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Higher Learning- IPM w/Matthew Gates, Introduction to Spider Mites

So we all know that feeling when you’ve spent the better part of a month vegging your plants to the right size to transition. You’re almost ready to flip over to flower and start the fun part of growing your own cannabis, the flowers! Then you notice a tell-tale sign like spotted or yellowing leaves, and realize you have a problem in your garden that needs to be fixed ASAP. It could be one of a wide range of potential predators from a spider mite, to thrips and aphids just to name a few.

How do you identify exactly what type of pest has infiltrated the grow, and how do you treat and eradicate them for good? Thankfully for you, Beard Bros is here to help save the day along with Integrated Pest Management (IPM) specialist, Matthew Gates from Zenthanol Consulting .

Matt’s here to get you up to speed on the most common pests you will encounter (and they’re not all evil, some you even WANT to have), how to deal with removing them in a safe and environmentally friendly manner, as well as some best practices you can implement in your own grows. Whether you’re in a small closet home grow or a new commercial facility, we’re here to help you succeed.

 

Two-Spotted Spider Mite

One of the most common pests in agriculture in general is the Two-Spotted Spider Mite, so-named because two sections of its digestive tract are typically visible when feeding, appearing as two dark spots on its backside, though sometimes there are more or none visible. There are many dozens of spider mite species, but this species, known scientifically as Tetranychus urticae, is documented on more than 10,000 plant hosts, making it one of the most speciose diets of herbivores known. Much of their biology is devoted to suppressing and neutralizing a diverse array of plant defenses and this honed edge cuts deep into Cannabis cultivation as even the most vibrant and healthy plants can be vulnerable.

twospotted spider miteSpider mite under microscope

Detection:

Sometimes called the “red mite” in English or equivalent phrases in other languages, Two-Spotted Spider Mites are fairly variable in color and can be red, pink, orange, and oftentimes green–this is due the amount of pigments called carotenoids produced across populations. Spider mites like other arachnids have eight legs as adults, and their sparsely-haired ovoid body is about 0.4 millimeters in length. Visible, but small enough that a few may go unnoticed especially in dense foliage as they prefer to feed on the underside of leaves.

However, their speckled damage, referred to as stippling, is much more visible and is the result of drinking the leaf juices cell-by-cell, leaving small chlorotic patches. When scouting for pests, catching a small population before it spreads is usually predicated on the observation of this cue and positive identification of the spider mite coupled with rapid treatment. Similar damage is caused by other pests like Western Flower Thrips making investigating any such damage proactive.

As a colony grows, stippling intensifies with multiple leaves becoming alarmingly damaged, hosting several generations that often crowd together in silk patches and may traverse leaf tops, especially if silk bridges have been spun between leaves. Silk allows spider mites to protect themselves from non-adapted predators, environmental stressors like ultraviolet radiation, and facilitates both travel and chemical communication.

Additional information: Featured Creatures- Twospotted Spider Mite

Treatment:

Ideally, preventative measures should be taken to reduce chance of spider mite ingress or success of colonization, failing that. Physical ingress occurs from commonly from nearby hosts and movement over ground, walls, or branches but long-distance movement through the air as aeroplankton is well-documented. For this reason, culling potential hosts, at least in the immediate areas of cultivation, can be helpful.

Some cultivators erect wind walls to divert windborne populations of pests but this is contextual and more complicated to execute. Possibly the most common vector are cuttings that have not been quarantined or vigilantly searched for evidence of pests. Not only for spider mites, but dozens of other pests, this is a highly common movement pattern in commercial and residential contexts. Checking incoming plants thoroughly and at best having a separate enclosed space for quarantine is essential to robust biosecurity procedure.

Simultaneously, regularly checking crop plants for signs of pests is necessary to catch spider mite colonies before they spread. Like many insects and mites, spider mites have a short development time that is usually positively correlated with temperature increase. At more optimal conditions of around 25 °C, spider mites can develop easily under 2 weeks, sometimes under 1 week, and only 1 female is necessary for a colony to form since unfertilized females produce males, and fertilized females produce females ensuring there are always compatible mating pairs. This temperature synergy makes hotter months more conducive to spider mite population blooms in the environment.

A fledging colony must contend with environmental and biological antagonism, and cultivators can leverage a considerable multifaceted approach. Biocontrol agents like predatory mites such as the Persimilis Mite (Phytoseiulus persimilis) and certain other predatory insects, are most effective when the colony is smallest: a sufficiently sized ambient population of compatible predators is likely to engage with and kill lone or small groups of mites as their primary defense against predation is silk shelter. Biocontrols can severely reduce and even eradicate dense spider mite colonies but require larger populations and more frequent applications which introduces logistical and financial burden.

Phytoseiulus Persimilis

Persimilis Mite

Applying a safe an context-appropriate spray application, especially one that isn’t too antagonistic for intentional biology, can sharply reduce spider mite population and reduce biocontrol application though this often comes at the cost of some off-target effects. Alternatively, culling dense colonies by carefully removing and containing leaves and branches before applying a spray or biocontrol can be effective damage control in hotspots but it is important to consider spider mites react to crowding by seeking less crowded plants to colonize so checking for new colonies is still necessary.

Looking for more info on sustainable growing methods? Check out our Higher Learning series on Regenerative Farming HERE!

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