SPIDER MITES

Spider mites (Tetranychidae family, with the most common found member being Tetranychus urticae) are one of the most damaging pests and can destroy plants quickly when left unchecked. They are tiny, eight-legged arachnids related to the spider and tick.

The term “spider mite” come sfrom their behavior of spinning fine silk webs on infested leaves and new growth.

Adults are normally green or yellow but turn red when day lengths shorten in the autumn. Adults have an oval body, with two red eye spots near the head end of the body. Females usually have a large dark blotch on each side and numerous bristles covering the legs and body. They look sort of like bristly black dots. Their eggs are visible too – they are very tiny, white and are laid closely grouped together.

Spider Mites attack plants by stabbing the underside of the leaves and sucking out the sap, causing the cells to collapse and die. The first sign of spider mite damage is fine stippling on leavesthat looks like many small holes poked with a needle which is clustered around the veins, especially the central vein. As their numbers increase, the number of white speckles on the leaf increases and damaged leaves will take on a whitish or grayish appearance before they die.

Once the spider mites begin reproduction, a distinctive `webbing’ forms, usually under the leaf, on the geniculum or petiole insertion point and then at the growing tip of the plant. The mites disperse from a plant of declining food quality on threads of webbing, and drift or are blown on to other plants.

What makes this pest truly difficult to control is its rate of reproduction. Each female will lay up to 12 eggs per day. Mating is not required for egg production. At 70º F, these eggs will hatch in as few asthree days and will develop into adults in only 14 days. Adult females also have the ability to go dormant for a time after the photo period (daily hours of light) shortens, then re-emerge to lay more eggs a few weeks after the photo period lengthens again. That’s one reason Spider Mites keep reappearing on indoor plants.

PLANT MYTH: High humidity deters spider mites

This myth is based on some truth, in that the ideal conditions for spider mites to develop are dry (this term is used in terms of tropical/greenhouse conditions, so it refers to 50-60% RH, which is likely already higher than most people’s ambient home humidities) and hot (25-27℃/77-80℉)

Under these conditions, spider mites can complete their life cycle from egg -> adult -> more eggs in 3 DAYS. HOWEVER, this does NOT mean they will not be successful in multiplying outside their preferred parameters

As was observed in this Nature paper, the normal RH levels found in most of our homes (30-70%) are more than suitable for mite survival and reproduction

Because of their ability to multiply rapidly in ambient home conditions, spider mites can seemingly appear out of nowhere, and develop resistance quicker than most pests. If you try and fail to kill mites all you are doing is breeding resistant super mites. This chart shows the activity of many commercially available pesticides

Miticides

 ●       Oberon SC or Forbid (Spiromesifen) - Forbid is a newer miticide and ovacide that has a unique mode of action that works against mites that have developed resistance to other more common miticides. It is active on eggs, larvae, and nymphs and adults. Like Avid it is both contact and translaminar and will provide resistance for 30-45 days. It is quite expensive normally around $220-240 for an 8oz bottle but can generally be found repackaged in smaller quantities (in-home applications require very little product)

●       Avid (Abamectin) - Avid is one of the most popular commercially available miticides. It will control mites and leaf miners as well as suppress aphids, thrips & whiteflies. It is active on larvae, nymphs and adults but is not active on eggs. It is both translaminar and active on contact. Translaminar pesticides are those in which the material (spray solution) penetrates leaf tissues and forms a reservoir of active ingredient within the leaf. It is semi-systemic and works its way through the leaf but is not transported throughout the plant. Avid retails for around $95-100 for an 8oz bottle

●       Floramite SC (Bifenazate) - Floramite is active on a wide range of mites and provides quick knockdown with 28 day residual suppression. It is active on all lifecycle stages from egg through adult. It is contact only and not translaminar so you must make sure to thoroughly cover the bottoms of the leaves with this one. Floramite retails for around $85-120 for an 8oz bottle

There are a few non-pesticide options that can control light infestations if caught early but once they have established themselves you should use miticides designed specifically for spider mites or you are in for an uphill battle.

Non pesticide options

 ●       Horticultural oil / dormant oil - This is a mechanical pesticide. It kills mites by suffocating them and so there is no way to develop resistance to it. It is effective if applied routinely several days apart since it only suffocates the mobile stages and not the eggs. It can also be mixed with many other pesticides to increase their effectiveness. It is direct contact only and has no residual effects. Horticultural oil and other oil-based pesticides can damage leaves. They can cause burning or other damage to leaves if applied when temperatures are high (80-85F or higher) or if the leaves are sensitive. Always test it on a couple leaves first before spraying a whole plant

●       Soapy water & peroxide - Real soap like dr. Bronner’s or safer insecticidal soap + 3-4 TBSP of 3% hydrogen peroxide per cup of water. Spray on and then use a soft, damp cloth to physically wipe the mites off the leaves. Rinse with plain water. Direct contact only with no residual effect