Spider mites are one of the most destructive pests around when it comes to greenhouse and indoor cultivation. Their characteristic webbing can seem to appear all over your plants in a matter of just a few days. This webbing helps them form colonies and protects them while they drain the life from your plant's leaves and stems.

These insects are tiny, but their destructive potential should not be underestimated.

Spider mites

What are spider mites?

Just as their name suggests, spider mites are related to spiders. They are in the arachnid family and share similar characteristics. Just like their spider brothers, they have eight legs and spin webs. Spider mites use their webs to build bridges and large networks between colonies to travel all over the canopy of your plants.

There are many different kinds of mites, and all are actually related to spiders in this way, but it is because of the tendency for spider mites to spin large networks of webs that give them their common name.

Spider mites (Tetranychidae) consist of about 1200 species. A few hundred of these species are well known horticultural pests. They feed by sucking the juices directly out of the leaves. Although spider mites are tiny, they form large colonies very quickly and can destroy a crop in as little as 2-4 weeks.

How to identify Spider Mites

Identifying spider mites is easy, seeing them, however, is difficult. They are less than a mm in length and width making them very hard to spot with the naked eye. In most cases, the first signs of spider mite infestation are actually their resulting leaf damage. The leaves will begin to turn yellow and develop a spotted texture. These light colored spots are the feeding sites where a mite has previously sucked the juices from the plant leaf. In more advanced infestations, webbing will be seen forming around the nodes of the leaves and flowers. In a late stage infestation, webbing can be found all over the tops of your plants, and you will see thousands of tiny spider mites running around the leaves and webbing. Leaves will be seen yellowing and dying in large numbers at this point.

Spider mites come in a variety of colors including red, black, and white.

The Life Cycle of a Spider Mite

Spider mites begin their lives as tiny eggs hidden in the bark and on the underside of leaves. They usually overwinter during this stage and come out in the spring as the temperatures warm up.

After hatching, they begin feeding as tiny six-legged larvae. After just a few days they molt and enter the first nymphal stage. These nymphs have 8 legs and more closely resemble adult spider mites. After 2 more molts, they become adult spider mites where they go on to reproduce and start the process all over again.

Spider mites feed at every stage of development (except egg of course). This means that all stages can damage crops. The adult stage it the most mobile, and is the longest living stage.

After mating, female spider mites lay up to 300 eggs in a matter of 2-4 weeks. It’s very easy for a spider mite population to grow out of control in a very short amount of time, especially in warm and dry temperatures. Temperature has such a profound impact on spider mite development that in their optimal warm dry conditions they can double their rate of growth, resulting in just 5 days time from egg to adult! This becomes an important factor to consider when combatting large-scale spider mite infestations.

How to combat spider mites

Spider mites can be very hard to kill. In outdoor growing, the main problem is that any pesticides you use will have far more impact on the beneficial insects that may be present than the spider mites. This is not as much of a problem for indoor grows since there is not generally any beneficial insects present, however.

With indoor grows, the problem is the lack of competition already present which can allow spider mite colonies to grow uncontrollably. There are no natural predators in an indoor grow to keep these pests in check. It then becomes the grower's responsibility to become this predator and take matters into their own hands before it’s too late.

Spider mites (and other mites) are well known to develop resistance to insecticides very quickly. This makes them especially hard to kick completely and makes it important to be aggressive to ensure that all traces of these insects are wiped from the grow room. If any are missed, they will likely pass on these resistive genes to their kin, which will result in an even stronger and more resistant colony a few weeks down the line. Since it is especially hard to see spider mites when there is not yet a large colony, it is incredibly difficult to ensure their departure completely.

The takeaway from this is understanding that the key to getting rid of spider mites is to be aggressive and to continue the treatment for a few weeks AFTER the problem has dissolved.

Keeping the treatment going longer than it seems necessary will ensure that all traces of the mites have been killed and prevents insecticide-resistant mites to restart the colony.

A Step By Step Treatment Plan:

1. Trim all heavily infected leaves first.

This includes leaves that are completely covered in webbing. The webbing will add protection to the mites and is nearly impossible to clean off completely. Just cut your losses and remove these heavily infested leaves.

2. Rinse the rest of the plant in water.

By spraying your plants with streams of water it will wash away a large portion of the mites living on the leaves. Try to get the undersides of the leaves especially, as this is where they prefer to hang out.

3. Use insecticidal soaps or sprays

Use sprays such as pyrethrum, neem, or insecticidal soaps. All will work, but the key is to use them consistently. For this reason, pyrethrum or its analogs are going to be the best options. Neem and insecticidal work great, but are not the best for all applications because any residue left on the leaves can burn the leaves under the heat of the grow lights. Pyrethrum is a chemical extracted from chrysanthemum, that suffocates and kills the mites, and then breaks down after a short period of time. This insecticide is safe to use on food crops and is non-toxic to humans.

Spray your plants top to bottom with the pyrethrum and allow them to sit in the dark for 2-3 hours before rinsing it off with water. Again, this insecticide is safe to use on food crops.

Repeat this every 3 days (the average length of time it takes a spider mite egg to hatch), for about 2 weeks. In severe infestation is may be necessary to go for up to a month.

There are more organic options available too including predator insects such as predatory mites which are small enough to maneuver the webs of spider mite colonies to attack and destroy the spider mites one by one.

Less organic options such as foggers and insecticidal nukes can be used to wipe out all traces of insect life in the grow room.

4. Adjust the environmental controls

Spider mites thrive in warm and dry conditions. By decreasing the temperatures, or raising humidity, you will make your grow room less of an oasis for spider mites to thrive and grow happily, and can prevent them from recovering or resisting your treatment.

5. Improve the overall health of your plants

After a spider mite infestation, your plants will be left weakened. It is important to improve the health of your plants once again at this stage by providing ample nutrition, and perhaps supplementing with a product such as GHE Flora Gro. This high potassium supplement will help to overall strengthen your plants and return its immune system to proper health.