1. Aphids are the most common insect vectors of plant viruses, accounting for transmission of 50% of insect-vectored viruses. The majority of aphid vectors belong to the subfamily Aphidinae. 2. Aphids are successful virus vectors due to their polyphagous nature, parthenogenetic reproduction, needle-like stylet capable of piercing plant cells without damage, and global distribution with over 200 identified vector species. 3. Aphid-transmitted viruses are categorized based on their mode of transmission within the vector as nonpersistent, semipersistent, or circulative propagative/nonpropagative. The viral capsid protein and presence of helper components influence

1. Safeena Majeed, A. A.,
PA1TAH082,
Agril. Entomology.

2. Introduction
• A majority of plant viruses are dependent on vectors for
their transmission and survival.
• Insects, mites, nematodes and protists - mediate
transmission of plant viruses.
• Insects are the most common of the vectors
• Aphids alone account for the transmission of 50% of the
insect-vectored viruses
• The majority of aphid vectors belong to the subfamily
aphidinae –other in 9 other subfamilies-small proportion
transmit viruses.
(Brunt et al., 1996; Nault, 1997)

3. 48%
9%
4%
11%
24%
4%
Bacteria
Fungus
Nematodes
Mycoplasmas
Virus
Protozoa
53
%23
%
13%
11%
Mechanically Transmitted
Transmitted by Arachanids
Data Source-Diseases: An Emerging Threat to Human, Animal & plant Health:Workshop Summary, 2011
Viral Diseases: An Overview
Insect Transmitted
Nematode Transmitted

4. Features - success of aphids as vectors
1. Polyphagous
• nature for some aphid species that allows them to feed on a wide
range of plant hosts
• dissemination of viruses infect a large number of plant speciese- M.
persicae
2. Parthenogenetic reproduction
• facilitate the rapid production of large quantities of offspring
• populations can increase at extraordinarily high rates,
• potentiating disease epidemics
• short- and long-distance spread of viruses.
3. Needle-like stylet
• capable of piercing plant cell walls
• delivering viruses into host cell
• without causing irrevocable damage.
4. Globally distributed & there are > 200 vector species identified

5. Vector potential
• Feeding behaviour and host plant selection- affects
potentiality
• The extent of these factors influence on virus
transmission- depends on specific virus and its mechanism
of transmission- positively or negatively
• Understanding the spread and control of viral diseases
requires,
• understanding of the vector and its behaviour
• vector transmission is paramount to epidemiology.
(Harris and Maramorosch, 1977)

6. Aphids entering a field and landing in a crop are of 4
categories based on,
relationship between the aphid’s colonising potential & the
way it is harmful to the crop.
1. Transient non-vectors
2. Transient vectors
3. Colonising non-vectors
4. Colonising vectors
Categories of aphids
Fereres & Moreno, 2009

7. 1. Transient non-vectors - land and probe but do not colonize the
crop and do not transmit virus
2. Transient vectors - land and probe without colonising the crop
but able to transmit virus
• Often the main vectors -transmitted quickly during brief probes
• e.g. nonpersistent and some semipersistent stylet-borne viruses
3. Colonising non-vectors -land, stay and reproduce on the crop
but do not transmit virus
4. Colonising vectors- land, stay and reproduce on the crop and
able to transmit virus
• Require longer feeding times & direct contact with vascular tissues
• e.g. persistent and some semipersistent foregut-borne viruses).
Fereres & Moreno, 2009

8. Host plant recognition
• Follow a series of steps to search, find their host
plants & identify feeding sites.
• Successive events that culminate in sustained sap
ingestion once the host plant is recognized as an
acceptable source for feeding.
• This can be categorized into,
1. Pre-alighting (before landing) behaviour
2. Plant contact and assessment of surface cues after
landing
3. Probing on superficial tissues.
4. Location and insertion of stylets at the appropriate
feeding site.
5. Salivation followed by committed sap ingestion.
Fereres & Moreno, 2009

9. 1. Pre-alighting behaviour
• Pre-alighting behaviour of insects involves phototactic
response to visual cues
• Alate female summer migrants of Myzus persicae have 3
types of photoreceptors in the compound eye.
1. Most sensitive to the green region: 530 nm
2. Second peak was in the blue-green region: 490 nm
3. Third peak was in the near UV: 330–340 nm
• λ in the UV region influence insect behaviour- orientation,
navigation, host finding & feeding
• Eg: During flight, aphids respond strongly to visual stimuli &
locate host plants from the contrast between the soil
background and the green colour of plant foliage
Fereres & Moreno, 2009

10. 2. Plant contactand selection of
probing siteafter landing
• Mediated both by tactile stimuli and olfactory cues.
• When aphids land on a plant they tend to walk and move
their antennae from side to side searching for odours
and assessing substrate texture before stylet
penetration - topology, trichome exudates, epicuticular
waxes & other surface cues.
• Sensilla located in the antenna, tarsi or labium act as
chemoreceptors and mechanoreceptors
• Smooth surfaces such as leaves will induce probing
activity of aphids, whereas rough surfaces such as the
soil may be easily distinguished from leaves and no
probing occurs

11. 3. Probing on superficial tissues: acquisition
& inoculation of stylet- borneviruses
• Stylet penetration is attempted as a reflex response once they land
on any smooth solid surface- even in the presence of strong repellents
such as dodecanoid acid
• Eg: M. persicae, on a glass Petri dish salivated and excreted potyvirus-
like particles
• Require >1 min to penetrate beyond the epidermis- initial exploratory
probes made by aphids are limited to the epidermis.
• Need to ingest small quantities of plant sap during these brief
superficial probes to discriminate between host and non-host plants.
• Absence of gustatory sensilla in the labium, stylets or antennae and
the location of their gustatory organ in the foregut, very close to the
precibarial valve

12. 4. Locationand styletpenetrationof targetfeeding
site followed bysalivation: inoculationof phloem-
restrictedviruses
Intracellular punctures
stylets penetrate deeper through intercellular spaces - vascular
tissues-extracellular salivation & intracellular penetrations
reach the phloem & puncture several sieve elements
long periods of salivation that usually last from 5s to 30min
• Salivation phase –overcoming plant wound reactions by preventing
phloem proteins from clogging the sieve elements and sealing the
sieve
• Some viruses are unable to move outside phloem tissue –phloem
feeding vectors - Luteovirus, Polerovirus, Enamovirus &
Closterovirus

13. 5. Acceptanceof feeding site & sustained sap
ingestion: acquisitionof phloem-restrictedvirus
• When watery saliva is injected for about 1min into the
sieve elements a subsequent period of passive phloem
ingestion
• Usually starts and lasts from a few minutes to several
hours.
• During sap ingestion, watery saliva continues to be
ejected, but it never reaches the plant cell.
• Instead, these salivary secretions mix together with the
ingested sap in the common duct and both fluids flow
through the food canal due to the high hydrostatic
pressure of the phloem sieve elements

14. Fereres & Moreno, 2009

15. Virus transmission
A step-wise processes
1. Acquisition - uptake of virus from an infected source
2. Retention - stable retention of acquired virions at
requisite sites within the vector
3. Inoculation - the release of bound or retained
virions and their delivery to a site of infection

16. Mode of virus transmission
• The fundamental distinction in transimission- ingested
virions are circulative or noncirculative in the vector
• Focuses on the duration
• Site of retention of virions
• Route of movement within the aphid.
• Circulative viruses -are taken up into cells, cross multiple
membrane barriers, are transported within the vector
haemolymph and ultimately exit the aphid in its saliva.
• Noncirculative viruses- more superficial and transient
relationship with the vector and only associated with the
mouthparts and foregut

18. Noncirculativetransmission
• Among the aphid-vectored plant viruses, a majority
are transmitted in a noncirculative manner.
• Depending on the length of time that an aphid
remains viruliferous following feeding on an infected
plant.
• Nonpersistent - if aphid moults, virus is lost (does
not persist)
• Semipersistent - Acquisition within minutes, but
the efficiency increases with prolonged feeding

19. 1. Nonpersistentvirus transmission
• Is characterized by very short acquisition and inoculation
times of seconds to minutes
• Aphids typically remain viruliferous for comparably short
periods of time: Once an aphid has left a virus-infected
plant and begun feeding on an uninfected plant
• Its ability to transmit virus is short-lived (minutes)
• Historically, this was referred to as ‘stylet-borne’- it was
thought that virus was merely contaminating the outside of
the stylet.
(Martin et al., 1997; Wang et al., 1996)

20. Virus destined for inoculation is
• retained at sites within the stylet and food canal or
foregut
• virions retained at the distal tip of the stylet bundle
are most likely to play a determining role in
transmission
One of the conundrums in transmission is that
• binding of virions within the vector must be readily
reversible
• food and salivary canals merge at the tip of the aphid
stylets
• salivation may function to enhance the release of
bound virions and their delivery into plant cells
(Martin et al., 1997; Wang et al., 1996)

21. Viral Capsid Protein (CP)
• A primary determinant of both aphid transmissibility
and specificity is the viral capsid protein.
• Eg:
1. Cucumber mosaic virus (CMV), wherein virions can be
reassembled in vitro from purified capsid protein
(CP) and genomic RNAs.
2. The CP of potyviruses is also a key determinant of
transmissibility. Spontaneous mutants of many
potyviruses have been recovered that are aphid
nontransmissible.
(Chen & Francki, 1990; Gera et al ., 1979)

22. Helper Component (HC)
• Contrasts with CMV, wherein virions alone will not be
transmitted; a second viral-encoded accessory protein is
required for transmission- ‘helper component’ (HC)
• This viral protein functions as a bridge and mediates the
binding of virions in the stylet of the aphid
• Eg: Potato virus Y & other potyviruses
• Thus, among the nonpersistently transmitted viruses, two
strategies have evolved
• helper-dependent transmission
• helper-independent transmission
(Pirone and Blanc, 1996)

23. 2. Semipersistentvirus transmission
• Acquisition can occur within minutes, but the efficiency increases
with prolonged feeding and retention period is of hours to days.
• Eg: Caulimoviruses, Closteroviruses.
• Helper transmission strategies have evolved in viruses with both
RNA and DNA genomes.
• Because HC and virus particles can be acquired sequentially, it is
possible for one virus to assist in the transmission of a second
virus.
• This phenomenon, termed HC transcomplementation, has been
observed in a number of families
• Influence the epidemiology and spread of plant viruses
(Palacios et al ., 2002; Froissart et al ., 2002)

24. Circulative transmission
• Requires acquisition periods of hours to days.
• The longer acquisition and retention periods are
attributed to the
• phloem-limited nature
• extensive path they have to navigate through the aphid
• Depending on whether a virus replicates in the
vectoring aphid, transmission is described as
• Circulative nonpropagative
• Circulative propagative
• Aphids remain viruliferous for extended periods; the
virus persists and this mode of transmission has
historically been referred to as ‘persistent’.

25. Circulative Nonpropagative Viruses
• Remain viruliferous for weeks and through moulting.
• Seen in a number of genera in the family Luteoviridae
• Eg: Barley yellow dwarf virus (BYDV) and related
luteoviruses.
• Following ingestion from infected phloem cells, viruses
traverse the food canal →foregut → midgut → hindgut →
across gut epithelial cells → haemocoel.
• Virions circulate in the haemolymph → accessory salivary
gland cells and are ultimately inoculated into plants with
salivary gland secretions
(Gray and Gildow, 2003)

26. (Van Regenmortel et al., 2000)

27. Vector specificity
• Transmission of a virus by one or a few specific aphid
species but not by others, is a prominent feature of
luteovirus transmission.
• Transmission is a function of compatible interactions
between virions and factors in the aphid.
• Barriers to transmission reside in regions of both the
• gut
• accessory salivary gland of the vector
• or both
(Gray and Gildow, 2003)

28. 1. CP (capsid protein) - primary determinant of transmission for
circulative, nonpropagatively vectored virus- luteovirus
2. Symbionin
• A protein homologue of the Escherichia coli chaperonin GroEL
• Produced by the aphid endosymbiotic bacteria Buchnera spp. and
released into the haemolymph
• Hypothesized to be an essential factor in transmission functioning
to stabilize virions in the aphid haemolymph
• Unlikely to play a role in vector specificity in luteoviruses bind
symbionins of both vector and nonvector
3. NO. of aphid-specific proteins that bind luteoviruses have been
identified in immunoblots with virus (BYDV-MAV) overlay assays.
Using proteins extracted from the head of the aphid Sitobion
avenae
(Filichkin et al., 1997; Van den Heuvel et al., 1994, 1997; Lio et al., 2001 )

29. CP protein & helpervirus
• Another biological property of luteoviruses- helper/ assistor viruses
• Helps in the transmission of viruses belonging to genus Umbravirus
• Umbraviruses are mechanically transmissible, but their genomes do
not encode a CP and they are aphid-nontransmissible.
• They achieve aphid transmission by associating with luteoviruses in
plants dually infected by both viruses.
• The luteovirus helper virus contributes to the transmission of the
umbravirus by encapsidating the latter’s genomes with its CP.
• Allows the umbravirus to take on the aphid transmission characteristic
exhibited by the luteovirus helper
(Robinson and Murant, 1999; Taliansky et al., 2000)

30. Importantaphidtransmittedviraldiseases
SL
NO
NAME OF
DISEASE
VECTOR
LOSS
(%)
REFERENCES
1 Papaya Mosaic Aphis craccivora 30-40 Vimala et al. 2017
2 Cucumber mosaic
Myzus persicae
50-60
Gad Loebenstein,
2012
3
Potato Leaf Curl
roll
Myzus persicae 38-42 Jeffries, 2014
4 Banana Bunchy Top Pentalonia nigronervosa 70-100
Selvarajan and
Balasubramanian,
2014
5 Banana mosaic Pentalonia nigronervosa 20-30
Selvarajan and
Balasubramanian,
2014
6
Cardamom foorkey
disease
Pentalonia nigronervosa 28-37 Paudel et al., 2018
7 Groundnut rosette Aphis craccivora 39-41 Naidu et al., 2015
8 Sugarcane mosaic Rhopalosiphum maidis 20-30 Fuchs et al., 2013
9 Papaya mosaic Aphis craccivora 30-40 Kabir et al., 2017

31. Green peach aphid, Myzus persicae
• Peach- potato aphid
• small green aphid
• most significant aphid,
• cause decreased growth
• shriveling of the leaves and
the death of various
tissues.

32. Transmitted diseases
1. Sugar beet yellow mosaic 2. Cucumber mosaic virus
• CMV-characteristic
hexagonal viral inclusion
bodies on fruits and
sometimes on leaves too.
• BYV -vein clearing or vein
yellowing 3-4 weeks’ post-
infection, then leaves
become thick, brittle and
yellow with necrotic spots.

33. 3. Potato leaf roll 4. Potato virus X & Y
• Genus- Poliovirus
• Family- Luteoviridae.
• Phloem limited +ve sense
RNA virus
• Infects potatoes & other members
of the family Solanaceae.
• Family- Alphaflexiviridae
• Order- Tymovirales.
• Found mainly in potatoes

34. Banana aphid, Pentalonia nigronervosa
Sucks sap mainly from Musa species but
found associated with many other plant
spp.

35. Global distribution
(source: CPC 2007)
Putative area of origin: Papua New Guinea, Eastern Indonesia
Little is known about timing invasion outside of its area of origin
International Institute of Tropical Agriculture – Institut international d’agriculture tropicale – www.iita.org

36. Transmitted diseases
• BBTV-ssDNA virus
• First identified in
Fiji in 1891
• Has spread around
the world since
then.
Banana leaf
showing light
and dark green
(mosaic)
patterns
Banana bunchy top
• Stunting & formation
of numerous minute
tillers
• Fail to form
inflorescence.
• Tillers do not grow
beyond a few inches in
height & appear bushy
Foorkey disease
of CardamomBanana mosaic

37. Second
Third
Fourth
(1.5 mm)Winged
(migrant)
Up to 30 generations
per year;
Around 25C is best temp
for growth
Life cycle of the
banana aphid
Live birth
First
International Institute of Tropical Agriculture – Institut international d’agriculture tropicale – www.iita.org
Parthenogenetic

38. Control options
 Very few attempts at biological control – no
serious efforts at classical biological control
 Discouraging results from releases of two non-
naturally associated-(Aphidius colemani,
Lysiphlebus testaseipes
 No serious attempt to develop entomopathogens
 Host plant resistance – not explored
 Several insecticides have been recommended:
 Several organophosphate and carbamate
 Imidachloprid – non-systemic
 No option for small growers; cause
secondary pest outbreaks (e.g.,
mealybugs)
 Other methods (hot water treatment of suckers)
International Institute of Tropical Agriculture – Institut international d’agriculture tropicale – www.iita.org

39. Groundnut Aphid and WhorlAphid
• Aphis craccivora
• cowpea aphid/ groundnut
aphid/ black legume aphid
• Originally of probable
Palearctic origin
• Invasive species of
cosmopolitan distribution
• Rhopalosiphum maidis
• Corn leaf aphid/ whorl
aphid/ corn aphid
• Insect pest of maize and
other crops.
• Distributed worldwide in
steppe & forest-steppe

40. Transmitted diseases
• Peanut pathogenic virus
• Found in Sub-saharan Africa
• Transmitted by GN aphid
Sugarcane mosaic
• Symptoms-
mosaic & necrosis
• Observed on
leaves, roots,
sometimes
the stems.
• Some times whole
plant is stunted.
Groundnut rosette virus

41. Papaya mosaic
 Mechanically transmissible viruses
 But often associated with groundnut aphid, Aphis
craccivora
 Causes leaf mosaic and stunting in papaya.
 Young seedlings in greenhouse shows vein clearing &
downward curling

42. Conclusion
• Aphids are the most common vector of plant viruses.
• Mechanisms of transmission are best understood by
considering
• routes of virus movement in the aphid (circulative versus
noncirculative)
• sites of retention or target tissues (stylets, salivary glands).
• Capsid proteins are a primary, but not necessarily sole,
determinant of virus transmission.
• A relatively unexplored area of research in plant virology is
that of early events of infection
• Salivation- essential role in release of virions- understanding
of virus entry ultimately require a better understanding of
salivation events of infection.

43. Thank you