Plant Virus Disease Spread Through Insect Vectors and Their Management 149
Inoculation Feeding Period
The time for which the virus-carrying vector ap-
pears to be feeding on virus-free plants.
Latent Period
The time for the beginning of acquisition feeding
until the vector can infect healthy plants with the
virus. It is also called as preinfection period or
incubation period.
Transmission Threshold Period
The minimum total time that a vector needs to ac-
quire a virus and inoculate it to a virus-free plant.
Infective Capacity or Retention Period of
Vector
This is the period for which an insect carries/
retains/transmits the virus to host plants and re-
mains viruliferous.
Persistence
The time for which a vector remains infective
after leaving a virus source. Persistence is further
divided into three main but integrating catego-
ries.
A. Nonpersistence Virus persisting for few (usu-
ally for few seconds, minutes, or less than an
hour) hours at about 20 °C.
B. Semipersistence Virus persisting for 10–100 h.
C. Persistent Persistence for more than 100 h,
in some instances for the life of the vector.
Recently, there has been an attempt to replace
these somewhat arbitrary categories of persis-
tence by categories based on the behavior of the
virus in the vector during transmission or mecha-
nism of virus transmission by vector.
Mechanism of Virus Transmission
Stylet-Borne Viruses
These are in fact nonpersistent viruses which are
carried in the stylets of the vector to the site of
inoculation. The term was proposed by Bradley
( 1964 ) based on a series of experiments. Stylet-
borne viruses adhered to tips of stylets, are im-
mediately acquired by vector during feeding and
are transmitted by vectors soon after acquisition.
Viruliferous insects can transmit these viruses
for a limited period, rarely for a few days but
never forever or after moulting. Thus, they can
infect only a limited number, sometimes only one
or two plants. These viruses are transmitted by
aphids. They are acquired mainly from the epi-
dermis within the first few seconds of probing.
Many viruses are stylet-borne viruses and a great
majority of them induce mosaic symptoms and
are sap transmitted, e.g., Cucumovirus, Carlavi-
rus, and Potyvirus are aphid stylet-borne viruses.
All aphids and leaf hoppers feed on plants
and possess piercing and sucking mouth parts,
consisting of pairs of stylets, a labium, a large
slender rigid organ with a deeply concave ante-
rior surfaces forming the channel of beak, a la-
brum, mandibles, and maxillae. The two pairs of
stylets form a compact bundle or fascicle with
slides in the groove of labium and constitute the
piercing organ. Piercing organ has two channels;
through one (salivary channel), saliva is injected
into a plant and through the second (food chan-
nel), plant sap is sucked. Mean length of 75
single stylets of adult Myzus persicae (Sulzer) is
496 ± 17 μm (Forbes and Mac Carthy 1969 ).
Aphids and leafhoppers secrete two types of
the salivary sheath material, which coagulate rap-
idly and form a salivary sheath or stylet sheath in
the path of stylets and water-soluble saliva (Miles
1968 ). Most of the aphids and leafhoppers form
this sheath around their stylets during the feed-
ing process. Stylet sheath is laid by the vector in
tissue of the host and it stays there even after the
withdrawal of stylets. It shows the intercellular
(in majority of aphids) or intracellular (in major-
ity of leaf hoppers) path that the stylet follows
during its passage through the tissues and also
the point where it terminates. Stylets move fairly
rapidly within this sheath but are subsequently
extended beyond the sheath for ingestion of food
material from host cells. Most aphids are surface
feeders and feed in parenchyma and mesophyll
and hence, induce mosaic symptoms while most
leafhoppers feed on phloem.