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Popular Kheti
Volume -2, Issue-2 (April-June), 2014
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© 2014 popularkheti.info
ISSN: 2321-0001
Insects as a Vector of Plant Diseases
Pooja Kapoor* and Abhishek Katoch
Department of Plant Pathology
CSK HPKV Palampur-176061, Himachal Pradesh, India
*Email of corresponding author: [email protected]
Insects are the most important vectors in transmission of plant viral diseases.
Approximately 80% of the plant viruses depend on insect vectors for transmission (other
vectors can be nematodes and fungi), and the plant virus vector interactions are very
specific. Insect vectors that transmit most plant viruses are aphids, whiteflies,
leafhoppers, thrips, beetles or weevil, mealybugs, and mites. Plant viruses can be
transmitted by insects in various ways such as non-persistent, semi-persistent, and
persistent. This article briefly summarize role of insects in causing plant diseases having
viral, fungal and bacterial origin.
Introduction
Insect vectors of plant viruses are found in seven orders of the class Insecta. The majority of vectors
are found in the two orders of insects i.e: Thysanoptera and Hemiptera. Some species of insects in
orders: Orthoptera, Dermaptera, Coleoptera, Lepidoptera and Diptera, also act as vectors of diseases.
Vectors of plant viruses are taxonomically very diverse and can be found among arthropods,
nematodes, fungi, and plasmodiophorids. Insect vectors that transmit most plant viruses are aphids,
whiteflies, leafhoppers, thrips, beetles or weevil, mealy bugs, and mites. Sucking insects that feed on
plants, vascular tissues (xylem and phloem) appear to be the most common vectors of plant viruses.
The order Hemiptera contains the greatest number of insect vectors of plant viruses. The most
common being aphids with more than 200 identified vector species (Ng and Perry, 2004). More than
half of the nearly 550 vector transmitted viruses recorded so far are disseminated by aphids (55%).
Leafhoppers, beetles, whiteflies and thrips transmit about 11%, 9%, 7% and 2 % viruses, respectively
(Astier et al. 2001). Plant viruses can be transmitted by insects in various ways. These have been
classified as non-persistent, semi-persistent, and persistent. Potyviruses and caulimoviruses encodes
HC component for virus transmission by aphids e.g. Potato Acuba Mosaic virus is transmitted by the
aphid vector by the involvement of PVY which act as helper virus. The coat protein (CP), or its
derivatives, and non-structural proteins, including a helper component (HC) or a transmission factor,
are involved in transmission specificity. For Cauliflower mosaic virus, the CP, protein P2, which is an
aphid transmission factor, and protein P3, which bridges protein P2 and virions, are all necessary for
virus transmission.
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Vector–virus transmission: It consists of several successive steps:
1. Acquisition of virions from an infected source by vector.
2. Stable retention of acquired virions at specific sites through binding of virions to ligands.
3. Release of virions from the retention sites upon salivation or regurgitation.
4. Delivery of virions to a site of infection in a viable plant cell.
5. Each step of this sequence is needed for transmission to be successful.
Mode of Transmission: Viruses can be transmitted by insects in various ways:Non-Persistent Transmission: In this type of transmission, Virus replicates in epidermal cells
and/or parenchyma - accessed rapidly. Insect can inoculate a healthy plant in a few seconds. It
takes less than one minute of feeding for an insect to acquire the virus. Retention lasts for only a
few minutes to several hours. If aphid remains on a single plant for a few minutes, it loses the
ability to transmit. Example-Stylet-borne most aphid-vectored viruses. e.g: Papaya ringspot
virus
Helper Strategy (Indirect): In most non-persistent viruses, helper components plays very
important role in virus transmission. Potyviruses and caulimoviruses encodes HC component for
virus transmission by aphids. The HC is a non-structural protein, to bind to both to virion and to
the cuticular lining of aphid mouthparts (Bridge hypothesis), thus retaining the virion within the
food canal of the aphid stylets. e.g. Potato Acuba Mosaic virus is transmitted by the aphid vector
by the involvement of PVY which act as helper virus.
Capsid Strategy (Direct Strategy): In some aphid transmitted viruses under the genera
Cucumovirus, Alfamovirus, and Carlavirus, purified viral particles are readily acquired and
transmitted by the vector. This indicated that coat protein (capsid protein) of these viruses must
be capable of direct attachment to vector receptor and thus leading to their transmission. e.g.
Cucumber mosaic virus
Semi-Persistent Transmission: These aphids require a long time to acquire virus. The
probability of transmission increases with feeding periods 12-24 hours. Once acquired, it
retained for a longer period. Virus appears to accumulate in the anterior gut - possibly binding at
specific protein site. Virus present deep in plant tissue – phloem. Aphids, whiteflies and
leafhoppers of various genera and families are recognized as vectors transmitted in
semi-persistent manner e.g. Beet pseudo yellows virus
Persistent transmission: It takes several hours of feeding for an insect to acquire the virus. The
virus must circulate through the insect’s body to the salivary glands before transmission can
occur. There is a latent period during which transmission cannot occur (while the virus particles
travel through the insect’s body). When the latent period is completed, the insect can then
transmit the virus for many weeks or the rest of its life without needing to obtain more viruses
from an infected plant. e.g. Tomato spotted wilt virus
There are two types of persistent transmission.
1. Non-propagative—virus circulates through the vectors body, but does not multiply.e.g Potato
leaf roll
2. Propagative—virus needs to multiply in the cells of the insect vector before transmission can
occur. The transmission latent period is normally longer with these viruses. e.g. Tomato
spotted wilt
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Plant Viral Diseases Transmission by Insects
Cauliflower mosaic virus: Transmission of CaMV by cabbage aphid (Brevicoryne brassicae)
occurs in a semi-persistent manner. For Cauliflower mosaic virus (CaMV), the CP, protein P2,
which is an aphid transmission factor, and protein P3, which bridges protein P2 and virions, are
all necessary for virus transmission. Aphids first probe the prospective food source by short,
only seconds lasting intracellular punctures in epidermis and mesophyll cells. After these
exploratory punctures and when they judge the plant as suited, the aphids insert their proboscis
like mouthpieces into the phloem and feed from its sap for time spans that may exceed several
hours.
Tomato spotted wilt virus: Tospoviruses is transmitted by various species of thrips, including
the western flower thrips, Frankliniella occidentalis, the onion thrips, Thrips tabaci, and the
chili thrips, Scirtothrips dorsalis. Nymphs that acquire the virus by feeding on infected plants
will retain the ability to transmit it for the remainder of their lives. Inoculation feeding period
must be 30 min or longer. The virus circulates in the vector and replicates throughout the life of
viruliferous thrips.
Potato leaf roll virus (PLRV): It is a phloem-limited Luteovirus which is transmitted by aphids
in a persistent manner. PLRV takes longer to be acquired (10-30 minutes) and transmitted (24
to 48 hours) by aphids, since the virus needs to move into the gut, through the body and back
out through the salivary system of the aphid. The virus persists throughout the aphid's life and
can be carried over long distances.
Rice dwarf virus: Rice dwarf virus is the best-known virus disease in the world although the
distribution of the disease seems to be limited to Japan and Korea. It was the first plant virus
disease found to be transmitted by an insect; it provided the first evidence for the multiplication
of a plant virus in an insect. Transmitted by three species of leaf hoppers: Recilia dorsalis,
Nephotetix cincticeps, in most areas N. cincticeps plays the major role in transmission of the
disease. Transovarial transmission of a plant virus by its insect vector was first described by
Fukushi in 1933, who showed that rice dwarf virus, was transmitted for several generations
through the egg of the leafhopper vector Nephotettix apicalis. Transovarial transmission has
been often associated with replication of the virus in its vector.
Bean common mosaic virus (BCMV): Bean common mosaic virus is a potyvirus, transmitted
by many aphid species in the non-persistent manner. More than 20 aphid spp. can transmit the
virus in a non-persistent manner, especially Acyrthosiphon pisum, Macrosiphum euphorbiae,
Myzus persicae and Aphis fabae.
Papaya ringspot virus (PRSV): Papaya ring spot was first coined by Jensen in 1949. It belongs
to Genus Potyvirus and family Potyviridae. Aphids are the predominant means by which PRSV
is transmitted in a non-persistent manner. Non-persistent viruses are transmitted quickly and
easily between plants. Many species of aphid can transmit PRSV, particularly the Peach Aphid
and Melon Aphid. Virus is grouped: papaya infecting type (PRSV-P) - affects both papaya and
cucurbits and cucurbit infecting type (PRSV-W) which affects cucurbits but not papaya. This
disease severely affected the papaya industry in Puna district in 1950s.
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Rice tungro disease: It is one of the most destructive diseases of rice in South and South east
Asia, where epidemics of the disease have occurred since the mid-1960s. Tungro means
degenerated growth. Tungro is caused by two viruses, an RNA virus, the rice tungro spherical
virus (RTSV), and a DNA virus, the rice tungro bacilliform virus (RTBV). RTBV depends on
the helper produced by RTSV for its transmission. It is the RTBV which is mainly responsible
for the severe tungro symptoms. Tungro virus disease is transmitted by leathoppers, particularly
the green leafhopper (GLH), Nephotettix viriscens in semi-persistent manner. The insect
acquires the virus by feeding on the plant for a short time, and can transmit the virus
immediately after feeding.
Citrus tristeza virus (CTV): It is a viral species of the Closterovirus genus that causes the most
economically damaging disease to its namesake plant genus, citrus. The disease has led to the
death of millions of Citrus trees all over the world and has rendered other millions useless for
production. Farmers in Brazil and other South American countries gave it the name "tristeza",
meaning sadness in Portuguese and Spanish, referring to the devastation produced by the
disease in the 1930s. The brown citrus aphid (Toxoptera citricida) is by far the most efficient
vector of CTV, followed by the melon aphid (Aphis gossypii). CTV is transmitted
semi-persistently by vectors that penetrate the phloem to extract sap, mostly the aphid species
that colonize the crop.
Fungal Diseases Transmitted by Insects
Dutch elm disease: It is caused by the vascular wilt fungus, Ophiostoma ulmi (formerly,
Ceratocystis ulmi). This fungus is vectored via two species of elm bark beetles: the smaller
European elm bark beetle and the native elm bark beetle. The primary vector of Dutch elm
disease (DED) is the native elm bark beetle (Hylurgopinus rufipes). This tiny beetle is only 2 to
3.5 mm long, about the size of a pin head. The sticky spores of the DED fungus attach
themselves to the beetle during its feeding, breeding, and over-wintering activities. As the beetle
moves around it spreads the spores from infected elms to healthy elms. Spores of O. novo-ulmi
are stored in xylem vessels and reproduce through budding. The beetles overwinter as larvae in
their tunnels, pupate in the outer bark and begin to emerge in May. They can be found
continuously from May to September. Elm bark beetles will only lay eggs in dead or dying elm
tissue. Dispersal of spores is via bark beetles that burrow under the bark and lay their eggs in
wood galleries. Elm bark beetles distribute O. novo-ulmi locally and over distances of several
miles while the fungus may be distributed over longer distances in elm logs and in firewood.
Bacterial Diseases Transmitted by Insects
Citrus greening disease: Citrus greening disease was presumed to originate in China during the
1890’s as a “yellow shoot disease”. The disease is widely distributed in Asia and Africa.
Transmission is by the Asian citrus psyllid, Diaphorina citri or, in Africa, by Trioza erytreae,
the African citrus psyllid, also known as the 2-spotted citrus psyllid. The disease was first
described in 1929 and first reported in China in 1943. Transmission of CGD happens in a
persistent manner, i.e. bacteria multiply in the psyllids. The greening bacterium can be found in
the haemolymph of the vectors. The acquisition latent period is 24 hours for African psyllid and
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transmits greening 7 days later and can infect with the exposure time of less than 1 h. Pyslla are
strongly attracted by yellow green of wavelength 550 nm, making diseased trees attractive
targets. Fourth and fifth instars of D. citri can acquire the pathogen and adult from these
nymphs transmit the disease.
Stewart's Wilt: It is a serious bacterial disease of corn caused by the bacterium Pantoea
stewartii. Chaetocnema pulicaria, the primary vector for Pantoea stewartii, overwinters as
adults and will begin feeding on corn seedlings early in the spring season. The bacterium
overwinters in the gut of the adult corn flea beetles. Warmer winter temperatures will allow for
greater beetle survival and in effect, higher populations come spring. Emerging beetles in the
spring transmit the bacteria into corn leaf tissue through feeding. The corn flea beetles wound
the leaf and contaminate the wounds with insect frass (excrement), which additionally contains
the bacteria. Once the bacteria are inside the plant, they multiply and fill the xylem and
intercellular spaces of the leaf.
Sudden wilt of cucurbits: It is a serious disease of cucurbits. It mainly affects bottle gourd and
caused by bacteria Xanthomonas. The bacteria are transmitted by cucumber beetles. Beetles
feed on all parts of plants including the flower, fruit, stems and leaves. In spring, beetles
inoculate the disease into the interior of the new plants. Only deep feeding wounds which
expose the water-conducing tissue will be infected. Cucumber beetle larvae feed on the roots
and bore into both roots and stems of cucumber plants. Primary infection is produced when
beetles feed upon young leaves or cotyledons. The bacteria multiply at the wound site, enter the
xylem vessels and then move down the petiole and stem. Vascular plugging by masses of
bacteria and formation of gums and resins are the primary mechanism of wilting.
Fire blight of apple and pear: Erwinia amylovora is a native pathogen of wild, rosaceous
hosts in eastern North America. It was the first bacterium proven to be a pathogen of plants.
Fire blight of apple and pear is caused by Erwinia amylovora. Cells of E. amylovora excrete
large amounts of an extracelluar polysaccharide, which creates a matrix that protects the
pathogen on plant surfaces. Erwinia amylovora overwinters in a small percentage of the annual
cankers that were formed on branches diseased in the previous season. These overwintering
sites are called “hold over cankers”. As temperatures warm in spring, the pathogen becomes
active in the margins of holdover cankers. Free bacterial cells are released onto the bark surface,
sometimes as visible ooze. Insects attracted to the ooze (e.g. flies) or rain disseminate the
bacteria from the canker to blossoms.
Conclusion
Insects have a major role in the transmission of plant viral diseases. Aphids, whiteflies,
leafhoppers, thrips, beetles or weevil, mealy bugs, and mites are the important insect vectors of
plant viral diseases that transmit viruses in non-persistent, semi-persistent, and persistent
manner.
References
Astier S, Albouy J, Maury Y and Lecoq H. 2001. Principes de Virologie Végétale. INRA, Paris,
France
Ng JCK and Perry KL. 2004. Transmission of plant viruses by aphid vectors. Molecular Plant
Pathology 5: 505-511
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