Cannabis has a long but relatively recent history, possibly close to 30 million years according to some studies, and for most of this time populations expanded across Eurasia subjected to environmental stresses of different locations from arid deserts to broad-leaved forests to steppe plains. Parasites and even mutualists like various insects, mites, fungi, and bacteria moved with cannabis and some species like the Cannabis Aphid (Phorodon cannabis) or Hemp Russet Mite (Aculops cannabicola) have adapted to it intimately, only ever feeding on it to the exclusion of others. Different pests like various species of fungal Powdery Mildews or viruses such as the Beet Curly Top virus and infamous Hop Latent Viroid are more prolific in host range and can be vectored from nearby plants that host them to cannabis crops. In the last 10 millennia or so, human captivation and subsequent cultivation has exposed this plant to many new environments replete with both conducive factors like beneficial microbes as well as detrimental threats like generalist herbivores.
Through breeding, many traits changed: flowers that were naturally small and sparsely populated by trichomes containing cannabinoids and other secondary metabolites such as esters, terpenes, and phenols exploded in size and trichome density with various chemical permutations.
Understanding cannabis pest mitigation starts with understanding the plant’s history, that pests are naturally occurring organisms, cultivated populations are the product of tumultuous artificial selection pressures, and surviving in nature didn’t require nor bestow adaptations for the level of pest absence as desired in human contexts, so we as growers and enjoyers must not forget the great effort needed to protect and produce acceptable—let alone exceptional—results in yield and potency.
As parasites, cannabis pests benefit from their symbiotic relationship at the expense of the host plant, usually by directly damaging the structure of tissues and the acquisition of resources. For this reason, pests are often found in close proximity to the plant either in the branches and leaves (phyllosphere) or root system (rhizosphere). Dozens of pests of various shapes and sizes exist that require different strategies for prevention and treatment but some of the most common species are well researched. Examples include:
Two-Spotted Spider Mite
Documented feeding on ~4,000 species of plants across dozens of plant families, this 0.4 millimeter long mite is extremely common globally and may be the most prolific herbivore. This is mainly due to a physiological smörgåsbord of genetic adaptations, the genes of which originally belonged to bacteria and fungi in some cases that allow them to resist plant as well as human chemistries meant to disrupt them. Biocontrol agents like the predatory mite Phytoseiulus persimilis are commonly used at small and large scale. Although incompatible with insect and mite biocontrols, wettable sulfur is also effective during vegetative growth but not flowering period. It should be noted that the “spots” of the Two-Spotted Spider Mite are not always visible as they are intestinal pouches that can be obscured by red or green pigmentation that covers the body.
Hemp Russet Mite
A specialist of cannabis specifically, these russet mites measure 0.15-0.2 millimeters in length and have a somewhat conical, worm-like appearance. Generally true for russet mites is an extremely selective host range of one or a few closely related species of plants. While other russet mite species produce large structures called galls by manipulating leaf development, the Hemp Russet Mite uses toxic saliva that crinkles and curls leaf tissue giving them better shelter from the environment and predators like Amblyseius swirskii or Neoseiulus cucumeris often used commercially. Most commonly, they are spread on the wind or cuttings.
Hops and cannabis diverged from a common ancestor around 30 million years ago according to some estimates, and the Cannabis Aphid has specialized on this plant to the exclusion of others which is the most common lifestyle of aphids generally. Also like other aphids, they feed on plant sap called phloem, adults reproduce with live-birthed clone-like offspring that are born pregnant. At about 2-3 millimeters in length, it has a pale-green or light yellow coloration with a long, piercing needle-like mouthpart, antennae on the head that point backwards, and a distinct pair of protruding structures near the abdomen called “cornicles” that secrete alarm or defensive compounds when threatened. When colonies become dense, winged adults are often produced which fly to new plants and may lay eggs or more clonal offspring. Quick detection is important for treatment to be most efficient; biocontrols like Green Lacewing larvae Chrysoperla carnea the parasitoid wasps in the group Aphidius have had good success when deployed correctly. Botanical insecticidal agents like Pyrethrin or Azadirachtin are also highly effective against target and non-target insects alike, while still being safe to use on plants.
Rice Root Aphid
The vast majority of aphids have co-evolved specialist lifestyles with specific plant species but the Rice Root Aphid is part of a small fraction that can feed on a wide assortment of plants, and switch between herbaceous plants including wild grasses in the spring and summer and woody trees like almonds and other Prunus species in the autumn and winter. Vexingly, they also tend to congregate in the root system of plants, especially cuttings, and often build up in population covertly as they feed on plant sugars, finally traveling up the stem and onto leaves after dozens have been produced and crowding occurs, which makes inspection of plants more laborious. With the typical teardrop aphid body shape, abdominal cornicle structures, and back-facing antennae these aphids are often confused for Fungus Gnats especially when adults develop wings, but their bulbous body can be a helpful way to discriminate them from their daintier, more nimble fly comparisons. In color the Rice Root Aphid is often a dark color, sometimes mottled with green or a reddish spot at the tip of the abdomen. Underground populations can be targeted with pathogenic fungi like Beauveria bassiana and both above and below-ground populations can be targeted with soft botanical chemistries like Azadirachtin which greatly reduces aphid defenses against Beauveria and similar pathogens.
Western Flower Thrips
In recent years, of all 5,000+ species of thrips, the Western Flower Thrips is the most researched. It is an incredibly successful generalist plant parasite and is often the most common species encountered in various crops, causing a stippling or silvering damage demarcated by black liquid spots of excrement. Larvae and adults tend to have a thin, cigar-shaped yellowish-cream colored body that reaches ~2 millimeters in length. When larvae develop enough, they enter a suspended state before developing into adults, and often drop to the ground during this period, making them vulnerable to Steinernema feltiae nematodes, predatory mites like Hypoaspis miles, and other treatments. The main challenge of thrips is their sheer persistence in the ambient environment that is intensified in contexts where surrounding plant life is desiccating, or the environment starts to increase in temperature as adults become more active in their search for suitable hosts.
Hop Latent Viroid
The cannabis virome—all the viruses known to associate with cannabis—is poorly characterized currently. Cannabis cultivators are often not aware of these conventionally incurable parasites and lack infrastructural support and biosecurity options at all scales from private to commercial for solutions. Similar to viruses, viroids are persistent replicating parasites but their unique RNA-structure means they do not code for any genes; the viroid is recognized as cellular RNA and uses plant cellular processes to create more of itself, in the process fragmenting into small pieces that bind with other genetic material and stalls their production which leads to symptoms like stunting, brittle stems, and low metabolite production. Insidiously invisible and prone to transmission through touch and equipment, HLVd testing is paramount for serious prevention strategies.
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