Spider mites are highly polyphagous pests feeding on a very wide host range of protected and unprotected crops. They can be a major pest in the cultivation of medicinal cannabis.
Spider mites are easily spread through the crop on airborne web-threads, as well as on tools and crop workers’ clothing. The populations can develop fast and colony sizes increase rapidly, especially in warm and dry conditions.
What to look out for
The first signs of spider mite damage are patches of light-yellow stippling on leaves, caused by the larvae, nymphs and adults feeding.
Learning to spot these early symptoms on leaves is key as, without intervention, these chlorotic patches can grow quickly with characteristic webbing starting to develop as the pest population colonises the plant.
Preventative and curative controls
The two key biocontrol products used in medicinal cannabis for spider mite control are Californicus-System and Phytoseiulus-System.
Amblyseius californicus is a predatory mite best used as a preventative control for spider mites. It is able to colonise plants and survive in the absence of the pest by eating pollen and other sources of nutrition - such as fungal spores and hyphae. It controls spider mites by locating small populations and predating upon them - containing and slowing their development; however, it is not so effective at operating within webbing or dense colonies.
In this situation, the predatory mite Phytoseiulus-System is an excellent curative choice. Able to move easily through webbing, it can entirely and quickly eradicate a population of spider mites if the outbreak is caught in time.
Regular scouting is important, as while Californicus can dramatically slow down a spider mite outbreak and reduce it from spreading, detecting hotspots early and treating with a curative predator - such as Phytoseiulus - is also important to maintain control.
Phytoseiulus-System is easy to observe working immediately upon release due to its bright red colour whereas Californicus-System is beige in colour.
Additional predator for humid conditions
Another curative control is Feltiella acarisuga, which works extremely well in more humid greenhouse climates. Feltiella-System is a Cecidomyid midge whose predatory larvae eat spider mites and, since the adult can fly, is able to search out hotspots in the crop for itself. It can also fly into spider mite colonies on buds and in sticky sugar leaves - which predatory mites cannot access.
Optimising predator conditions
It is critical to understand the optimal conditions in which both the pest and the predators thrive - not just the ambient conditions in the greenhouse, but the delicate veil of micro-environmental conditions on the leaf surface.
During the day, growing plants transpire water through stomata on the undersides of leaves creating a humid microclimate under the leaves and within the canopy. This humidity is important for predatory mites, particularly Phytoseiulus. It is important to understand that air movement, if too fierce, can affect the microclimate and disrupt the humidity layer. Leaf density in the crop also plays an important role in maintaining suitable conditions for predators.
On the other hand, spider mites thrive in dry, hot conditions and are even susceptible to some fungal pathogens in humid conditions.
So, the key messages are:
- Pest prevention is key
- Weekly monitoring and mapping recommended
- Californicus-Breeding-System (sachets) in the vegetative growth phase and flowering phase
- Phytoseiulus-System if any damage seen
- Feltiella-System for additional support
- Consider micro-environmental conditions and manage to suit predators
- Spot sprays permitted products only if necessary