Pests and Diseases

All About Spider Mites

Spider mites are one of the most feared and loathed enemies of all indoor gardeners. If you’ve not come across this feisty garden pest yet then be sure to do everything you can to prevent an infestation! Here we learn what they are, what they do to your plants, and how to get them out of your life.

Spider mites have been known to make grown men cry. Sooner or later, every indoor gardener is likely to get them. Once you've been infiltrated by spider mites you'll definitely take pest prevention more seriously.

The Golden Rule

Spider mite as viewed under microscopeAlas, the golden rule is often broken because it's a real party pooper. Here it is: Don't accept plant gifts from your friends! If their plant genetics are just too tempting to pass up you should make a special quarantine area for imported cuttings and keep them there for a minimum of a few months. If this sounds harsh then remember: most gardeners report that they discovered mites a short time after they accepted plants from a fellow grower. It's a huge gesture of trust (blind faith more like) to accept cuttings from other growers. In essence you are banking on the hope that their indoor garden is as clean as yours. Remember: All self-respecting, professional and commercial indoor growers are obsessive about cleanliness!

Why are Spider Mite Attacks so Damaging and Distressing?

Close-up of a major spider mite infestation.Spider mites are particularly vicious because they cause severe injury to your plants in a very short time.

Normally, a healthy plant regulates its water retention and transpiration through its leaves. A leaf contains a vast array of stomata that open and close like valves according to environmental conditions, allowing water to escape or to be retained. This moisture regulation is absolutely vital to a plant's health. When the stomata are closed, the surface of a leaf is highly resistant to water loss. This is part of your plant's life-support system. It's this "life support system" that spider mites attack.

Telltale signs of spider mite damage.Spider mites feed by piercing the leaf surface and extracting leaf cells and fluid. What was once the plant's protective, waterproof casing is now punctured with thousands of tiny holes. Your plants sense they are losing too much moisture all of a sudden and close their stomata - but it's no use as the moisture is still being lost through the holes made by those evil critters! The injured leaves continue to become dehydrated and lose significantly more water. The effect snowballs as the plant's ability to photosynthesize and repair itself diminishes through sustained attack. Leaves die and fall off. The plant becomes weaker and weaker until it eventually gives up the ghost. Major infestations show up quickly given the right conditions. Plants that looked healthy two days ago can suddenly be covered in fine spider webs.

Leaves that have been attacked by spider mites are usually dry, brittle and discolored. Even a minor spider mite infestation can have a significant impact on a plant's productivity. But it doesn't stop there. The moisture stress caused by spider mite feeding actually makes the leaves taste better to the mites! Stressed leaves are sweeter and contain higher levels of soluble nitrogen. Both sugar and nitrogen are sought-after delicacies on the spider mite's menu.

Difficult to Spot ... Until It's Too Late!

Established spider mite infestations produce webbing on plants.Novice growers are continually astounded that something so small can wreak such a huge amount of havoc. Yes, spider mites are small-really, really small. The female is only 1/50 of an inch long and the male is even smaller. They are mere specks when seen crawling on the undersurface of leaves. The adult has eight legs and is usually pale green or amber / yellow. Under a microscope they appear to have two (occasionally four) black spots. Young mites are six-legged. After two periods of molting and resting, they become adults and have eight legs. Generations may be completed in 5 to 40 days. If you don't own a jeweler's loupe, you should consider buying one. They are fairly inexpensive.

Spider mites are so tiny that they can float in on a breeze! They can therefore easily migrate from other plants that you may have in your house to your indoor garden. Mites will go dormant, even when pregnant. They will hitch a ride on your shoes and clothes and on your pets. Savvy growers cover all air intakes with a bug screen. Often you can buy these at your local indoor gardening store, or you can fashion one yourself with an ultra-fine mesh silkscreen or stainless steel screen. Just make sure it is 180 microns or less.

Tech Tip

Air intakes should use a bug screen so that you don't inadvertently suck bugs into your garden.Before you go cover all your intake and exhausts with 180-micron stainless steel screen, know that the screen is only 33% open area and so generates a great deal of air resistance. If you don't take this into account you will cut down your CFMs of airflow and up the static pressure of your room's air handling system(s). In order to get to what is called (Net Free Air) or (CFMs corrected for louver / screen / etc impingement) air flow you need to oversize the penetration / duct, so do the math. I usually like to oversize and go four times bigger than the ductwork for the 180-micron screened vent penetration.

Prevention, Control and Cure

"So how do I get rid of spider mites?" - this question is asked time and time again and the answer is an issue of hot debate and contention among indoor gardeners. There are so many different approaches and philosophies when it comes to pest management, with some growers invariably being willing to resort to more extreme measures than others.

Obviously, as with all pests, prevention is better than cure. But tell that to somebody who's just discovered an infestation and this little gem of wisdom is not likely to be greeted very philosophically. The choice of product you use also depends on where your plants are in terms of their lifecycle. One product that is good for vegetative plants may not be advisable to use if your plants are heavily flowering. If you are a couple weeks away from harvest, even if you have the serious infestation and have the web thing going on, knock the bugs down with cold water and finish the crop. Some studies have even shown increased yields with moderate levels of insect stress.

How To Deal With Spider Mites: Know what they Like, and What They Don't!

Spider mites thrive in hot, dry conditions. Cooler, more humid conditions slow reproductive rates considerably. Outdoors, spider mites are active in the spring and go dormant over winter. The risk of spider mites is always greater if you live in a region that does not freeze during the winter.

Do the Rest of Us a Favor!

Before you go running to your grow store with your spider mite woes, remember to change your clothes and shoes. Grow stores often have problems with spider mites and other insect problems due to customers dragging them in on their shoes and clothes.

Bombs Away!

Total Release FoggerMany growers who are not using predator mites use a total release fogger (aka 'bomb') to treat a spider mite infestation in an indoor garden. These products release an insecticide 'fog' using an aerosol propellant. If you are going to bomb your indoor garden, you may want to consider moving your houseplants into the room and bombing them too.

The active ingredient of a total release fogger is Pyrethrum. Pyrethrum will kill adult mites but it will not kill the eggs. The gestation period of spider mites is temperature and humidity dependant, but most growers deploy a strategy of setting off multiple bombs three to five days apart. This will usually kill the adults and then the juvenile mites before they have had a chance to reproduce. I advise growers to go for three or four consecutive bombings depending on how close you are to harvest and how angry you are. 

Another important thing to consider is all Pyrethrums are not equal. Natural Pyrethrum bombs are made from Chrysanthemum plants; they are suitable for food crops. Synthetic Pyrethrums are not suitable for food crop production. Read the fine print on the label and FOLLOW THE INSTRUCTIONS. UV (Ultraviolet) light reduces the insecticidal qualities of pyrethrum (most manufactures say in 14 days). However, if you are using High Pressure Sodium (HPS) lamps these put out virtually no UV light anyway.

Remember, the bomb's propellant will harm your plants if you do not position it correctly. Do not push the plunger of a bomb and set it in front of an oscillating fan, which is going to blow the propellant all over your plants. Before you set the bomb off, shut off your grow-lights and fans, place a small piece of cardboard under the bomb, hold your breath, hit the button, run out of the room and go for some drinks. It is safe to come back in a couple hours to turn the fans on.

I tend to set off bombs as a preventative measure, when I flip (clean) the room in between crop cycles or … just if I feel like mites may be a threat.

CAUTION: Systemic Pesticides and Consumable Crops

If you are growing crops that you intend to consume, make sure that your pesticide is designed to be used for human consumption. Some systemic and residual pesticides are not designed for food crops. Yes, they will kill mites and spider mites' eggs, but the insecticide remains inside the crop; Systemic means it stays in the system of the plant and does not go away after time.

Spider mites attacking a strawberry plant.

Mother Nature's Controls

You can ask your grow store to purchase predatory bugs for you. Since they are live bugs most stores will require you to pre pay so they don't get stuck with dead bugs if you forget show up in a couple days.

Ladybugs will eat spider mites if there are no other insect treats around (such as aphids); if you drop several thousand ladybugs in your garden they will eat everything, including each other. I have seen desperate growers that have gone this route. The aftermath of this is dead ladybugs everywhere.

Last but not least...

The best overall advice I can offer is that healthy plants will repel insect attacks. I'm pleased to report that I've experienced no spider mite infestations in over a year, and that was on an ornamental banana tree. Treat your plants right and they'll do the preventative work for you.