Problem: Flies, Mites, and Aphids in Your Marijuana Plants
Symptoms: Yellowing leaves; tiny, white flying insects (whiteflies); webs on the bottoms of leaves or the flowering tops (spider mites); wilting plants (aphids).
Solution: I’m going to show you how to fight and conquer the indoor and outdoor pests that frantically want to molest your fast-maturing female marijuana plants with their flowering tops, so the most important thing first: I prefer mechanical solutions over chemical ones. The most commonly encountered indoor growing pests are whiteflies, spider mites, and aphids. They are all vegetarian vampires and they want to suck the life out of your marijuana plants. Indoor infestations are the worst, because the pests won’t have the natural predators that they would outdoors.
WHITE FLIES: Whiteflies are harmful because they suck juices from plants, causing the leaves to shrivel, turn yellow, and drop. They can carry viruses as well. They fly from one plant to another, carrying disease with them, and quickly attack all of the plants in your indoor marijuana garden. These pests are white and about 1 / 16 inch long. Adult and young whiteflies feed on the underside of leaves. Organic insecticides have only a limited effect on whiteflies, as they quickly build up resistance to them, and most are not very effective in garden situations. Insecticidal soaps and oils are only marginally effective. Physical removal of the flies is the best solution. You should move infested indoor marijuana plants to the outside, lay them on their sides, and use the garden hose to spray the underside of the leaves to physically remove the flies and their nymphs. In the grow room or bloom room, hang flypaper ribbons without insecticide and trap the flies.
SPIDER MITES: I despise spider mites more than any other pest because they are insidious. Under optimal conditions (about 80 degrees Fahrenheit), the two-spotted spider mite can hatch from an egg in as little as three days and become sexually mature in as little as five days. One female can lay up to twenty eggs per day and can live for two to four weeks, laying hundreds of eggs. A single mature female can spawn a population of a million mites in a month or less. Spider mites look like tiny moving red dots and are almost too small to be seen with the naked eye; however, they are easily seen with a 10-power hand lens. Adult females rarely exceed half a millimeter (.02 inches) in length. Spider mites live in colonies, mostly on the undersurfaces of leaves; a single colony may contain hundreds of individuals. The name “spider mite” comes from the silk webbing most species produce on infested leaves. The presence of webbing is an easy way to distinguish them from all other types of mites. So, spot the webs, not the mites. The webs reflect light, so turn off your grow lights and use a narrow-beam flashlight and a magnifying glass to inspect the plants. Look beneath the lower, large fan leaves first, because those are the most vulnerable to early mite infestations.
Mites cause damage by sucking cell contents from leaves and tops. At first, the damage shows up as a stippling of light dots on the tops of the leaves; sometimes the leaves take on a bronze color. As the feeding continues, the leaves turn yellow and drop off. Often leaves, twigs, and fruit are covered with large amounts of webbing. Damage is usually worse when compounded by water stress. They can kill the plant if left uncontrolled.
Miticides are not recommended for controlling spider mites because they will be inhaled with the smoke of any tops or leaves that you smoke. In their early stages, spider mites can be eliminated by washing the marijuana plants with a mild soap solution, then spraying the plant with water, including the undersides of leaves where the mites usually harbor. Repeat this process every three or four days until the mites are gone. If spider mites attack the marijuana plants in your bloom room and you don’t want to subject the tops to water spraying that might damage or destroy trichomes, your only solution is to harvest all the plants immediately.
APHIDS: Aphids are usually brown or light green and soft-bodied. They cluster on leaf undersides or on stems sucking plant juices and excreting a sticky liquid called honeydew, which creates spots on the foliage. A black fungus called sooty mold may then grow on the honeydew. Severe infestations can cause wilting, stunting, curling, and leaf distortion. Normally aphid predators such as lady bird beetles (ladybugs) and their larvae (black and orange segmented creatures about 1 / 8 inch long) keep the aphid numbers low, but the aphids can multiply quickly. Spray the aphids with soapy water and then rinse them away as a mechanical means of removal.
Problem: Heavy Pest Infestation Beyond Mechanical Removal
Symptoms: Webs totally covering your flowering tops (spider mites); clouds of whiteflies; aphids so thick that the plants are dying.
Solution: It’s inconvenient to constantly wash your plants, but it is an inconvenience you must occasionally put up with. Remember, the earlier you catch the infestation, the easier the treatment will be.Many marijuana growers, who are inexperienced in recognizing the early signs of spider mites, had such a severe attack on three nearly mature plants in his bloom room that he had no choice but to harvest immediately. After removing infested plants, many growers opt for fumigation of the room to kill any lingering pests. I have used this technique in my winter greenhouse and discovered that Raid Flying Insect Killer is particularly effective— especially for whiteflies and aphids. I’ve never found any insecticide effective against spider mites, although neem oil, made from the neem tree, seems to be an effective treatment for repelling, rather than immediately killing, the pests. The oil is nontoxic to humans and pets, and should be diluted with water and an emulsifier like soap before being sprayed on infested plants. Neem oil spray, however, may alter the taste of the marijuana tops.
(source; Growing Medical Marijuana by Dave DeWitt)