Cannabis as repellent and pesticide

John M. McPartland

53 Washington Street, Middlebury, VT 05753 USA

        McPartland, John M. 1997. Cannabis as repellent and pesticide. Journal of the International Hemp Association 4(2): 87-92 Cannabis has been used as a pest repellent and pesticide in a variety of formulations. It has been planted as a companion crop to deter insects, nematodes, fungi, and weedy plants. Dried leaves and flowers have repelled or killed insects, mites, nematodes, and weeds. Plant extracts (either aqueous or polar organic solvent extracts) have killed or repelled insects, mites, nematodes, fungi, weedy plants, bacteria, and protozoans. Pure cannabinoids reportedly inhibit or kill bacteria, fungi, and insects. The validity of some of these reports is debated. Most of the scientific literature describes in vitro experiments, few studies concern field work. Utilizing left-over Cannabis leaves against pests appears to be a possible use for this harvest residue.

        Certain plants with repellent and pesticidal properties have long been used to rid our crops, homes, and bodies of various pests (Grainge and Ahmed 1988). Famous examples include garlic (Allium sativum L.), castor (Ricinus communis L.), marigold (Tagetes patula L.), tansy (Tanacetum vulgare L.), neem (Azadirachta indica L.), pyrethrum (Chrysanthemum cinerariifolium (Trevir.) Vis.), tobacco (Nicotiana tabacum L.), and strychnine (Strychnos nux-vomica L.). Cannabis has also been used as a pest repellent and pesticide. This paper documents these uses from the scientific literature.

Materials and methods
        The following data bases were searched with the keywords Cannabis, hemp, marijuana, cannabinoids: AGRICOLA (1990-1996), Biological and Agricultural Index (1964-1990), Review of Agricultural Entomology (1913-1996, a continuation of Review of Applied Entomology, Series A), Review of Plant Pathology (1922-1996, a continuation of Review of Applied Mycology), MEDLINE (1984-1994), and Index Medicus (1964-1994).
     Cataloged holdings at the following libraries were searched for texts concerning allelopathy, biopesticides, botanical pesticides, and biological control: Dartmouth, Harvard, Michigan State University, National Agriculture Library, National Library of Medicine, Pennsylvania State University, Stanford University, University of Illinois, University of Missouri, University of Michigan, University of Pennsylvania, University of Vermont. All information regarding Cannabis was scanned for supporting citations, and antecedent sources were retrieved.

        Cannabis has been utilized as a pest repellent or pesticide, in a variety of formulations. These formulations include dried plant parts, plant extracts or pure cannabinoids, as well as use of the genus as a "companion plant".

Companion plants
        Companion plants constitute a form of biological control - the use of living organisms to manage unwanted pests and disease organisms. Cannabis plants have been grown as companion plants alongside crops which require this protection. Riley (1885) noted that Cannabis sativa growing near cotton exerted a "protective influence" against cotton worms (Alabama argillacea, then called Aletia xylina). Similarly, hemp grown around vegetable fields safeguarded the fields from attack by a cabbage caterpillar, Pieris brassicae (Beling 1932); potato fields were protected against the potato beetle, Leptinotarsa decemlineata (Stratii 1976); wheat suffered less damage by the root maggot, Delia coarctata (Pakhomov and Potushanskii 1977); and root exudates of Cannabis repelled underground larvae of the European chafer Melolontha melolontha (Mateeva 1995). Some of these reports have been refuted in subsequent studies (Ziarkiewicz and Anasiewicz 1961, Mackiewicz 1962, Kurilov and Kukhta 1977).
         Cannabis suppresses the growth of neighboring plants, whether they are noxious chickweed, Stellaria media (Stupnicka-Rodzynkiewicz 1970) or valuable crops such as lupine, beets, brassicas (Good 1953) and maize (Pandey and Mishra 1982). Hemp has been interplanted with potatoes to deter the potato blight fungus, Phytophthora infestans (Israel 1981).
     Hemp has been rotated with potatoes to suppress the potato cyst nematode, Heterodera rostochiensis (Kir’yanova and Krall 1971). Hemp rotations also suppressed soil populations of the root knot nematode, Meloidogyne chitwoodi (Kok et al. 1994). Some cultivars of Cannabis are resistant to Meloidogyne hapla (de Meijer 1993). Scheifele et al. (1997) assessed the soil populations of several nematodes, before and after a hemp crop (using cultivars ‘Unico B’ and ‘Kompolti’) in Ontario, Canada. The hemp crop suppressed soybean cyst nematodes (Heterodera glycines), but increased the populations of spiral nematodes (Heliocotylenchus or Scutellonema species) and root knot nematodes (Meloidogyne incognita).
     Mateeva (1995) studied an unspecified Meloidogyne on four different crops growing in Bulgarian soil. After 30 days, cucumber plants averaged 56 root knots per plant and 396 Meloidoygne larvae were found in the surrounding soil. Tomato plants averaged 42 root knots and 318 larvae, Cannabis plants averaged 5 root knots and 21 larvae, and marigolds averaged 1 root knot and no larvae. Mateeva concluded, "by including unfriendly plants in the rotation scheme with tomato and cucumber, it is possible to obtain a soil completely cleared from root knot nematodes."

Dried plant parts
        Grewal (1989) suppressed mushroom-eating nematodes (Aphelenchoides composticola) by mixing 3 kg of dried Cannabis leaves into 137 kg of the compost used to cultivate edible Agaricus mushrooms. He also measured suppression of most mesophilic fungi in the mixture - especially Fusarium solani, Trichoderma viride, and Verticillium sp. Storage fungi such as Aspergillus spp. and Penicillium spp. demonstrated little suppression.
     Dressing Cannabis seed cakes into soil successfully reduced populations of the soil nematode Meloidogyne incognita (Goswami and Vijayalakshmi 1986). Hemp straw mixed into soil depressed the growth of quack grass, Agropyron repens (Muminovic 1990), and rice, Oryza sativa (Vismal and Shukla 1970).
     Dried leaves have been used to repel weevils in stored grain (Riley and Howard 1892, MacIndoo and Stevers 1924) and woolen cloths (Bouquet 1950). Scattering a 2 cm layer of dried, powdered leaves over piles of potatoes protected them from the tuber moth, Phthorimaea operculella, for up to 120 days (Kashyap et al. 1992). Khare et al. (1974) repelled Sitophilus oryzae weevils by mixing powdered Cannabis leaves into wheat, 1% w/w. Prakash et al. (1987) mixed dried leaves into rice, 2% w/w, to control S. oryzae weevils in the laboratory, but this dose failed to provide adequate protection under natural storage conditions (Prakash et al. 1982).
     Dried leaves or juice squeezed from fresh leaves have removed vermin from the scalp and ears (Culpeper 1814, Indian Hemp Drugs Commission 1894), driven off bedbugs when placed under mattresses (Chopra et al. 1941), and killed larvae of the ticks Ixodes redikorzevi, Haemaphysalis punctats, Rhipicephalus rossicus, and Dermacentor marginatus (Reznik and Imbs 1965). In the latter study, powdered leaf material killed larvae in 10-12 minutes, whereas exposure to fresh, whole leaves killed larvae in 50 to 72 minutes.

Plant extracts
        Plant extracts are a popular pesticide formulation. Extracts are produced by soaking fresh or dried plant material in a solvent. The plant material may be mashed or soaked whole. After an appropriate period of time, the solid material is filtered out, leaving the liquid extract. Aqueous extracts are the most popular. Polar organic solvents, such as alcohol or ether, are sometimes used to extract lipids from plant material.
     Many reports describing the use of Cannabis extracts have omitted important information. For instance, the plant part harvested, and when it was harvested (what stage of the life cycle)? How much plant material (by weight) was soaked in the solvent (by volume)? How long was the material extracted, and at what temperature?

        Inadequately described extracts have killed insect pests (Bouquet 1950, Abrol and Chopra 1963) and the mite Tetranychus urticae (Fenili and Pegazzano 1974). Metzger and Grant (1932) sprayed Japanese beetles with a United States Pharmacopoeia (U.S.P.) 80% ethanol extract, diluted to 1/64 strength. The spray weakly repelled adult beetles. (U.S.P. extracts are no longer available.)
     Stratii (1976) boiled flowering hemp plants in water and sprayed the decoction on potato plants to kill the potato beetle (Leptinotarsa decemlineata). Jalees et al. (1993) killed mosquito larvae (Anopheles and Culex species) with an ethanol extract. Bajpai and Sharma (1992) sprayed a 20% w/v cold water extract of "bhang" on crop plants to reduce egg laying by Chilo partellus, a lepidopteran borer. A 20% w/v petroleum ether extract killed 40% of the borers, and this toxicity persisted for four days.

        Mojumder et al. (1989) ground up 100 g of fresh Cannabis leaves in 25 ml water and filtered it through muslin cloth. This extract killed J2-stage juvenile nematodes (Heterodera cajani) in 6 hours. A 5% solution of the extract also killed nematodes after 24 hours. Haseeb et al. (1978) macerated 10 g of either roots or above-ground shoots in a Waring blender for 10 seconds and extracted the mash for 24 hours in 75 ml distilled water. The shoot extract did slightly better than the root extract, killing a variety of plant-pathogenic nematodes (Hoplolaimus indicus, Rotylenchulus reniformis, Tylenchorynchus brassi-cae). When diluted to 10% of its original strength, the root extract caused greater mortality than the shoot extract.

        Ferenczy et al. (1958) extracted an Italian hemp (cultivar ‘Bologniensis’) in "organic solvents and alkali". The extract did not inhibit in vitro growth of yeasts (Saccharomyces, Rodotorula, Hansenula spp.) or filamentous fungi (Aspergillus, Penicillium spp.). Ethanol extracts of leaves inhibited spore germination of Ustilago species (Misra and Dixit 1979, Singh and Pathak 1984) and Neovossia indica (Gupta and Singh 1983).
     Upandhyaya and Gupta (1989) inhibited Curvularia growth on petri plates with an aqueous extract, prepared by extracting 10 g of Cannabis leaves in 100 ml cold water. Greater inhibition was achieved with an ethanol extract, mixing 5 g leaves in 100 ml of 80% ethanol. In a similar experiment, Kaushal and Paul (1989) inhibited growth of Colletotrichum truncatum, but not Septoria glycines or Ascochyta phaseolorum. Neither aqueous nor ethanol extracts inhibited growth of the human pathogen Trichophyton rubrum or the opportunistic pathogen Aspergillus niger (Gupta and Banerjee 1972).
     Pandey (1982) crushed 10 g of leaves in 100 ml water and filtered the mash with filter paper. A 40% solution of the extract inhibited 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. An aqueous extract of hemp protected pine seedlings from a disease caused by an unnamed Fusarium species (Vysots’kyi 1962).

         In vitro experiments show that whole seeds inhibited growth of gram (+) Bacillus cereus (Ferenczy 1956). Subsequent research by Ferenczy et al. (1958) disclosed that the flowering tops and "resinous organs" of hemp inhibited growth of gram (+) bacteria (B. cereus, B. subtilis, Staphylococcus aureus), but did not affect gram (-) bacteria (Escherichia coli, Pseudomonas aeruginosa, Salmonella paratyphi, Shigella species). Ferenczy and coworkers concluded, "the antibacterial activity is ever proportionate to the intensity of the hashish reaction [potency]."
     In contrast, Radosevic et al. (1962) stated, "antibiotic activity decreases with...the increase of hashish activity." They examined resins extracted from 11 different varieties, including tropical hashish varieties (from Brazil) and temperate hemp varieties (from several European countries and Canada) and assayed the effects of these ethanol extracts (using 60 mg Cannabis resin per 1 ml ethanol) on growth of B. subtilis. concluding that cannabidiolic acid was the primary antibiotic agent.
     Ethanol extracts of Cannabis tissue cultures inhibited gram (+) S. aureus, Bacillus megaterium, and gram (-) Escherichia coli, but did not affect gram (-) Pseudomonas aeruginosa (Veliky and Genest 1972). In a subsequent study (Veliky and Latta 1974), thin layer chromatography located two active Cannabis ingredients at R f 0.87 and R f 0.61 on the chromatography plates.
     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 (Zelepukha 1960). The extract worked against gram (+) Corynebacterium species and gram (-) Pseudomonas and Agrobacterium species (Bel’tyukova 1962). Aqueous extracts also inhibit gram (-) Erwinia carotovora and other bacteria causing soft rot of potatoes (Vijai et al. 1993).

        Srivastava and Das (1974) prepared an aqueous extract by crushing 100 g of Cannabis leaves and stems in 10 ml of water at 30 o C. The extract inhibited seed germination of purple nut sedge, Cyperus rotundus. Stupnicka-Rodzynkiewicz (1970) used aqueous extracts to inhibit weed seed germination of false chamomile (Matricaria recutita) and lemon grass (Lepidium sativum).

        Nok et al. (1994) prepared a seed extract by crushing 10 g of dried seeds, mixing the powder in 200 ml petroleum ether, stirring continuously for 1 hour, filtering, and then washing the filtrate with 100 ml portions of N/10 NaOH. The ether extract had a profound trypanocidal effect on Trypanosoma brucei tested in vitro. Mice infected by T. brucei were injected with the extract (50 mg/kg/d) and cured in 5 days.

Pure cannabinoids
        Cannabinoids are a family of C 21 terpenophenolic compounds uniquely produced by Cannabis (Turner et al. 1980). Some studies reportedly working with pure cannabinoids are probably erroneous, such as those working with aqueous extracts (Bajpai and Sharma 1992). Aqueous extracts should contain little cannabinoid, as these compounds are not very water-soluble. In a study using ethanol extracts, Veliky and Latta (1974) located two zones of bacterial inhibition at R f 0.87 and R f 0.61. These R f values did not correspond to the tetrahydrocannabinol (THC) and cannabidiol (CBD) values they had determined in a previous study (Veliky and Genest 1972). Nok et al. (1994) assumed that the ether extract they successfully tested against Trypanosoma brucei contained cannabinoids. But they had extracted Cannabis seeds, which do not contain cannabinoids except sporadically as contaminants from bract debris clinging to the outside of the seeds (Máthé and Bócsa 1995).

        Schultz and Haffner (1959) inhibited gram (+) S. aureus and B. subtilis with a dilute solution (1:100,000) of CBD. Inhibition of gram (-) E. coli required a much stronger solution (1:1,000) of CBD. Gal et al. (1969) tested cannabinolic acid as a fruit juice preservative and reported its effectiveness against gram (+) B. cereus, Lactobacillus plantarum, and Leuconostoc mesenteroides. Klingeren and Ham (1976) inhibited gram (+) S. aureus, Streptococcus pyongenes and S. faecalis growing in broth with THC and CBD at minimum inhibitory concentrations of 1-5 µg/ml. Both compounds were also bactericidal. Gram (-) organisms (E. coli, Salmonella typhi, Proteus vulgaris) were resistant to THC and CBD at the highest concentration tested (100 µg/ml).

        Flowering tops, where cannabinoid concentrations are highest, are less susceptible to some fungal pathogens (Charles and Jenkins 1914, McPartland 1983). McPartland (1984) extracted flowering tops with petroleum ether, then eluted the extract into its separate components with thin-layer chromatography (using silica gel on glass plates). He sprayed eluted plates with spores of Phomopsis ganjae suspended in a nutrient solution. Spore germination was inhibited by three components of the extract, located by thin-layer chromatography at R f 0, R f 0.20 and R f 0.68. The latter two zones consisted of CBD and THC, respectively.
        Dahiya and Jain (1977) extracted THC and CBD using adsorption column chromatography, (affirming correct identification with thin-layer chromatography and gas chromatography), and assayed effects of these cannabinoids against in vitro growth of 18 fungi. Generally, THC inhibited human pathogens (e.g., Microsporium and
Trichophyton species) more than CBD. Conversely, CBD inhibited plant pathogens (e.g., Alternaria alternata, Curvularia lunata, Fusarium solani, Trichothecium rose-um) more than THC. Two fungi were completely resistant to THC and CBD - Aspergillus niger and Penicillium chrysogenum. Interestingly, these two species are frequently isolated from moldy marijuana (McPartland and Pruitt 1997).

        Rothschild et al. (1977) conducted fascinating experiments with tiger moths (Arctia caja). Tiger moth caterpillars, like monarch butterfly caterpillars, feed on poisonous plants and store plant poisons in their exoskeleton. The stored poisons repel caterpillar predators, providing an evolutionary advantage. Rothschild et al. (1977) fed the caterpillars a Mexican plant rich in THC. The caterpillars stored some of the THC in their exoskeleton. But caterpillars fed a pure diet of high-THC plants were stunted and did not survive beyond the third instar. Nevertheless, Tiger moth caterpillars preferred eating the deadly high-THC varieties to low-THC plants. Rothschild noted, "...should these compounds [cannabinoids] exert a fatal fascination for tiger caterpillars, it suggests another subtle system of insect control by plants."

        Research demonstrates the moderate efficacy of Cannabis as a repellent crop or botanical pesticide. We are faced, however, with a paradox: some of the pests controlled by Cannabis are also known to attack hemp and marijuana crops. If Cannabis serves as an effective pesticide, then how can the pests infesting Cannabis survive? Perhaps they intersperse marijuana meals with less-toxic lunches on other plants. Caterpillars of Spilosoma obliqua, for instance, eat Cannabis leaves and female flowers (Nair and Ponnappa 1974). But when Deshmukh et al. (1979) force-fed S. obliqua caterpillars a pure Cannabis diet, they died after 20 days.
     Obviously Cannabis is not a cure-all poison; a highly toxic botanical insecticide could not also serve as a human medicament, as Cannabis does (Clarke and Pate 1994, McPartland and Pruitt 1997). Indeed, plants that are never infested by insects are considered dangerous by traditional Ayurvedic healers in India (Thatte et al. 1993). What active ingredients in Cannabis work so well against pests? Is THC the primary pesticidal ingredient? Characterizing THC as a powerful pesticide would put it in the same category as nicotine, which is highly toxic to arthropods, fish, birds, and mammals. Although THC and other cannabinoids have proven activity against bacteria and fungi, their activity against insects is questionable. The study by Rothschild et al. (1977) assumed the sole difference between high-THC and low-THC plants was the level of THC. Recent work has shown that high-THC plants produce 3 to 6 times more limonene and pinenes than most low-THC plants (Mediavilla and Steinemann 1997). Cannabinoids are very safe to mammals. The oral LD 50 of THC in mice is greater than 21,600 mg/kg (Loewe 1946), safer than neem oil.
     Cannabinoids probably play a small role in Cannabis’ pesticidal activity, but aqueous Cannabis extracts work very well as pesticides, even if they contain little cannabinoid. Cannabis contains more than 400 chemicals (Turner et al. 1980). Their leaf glands ooze dozens of volatile compounds, such as terpenes, ketones, and esters which produce the characteristic odor of the plant (Ross and ElSohly 1996). The limonene and several pinenes which comprise over 75% of volatiles detected in the "headspace" atmosphere surrounding Cannabis leaves (Hood et al. 1973, Ross and ElSohly 1996) are powerful insect repellents. Methyl ketones present in Cannabis (Turner et al. 1980) also repel many leaf-eating insects (Kashyap et al. 1991). A synergistic combination of these many compounds may serve as the "active ingredient" in Cannabis.