A vector is an organism capable of transmitting pathogens from one host to another. The insects, besides directly damaging the crops, sometimes become responsible for the spread of pathogens in plants. All those insects which acquire the disease causing organisms by feeding on the diseased plants, or by contact and transmit them to healthy plants are known as insect vectors of plant diseases.
Insect Vectors of Plant Diseases:
Plant pathogens are transmitted either through contact, by contamination through soil or other biological agencies. Majority of the plant diseases are transmitted by insects and a few by other arthropods like mites and only a small percentage by mechanical means or contamination of the soil.
Insects having both piercing and sucking mouthparts and biting and chewing mouthparts are associated with disease transmission. Most the insect vectors belong to the order Hemiptera (aphids, leafhoppers, whiteflies and mealy bugs), but a few others belong to Thysanoptera (thrips), Coleoptera (beetles), Orthoptera (grasshoppers) and Dermaptera (earwigs). Homopteran insects alone are known to transmit about 90 per cent of the plant diseases.
The salient features of homopterans (aphids and leafhoppers), which make them efficient vectors are as follows:
i. They make brief but frequent probes with their mouthparts into host plants.
ii. As the population density reaches a critical level, winged migratory individuals are produced.
iii. In many species, winged females deposit a few progeny on each of the many plants.
iv. These insects do not cause wholesome destruction of cells during feeding and viruses require living cells for their subsistence and multiplication.
A number of plant diseases caused by viruses, phytoplasmas, bacteria and fungi are transmitted by insects.
(i) Viruses:
A virus is a set of one or more nucleic acid molecules, normally encased in a protective coat or coats of protein or lipoprotein that is able to organize its own replication only within suitable host cells. It is ultramicroscopic in size and can be seen only with the aid of an electron microscope.
It is spherical or rod-shaped, nucleoproteinaceous (chief constituents being ribonucleic acid 5-35% and proteins 65-95% by weight) in composition and can live and multiply only in living cells. Viruses are responsible for many diseases in man (influenza, measles, mumps, polio, pox, etc.) and plants (mosaic, leaf curl, etc.).
Plant virus diseases have become more prevalent and destructive in recent years. This is mainly because of better recognition of the virus diseases, exchange of plant material from region to region facilitating spread of the virus to new areas, and distribution of many insect vectors in new regions in the world.
There are over 850 described plant virus species. About half of the insect vectors are aphids, a third are the leafhoppers. Mealy bugs and whiteflies transmit some viruses, and six are transmitted by thrips. The main aphid vectors are Myzus persicae (Sulzer), Aphis gossypii Glover and Aphis craccivora Koch.
In addition, whitefly, Bemisia tabaci (Gennadius) and leafhoppers are also responsible for transmission of plant viruses. Whitefly mostly transmits mosaics and leaf curls in pulses, vegetables and other crops like cotton, tobacco and papaya. The leaf- and planthoppers transmit tungro, yellow-orange leaf, grassy stunt and ragged stunt in rice.
Tomato spotted wilt is known to be transmitted by thrips. Mandibulate insects like grasshoppers, earwigs and chrysomelid beetles transmit turnip yellow mosaic. Several species of mites are also responsible for transmission of viruses of cereals and fruit crops (Table 31.2).
Types of Viruses:
On the basis of the method of transmission and persistence in the vector, viruses may be classified into three categories:
(i) Non-Persistent Viruses:
These are those viruses which are believed to be transmitted as contaminants of the mouthparts. Such viruses are also called stylet-born viruses and the type of transmission is mechanical. The vector is able to acquire the virus from a disease source and transmit to a healthy plant by feeding for a few seconds.
These viruses do not persist longer within the insect vectors which can transmit them soon after feeding on infected plant but the ability to transmit fresh infection soon disappears after the insect feeds on healthy or immune plants. The efficiency of transmission of non-persistent virus is greatly affected by modifying the time of feeding and by starving the vectors before and after feeding. Aphids are the vectors of a great majority of such viruses which are carried only in their stylets.
The following are the main features of the non-persistent viruses:
a. Vectors are optimally infective when they have fed for approximately 30 seconds on the infected plant.
b. Transmission is improved if vector is starved for a period before an infection feed.
c. If the vector is starved after an acquisition, it begins to lose ability to transmit within 2 minutes.
d. After acquisition feeding, infectivity is rapidly lost when the vectors feed on healthy plants.
(ii) Semi-Persistent Viruses:
These viruses are carried in the anterior regions of the gut of a vector, where they may multiply to a certain extent. Vectors do not normally remain infective after a molt, presumably because the viruses are lost when the foregut intima is shed. Several of the leafhopper transmitted viruses fall under this category.
(iii) Persistent Viruses:
Persistent viruses are those that persist longer within the infective agent, i.e., vector. These viruses, when acquired by a vector, pass through the midgut wall to the salivary glands from where they can infect new hosts. In case of these viruses, the insect has to feed on the source of virus for comparatively longer periods.
The insect, after such acquisition of virus, becomes infective only after a certain period, ranging from several hours to 10-20 days, which is called the incubation period or latent period. Such viruses may multiply within tissues of a vector, which retains the ability to transmit the virus for several days and in some instances the rest of its life. Therefore, the vector need not feed on the virus source again and again to retain its infective capacity.
Thus, the vector insect feeds on the diseased plant (acquisition feed), requires some time after acquisition feed to transmit the virus (latent or incubation period), feeds on healthy plant (inoculation feed) and in the process transmits the virus acquired earlier. Such viruses are also called circulative or circulative-propagative viruses and the type of transmission as non-mechanical. Many of the leafhopper transmitted viruses belong to this category.
Mechanism of Transmission:
For inoculation of virus into a plant by sucking insects, the puncture is initiated by a number of forward and backward movements of the inner pair of stylets. During the forward movements, the fluid flows into them, during the backward movements, saliva is ejected.
Generally, an insect injects by feeding on any part of the plant, but in some cases the virus is only found in the phloem and has to be injected into the phloem, the movement of which is perhaps controlled by the pH gradient between the mesophyll and the phloem. Some viruses are concentrated in the epidermal cells and others in the mesophyll or xylem.
The mandibulate insects like grasshoppers and beetles regurgitate during feeding. The regurgitated fluid containing the virus is brought into contact with the healthy plant, thus transmitting the virus.
Virus-Vector Relationship:
Irrespective of the type of transmission, virus-vector relationship is highly specific. Generally, one type of virus disease is transmitted only by insects belonging to one particular group, i.e., mosaics by aphids and leaf curls by whiteflies. In case of leafhoppers, among 110 species known to be vectors, about 100 species transmit only one virus.
Similarly, there are viruses which are transmitted by a particular species of an insect and not by others of the same genus. For instance, cabbage ring spot is transmitted only by M. persicae and not by M. ornatus. A vector can also acquire and transmit more than one virus to the respective hosts.
For example, the aphid, Pentalonia nigronervosa Coquerel, transmits banana bunchy top and cardamom mosaic. Similarly, the whitefly, Bemisia tabaci (Gennadius) transmits okra yellow vein mosaic, dolichos yellow mosaic, tomato leaf curl, papaya leaf curl, etc. Onion yellow dwarf is known to be transmitted by 60 insect vectors.
For transmission of viruses, activity of insect vectors is more important rather than their number. In case of aphids, it is the activity and number of migrant insects that is important in the efficiency of virus transmission rather than the number of apterous individuals which are, of course, important in respect of their direct injury to the crop.
(ii) Phytoplasmas:
Phytoplasmas (originally called mycoplasma-like organisms) are non-culturable degenerate gram-positive prokaryotes closely related to mycoplasmas and spiroplasmas. They are without a visible cell wall, whose place is taken by a thin elastic cytoplasmic membrane which cannot withstand osmotic pressure. Phytoplasmas are pleomorphic and may be spherical or oval, varying from 80 to 800µ in diameter.
Phytoplasmas are important insect-transmitted pathogenic agents causing more than 700 diseases in plants. The single most successful order of insect phytoplasma vectors is the Hemiptera. This group collectively possesses several characteristics that make its members efficient vectors of phytoplasmas.
a. They are hemimetabolous, thus nymphs and adults feed similarly and are in the same physical location-often both immatures and adults can transmit phytoplasmas.
b. They feed specifically and selectively on certain plant tissues, which makes them efficient vectors of pathogens residing in these tissues.
c. Their feeding is nondestructive, promoting successful inoculation of the plant vascular system without damaging the conductive tissues and eliciting defensive responses.
d. They have a propagative and persistent relationship with phytoplasmas. They have obligate symbiotic prokaryotes that are passed to the offspring by transovarial transmission, the same mechanisms that allow the transovarial transmission of phytoplasmas.
Mechanism of Transmission:
Phytoplasmas are phloem-limited; therefore, only phloem-feeding insects can potentially acquire and transmit the pathogen. Most phytoplasma vectors are members of the family Cicadellidae (Table 31.3). Phloem-feeding insects acquire phytoplasmas passively during feeding in the phloem of infected plants.
The feeding duration necessary to acquire a sufficient titer of phytoplasma (acquisition access period), may range from a few minutes to several hours, the longer the period, the greater the chance of acquisition. The time that elapses from initial acquisition to the ability to transmit the phytoplasmas (latent period or incubation period) is temperature dependent and ranges from a few to 80 days.
During the latent period, phytoplasmas move through and replicate in the vector’s body. They can pass intracellularly through the epithelial cells of the midgut and replicate within a vesicle or they can pass between two midgut cells and through the basement membrane to enter the hemocoel. Phytoplasmas circulate in the haemolymph, where they may infect other tissues such as the Malpighian tubules, fat bodies and brain or reproductive organs.
The replication in these tissues, albeit not essential for transmission, may be indicative of a longer coevolutionary relationship between host and pathogen. To be transmitted to plants, phytoplasmas must penetrate specific cells of the salivary glands and high levels must accumulate in posterior acinar cells of the salivary gland before they can be transmitted.
Vector-Phytoplasma Relationship:
The interaction between insects and phytoplasmas is complex and variable. The complex sequence of events required for an insect to acquire and subsequently transmit phytoplasmas to plants suggests a high degree of specificity of phytoplasmas to insects. However, numerous phytoplasmas are transmitted by several different insect species.
In addition, a single vector species may transmit two or more phytoplasmas, and an individual vector can be infected with dual or multiple phytoplasma strains. Vector-host plant interactions also play an important role in determining the spread of phytoplasmas. Polyphagous vectors have the potential to inoculate a wider range of plant species, depending on the resistance to infection of each host plant.
It has been found that leafhoppers are not able to acquire equally phytoplasmas from different infected plant species. Chrysanthemum yellows (CY) phytoplasma is successfully transmitted by three leafhoppers, viz. Euscelidius variegatus, Macrosteles quadripunctulatus and Euscelis incisus. All three species acquire from and transmit to CY-infected chrysanthemum and uninfected chrysanthemum, respectively.
However, only M. quadripunctulatus and E. variegatus acquire CY after feeding on CY-infected periwinkle and subsequently transmit CY to uninfected plants. None of the leafhoppers acquire the phytoplasma from CY-infected celery, a dead-end host. Dead-end hosts are plants that can be inoculated and subsequently become infected with phytoplasma, but from which insects cannot acquire phytoplasma.
(iii) Bacteria:
Bacteria are microscopic single celled organisms increasing by fission; they have a cell membrane, a rigid cell wall and often one or more flagella. Of the total about 1800 known bacterial species, most are saprophytes living on dead plant or animal tissues or organic wastes. There are about 200 species of bacteria which are parasitic on plants and many of them consisting of numerous pathovars.
Bacterial diseases fall into three categories:
(i) Wilting, due to invasion of the vascular system or water-conducting vessels, e.g., cucumber wilt.
(ii) Necrotic blights, rots and leaf spots, where the parenchyma is killed, e.g., fire blight.
(iii) Hyperplasia or over growth, e.g., crown gall.
Pathogenic bacteria apparently cannot enter plants directly through unbroken cuticle but get in through insect or other wounds, stomata, hydathodes, lenticels and flower nectaries. The fact that the plant pathogenic bacteria are unable to penetrate plant tissue without a court of entry has led to the realization of significance of the role of insects in transmission.
The insect contributes through feeding and oviposition wounds, as a mechanical carrier of the organism on its body and in some cases, by virtue of a mutualistic relationship between the organism and the insect which insures a continuing association among pathogen, insect and the host plant.
A number of plant diseases caused by bacteria are known to be transmitted by insects (Table 31.4). Fire blight of apple and pear, caused by Erwinia amylovora is carried by aphids, leafhoppers, etc. Potato blackleg, caused by Erwinia carotowora, is transmitted by seedcorn maggot, Hylemyia cilicrura (Rondani).
Bacterial wilt of cucurbits, caused by Erwinia tracheiphila is transmitted by cucumber beetle, Diabrotica duodecimpunctata (Olivier). Bacterial wilt of maize, caused by Xanthomonas steward is transmitted by the flea beetle, Chaetocnema pulicaria (Meisheimer). Black rot of crucifers, caused by Xanthomonas campestris is transmitted by several insects and slugs.
(iv) Fungi:
Fungi are organisms having no chlorophyll, reproducing by sexual and asexual spores, not by fission like bacteria and typically possessing a mycelium or mass of interwoven threads (hyphae) containing well marked nuclei. There are about 4300 valid genera of fungi and about 70,000 species living as parasites or saprophytes on other organisms or their residues. More than 8,000 species are known to cause plant diseases.
There are several insects associated with the spread of fungal diseases (Table 31.5). Many flies mechanically transmit the ergot of cereals caused by Claviceps purpurea. The ergot disease of bajra, caused by Sphacelia microcephala, is mechanically carried by insects that visit the flowers attracted by the sugary secretion found on the fungus infected earheads.
The cotton wilt, caused by Fusarium vasinfectum, is transmitted through the faecel pellets of many grasshoppers like Melanoplus differentialis (Thomas), after they have fed upon infected plants. The common sooty mould fungus (Capnodium spp.) grows on the honeydew excreted by several homopteran insects like aphids, leafhoppers, mealy bugs, whiteflies, etc.
Control of Vectors and Diseases:
Among the various types of plant diseases transmitted by insects, virus diseases are considered to be the most serious. Hence a multi-pronged strategy needs to be adopted to manage the vectors and virus diseases. Some of the important components of such a strategy should involve selection of healthy seed, cultural practices, biological measures, resistant varieties and use of chemicals.
(i) Healthy Seed:
Management of virus diseases starts with obtaining healthy seed, cuttings or plants. Care should be taken to obtain only certified seed, i.e. seed obtained from the plants which have been inspected during growing season and found free of certain diseases. Virus-free foundation stock can be built up by heat treatment, i.e., growing plants at high temperatures for weeks or even months.
The production of virus-free stocks can also be achieved by taking advantage of the fact that some plants grow and elongate faster than the virus can occupy the new tissue. Therefore, the virus can be eliminated by using meristem or tip cultured plants. Virus free stock is tested by indexing (growing a part of the cutting or plant in a pot or greenhouse and recording its condition with respect to disease symptoms), bioassays and/or serological assays.
(ii) Cultural Control:
Several cultural practices have proved to be helpful in reducing the incidence of vectors and vector-borne diseases. Intercropping with a barrier crop has provided encouraging results to reduce the incidence of several diseases. For example, the incidence of yellow vein mosaic of okra is reduced by intercropping with soybean. Similarly, intercropping of tomato with coriander and lobiabean reduces the incidence of tomato leaf curl virus in tomato.
Plant spacing such as close spacing reduces the incidence of French bean crinkle stunt disease. Manipulation in planting dates is another way of reducing the disease incidence. Rogueing also helps the removal of the source of disease causing organism. Removal of weeds and alternate hosts of viruses and vectors helps to reduce the incidence of diseases.
(iii) Resistant Varieties:
Growing resistant/tolerant varieties is another effective way of managing vectors and vector- transmitted diseases. A number of genotypes have been identified under various All India Coordinated Research Projects, sponsored by the Indian Council of Agricultural Research and various State Agricultural Universities, which have resistance against virus borne diseases in pulses, tomato, cotton, etc.
(iv) Biopesticides:
The use of biopesticides such as parasitoids and predators, microbials and plant extracts is an eco-friendly approach to manage the vectors and vector born diseases. A fungus, Paecilomyces farinosus has been found parasitic on Bemisia tabaci (Gennadius).
Several neem-based formulations have provided effective control of B. tabaci on cotton. Aqueous extracts of leaves of Clerodendron frageans and Aerva anguinolenta and roots of Boerhavia diffusa, sprayed at 4 per cent concentrations at 3-4 days starting from germination, was found to reduce yellow mosaic incidence in blackgram and mungbean.
(v) Chemical Control:
The control of insect vectors by application of insecticides appears to be a difficult task as few survivors would be able to transmit the disease. Still insecticidal control of insect vectors is the most practicable method of control of plant viruses. The timely application of insecticides restricts the spread of the disease by reducing the vector population.
Several systemic and non-systemic insecticides have been reported to control the insect vectors. The prominent insecticides reported to be effective are sprays of oxydemeton methyl, malathion, dimethoate, carbaryl, dichlorvos, fenitrothion, phosphamidon, monocrotophos, triazophos and ethion in doses ranging from 300 ml to 1-5 litres per ha. The soil application of carbofuran and phorate granules @ 10-12 kg per ha has also proved useful.