This is a short overview of one not very popular cannabis application – as repellent and pesticide.
Cannabis has been planted as a companion crop to deter insects, nematodes, fungi and weedy plants. Interestingly dried leaves and flowers, plant extracts and pure cannabinoids are known to repel and kill insects, mites, nematode, fungi and weeds. Companion plants constitute a form of biological control – the use of living organisms to manage unwanted pests and disease organisms.
What do we know about cannabis as repellent?
In 1885 Riley observed that when growing Cannabis Sativa near cotton it has protective effect against cotton worms (Alabama argillacea). Similar effects were observed when growing hemp around vegetable fields (protection against cabbage caterpillar), potato fields (protection against potato beetle, potato blight fungus and potato cyst nematode), wheat fields (protection against root maggot).
Cannabis was known to suppress the growth of chickweed, lupine, beets, brassicas and maize.
How can we use dry hemp plant parts?
Reports from 1892 and 1924 show that dried cannabis leaves have been used to repel weevils in stored grain and woolen cloths. Scattering a 2 cm layer of dried, powdered leaves over piles of potatoes protect them from tuber moth for up to 120 days. Mixing powdered cannabis leaves repel weevils from wheat.
Dried leaves or juice squeezed from fresh leaves could remove vermin from the scalp and ears and killed larvae of the ticks in 10-12 min.
How can we use plant extracts?
80% ethanol extract of cannabis plants repel Japanese beetles when sprayed weekly. Boiled flowering hemp part in water could kill potato beetle, while ethanol extracts kill mosquito larvae. 20% petroleum ether extract kills 40% of the lepidopteran borer and the toxicity persist for 4 days.
100 g of fresh cannabis leaves in 25 ml water filtered in muslin cloth can kill juvenile nematodes in 6 hours. Variety of plant-pathogenic nematodes can be killed with 10 g roots in a Waring blender for 10 seconds and extracted the mash for 24 hours in 75 ml distilled water.
40% solutions of the extract from crushed 10 g of leaves in 100 ml water and filtered the mash with filter paper inhibits 25 different species of fungi that infested stored seeds of finger millet (Eleusine cora-cana L.), including species of Aspergillus, Penicillium, Cladosporium, Drechslera, Fusarium, Cephalosporium, Rhizopus, Mucor and Curvularia.
Soviet agronomists sprayed an aqueous extract of hemp and wild hemp, called “cansatine 4” or “konsatin,” on potato crops and tomato seeds to kill plant-pathogenic bacteria. The extract worked against gram (+) Corynebacterium species and gram (-) Pseudomonas and Agrobacterium species. Aqueous extracts also inhibit gram (-) Erwinia carotovora and other bacteria causing soft rot of potatoes.
Research with CBD extracts show inhibition of gram (+) and (-) bacteria.
An aqueous extract by crushing 100 g of Cannabis leaves and stems in 10 ml of water at 30 C inhibits seed germination of purple nut sedge, Cyperus rotundus. Aqueous extracts inhibit weed seed germination of false chamomile (Matricaria recutita) and lemon grass (Lepidium sativum).
A research from 1977 showed that THC inhibits more human pathogens than CBD and CBD was more active against plant pathogens than THC. Interestingly 2 fungi were completely resistant to THC and CBD and they are frequently isolated from moldy cannabis plants – Aspergillus niger and Penicillium chrysogenum.