162 N. Nagaraju et al.
between viruses and vector components (Hogen-
hout et al. 2008 ).
Many plant viruses are transmitted from plant
to plant in nature by invertebrate vectors and the
plant virus vector interactions are very specific
(Muniyappa and Veeresh 1986 ; Hohn 2007 ).
Members of the class, Insecta and Arachnida
from the phylum Arthropoda and the members of
the order Dorylaimida from of the phylum Nem-
atoda are the major vectors of plant viruses. The
Homopterans feed by sucking sap from plants
and are numerically the most important suborder
containing plant virus vectors. The tomato yel-
low leaf curl virus tomato yellow leaf curl virus
(TYLCV) and its whitefly vector Bemisia tabaci
Genn. have been of increasing importance re-
cently in many regions with tropical, subtropical
and arid Mediterranean climates due to a rapid
expansion in geographic distribution and host
range of the virus and its vector (Pico et al. 1996 ).
In this review, vectors that transmit viral dis-
eases on important crops are discussed (Table 3 ).
Viruses cause serious diseases of crop plants
reducing both quality and quantity of final produce
and keeping quality of plant produce. Some of the
important viral diseases are: mosaics in chilli and
cucumber, tomato mosaic, tomato spotted wilt,
potato virus Y in potato, papaya ringspot virus in
papaya and cucurbits, citrus tristiza, chilly leaf curl,
banana bunchy top, etc. Aphids constitute the larg-
est vector group transmitting plant viruses com-
pared to whitefly, thrips leafhoppers, psyllids and
mites. The number of transmission patterns that
have evolved are unique in their variety compared
to those found with other vector groups. In India,
the viruses transmitted by aphids, mainly Aphis
craccivora, Myzus persiacae, Aphis gossypii, are
more important in transmitting many viruses com-
pared to other South Asian country infecting varied
crop plants ranging from cereals, vegetables, fruit,
and ornamental crops. mMst of them are major vi-
ruses which are not so important in countries like
Pakistan, China, and neighboring countries.
Additionally Rhopalosiphum padi, R. maidis
and Aphis gossypii are found important in trans-
mitting sugarcane mosaic and chilli veinal mottle
virus (ChiVMV). The occurrence of M. persicae
in many South Asian countries on transmission
of several viruses in nonpersistent manner can
be observed. However, viruses like tomato leaf
curl virus, yellow mosaics in legumes, Bhendi
are transmitted through whitefly in a semiper-
sistent manner. The tomato yellow leaf curl virus
(TYLCV) and tomato yellow leaf curl China
virus (TYLCCNV) transmit through B. tabaci are
major viral diseases in China and Nepal, and dif-
ferences can be found in transmission, symptom
expression, and molecular description compared
to Indian ToLCV. In recent years, tomato spotted
wilt (Tospo) virus in tomato, capsicum and let-
tuce has become serious. In general, viruses do
not exist and survive in nature without another
living organism. In Thailand, Ceratothripoides
claratris is a major thrip vector which is trans-
mitting capsicum chlorosis virus in capsicum
and other solanaceous crops causing huge loss.
Leaf hoppers are found to be successful transmit-
ters of many viral diseases in cereals compared
to vegetables particularly phytoplasma diseases
viz. southern rice black-streaked dwarf virus, rice
gall dwarf virus, grassy stunt virus in a persis-
tent propagative manner. Mites Aceria tulipae
and Aceria tosichella are also important virus
vectors, transmitting wheat streak mosaic and pi-
geon pea sterility mosaic virus. Plant viruses can
be transmitted by insects in many ways. These
have been classified as nonpersistent, semiper-
sistent, and persistent, depending on the vector
that can harbor infectious particles, ranging from
minutes to hours (non-persistent) to days (semi-
persistent) and to life-time and even inheritance
by the insect progeny (persistent) (Hohn 2007 ).Aphids Aphids are among the most destruc-
tive insect pests on cultivated plants in temper-
ate regions. The damage they cause to plants has
made them enemies of farmers and gardeners the
world over, though from a zoological standpoint
they are a highly successful group of organisms.
Their success is due in part to the asexual repro-
ductive capabilities of some species. About 4400
species of 10 families are known. Historically, far
fewer families were recognized, as most species
were included in the family Aphididae. Around