outside during the summer and then bring them inside in the fall. They will often bring pests inside with them. Outside a few spider mites weren’t a problem as there was always a lady bug or other predator nearby to eat them, but inside those few mites that rode in on your plant can become a major problem in a month’s time.
Under warm dry conditions (like in that cozy living room of yours) they can complete their full life cycle in less than a week. This means they can get out of hand FAST!
So what is a discerning gardener to do about this terrible pest?
First off, monitor your plants. This is particularly important when transitioning plants indoors for the winter. Make it a habit to take a quick glance at the backs of a few leaves when you water. Its essential to turn leaves over to check the backs, particularly where the petiole (leaf stem) is attached. This is where spider mites prefer to hide.
Also, avoid spraying pesticides unnecessarily outdoors, particularly broad spectrum ones. This can inadvertently kill off beneficial insects and cause outbreaks of pests such as spider mites. This is probably the most common reason for a severe outdoor spider mite problem. Typically there are enough natural spider mite predators in an outdoor garden to prevent spider mite outbreaks. However, frequently spraying pesticides in your garden will kill all the beneficial insects causing the spider mite population to explode. Why? Because of a seemingly unimportant, but key detail briefly mentioned earlier. Spider mites are arachnids not insects. The commonly use broad spectrum insecticides do not kill them, but they absolutely do kill everyone who wants to eat them.
Purchased spider mite predators can help
There are many natural predators for spider mites. Some of which are even available for purchase to release in your garden. If you have ongoing spider mite problems in your garden, consider purchasing spider mite predators to release. You must select species that fit your situation and climate, and you must cease making frequent pesticide applications so that they can establish a breeding population. In the absence of pesticides spider mite predators can and will establish populations in a garden preventing future outbreaks. Biological control of pests is a topic we will explore further in a future blog post.
Other things you can do to help include spraying the plants with water. Remember spider mites thrive in hot, dry, low humidity environments. Increasing humidity can help make the area less desirable. Additionally stressed plants, particularly water stressed plants are more susceptible to being attacked and less able to withstand a spider mite attack, so keeping plants well watered, fertilized and rapidly growing can help.
If you are unfortunate enough to have spider mites show up on a houseplant, isolate it from other plants, and consider discarding it if it is severely infested. If you don’t wish to discard it, or the infestation is mild, remove and destroy severely infested leaves, and leaves that have webbing on them if at all possible. The webbing protects the mites from your attempts to kill them. Removing these web covered, severely infested leaves and destroying them will give you the best chance of saving the plant.
So how do we kill these menaces?
If you haven’t succeeded in preventing or controlling an outbreak with other methods, it is time to consider using a pesticide.
However, before you run to the store and grab the first pesticide you see. Remember that most pesticides do not kill spider mites. As mentioned above most severe spider mite outbreaks FOLLOW the application of pesticides because they kill natural predators but not the spider mites. In fact some very common pesticides like Seven (carbaryl), pyrethroids, and organophosphates have been shown to actually increase the reproduction rate or favor treated populations. Severe outbreaks can occur within days of treating with these chemicals during hot dry weather.
What pesticide should I use?
Whether your plants are inside or outside, we would recommend using the same treatment. Our recommendation would be to use an insecticidal soap or horticultural oil. If your plant has obvious signs of spider mites being present, you will likely have to treat it several times before they are completely killed. You may also achieve control by using spider mite predators. However, if you have a severe infestation it is better to treat the plants with Neem Oil to lower the mite population prior to releasing predators.
There are some other chemicals available that are labeled for spider mite control. But, commercial growers have discovered that spider mites are good at becoming resistant to chemicals quickly, so it is best to use an oil to kill them. OIls are also typically less toxic, safer, and more easily accessible to home gardeners.
Please keep in mind before you spray that oils can burn plant leaves. Do not spray on a water stressed plant, on a hot day, or in direct sun. In hot weather, it is best to spray on a cloudy day. If you can’t do that spray in the morning/evening so that the leaves will be dry before the sun gets onto them. This will prevent the sun from burning your plant with the oil. It is also a good idea to test on a few leaves before spraying the whole plant if you are unsure of how the plant reacts to being sprayed with a horticultural oil.
Our pesticide of choice is Neem Oil. It is a plant extract and considered organic. It is is quite safe for humans, pollinators and the environment. It also works well to kill spider mites with the caveat that thorough coverage is essential. You must spray the leaves front and back covering every inch of the plant. It needs to contact the spider mites to kill them. You will likely need to repeat applications to completely clear up an infestation.
As with any pesticide, please be sure to read the entire label and thoroughly follow the directions. We also encourage you to consult with local extension agents if you have questions about using pesticides.
Key points to remember
Spider mites are an aggressive plant pest that can disfigure or kill your plants if left unchecked. They are sneaky little invaders who love indoor conditions and can hide from you until their population has gotten huge.
They are arachnids NOT insects and cannot be killed by most pesticides. An outbreak can be caused or worsened by common pesticides such as Seven.
You can control them using predatory insects, cultural methods, and organic pesticides such as Neem Oil.
Now that you’re familiar with spider mites, and the havoc they can cause, you’re fully prepared to prevent them from invading your plants!
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