150 V. M. Chavan
Foregut-Borne Viruses
An ingestion–egestion mechanism for transmis-
sion of both nonpersistently and semipersistent-
ly transmitted viruses was proposed by Harris
( 1977 ), where virus enters the foregut, attaches to
the lining of the anterior portion of the alimentary
canal, and is inoculated when the vector egests
while probing a plant. Foregut-borne viruses have
no latent period in their vectors, cannot recover
from the vectors hemolymph and cannot be trans-
mitted after injection into the vector’s hemocoel.
Infectivity is retained only a few days and it is lost
after ecdysis, e.g., Rice tungro spherical virus.
Circulative Viruses
Kennedy et al. ( 1962 ) used the term circulative
for these viruses that after ingestion, pass through
the gut wall into hemolymph, and then pass
through the salivary glands to be discharged with
the salivary secretion. Circulative viruses can be
recovered experimentally from the vector’s he-
molymph, can be transmitted after injection into
the vector’s hemocoel, are not lost after ecdysis
and may be transmitted for weeks, sometimes for
the life of the vector (Sylvester 1980 ). These vi-
ruses are not transmitted mechanically or imme-
diately after acquisition but become transmissi-
ble only after a latent or incubation period within
the body of insects. The latent period may be sev-
eral hours or days and is temperature-dependent.
No circulative virus is transovarially transmitted
from ineffective females to their progeny. Circu-
lative viruses are mainly transmitted by aphids,
e.g., Barley yellow dwarf, beet curly top, and
beet western yellows.
Propogative Viruses
These viruses propogate or multiply within their
insect vector which transmits them for a long time
but vary upon as they live. These viruses possess
incubation period (one or more weeks) which
is presumed to be the time needed for them to
multiply and to reach a definite concentration to
become transmissible. Thus, they have a definite
biological relationship with their vectors. Some
propogative viruses are transovarially transmit-
ted from a viruliferous female to its progeny, e.g.,
Maize rayado fino virus, rice stripe virus.Transmission by Aphids
Aphids form the largest group of insect vector
both because of large number of species, about
370 involved and the large number of viruses,
about 300 they transmit. M. persicae alone is es-
timated to transmit about 100 viruses while other
aphids transmit more than 30 viruses each. Some
aphids on the contrary can transmit only one
virus each. Following three types of virus trans-
mission are observed in aphid vectors.Nonpersistent Viruses
The nonpersistent viruses transmitted by aphids
include Potyviruses, Carlaviruses, and alfalfa
mosaic virus. Specificity between the virus and
species of vector is not well developed and usu-
ally one virus is transmitted by several or many
aphid species. The transmission threshold period
can be as short as 2 min, and the virus can be ef-
ficiently acquired and inoculated during probes
of only 10–30 s indicating that the virus is ac-
quired and inoculated to cells of the epidermis.
Transmission of nonpersistent virus is favored by
making the aphids fast before giving acquisition
feeding. Experimental results show that nonper-
sistent viruses are generally stylet borne. Some
workers have suggested that a virus is held in the
food canal and still others opined that it is carried
in a plug of gelled saliva where the stylet punc-
tures the leaf.Semipersistent Viruses
A small heterogeneous group of viruses is ac-
quired by aphids or other insects during feeding
times from several minutes to several hours but
not usually by probing. Efficiency of transmis-