Plant Virus Disease Spread Through Insect Vectors and Their Management 151
sion increases with increase in both acquisition
feeding and inoculation feeding time but there is
no definite incubation period and fasting before
acquisition feeding has no effect on efficiency of
transmission. There is no good evidence on how
or where these viruses are held with the aphids
but they seem not to be retained through moult.
Persistent Viruses
Persistent aphid transmitted viruses have mini-
mum acquisition access and inoculation access
period of 10–60 min usually, have latent period
of 12 h or more and can be retained by the aphids
for at least a week. Infected plants mainly show
leaf rolling and yellowing symptoms and many
viruses are probably concentrated in the phloem
of the plant.
Transmission by Leafhopper and Plant
Hopper
Leafhopper and plant hopper make up the second
most important group of virus vectors. Most of
the vector species are leafhopper (Cicadellidae),
but nearly 20 are plant hoppers (Fulgoridae) and
the tomato pseudo curly top agent is transmitted
by the treehopper, Micratalis sp.
The viruses transmitted by leaf and plant hop-
pers cause yellowing and/or leaf rolling and only
a few are sap transmissible. The vector mainly
feeds from the phloem of plants. Most com-
monly, the viruses multiply in their vectors and
persist for a long periods and several of them are
transmitted through the eggs to the progeny.
Transmission by Whiteflies
Of the 1100 identified species of whiteflies in the
world, only three are recognized as vectors of
plant viruses. B. tabaci is considered as the most
common and important whitefly vector of plant
viruses.
None of the whitefly–transmitted (WFT) vi-
ruses is transovarially or seed transmissible. Only
a few of these viruses induce mosaic symptom.
Whiteflies generally acquire viruses more rapidly
from young leaves and recently infected plants.
Females are more efficient vectors of virus trans-
mission than males. Muniyappa ( 1980 ) divided
WFT virus diseases into four groups, i.e., yellow
mosaic, yellow vein mosaic, leaf curl, and mosa-
ic diseases. Transmission efficiency of whiteflies
increases with longer feeding periods, a short
incubation period occurs in most cases, viruses
retain in vectors from few days up to 20 days and
serial transmission is generally intermittent and
inefficient. Hence, relationship of WFT viruses
to their vectors is of circulative type. However,
Brown ( 1994 ) stated that Gemini viruses are
transmitted in a persistent manner by whitefly
as well as some WFT gemini viruses were ex-
perimentally transmitted by sap or mechanical
inoculation.Transmission by Thrips
Thrips tabaci (L.) and three species of Franklini-
ella are the only known vectors of tomato spotted
wilt virus (TSWV). T. tabaci is a cosmopolitan
species feeding on at least 140 species in 40 fam-
ilies of plants. It reproduces parthenogenitically
and feeds by sucking the content of the subepi-
dermal cells of the host plant. Only larvae can
acquire TSWV with a minimal acquisition time
of 15–30 min and with incubation period of 4–18
days. Before completion of incubation period, the
larvae become ineffective. Virus may be retained
for life with erratic transmission. No transversal
transmission. Probably the virus multiplies in the
vector (Matthews 1970 ).Transmission by Mealy Bugs
Several species of mealy bugs are recorded as
vectors of isolates of cacao swollen shoot virus.
The virus can be acquired in 1 h and inoculated
in 15 min. This virus persists for 3–4 days in
Planococcoides njalensis (Laing) and is retained
through the moult (Gibbs and Harrison 1976 ).