Everything you need to know about controlling pests of medicinal plants. Learn about:- 1. Types of Pests Infesting Medicinal Plants 2. Methods of Pest Control in Medicinal Plants 3. Biopesticides.
Population increases in arithmetic progression, whereas the agricultural produce is enhanced in geometric progression. What man has reaped must be protected. At the same time, future yields must be improved. But, unfortunately this is not so because, between the time a medicinal crop is harvested and consumed by man, considerable quantity of crude drug is wasted or destroyed by pest.
Also, loss in quality occurs when these pests are allowed to grow on produce. The overall losses due to pest infestation tune to millions of rupees every year. Pest is an undesired animal or plant species and pesticides are chemicals derived from synthetic and natural sources effective in small concentrations against pest.
Types of Pests Infesting Medicinal Plants:
The different types of pests infesting medicinal plants are:
1. Fungi and viruses
2. Insects,
3. Weeds, and
4. Non-insect pests including rodents.
1. Fungi and Viruses:
Different types of fungi are known to occur on medicinal plants.
Ascochyta atropae causes the formation of greyish-white irregular spots which further cause necrosis of leaves. The disease is called leaf necrosis. Cercospora atropae causes round to angular brown spots with chestnut coloured margins on both sides of leaves. It is called as leaf-spot. Phytophthora nicotianae is a dreadful disease occurring on belladonna and other plants in which dropping of young leaves and branches, yellowing of older leaves and drying of whole apical portion occur.
This disease is called as phytophthora root-rot. The association of Fusarium solanii and Pythium butleri causes damping off in young seedlings. The disease occurs on isolated branches of the roots of older plants. Phytophthora erythro-sceptica causes damping off in young seedlings and wilt in matured plants. It causes black colouration of root. The disease is called phytophthora rot disease.
Several other pathogenic fungi infest the medicinal and aromatic plants and cause diseases, like Pythium spinosum (pythium rhizome rot); Currularia prasadii (leaf blight); and Collectotrichum fuscum (anthracnose) on digitalis; Septoria digitalis and Phyllosticta digitalis (leaf spot); Ascochyta kashmiriana (leaf spot), Peronospora hyoscyami (mildew) and Thiclavia basicola (root rot) occurring on hyoscyamus, Fusarium oxysporum (wilt), Levellula taurica (powdery mildew), Cercospora rauuolfiae (leaf spot) and Alternaria tenuis (leaf blight and bud rot) occurring on rauwolfia; Sclerotium rolfsfi (wilt), Corticium solanii (root and root rot) and Alternaria tennussima (leaf spot) occurring on Datura species; Fusarium solanii (root rot); Sclerotinia miner (damping off), and Ramularia bellunensis (bud disease) occurring on pyrethrum; Puccinia dioscorea (bust) and Cercospora dioscorea (leaf spot) on Dioscorea; Phytophthora species (blight) on vinca; Erysiphe cinchoracearum (powdery mildew), Verticillium albotarum (wilt), Puccinia menthae (rust), Alternaria sp. (leaf blight), Curularia lunatus (leaf spot) and Sclerotium rolfsii (sclerotium rot) on Mentha species; Cercospora canescens (leaf spot) on Ocimum species; Cylindrocladium scoparium on Eucalyptus; Cercospora jasminicola (leaf blight) and Uremyces hobsonii (rust) on Jasminum species and Colectrotrichum graminicola on lemongrass.
Many different viruses are also the cause of some diseases occurring on plants. They are mosaic causing necrosis of leaves, petioles and stem on different solanaceous plants. Tobacco mosaic virus, cucumber mosaic virus and tobacco ring spot virus are observed on digitalis and a strain of cucumber mosaic virus is detected on hyoscyamus.
The viruses show disease symptoms on rauwolfia, tobacco, datura, vinca and eucalyptus. Other viruses reported on medicinal plants are yellow vein mosaic, graft transmissible virus, distortion mosaic, rugose leaf curl, and Ruga tabaci.
2. Insects:
It is found that the total number of insect species in the world is larger than the total number of species of all other forms of life put together. Throughout the world, about one million species of insects have been reported. These small creatures cause a drastic problem from which the plants must be properly protected.
Various insect pests which attack medicinal plants that can be enumerated here are Agrotis species; Heliothis armigera, and Odontotermes obesus. Flea beetle, Empoasca pteridis, Laphygma exigna and Odontotermes obesus and Phytomyza atricornis are the insect pests occurring on Mentha species.
Rauwolfia is attacked by Diaphania nilgirica, Indomia cretaceus, Plantia viridicolis and various beetles. Dill is affected by Papilio machon and Hyadaphis coriandri. Belladonna loses the leaves due to Gonocephalum species and Agrotis flammatra.
The other insect pests known to cause damage are caterpillar Lepidopterus larvae, cutworms, termites, weevil, Hessian fly, aphids, pyrilla, grass-hoppers, locusts, spiders, ticks, mites, etc.
All the insect pests belong to phylum Arthropoda of animal kingdom and they are further placed under two morphological groups, viz. (1) biting and chewing, (2) piercing and sucking insects.
In most cases, the adult form in the metamorphosis of these insects is damaging in nature, but in certain instances the earlier stage like larvae is also destructive to plants.
3. Weeds:
A weed is undesired plant. Weeds are considered as dreadful pests because losses due to them are estimated to be more than those occurring due to other pests and diseases combined together. If the problem of control of weeds is not handled properly, it leads to loss of nutrients, water, light and space, increase in cost of labour and equipment, low product quality and problems in marketability, enhanced chances for attacks of bacteria, fungi, viruses and insects. But among all these, losses of nutrients and moisture due to weeds are the problems of major concern to all plants yielding foods, medicines, fibres and other economic raw materials.
Some weeds cause allergies e.g. hay fever caused by ragweed, Medican tea, yellow dock, parthenium, etc. While the others, like corn cockle contains cyanophore glycoside and the seeds of this plant may cause fatal effects. Dermatitis is caused by poison ivy, western poison oak, varnish tree, poison sumac, etc. Some plants growing as weeds may be poisonous like Datura and Menispermus species, etc.
4. Non-Insect Pests:
They are grouped into two categories:
i. Vertebrates like rats, monkeys, birds, rabbits and hares, squirrels, antelopes, deer, pigs, etc.
ii. Invertebrates like nematodes, crabs, snails, mites, and symphylids.
The rodents have sharp and gnawing incisor teeth with which they cause considerable spoilage to stored crude drugs and also the faecal matter of such animals causes serious contamination of crude drugs.
Methods of Pest Control in Medicinal Plants:
Different techniques followed for an integral approach to pest control are briefly summarized as under:
(1) Mechanical Method:
It employs manual labour along with different devices for collection and destruction of pest.
The simple techniques used are hand- picking, pruning, burning and trapping of pests. A proper approach is made for collection and destruction of eggs, larvae, pupae and adults of insects.
The better way for protection from rodents like rats is construction of concrete warehouses. The warehouses should have metal reinforcement corners on window frames. The rat and mouse traps are also used.
The method adopted for trapping flying insects is flavoured attractants placed in funnel shaped container, which are formulated with rose oil, anise oil, etc. mixed with saw dust. The insects can easily get an entry in the trap, but find it very hard to come out.
(2) Agricultural Method:
It covers advanced plant breeding techniques capable of inducing genetic manipulations resulting in production of pest-resistant species. It has achieved much success in producing hybrid varieties, which are resistant to fungal and bacterial attack, as compared to limited success with insects. The systemic insecticides have been developed which are absorbed through the roots and reach to leaves by which all the foliage portion becomes distasteful for insects.
Another aspect in agricultural control is ploughing which should be sufficiently deep so as to eradicate weeds, as well as, early stages of insects. If a plant is found out to be favoured by insects as major source of food, the land under cultivation of such plant should be subjected to crop rotation. Another method for checking supply of food to insects is by changing the environment which in many cases, may lead to obstruction in their life cycle. Proper drainage serves this purpose to a great extent.
(3) Biological Control:
This method is practised by combating the pests, mostly the insects, with other living organisms. The latter is frequently the parasite form. If this method is properly designed, it may emerge as an effective, safe and economic method of pest control.
The chemical substances produced and released by some female insects are capable of eliciting a sexual response from the opposite sex, which could be properly exploited for biological control of pests. Such substances are called as sex pheromones, e.g. 7, 8 – epoxy – 2 methyloctadecane from gypsy-moth.
One of the experiments worth noting is about screw worms, which has been successfully carried out in U.S.A. The large number of laboratory reared male insects are sterilised by exposure to radioactivity with such a dose which shall not affect other physiological activities of that insect. Such irradiated insects are released in nature, in large number, so that they are enough to oust the normal males existing in that area during competition for mating with virgin females.
This leads to ineffective mating and sharp decline in progeny. There are several examples of biological control such as employing of Australian lady beetle ‘lady bug’ to feed on damaging insect called cottony cushion scale insect on citrus crop, rat terriers for rats, and various birds for insect pests. The larger harmful insects are, many a time, destroyed by hatching the eggs of certain types of flies and wasps.
(4) Chemical Control:
The control of pests is brought about with the use of chemical pesticides, which include insecticides, fungicides, herbicides and rodenticides.
Because of toxic effects of all such chemicals used as pesticides, their use is regulated by the Insecticides Act in India; Federal Insecticide, Fungicide and Rodenticide Act in United States and the Agriculture (Poisonous substances) Regulations in U. K.
The chemical pesticides are further classified as rodenticides, insecticides, acaricides, fungicides, herbicides, predacides, ovicides, bactericides, arboricides, etc.
(i) Rodenticides:
Warfarin, strychnine, arsenic trioxide, thallium sulphate, red squill, etc.
(ii) Insecticides:
D.D.T., gammaxine, methoxychlor, parathion, malathion, sodium arsenate, pyrethroids, rotenoids, carbamates, etc.
(iii) Acaricides (miticides):
Tetradifon, chlorobenzolate.
(iv) Fungicides:
Bordeaux mixture, chlorophenols, antibiotics, quaternary ammonium compounds, etc.
(v) Herbicides:
2, 4-dichlorophenoxy acetic acid, calcium arsenate, sulphuric acid.
The insecticides are applied to vegetative parts for protective or eradicant activity in the form of spray, aerosol, solution, suspensions and fine dust. The stomach poisons are protective in action, while contact poisons act as eradicants. The fumigation keeps away the insect from the area under the influence of insect repellents.
An ideal insecticide is required to satisfy following parameters:
(i) It should be non-toxic and non- injurious to medicinal plants and human beings.
(ii) It should be selective in action and highly toxic to insects in small concentrations.
(iii) The pesticides should be stable under ordinary conditions of storage, non- inflammable, non-corrosive and free from obnoxious odours.
(iv) It should be non-cumulative in soil and should possess stability on treated surface (residual effect).
The mode of action of different insecticides is given in Table 6.2.
Biopesticides for Crop Protection:
Biopesticides (biological pesticides) are pesticides derived from natural materials such as animals, plants, bacteria and certain minerals.
Biopesticides also include naturally occurring substances that control pests and micro-organisms that control pests (microbial pesticides).
Chemical pesticides are hazardous and cause grave damage to health, ecosystem and water.
Grandfather of Charles Darwin, Erasmus Darwin in 1800, predicted the use of natural predators to minimise the pests in his book ‘Philosophy of Agriculture and Gardening’. But the importance of Biopesticides early in the 20th century was realised. After 1950 only the management of pest control Biopesticides got the real impetus.
To boost up the food production, the use of chemical pesticides was favoured and to some extent it was useful.
But uncontrolled and unscientific use of the chemical pesticides has led to the disaster and has created several problems such as:
(a) Insecticide resistance
(b) Resurgence of secondary pests
(c) Environmental pollution and
(d) Residues in food, drinking water and even in the breast milk.
As per the Gazette notification of Central Insecticide Board, Ministry of Agriculture New Delhi, Dated 26/3/1999 Biopesticides have been put under Insecticide Act 1968.