154 V. M. Chavan
Effect of Viruses on Vectors
Harmful Effects
Deleterious effects induced by viruses and mol-
licute like organism (MLO) are identical and can
be grouped under four categories.
Cytopathological and Histological Effects
Fat bodies take up different shapes and their
cells contain scanty cytoplasm, decreased car-
bohydrates and numerous vacuoles. Their nuclei
become stallate or sharply irregular, sometimes
they first enlarge and then shrink and amount of
their DNA and RNA may damage. Mycetome
undergoes premature degeneration in Colladonus
montanus infected with western X MLO and be-
come abnormally hardened in Nephotettix sp. in-
fected with rice dwarf virus.
Effect on Age of Vectors
Cytopathological and histopathological effects
are liable to reduce life span of infected vectors.
Mean adult longevity of N. cincticeps infected
with rice dwarf virus decreased from 29 days life
of healthy insects to 20 days. Simultaneously,
nymph mortality increased from 21.7 to 35 %.
Effect on Reproductive Capacity of
Vectors
Viruses and MLO cause partial to total reduc-
tion in their egg laying capacity. N. cincticeps
infected with rice dwarf virus lays only 32–72 %
of eggs laid by healthy leaf hoppers.
Metabolic Effects
Viruliferous insects may show increased or de-
creased respiration of N. cincticeps infected with
rice dwarf virus.
Beneficial Effects
Virus-infected plant serves as better host for
aphids than healthy plant. Aphids fed on infect-
ed plant in certain cases have longer life span,
greater egg laying capacity, early attainment of
adulthood and rapid breeding. A. fabae breeds
more rapidly on sugar beet plant infected with
beet yellow mosaic virus than on healthy beets.
Similarly, maturation of aphids as winged forms
(alatae) was favored when cereal grain aphids,
Sitobion avenae F. and Rhopalosiphum padi (L.),
were reared from birth on barley yellow dwarf
virus (BYDV)-infected oats or barley (Gildow
1983 ).Management of Vectors of Virus
Diseases
Most approaches to control the vectors of virus
diseases are aimed at eradicating or altering one
or more of the primary participants in the trans-
mission process (vector, virus, and host plant) or
at preventing their coming together. Knowledge
of virus–vector relationship is essential to de-
vise suitable measures against vector-borne dis-
eases (Watson and Plumb 1972 ; Basu and Handa
1987 ). Broadly, management of vectors of viral
diseases can be done by adopting cultural con-
trol, biological control, and chemical control.Cultural Control Methods
The nonchemical methods of control are becom-
ing increasingly popular due to limitations of
pesticide in preventing disease spread, growing
problem of insect resistance, and recent aware-
ness of pollution problems. The control of vec-
tors of plant virus diseases by cultural practices
is not new. These methods include:- Use of disease-free seeds, seedings and tubers
for initial sowing/planting - Removal of weeds, volunteer crops, and crop
residues being alternate source for virus and
vector - Vector avoidance by crop rotation, crop isola-
tion or growing barrier crops - Use of reflective surfaces as crop mulches to
deter vectors from alighting on the crops - Manipulating plant distribution, density and
field size to minimize vector population and
there by checking disease spread