49

Cannabis pests

J. M. McPartland

AMRITA, 53 Washington Street,
Middlebury, VT 05753, USA

        McPartland, J.M. 1996. Cannabis pests. Journal of the International Hemp Association 3(2): 49, 52-55.
        Most Cannabis pests are insects.   Nearly 300 insect pests have been described on hemp and marijuana, but very few cause economic crop losses.  In hemp crops, the most serious pests are lepidopterous stem borers, predominately European corn borers (Ostrinia nubilalis) and hemp borers (Grapholita delineana).  Beetle grubs also bore into stems and roots (e.g., Psylliodes attenuata, Ceutorhynchus rapae, Rhinocus pericarpius, Thyestes gebleri, and several Mordellistena spp.).  In field crops, damage to leaves and flowering tops is usually caused by caterpillars (e.g., hemp borers and budworms), beetles (e.g., Psylliodes attenuata), bugs, and leafminers.   In marijuana crops, the predominant pests are insects with piercing-sucking mouthparts, such as aphids (Phorodon cannabis, Myzus persicae, Aphis fabae), whiteflies (Trialeurodes vaporariorum, Bemisia spp.), leafhoppers, and mealybugs.   The most important non-insect pests are mites (Tetranychus urticae, Aculops cannabicola).  Slugs, rodents, and birds are pests of seedlings and seeds.   The pest-repellent properties of cannabinoids are discussed and correlated to Cannabis pest profiles.

Introduction
      This article is about pests -- the organisms that cause injury, not disease.  It is the second half of a two-part series about diseases and pests of Cannabis. The first part (McPartland 1996) described diseases caused by fungi, bacteria, nematodes, and viruses.  The pests described in this second part include arthropods, mollusks, birds, and mammals.  Some of these pests predominate in fiber crops, others prevail in crops raised for seeds or flowering tops.  Some are specific to Cannabis, others are general feeders.  Some pests have many natural enemies so they only cause problems indoors under artificial lights, other pests cause problems anywhere.
        Cannabis has a reputation for being pest-free.  Actually, it is pest-tolerant. Many pests have been found around Cannabis, but they rarely cause economic damage.  The most common pests are arthropods.

Arthropods
      Six arthropod classes are particularly important to Cannabis agriculture: the Crustacea (including "pillbugs," with 5-7 pairs of legs), Symphyla ("garden centipedes," with 12 pairs of legs), Chilopoda (true centipedes, with 1 pair of legs per segment), Diplopoda (millipedes, "thousand-leggers," with 2 pairs of legs per segment and many segments), Arachnida (spiders and mites, with 4 pairs of legs), and the Class Insecta, with 3 pairs of legs.
        Insects are the largest class.   Twenty-seven orders of insects are currently recognized by entomologists, and half of them attack Cannabis. Mostafa and Messenger (1972) list 272 species of insects and mites associated with Cannabis! Of course, few of these species elicit serious concern.  Probably the worst pests are stem-boring caterpillars, especially in fiber crops.  Two economically important pests are the European corn borer (Ostrinia nubilalis), and the hemp borer (Grapholita delineana).
        European corn borers (ECBs) attract a lot of scientific attention thanks to their amazing appetite for corn plants.  ECBs are native to eastern Europe, where Cannabis sativa and Humulus lupulus (hops) served as original host plants.  ECBs switched to maize after Zea mays cultivation began in Europe two centuries ago (Nagy 1976, 1986).  About one century ago ECBs moved to North America and plagued American hemp, where they "nourished themselves upon the marrow within stalks" (Dodge 1898).  More recently ECBs have infested marijuana crops (Bush Doctor 1987).
        ECB feeding induces stem cankers, which are structurally weak.  Stems supporting heavily flowering tops often break at cankers.   Larvae boring into smaller branches cause wilting of distal plant parts.   Under heavy infestations entire plants collapse.  Emchuk (1937) states 5-12 larvae can destroy a hemp plant.  ECB entry holes in stems are essentially open wounds, providing access for fungi such as Macrophomina phaseolina. Other insects may also crawl in.  ECBs hatching late in the season may infest flowering tops instead of stems, where they spin webs and scatter feces.

Figure 1. Larva, pupa and female moth of Grapholita delineana (A) compared to larger Ostrinia nubilalis (B).  Both about 1.5x actual size. (G. delineana from Senchenko and Timonina 1978, O. nubilalis from Ceapoiu 1958.)

        Hemp borers (HBs) are smaller than ECBs (Figure 1).  HBs cause similar stem damage and are much more destructive in flowering tops.  HBs are also called hemp leaf rollers and hemp seed eaters.  In Russia, HBs have destroyed 80% of a crop's flowering tops (Kryachko et al. 1965).   Bes (1974) reports 41% seed losses in unprotected Yugoslavian hemp.  Each larva consumes an average of 16 Cannabis seeds (Smith and Haney 1973).  HBs appear host-specific on Cannabis (Mushtaque et al. 1973), so they have attracted attention as potential biocontrol agents against marijuana.  Baloch et al. (1974) determined that 40 larvae will kill a Cannabis seedling (15-25 cm tall) in 10 days.  As little as 10 larvae per plant cripple growth and seed production.
        Other Cannabis caterpillars feed as stem borers (e.g., Cossus cossus, Zeuzera multistrigata, Papaipema nebris, P. cataphracta, and Endocylyta excrescens).  Some caterpillars spoil leaves, seeds, and flowering tops (e.g., Mamestra brassicae, Autographa gamma, Melanchra persicariae, Spilosoma obliqua, Arctia caja, and Loxostege sticticalis).  However, few caterpillars cause as much damage as ECBs and HBs.   An exception is the budworm (e.g., Heliothis armigera and Heliothis viriplaca).  Budworms wreck havoc on flowering buds, but leave stems alone.
        Other insects may also bore into stems.   Examples include the grubs of flea beetles (Phyllotreta nemorum), tumbling flower beetles (Mordellistena micans and M. parvula), longhorn beetles (Thyestes gebleri), weevils (Ceutorhynchus rapae and Rhinocus pericarpius), and the maggots of gall midges (Melanogromyza urticivora).
        Beetle grubs and midge maggots also bore into roots and leaves.  The former includes the hemp flea beetle (Psylliodes attenuata), a serious pest in eastern Europe and China (Angelova 1968).  The latter are called "leaf miners."  Some leaf miners are beetle grubs (e.g., Phyllotreta nemorum), but most are tiny maggots (e.g., Liriomyza strigata, L. eupatorii, L. cannabis, Phytomyza horticola, Agromyza reptans).
        The most serious root pests are flea beetle grubs (Psylliodes attenuata) and white root grubs -- Japanese beetles (Popillia japonica) and chafers (Melolontha hippocastani and M. melolontha).   Minor root pests include root maggots (Delia platura), ants (Solenopsis geminata), termites (Odontotermes obesus), fungus gnats (Bradysia sp.), and wireworms (Agriotes lineatus).
        Seedling pests can completely annihilate a young crop before it makes a stand.  Cutworms are the most common.  There are many -- the black cutworm (Agrotis ipsilon), paddy cutworm (Spodoptera litura), beet armyworm (S. exigua), claybacked cutworm (A. gladiaria), common cutworm (A. segetum), and cabbage moth (Mamestra brassicae).   Crickets may also cut down young seedlings.  The worst are field crickets, Gryllus desertus and Gryllus chinensis, followed by house crickets (Acheta domesticus), and mole crickets (Gryllotalpa gryllotalpa).
        All the insects discussed above are distinguished by chewing mouthparts.  Marijuana crops are also frequently damaged by piercing-sucking insects.  Perhaps the stylet used by piercing-sucking insects enables them to bypass toxins on the leaf surface, so they can suck up sap from within the plant.  Cannabinoids are produced in surface glands and possess insecticidal properties (Rothschild et al. 1977).  These insecticidal properties have long been recognized and utilized, especially in India. Cannabis leaves have been scattered in grain bins to repel weevils (MacIndoo and Stevers 1924, Khare et al. 1974), and placed under mattresses to drive off bedbugs (Chopra et al. 1941).   Kashyap et al. (1992) protected piles of potatos for up to 120 days by coating them with a layer of Cannabis leaves.  Sprays made from hemp leaves kill many insect pests (Bouquet 1950, Reznik and Imby 1965, Stratii 1976, Fenili and Pegazzano 1974, Bajpai and Sharma 1992, Jalees et al. 1993).
        If Cannabis is so insecticidal, then how do insects eating it survive?  Perhaps they interspace marijuana meals with less-toxic feeding on other plants. Spilosoma obliqua, for instance, often eats Cannabis leaves and flowering tops (Nair and Ponnappa 1974).  But when Deshmukh et al. (1979) force-fed S. obliqua caterpillars a pure Cannabis diet, they died after 20 days.  Rothschild et al. (1977) conducted interesting 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 predators of the caterpillars.  But THC stunted caterpillar growth, and a high-THC diet killed caterpillars.  Nevertheless, Tiger moth caterpillars preferred eating deadly high-THC varieties to low-THC plants.   Rothschild notes, "Should these compounds [THC] exert a fatal fascination for tiger caterpillars, it suggests another subtle system of insect control by plants."
        The most common pests of high-THC plants are insects with piercing-sucking mouthparts, ostensibly enabling them to bypass surface cannabinoids.  Many piercing-sucking insects also do well in greenhouses under artificial light, such as aphids, whiteflies, leafhoppers, mealybugs, scales, and true bugs.
        Spider mites, while not insects, also suck plant sap, and are the most destructive pests of greenhouse-grown Cannabis (Bush Doctor 1986).  Outdoor crops may also become infested in warm climates -- Cherian (1932) reports 50% losses in field crops near Madras, southern India.  Two species are the worst: the two spotted spider mite (Tetranychus urticae) and the carmine spider mite (T. cinnabarinus). T. urticae and T. cinnabarinus are similar in appearance, in life cycle, and fecundity, but T. urticae thrives in cooler temperatures than T. cinnabarinus. The latter species only causes problems in semi-tropical zones with temperatures above 34°C.  The hemp russet mite (Aculops cannabicola) is equally destructive, but less commonly encountered.   Other mites cited on marijuana include the oriental mite (Eutetranychus orientalis), privet mites (Brevipalpus obovatus and B. rugulosus), and Typhlodromus cannabis.
      Six species of aphids ("plant lice") cause Cannabis problems (Bush Doctor 1985).  Some are fairly specific feeders, such as the bhang aphid (Phorodon cannabis) and hops aphid (P. humuli).  Others are general feeders, such as the green peach aphid (Myzus persicae) and black bean aphid (Aphis fabae).  Aphids congregate on the underside of leaves.  They cause leaf wilting and yellowing.  Entire plants may wilt and die.  Any survivors are stunted.  Some aphids also infest flowering tops, which become distorted and hypertrophied.  As an aphid feeds, it exudes small drops of a sugary sap from its anus.  This "honeydew" causes secondary problems--it attracts ants and can support a heavy growth of black sooty mold.   Besides feeding damage, aphids also spread plant diseases.  They vector viruses, bacteria and fungi from diseased plants to healthy plants as they feed and migrate.
        Whiteflies cause similar problems (Bush Doctor 1985).  The greenhouse whitefly (Trialeurodes vaporariorum), sweetpotato (or tobacco) whitefly (Bemisia tabaci), and silverleaf whitefly (Bemisia argentifolii) are vexing pests.  Leafhoppers and mealybugs can also become serious pests, especially the glasshouse leafhopper (Zygina pallidifrons) and the long-tailed mealybug (Pseudococcus longispinus).
        True bugs, like the Homopterans (aphids, leafhoppers, whiteflies), have piercing-sucking mouthparts and feed on plant sap.   They feed predominately on leaves, but also suck on stems, flowering tops, and unripe seeds.  Bugs, unlike most Homopterans, are outdoor problems.  The southern green stink bug (Nezara viridula) feeds on marijuana in India (Cherian 1932), hemp leaves in Europe (Sorauer 1958) and hemp seeds in the USA (Hartowicz et al. 1971).  Other examples include the tarnished plant bug (Lygus lineolaris), false chinch bug (Nysius ericae), and potato bug (Calocoris norvegicus). Liocoris tripustulatus has become an emergent pest in the Netherlands, where it feeds on pollen.
        Thrips are becoming a problem in modern greenhouses that use rockwool and hydroponics.  In old soil-floored greenhouses, damp conditions encouraged the growth of Entomophthora thripidum, a soil fungus which infects and kills thrips when they drop to the ground to pupate.  Now, with soilless growhouses, there is no damp soil and, therefore, no fungus to act as a natural biocontrol.  Pests include the greenhouse thrips (Heliothrips haemorrhoidalis), western flower thrips (Frankliniella occidentalis), and onion thrips (Thrips tabaci).  These are all general feeders. Oxythrips cannabensis is specific to Cannabis, but is rarely encountered (Bush Doctor 1989).

Non-arthropod pests
      Non-arthropods are minor pests compared with insects and mites.  Mollusks, such as slugs and snails, may cause horrific damage to seedlings in damp and cool bioregions (such as the northwestern and northeastern US).  Slugs cut down seedlings and shred leaves of older plants.
        Birds devour Cannabis seeds.   Early reports from Kentucky describe the passenger pigeon (Ectopistes migratorius) feeding on hemp seeds (Allen 1908).  McClure (1943) ranks wild hempseed as the most important food of mourning doves (Zenaidura macroura) in Iowa.  Captive doves thrive for long periods of time on hemp alone.  Bobwhite quail (Colinus virginianus) and ringtail pheasant (Phasianus colchicus) depend on wild hempseed in the American midwest (Hartowicz and Eaton 1971).  The dependence of these game birds on wild hemp has led wildlife agencies to oppose police eradicating wild hemp (Vance 1971).
        Birds were the most serious pest problem in the recent cultivation of hemp by the Hempstead Company in the Imperial Valley, and substantial losses have recently been registered in Tasmania (Lisson and Mendham 1995).   Sorauer (1958) cites many seed-eating European birds, including the hemp linnet (Carduelis cannabina), magpie (Pica pica), starling (Sturnus vulgaris), common purple grackle (Quiscalus quiscula), tree sparrow (Passer montanus), English sparrow (P. domesticas), nuthatch (Sitta europaea), lesser spotted woodpecker (Dendrocopus minor), and turtledove (Streptopelia turtur).
        Mammals may cause crop damage.  Some are domesticated -- Siegel (1989) describes cows and horses feeding on flowering tops in Hawaii.  Duke (1985) recounts the loss of several horses and mules who gorged themselves on illicit Cannabis in Greece.  Non-domesticated mammals cause more problems.   Such pests include deer in the USA (Siegel 1989, Frank and Rosenthal 1978), monkeys in South America (Siegel 1989), and dik-dik in Kenya.
        Hartowicz and Eaton (1971) found Kansas cottontails (Sylvilagus floridanus) feeding on wild hemp.  Siegel (1989) cites raccoons (Procyon lotor) eating Cannabis. In Europe, Sorauer (1958) reports field voles (Microtus sp.) and hamsters (Cricetus cricetus) feeding on sown hemp seeds.  Siegel (1989) mentions mice breaking into San Francisco police vaults to feed on seeds in confiscated marijuana!  Alexander (1984) describes mice eating young sprouts, and groundhogs ("woodchucks"), gophers, moles and voles destroying roots.  Groundhogs can also quickly destroy a young Cannabis stand by feeding on some plants and rolling themselves around in the rest.  Siegel (1989) adds Rattus rattus to this list.  Rats kill plants by stripping bark from stems to build nests.  In Western Europe, pre-adult Homo sapiens who confuse fibre hemp for marijuana are a notable pest (Van der Werf 1996).

Table 1. Common Cannabis pests

References

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This article is excerpted from "Biocontrol of Cannabis diseases and pests", publication due 1997.