Some nasty buggers
The most common insects that plague marijuana indoors and outdoors are thrips, mites, whiteflies, fungus gnats, budworms, and caterpillars. Of these, mites are most prevalent in indoor grow rooms; outdoor growers generally find that leaf-eating insects like caterpillars and budworms are their most threatening insect pests.
Mites are tiny, mobile, hardy, and vicious. They travel from grow room to grow room on infected clones and on the bodies and clothes of people who have come in contact with mites. Mites live in nature and on dogs. Approximately 60% of BC’s indoor grow rooms are infected with mites.
Mites puncture leaves and gorge themselves on plant juices. In the early stages of infestation, they produce little evidence of their presence, although they can be detected by careful examination of leaf surfaces with a magnifying glass. Look for yellow or white spots on both sides of the leaves. The spots radiate along leaf veins, and are evenly spaced.
In advanced stages, spider mite infestations are easily diagnosed, because the mites build white webbing which encapsulates plant sections.
Non-web leaf damage caused by mites can be mistakenly diagnosed as leaf spotting caused by thrips. Thrips are larger than mites with a more linear body shape. They bite leaves and create white or yellowish marks on them, but the marks are more clustered and solidly discolored than those left by mites. Thrips leave black fecal debris on leaves and in floral clusters.
Whiteflies are more easily seen than thrips or mites because they fly around plants, especially when plants are shaken by gardeners or air movement. As their name implies, whiteflies are pure white, and are about the size of the head of a pencil lead. Whiteflies drink plant juices and leave residue on plant surfaces that provides sustenance for harmful fungi.
Fungus gnats and related species such as the crane fly infest soil and root zones. They feed on roots, which causes a decrease in plants’ ability to uptake nutrients. Gnats are tiny, mobile, and dark. Crane flies resemble mosquitoes.
Budworms and caterpillars are problems primarily for outdoor growers. Budworms burrow into ripe floral clusters; they also spin webs around floral clusters, although this is infrequent. They eat floral clusters from the inside out, and their activities provide a vector for a pervasive cannabis fungal disease known as botrytis or gray mold.
Leaf-eating caterpillars are less difficult to detect than budworms because they spend most of their time on leaves. They often manipulate leaves before eating them; their feeding habits include eating small sections of leaf in between leaf veins, or eating large chunks of leaf outright.
No fun fungi
Although healthy root zones and leaves often contain beneficial fungi and bacteria, other forms of fungi and bacteria are damaging to cannabis plants.
The most pervasive fungal invader is botrytis, otherwise known as gray mold. This fungus is particularly dangerous because it is virtually undetectable in early stages, and because it attacks the ripest, phattest buds from the inside out.
Botrytis waits until cannabis flowers are dense and well developed, then it infests the buds near the core of the flower at stem level, and quickly produces gray garbage that destroys the bud’s vigor and appearance. Gray mold and other pathogens can also affect harvested cannabis, even when harvested material is placed in a freezer.
Gray mold causes problems indoors and outdoors, particularly where humidity levels are above 50%, with poor aeration of plant tops.