Two-Spotted Spider Mite
Nymphs, adult males, and reproductive adult females are oval and generally yellow or greenish in color and have one or more dark spots on each side of their bodies. Two-spotted spider mite eggs are about 0.14 mm in diameter and are usually laid on the undersides of leaves. They are spherical, clear, and colorless when laid but become pearly white as hatch approaches.
Two-spotted mites suck cell contents from leaves, initially stippling leaves with a fine pale green mottling. As feeding continues, the stippling increases and leaves turn yellow with bronzed or brown areas. Undersides of infected leaves may have many cast exoskeletons of mites, and the webbing on flowers and foliage is a sign of a serious infestation. Yield and quality are severely affected when large mite populations are allowed to proliferate unchecked.
Powdery Mildew of Cannabis
Powdery mildew is one of the most common pests of Cannabis and is one of the easier plant diseases to identify, as its symptoms are quite distinctive. Infections occur when powdery mildew spores (conidia) contact a suitable host and environmental conditions are favorable. Initial symptoms are small, circular, powdery, white spots which expand and eventually join as infections progress. Shaded and crowded leaves are often the most affected, but powdery mildew can appear on any above-ground part of the plant.
Powdery mildew fungi grow as thin layers of mycelium (fungal tissue) on the surface of the affected plant part. Specialized absorption cells, termed haustoria, extend into the plant epidermal cells to obtain nutrition. Spores, which are the primary means of dispersal, make up the bulk of the white, powdery growth visible on the plant’s surface and are produced in chains that can be seen with a hand lens. Powdery mildew affects the rate of photosynthesis, thus reducing plant brix levels as well as plant health. Powdery mildew is common on Cannabis grown indoors, in greenhouses and outside.
Rice Root Aphid
(Rhopalosiphum abdominalis or rufiabdominalis)
Infestations generally begin with a pear-shaped wingless form of the rice root aphid, which are dark purplish, with tints of orange, dwelling on the root ball. When populations exceed capacity in the rhizosphere (root zone), root aphids can be seen crawling on the surface of the substrate and winged adults are produced, which fly to look for more host plants to feed on. Winged root aphids are attracted to light, so they are often found dead on grow light fixtures. The winged version can also be caught in yellow sticky cards. To the naked eye, they look much like fungus gnats, but with magnification, they have shorter, hairy antennae while aphids have a longer filament type antennae. Also, fungus gnats have one pair of wings while aphids have two.
Root aphids suck plant sap from the roots, often leading to mineral deficiencies in the plant when populations are high and left untreated. Root aphids in high populations can reduce plant yield. Loss is quality generally stems from flyers caught in sticky buds.
Dark Winged Fungus Gnat
Fungus gnats are dark, delicate-looking flies similar in appearance to mosquitoes. Adult fungus gnats have slender legs with segmented, hairy antennae that are longer than their head. Fungus gnat adults are generally about 1.59 mm to 3.18 mm long. They have a single set of light grey to clear wings.
Adult fungus gnats generally don’t directly damage plants. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in young plants. Unrooted cuttings are also particularly vulnerable, with larval feeding interfering with callus formation and thus root initiation. In addition, adults and larvae have been implicated in the transmission of plant fungal diseases, including Pythium and Fusarium. Larvae may transmit fungal plant diseases via their excretions. Adults may transmit these diseases via excretion or by carrying spores of disease organisms on their bodies. Severe infestations of fungus gnats can lead to flying adults getting caught in sticky buds, thus reducing the marketability of the crop.
Western Flower Thrips
Thrips are often found in the flower buds. These flowers may be tapped or lightly shaken over a white sheet of paper (a drop cloth, of sorts) where you will be able to see their slender, small bodies racing around. The adult male is about 1 mm long; the female is slightly larger, about 1.4 mm in length. Most western flower thrips are female and reproduce by live birth. Although females can produce males from unfertilized eggs, females arise only from fertilized eggs. Males are rare, and are always pale yellow, while females vary in color, often by season, from red to yellow to dark brown. Each adult is elongated and thin, with two pairs of long wings. The eggs are oval or kidney-shaped, white, and about 0.2 mm long. The nymph is yellowish in color with red eyes.
A thrips (indeed, thrips, with an “s” is the singular and plural versions of this pest’s name) is not only known for the direct plant damage it causes, but also for the plant diseases it vectors. Damage to the leaves results in light colored silvery patches or streaking, often with blackish thrips droppings in the vicinity. Each female may lay 40 to over 100 eggs in the tissues of the plant, often in the flower, but also in the leaves. The newly hatched nymph feeds on the plant for two of its instars, then falls off the plant to complete its other two instar stages in the soil or substrate. The insect damages the plant in several ways. The major damage is caused by the adult oviposition in the plant tissue. The plant is also injured by feeding, which leaves holes and areas of silvery discoloration when the plant reacts to the insect’s saliva.
Hemp Russet Mite
A single russet mite is too tiny to be seen by the human eye without magnification of 14x and higher. Their near invisibility makes these mites a particular threat to become established in your commercial grow before you realize it. Unlike spider mites, these voracious plant pests leave no webbing or other secretions when present. Visible damage to the plant is the first indication of their presence. The first sign is generally curled leaflet edges, just above the petiole. Russet mites tend to start at the bottom of a plant and work their way up the plant. Seen through a lens or microscope, russet mites are torpedo-shaped, translucent/yellow-tinted, maggot-looking creatures, especially in groups. Unlike most varieties of mites, eriophyid, including russet mites, have only two pairs of legs.
Russet mites live and feed mainly on leaves and petioles. They puncture the plant epidermal cells to feed on the plant cell contents. Feeding damage causes leaflets to curl at their edges, followed by a rusting discoloration and necrosis. Leaves and petioles become brittle. Flower buds on the bottom of infected plants will often be much smaller, discolored and significantly less developed than they would normally be. Serious infestations can turn leaves beige with hemp russet mite bodies. If left unchecked, hemp russet mites will often completely destroy a Cannabis plant over the course of an 8-week flowering cycle. They congregate on the upper part of the plant as it dies. Hemp russet mites can also be the vector of viruses.
Pythium Root Rot
Pythium root rot can develop at any time during the growing season if the soil remains saturated, warm, or depleted of oxygen for prolonged periods. Pythium root rot is a persistent problem in areas that are poorly drained or over-irrigated. From a distance, root symptoms can appear orange, yellow or dark and greasy. Slowed growth and soft plants (low turgor pressure) is often the first sign that will be seen. We recommend lab tests to confirm the species so treatment plans can be tailored.
Pythium attacks either juvenile tissues, such as the root tip, or roots damaged by mishandling or invertebrate pest damage. After gaining entrance to the root the fungus may cause a rapid, black rot of the entire primary root and may even move up into the stem tissue. As the substrate dries, new roots may be produced and the plant may recover or never show symptoms of disease. The depth and density of roots will be drastically reduced in affected areas. Under continued anaerobic conditions, more and more roots are killed and the plant may wilt, stop growing, or even collapse and die.
“It is not a question of “will you have pests?” – it is just a matter of when, what type, and how much damage they will cause.”