by Ed Rosenthal
CONTROLS
+ Traps and poison provide protection. Hardware and garden stores carry a variety of effective rat traps. Place traps around the plant cage, and use an attractant bait such as peanut butter. Once a few rats have been trapped, predators may come to enjoy the carrion, making the area unattractive to the rodents.
+ Rats have begun to develop resistance to warfarin, the classic anticoagulant “rat poison.” Another problem with this poison is that it can kill predators that eat the dead rats. A newer poison is cholecalciferol (Vitamin D3). Another is zinc phosphide. Both are available in various brand name rodent baits. Place any such poison bait in a tamper-proof bait station.
SLUGS AND SNAILS
Snails and slugs occasionally attack outdoor gardens, especially young plants with leaves close to the ground. They are rare indoors.
Slugs range in color from pale gray to tan, and grow to as long as 2 inches (5 cm) long. Their bodies are soft and fleshy, and glisten with a clear slime that the slugs secrete to retain moisture and help their movement. Two small “horns” atop the slug’s head are actually the slug’s “eyes”, which sense light, (slugs have no sense of sight).
Snails are slugs with shells. They are built almost identically to slugs, except for a coiled shell of calcium carbonate that protects most of a snail’s body. Snails can withdraw completely into their shells when threatened. Shells of common garden snails can reach up to 1.5 inch (3.75 cm) in diameter, and are colored gray, brown, and black.
Snails and slugs are found on the leaves and edges of leaves and flowers when it is moist out, usually after dusk. When it is dry or light they hide in dark moist areas such as piles of fallen leaves, crevices and moist shady areas.
Snails and slugs eat leaves. Holes in leaves and/or clipped edges of leaves and flowers, accompanied by a silvery, slimy trail, indicate snail or slug damage. A single snail can savage many small plants in one night.
These pests thrive in moist, dark environments. They hide in mulch, short and stubby plants, under boards, in soil, and they avoid sunlight, so they are seldom seen during the day, but come out to feast at night.
There is one particular kind of snail that you should leave alone. Decollate snails sometimes attack plants, but their main food is other snails and slugs. The fastest way to tell a “good” snail from a plant-eating pest is the shape of the shell: common garden snails usually have round shells that coil in a simple spiral. Most species of decollate snails have cone-shaped shells. If these are the only snails you ever see in your garden, then go ahead and get rid of them, because they eat plants if there is no other food in their habitat. But if you have other snails as well then the decollate snail is your friend.
Reproduction rate and life cycle: Slugs and snails are hermaphroditic so every individual lays eggs in clutches of 30 to 120 eggs, 1-2 inches (2.5 to 5 cm) deep in moist soil. When conditions are suitable (not too dry or too cold) slugs and snails can lay eggs as often as once a month, so their numbers can increase rapidly during damp spring and fall weather.
PREVENTION & CONTROL
+ The best way to prevent and kill snails and slugs is with iron phosphate, sometimes called ferric phosphate. It is completely effective and requires little effort. It comes as a powder or granules and is not harmful to plants, pets or humans. Sprinkle on the ground as directed. Many brands are available including Escar-Go, Sluggo, Slug Magic and Worry Free.
+ Copper wire, tape or mesh protects the garden or the base of the plants. Copper shocks the pests and deters them. Dimes and quarters contain copper so they can be used as well. When enclosing the garden or plants with copper, make sure not to trap snails inside.
+ Reduce damage dramatically by watering in the morning instead of the evening. The soil has time to dry out and become less attractive to the pests.
+ Snails and slugs can be trapped. Construct a cool moist area for them to retreat to during the heat of the day and then collect the refugees.
+ Firefly larva, toads, frogs, fireflies, snakes, birds, and black iridescent beetles prey on snails.
SPIDER MITES
Spider mites are very common and are the most serious pests in the Cannabis garden.
Spider mites are barely visible to the naked eye since they are only 0.02 inch (0.4 mm) long. They are arachnids (relatives of spiders), and like other arachnids they have four pairs of legs and no antennae. Unlike spiders, though, mites have only a single body segment. Their color range includes red, brown, black, yellow and green. It depends on the food they are eating, species, and the time of year. Spider mites are so tiny that most of these details are visible only with a magnifier.
They live on the plants, mostly on the underside the leaves, but can also be found around the buds. They can also be found moving from leaf to leaf and plant-to-plant using their silvery webbing as highways.
Spider mites pierce the surface of leaves, then suck plant juices from them. These punctures appear on the leaves as tiny brown spots surrounded by a yellow ring.
As the population grows the mites produce webbing that they use as a pedestrian bridge between branches or plants.
Spider mites are also vectors for disease, since they travel from plant to plant.
Spider mites are sap-feeders, like many other garden pests. They are more of a threat than most though, due to their high rate of reproduction.
Spider mites are by far the most fearsome of all plant pests. They suck plant juices, weakening the plants. Spider mites multiply quickly. They are most active in warmer climates than cold ones.
Reproduction rate and life cycle: Newly hatched mites have a 3:1 ratio of females to males, and each female lays up to 200 eggs. This life cycle can repeat as often as every eight days in warm, dry conditions–such as a grow room. This means that a spider mite population can explode with shocking speed, and this rapid reproduction is what makes them so troublesome.
SPIDER MITE, photo courtesy Nature’s Control
SPIDER MITE
Mite Webbing
PREVENTION
+ Almost all spider mite infestations enter the garden on an infested plant or through the ventilation system, or are introduced by gardeners who carry the hitchhikers into the garden.
+ Quarantine all plant introductions.
+ Use a fine dust filter in the ventilation system so all incoming air is cleaned. Never enter the grow space wearing clothing that has recently been outdoors, especially in a garden.
+ Look for webbing and for yellow-brown spots that result from mites sucking. Infected mother plants transmit mites to their clones, so it is especially important to watch for mites in a mother room. When you spot mite symptoms take action immediately.
+ Neem oil is often used as a preventative, but it leaves an oily residue that builds up with regular spraying.
CONTROL
+ Spider mites thrive in dry climates. High humidity slows spider mite development and reproduction. This can be used in vegetative and early flowering stages to slightly slow population increase.
+ Insecticidal soap smothers many of the mites, lowering the population and the damage, but does not eliminate the population. Even using it twice a week, the mite population may increase.
+ Pyrethrum is effective against some mite populations, but others have developed an immunity to it.
+ Cinnamon-clove tea can be brewed using powdered cinnamon and cloves. Start using about an ounce of each per gallon of water (0.75 ml per liter). Boil water. Let stand a couple of minutes. Add the herbs and let brew until the water is cool. Strain.
+ Predator mites: There are many varieties. Get those best suited to the environment in your garden. Apply predator mites at the earliest sign of infestation. Most predator species reproduce faster than spider mites, but if the mites get a good head start the predator population can never catch up. Even in optimal conditions control with predator mites is very difficult.
+ Beauveria bassiana (beneficial fungi) are somewhat effective—brands are Mycotro
l and Naturalis.
+ Herb and spice pesticidal oils are effective mite killers. They can be used as a protective spray to keep the plants from being attacked and fully evaporate leaving no residue. There are several products available. Ed Rosenthal's Zero Tolerance® blends cinnamon, clove, rosemary and thyme oils for fast results.
+ Fish Oil is an effective mite killer but it leaves a slight residue. One brand is Organicide® which is composed of edible fish and sesame seed oils.
+ Neem oil comes from the neem tree, native to west Asia. It has many protective qualities including both repelling and killing arthropods, including mites. It can be used as a protective spray to keep the plants from being attacked.
+ Sesame oil is toxic to spider mites. Use 1 tablespoon to a quart (15 ml per liter) of water. To facilitate the mix, add 1 teaspoon (5 ml) dry lecithin granules to the water and mix them in a blender. The spray is then ready to use.
+ Sulfur has been used to kill mites for many years. Sulfur candles and boilers produce vapors that kill mites and other pests. It is very effective but should not be used near people. The space should be aired out before people re-enter. Do not use it near residences.
THRIPS
Thrips are not commonly considered pests of marijuana. However, in some greenhouse conditions they can be serious pests.
Thrips are tiny, no more than 0.06 inch (1.5 mm) long, but can still be seen by the naked eye. Adults have wings but do not fly well; they jump when startled. The head and body range from yellow to dark brown. The larvae are about half the size of adults, lighter in color, and wingless.
Thrips attack the leaves and are usually found on the top surface of the leaf.
Thrips use a saw-like structure to pierce and scrape the flesh until sap begins to flow. They then suck up the juices, and leave a surface of patchy white or silvery scrapes. The leaf surface looks scarred or scabby. Eventually the leaves look like all the chlorophyll has been drained, and they turn white. Thrips leave behind greenish black specks of poop on and under leaves. The scar tissue shows up in silver patches. Thrip damage can resemble that of spider mites or leaf miners at first, but more severe cases result in the color-stripped leaves.
Damaged leaves can’t be healed and their ability to absorb light is compromised. If the thrips are not controlled the plants die. Thrips also carry pathogens that they transfer.
Outdoors, thrips hibernate over the winter in soil and plant debris. Thrips become active when the temperature climbs above 60 °F (16 °C). The warm, stable temperatures of indoor gardens allow them to be active year-round. Thrips are a more serious problem indoors because of this, and also because a natural soil-dwelling fungus that infects thrip pupae is not present indoors.
Reproduction rate and life cycle: Females lay eggs (anywhere from 40 to 300 depending on species) in plant crevices or actually insert them into the leaves and stems. The larvae feed until they enter the pupal stage, when they fall to the ground (and when the soil fungi provide some biocontrol outdoors). Depending on the species and temperature (optimum is 77 to 82°F (26 to 28°C), the larval thrips hatch, pupate, and mature into egg-laying adults in 7 to 30 days.
PREVENTION
Thrips are drawn to the colors blue and yellow, so it’s best to avoid having yellow walls or items around your Cannabis gardens. Yellow and blue sticky cards can be used as indicator traps to detect an infestation of thrips. Use garlic in outdoor gardens to deter/repel thrips.
CONTROL
+ Barrier: Thrip pupae live in the soil after they drop from the plant. By placing a barrier around the top of the container, the pupae can’t get to the soil and they die. As with fungus gnat larvae, a layer of diatomaceous earth on top of the soil also helps to destroy the thrips pupae.
+ Beauveria bassiana (beneficial fungi)
+ Beneficial Nematodes: Beneficial nematodes attack the pupae in the soil.
+ Capsaicin
+ Herb and Spice Oil Pesticides such as Ed Rosenthal's Zero Tolerance®
+ Insecticidal Soaps
+ Minute Pirate Bugs (Orius): these tiny insects attack adult thrips. They work well in combination with beneficial nematodes.
THRIPS, photo courtesy Nature’s Control
+ Neem oil
+ Predatory mites (amblyseius cucumeris) are a mite that loves to feast on thrips. Use large numbers to make sure the predators control the pests quickly.
+ Pyrethrum is an effective pesticide against thrips. Spray the leaves thoroughly.
+ Saccharopolyspora spinosa (Spinosad)
WHITEFLIES
Whiteflies are a common pest indoors and outdoors. Whiteflies resemble tiny moths but are neither moths nor true flies. They are relatives of aphids and scales. They are 0.04 inch (1 mm) long and their soft bodies are covered in a powdery wax which gives them protection and their white color.
Whiteflies infest the undersides of leaves. If the plant is disturbed they take flight and a mass of tiny white flies can be seen fluttering around the plant.
They suck sap from the plants, and are vectors for viruses. The plants release sticky honeydew and this can contribute to mold problems on the plants. Leaves appear spotty, droop, and lose vigor.
Whiteflies are sap-feeders, like their relatives, aphids and scales.
Whiteflies are pests that quickly build up a large population and suck the life out of the plants, but are not difficult to get rid of. If you think the plants might have whiteflies shake them. If they are present they’ll fly off, then settle right back onto the leaves.
Reproduction rate and life cycle: Females each lay about 100 tiny eggs on the undersides of leaves. Eggs hatch in about seven to ten days, and the larvae drain sap from leaves. Larvae mature in 2 to 4 weeks and the adults live for 4 to 6 weeks after that. The reproductive rate is temperature dependent: most whitefly species do best in a temperature range of 80 to 90°F (27 to 33°C).
WHITEFLIES, photo courtesy Nature’s Control
WHITEFLY PARASITE, photo courtesy Nature’s Control
PREVENTION
+ Keep the temperature of the garden below 80°F (27°C) to slow whitefly reproduction. Clear out plant debris quickly. Install a fine dust filter in the air intake for the grow space to prevent whiteflies from entering through the vents.
CONTROL
+ Yellow Sticky Traps- Whiteflies are attracted to yellow sticky cards. These trap a small proportion of the population, but are good indicators of infestations.
+ Shake plants to dislodge whiteflies and then suck them out of the air with the hose attachment of a vacuum cleaner. Heavily infested plants should be removed from the garden or grow room before treatment.
+ Encarsia formosae are tiny wasps that lay their eggs inside immature whiteflies. They are antisocial, they don’t make hives and are so small that once they are released you may never see them again. Indoors and in greenhouses one introduction may be all that is needed because their numbers increase much faster proportionally than their prey.
+ Beauveria bassiana (beneficial fungi)
+ Beneficial insects (lacewing larvae, minute pirate bugs)
+ Capsaicin
+ Carbon dioxide
+ Herb and spice oils
+ Insecticidal soap
+ Neem oil
+ Pyrethrum
+ Sesame oil
DISEASES
Disease can strike marijuana plants at any stage. Most diseases that affect marijuana fall into two broad categories: fungal and bacterial. The spores and bacteria that cause plant diseases are ubiquitous. A garden’s susceptibility to disease is often traceable to environmental imbalances in temperature, moisture, light conditions, airflow, and pH, among others.
Fungus grows when it finds the right levels of moisture, temperature (the range varies by species), acidic conditions, and a reliable source of food.
Bacteria are much more likely to invade when the environment has been compromised, in conditions such as oxygen deprivation, which make their attack more successful.
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br /> Once disease hits, it is important to act quickly and restore balance to the environment. However, prevention by providing a balanced environment for the plants is the best solution.
AIR FILTRATION AND SANITATION
Fungal spores often enter a grow space on air currents. Use a hepa filter in the air intake system to capture these spores and reduce the chance of fungal infections. Another option is a UVC lamp in the intake duct. The light from these lamps kills microbes and destroys spores. These two methods can be combined.
ALGAE
Algae are plant like microscopic organisms. They conduct photosynthesis like larger plants, but lack stems, roots and leaves.
Algae occasionally grow in hydroponic and aeroponic systems. They thrive in environments that provide warm temperature, light and nutrients. The water temperature should be 72° F (22° C), which is best for maximum plant growth. Unfortunately it is also a good temperature for algae.
When light shines on nutrient-rich water, algae are almost guaranteed to grow. Algae can grow anywhere in your system. Algae are commonly found inside tubes, nutrient reservoirs, buckets, on exposed rock wool cube surfaces, and DWC buckets (mainly if the containers allow light in).
All containers should be opaque.
Some algae attach to surfaces such as tubes and reservoir surfaces as well as roots. They form a green film that looks and feels either velvety or slimy. When it covers the roots it starves them of oxygen. It also competes for nutrients and can clog your system lines, motors, and sprayers in aeroponics systems.
Other algae are free-swimming and don’t attach themselves to surfaces. These algae can also clog up tubing.
Both kinds of algae photosynthesize during the day, using CO2 dissolved in the water and releasing oxygen (O2). However, during the dark period algae use oxygen dissolved in the water and release CO2. This depletes the oxygen in the water, which the roots require to maintain health.