Spider Mites

Marijuana Horticulture

by Jorge Cervantes

Identify

The spider mite is the most common pest found on indoor plants and causes the most problems. Spider mites have eight legs and are classified as spiders rather than insects, which have six legs. Find microscopic spider mites on leaf undersides sucking away life-giving fluids. To an untrained naked eye, they are hard to spot. Spider mites appear as tiny specks on leaf undersides; however, their telltale signs of feeding – yellowish-white spots, stippling – on the tops of leaves are easy to see. Careful inspection reveals tiny spider webs – easily seen when misted with water – on stems and under leaves as infestations progress. A magnifying glass or low-power microscope helps to identify the yellow-white, two spotted brown or red mites and their translucent eggs. Indoors, the most common is the two-spotted spider mite. After a single mating, females are fertilized for life and reproduce about 75 percent male eggs. Females lay about 100 eggs.

Damage

Mites suck life-giving sap from plants, causing overall vigor loss and stunting. Leaves are pocked with suck-hole marks and yellow from failure to produce chlorophyll. They lose partial to full function, and leaves turn yellow and drop. Once a plant is overrun with spider mites, the infestation progresses rapidly. Severe cases cause plant death.

Controls

Cleanliness! This is the most important first step to spider mite control. Keep the grow room and tools spotless and disinfected. Mother plants often have spider mites. Spray mothers regularly with miticides, including once three days before taking cuttings. Once mite infestations get out of control and miticides work poorly, the entire grow room will have to be cleaned out and disinfected with a pesticide and 5 percent bleach solution. Steam disinfection is also possible but too difficult in most situations.

Cultural and Physical Control

Spider mites thrive in a dry, 70-80F climate, and reproduce every five days in temperatures above 80F. Create a hostile environment by lowering the temperature to 60F and spray foliage, especially under leaves, with a jet of cold water. Spraying literally blasts them off the leaves as well as increases humidity. Their reproductive cycle will be slowed, and you will have a chance to kill them before they do much damage. Manual removal works for small populations. Smash all mites in sight between the thumb and index finger, or wash leaves individually in between two sponges. Avoid infecting other plants with contaminated hands or sponges.

Remove leaves with more than 50 percent damage and throw away, making sure insects and eggs do not reenter the garden. If mites have attacked only one or two plants, isolate the infected plants and treat them separately. Take care when removing foliage not to spread mites to other plants. Severely damaged plants should be carefully removed from the garden and destroyed.

Smear a layer of Tanglefot around the lips of containers and at the base of stems to create barriers spider mites cannot cross. This will help isolate them to specific plants. Smear a layer of Tanglefoot at each end of drying lines when hanging buds t contain spider mites. Once foliage is dead, mites try to migrate down drying lines to find live foliage with fresh, flowing sap.

Biological

Neoseiulus (Amblyseius) californicus and Mesoseiulus (phytoseiulus) longipes, are the two most common and effective predators. Phytoseiulus persimilis, Neoseiulus (Amblyseius) fallacius, Galendromus (Metaseiulus) occidentalis, and Galendromus (Typhlodromus) pyri predators are also available commercially.

When properly applied and reared, predatory spider mites work very well. There are many things to consider when using the predators. First, predators can eat only a limited number f mites a day; the average predator can eat 20 eggs or 5 adults daily. As son as the predator’s source of food is gone, some mites die of starvation while others survive on other insects or pollen. Check with suppliers for release instructions of specific species. A general dosage of 20 predators per plant is a good place to start. Predatory mites have a difficult time traveling from plant to plant, so setting them out on each plant is necessary. Temperature and humidity must be at proper levels to give the predators the best possible chance to thrive. When spider mites have infested a garden, the predatory mites cannot eat them fast enough to solve the problem. Predatory mites work best when there are only a few spider mites. Introduce predatory mites as son as spider mites are seen on vegetative growth., and release them every month thereafter. This gives predators a chance to keep up with mites. Before releasing predators, rinse all plants thoroughly to ensure all toxic spray residues from insecticides and fungicides are gone. The fungus, Hirsutella thompsonii, trade name Mycar, kills spider mites.

Sprays

Homemade sprays often lack the strength to kill infestations but work as a deterrent by repelling mites. Popular homemade sprays include Dr. Bonner’s soap, garlic, hot pepper, citrus oil, and liquid seaweed combinations. If these sprays do not deter spider mites after four to five applications, switch to a stronger spray: neem oil, pyrethrum, horticultural oil, or nicotine sulfate, cinnamaldehyde.

Insecticidal soap does a fair job of controlling mites. Usually two or three applications at five to ten day intervals will do the trick. Horticultural oil smothers eggs and can be mixed with pyrethrum and homemade sprays to improve extermination.

Heavy duty chemical miticides are available but are not recommended on plants that will be consumed by humans. If using any chemical miticide, be sure it is a contact poison and not systemic.