Common Hemp Crop Pests

Despite claims that growing hemp requires no pesticides, the plant does have its pest problems. With care, however, even these can be avoided without the use of chemicals. There are no pesticides or herbacides registered for hemp growing. Although most hemp crops do not suffer damage from animal pests, it does happen, causing serious economic loss.

The term “pests” generally refer to insects, diseases, weeds, nematodes, slugs, and mice. Pest problems are always aggravated by large areas where hundreds or thousands of acres are grown in one place.

Because of the risk of bringing disease or insect pests into the country, hemp seeds must be certified to be free of at least four diseases and one parasitic weed species. Hemp is known to have many insect pests, including the hemp stem borer and bertha army worms.

As far as hemp is concerned, the specified diseases are all rare. Specialized literature describes a number of other pathogens in addition to those mentioned here; but these are rarely, if ever, a threat to hemp cultivation.

Weeds
Among all the weeds and parasitic plants only one is important to hemp cultivation.

Broomrape (Orobanche ramosa): It damages fiber hemp; but this only applies to Chinese varieties in seed-bearing stock, which is not cultivated in Germany or western Europe. The roots of broomrape penetrate hemp roots and extract nutrients. Modern seed-cleaning methods have significantly reduced the risk of damage from broomrape, but the cultivation of resistant hemp varieties is still recommended.

Insect Pests

Hemp Flea (Psylliodes attenuata): It is the most dangerous insect pest to industrial hemp. Damages are noticed only when the temperature of the top layer of soil and the air rises to 10-15°C (50-60°F) and the weather is dry. The hibernating fleas then crawl from the soil and feed on the cotyledons and on the hypocotyl (the seedling stem below the cotyledons). If the plants have already developed, the fleas will even chew small holes in the leaves; and, if they have reproduced extensively, can consume the complete hypocotyl right down to the topsoil. More than 50% damage to the leaf surface will kill the plants; but they can survive with less damage, although development will be much slower and overall crop yield will be reduced. Fighting the flea is a challenge because it is not a threat every year.
Hemp Borer (Grapholita delineana): Also known as the hemp moth, it can have two or three generations per year. It first proved to be a problem in the late 1960s. The first generation larvae damage fiber hemp. During the winter, they hibernate in the stalk and stubble. After mid-May the following year when temperatures reach 15°C (60°F), the larvae spin cocoons around themselves. A week or two later, the moths are fully developed and emerge from the cocoons. The moths are long and greyish-brown with three or four white stripes on its back.
Hemp Moth: It is a skilled flyer and searches for fiber hemp fields, using its excellent sense of smell. The female lays 100-200 eggs, scattering them beneath the growth apex of the stalk (the youngest part of the plant). The tiny, hatched catepillars feed on the backside of the leaves during the first larval stage. In the second stage, after preparation in the ground, they feed in the cavity of the stem medulla where they continue to develop until they reach the 5th larval stage, or until the wood tissue has developed to the point where the larvae can feed on it by chewing with their teeth. They proceed to crawl through the hole and begin feeding on the younger, more tender sections of the stalk. In the fifth larval stage, they spin cocoons around themselves and develop into moths. This generally occurs at the end of June or beginning of July. The damage done by the borers is visible as trails on the stalk and the galls. Such hemp stalks have low fiber yield, and quality does not meet industry standards.
The second generation of hemp moths appears at the end of July. Since harvesting has usually begun, the larvae of this generation cannot fully develop on the plants. Therefore, they must move from the harvested hemp and crawl into the moist soil to hibernate. In Hungary, hemp farmers burn the stubble and stalk remnants as a preventative. The field is plowed with a deep furrow and the wild-growing hemp along the fields and roads is destroyed. In Germany, burning is prohibited because of the risk of uncontrolled fires, heavy air pollution, and damaging effects to wildlife.
European Corn Borer (Ostrinia nubilalis): Damage to fiber hemp crops by the corn borer occurs primarily in eastern and southeastern Europe. The last such damages worth noting were registered in the late 1950s. Damage occurs in the form of bore holes created by the caterpillars as they bore through the stalk, but no gall is formed. Since the European corn borers are larger insects and consume more than the hemp borers, they devour larger pieces of wood, causing the hemp stalk to break in the wind. Preventative measures have been adopted in an attempt to keep European corn borers from reproducing. A system for fighting these pests with the use of chemicals has never been generated, but if the European corn borer multiplies, the methods used against the hemp borer (G. delineana) appear to be promising.

Other Pests

Hemp Greenfly (Phorodon cannabis): Fiber hemp can be damaged by this pest, but it has never occurred in large numbers.
Northern Rootknot Nematode (Meloidogyne hapla): This parasite has recently been reported in the Netherlands. It can cause significant damage to crops in countries with an oceanic climate, as Belgium, Luxembourg, the Netherlands, England, and western sections of Germany. The nematodes can presently be controlled only by using resistant plant varieties.

Fungal Diseases

Pythium Disease (Pythium debaryanum): This fungus, commonly referred to as ‘the young plant disease’, attacks both the seeds and the sprouting plants. It is a nonspecific parasite that attacks many different varieties of sprouting plants. The pathogen breeds in wet, poorly aerated soil and can be combatted with appropriate agrotechnical actions. The fungus appears mainly during the germination stage or shortly thereafter, and causes the plants to topple over.
Hemp Canker (Sclerotinia sclerotiorum): This disease occurs in wet soil or during years of particularly high amounts of precipitation. The fungus attacks the plant tissue at the base of the root and eventually kills the plant. Damage to the plant occurs in the form of a yellowish-brown discoloration on the lower portion of the stalk. Later, the leaves turn yellow and fall off, and the plant dries out. The best method of control is to avoid fields with high water tables and to improve the water balance of the soil by means of technical cultivation methods.
Grey Mold (Botrytis cinerea): Affected plants develop blotches, causing the stalks to break and topple over. Grey mold can also affect the crop during field retting if the stalks get wet repeatedly from recurrent rainfall. Presently, no method has been developed to fight grey mold, which causes crop damage primarily in western Europe. It is possible to fight grey mold by means of resistance breeding, crop rotation, and soil rejuvenation. This disease is practically nonexistent in the dry climates of southeastern Europe.
Hemp Rust (Melampsora cannabina): This pathogen attacks the plant fiber. Its presence can be detected when, on both sides of the leaf, orange-colored blotches appear, from which yellow spores fall. The control method is to spray with a solution containing thiocarbamate (Zineb-Maneb). Specific conditions must be met when using Maneb because this chemical has high aqueous and fish toxicities. A minimum distance of 20 m (65 feet) has to be kept away from lakes, rivers, or streams. Furthermore, Maneb cannot be used in areas where there is a significant danger of runoff or erosion. Hemp rust does not result in infestation epidemics.

Other Causes for Damage

Triazine Derivatives: Damaging effects of such residues (Atrazine) have occasionally been detected on smaller plots. In every case, the first noticeable damage appears during the first growth stage and first on the more developed plants, in the form of drying leaves and deformation of the stalk. Such a plant ultimately dies. This process begins in patches, then in smaller sections of the crop. The soil in these areas has been found to contain triazine derivatives, even after the stipulated waiting period had elapsed, which shows that hemp is extremely sensitive to residue from triazine derivatives. This residue also regularly appears when the previous rotational crop has been corn that was repeatedly treated with triazine.
Leaf Herbicides: In several farms in Hungary during the past few years, deformed leaves and stalk portions have occurred with greater frequency in fiber-hemp crops in which plants had already reached a height of 40-60 cm (16-24 inches). The damage could be shown to be a direct result of the use of leaf herbicides applied with airplanes onto neighboring cereal crops. In all cases, the prescribed techniques were obeyed; but air movement caused by the planes transported very small droplets of the herbacide spray to neighbouring hemp fields. In smaller quantities, the derivatives do not completely destroy hemp plants; but the leaves, petioles, and stalks become deformed, growth is halted, and intensive growth does not resume until two or three weeks later. The stem segments branch out like a fork and the nodes become elongated. Such stalks no longer have any industrial value. Similar experiences with hemp’s herbacide sensitivity have also been referenced in Germany. For example, beet breeders who have grown hemp as a buffer crop have noted its sensitivity to herbacides. The damage caused by herbicides can be avoided with proper handling and application, but any damage done is irreversible.
Weather Damage: Hail can cause severe damage to a hemp field just as it can to any other crop. Even small hailstones can break the stalks of young plants, which may die or develop secondary shoots not capable of surviving. The stand can be thinned out by numbers of dead plants, increased spacing between the plants, thicker stalks, and worsening quality. If branches develop on a stalk, that plant is no longer suitable as fiber hemp. Only fiber-hemp crops are damaged by hail which does not affect that cultivated for its cellulose (paper). Minor damage occurs in widely spaced hempseed crops because the injured plants branch out and produce more seeds. However, more densely spaced seed crops can suffer damage from hailstorms.
Drought: It can reduce the yield of fiber-hemp just as it can lessen the yield of any other cultivated plant. The hemp stalk remains smaller; and if the stalk length is near a quality-class limit, it is likely the fiber quality that will be classified in a lower group.
Water: It can damage hemp crops even though it is vital for its growth. Fiber hemp should not be cultivated near inland waterways or in areas prone to high water tables because hemp will not survive if exposed to too much water over a 24-hour period. The same holds true for long, hard rainfalls if the water remains in the field for longer than 12 hours. In such cases, the development of the plant will be halted so that it turns yellow and may eventually die.