Aphid Transmission of Potato Viruses

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These aphids cause more damage by transmitting viruses than byfeeding on plants. The most important aphid vector is Myzus persicae.

Aphids transmit potato viruses in two ways (Figure 1):

non-persistent transmission

persistent transmission

Figure 1. In non-persistent virus transmission (left), aphids acquireviruses during brief periods of probing on epidermal tissues. In persistent

transmission (right), an aphid must feed on the phloem.

Non-persistent transmission. Aphids can acquire viruses during briefperiods of probing or feeding on the epidermal tissues of infected plants.It may take only a few seconds for the mouth parts to becomecontaminated, and then the aphids can transmit viruses immediately toother plants. Aphids remain infective (viruliferous) for a short period,usually for not more than two hours, and viruses can only be carried overshort distances. Except for potato leafroll virus, all aphid-borne potato

viruses are non-persistently transmitted. These include potato viruses Y,A, and M (PVY, PVA, PVM), and some strains of PVS. Of this group,PVY is the most important virus.

Persistent transmission. Viruses that are persistently transmitted arelocated in the phloem tissue of plants. To acquire them, an aphid mustfeed on the phloem, not just probe on the leaf surface. This may take 20to 30 minutes. The virus enters the body of the aphid and, during anadditional latency or incubation period lasting several hours, aphidsusually remain non-viruliferous. The virus persists throughout the aphid'slife and can be carried over long distances. The best known persistentlytransmitted potato virus is potato leafroll virus (PLRV), which is also themost important one.

2. Aphid Biology

Aphids survive the winter or unfavorable seasons in the egg stage. Inspring, the eggs hatch and the nymphs develop into first-generationadults. During the following generations, females produce live(viviparous) offspring without fertilization (parthenogenetically). Towardautumn, or in seasons that do not favor aphid reproduction, males and


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egg-producing (oviparous) females may appear again. In tropical zones,aphids can reproduce parthenogenetically throughout the year.

Aphids can reach pest proportions rapidly. One aphid can bear 30nymphs and, during a favorable season, eight successive generationsmay follow. Thus, one aphid can theoretically give rise to 30

8individually,

that is, more than 6 x 1011

aphids. Fortunately, nature limits aphidpopulations through unfavorable climatic conditions, parasites, predators,and nutrient deficiencies in the host plant.

Many aphid species live on only one host plant; these are said to beautoecious. Others migrate between primaryoverwintering hosts, whichare usually woody plants, and secondary hosts, which are usuallyherbaceous plants. These aphids are heteroecious. Myzus persicae, forexample, may overwinter on peach trees (Prunus persicae); therefore,the name "green peach aphid." During subsequent reproductive cycles, itinvades many secondary hosts, including potatoes.

After overwintering, winged (alate) aphids appear that may migrate fromprimary to secondary hosts. During the following generations, winged as

well as wingless (apterous) forms may be produced simultaneously.

Aphids usually do not fly when temperatures are below 13C. Hightemperatures and low rainfall are ideal for their multiplication. As thetemperature rises to 30C, their activity increases. Low wind speed ofless than 1 km/h favors aphid movement. Warm air currents can liftaphids several hundred meters into the air, and they have been found atmore than 1000 km from the nearest possible origin.

Since aphids are sucking insects that feed on plant sap. After landing,they probe to determine whether the plant is suitable as food. If the plantis suitable, an aphid may take several minutes or sometimes hours toinsert its mouth partsthe stylistinto the phloem.

Infestation of a plant depends on its physiological state. Droughtincreases the concentration of solutes in the plant sap, which is preferredby the aphid. Deficiencies of nitrogen and potassium influence the plantmetabolism and osmotic conditions of plant cells, thus predisposingplants to increased infestation. Leaf composition changes during plantdevelopment. Myzus persicae prefers lower leaves, which are moremature. Other aphids, such as Macrosiphum euphorbiae, prefer youngleaves and are found on the upper portions of the plant. Plants showing ayellow appearance caused by diseases or deficiencies may be more

attractive to winged aphids than healthy-looking plants.


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Figure 2. Life cycle of Myzus persicae, which can overwinter on peachtrees. During subsequent reproductive cycles, it invades many

secondary hosts, including potatoes. In tropical zones, aphids

may reproduce parthenogenetically throughout the year.

3. Aphid Identification

Aphid species differ in their capacity to transmit viruses. In temperatezones, the only significant correlation between the dissemination ofpotato viruses and aphids appears with Myzus persicae. Therefore, aphididentification, population studies, and control should concentrate on thisspecies. However, because of a lack of data from tropical countries,other prevalent species should be observed as well, especially in areasthat are new to potato production.

Aphids are soft-bodied insects that measure a few millimeters in length.

They possess up to 50 identifiable characteristics. In practice, only a feware importantthe body, head, wings, cornicles, and cauda (Figure 3).These can be observed under 6x to 10x magnification with a good handlens or stereoscopic microscope.

Body. The body of an aphid consists of three parts: head, thorax, andabdomen. These parts have a distinctive size, shape, and color indifferent aphid species.


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Head. The shape of the head is determined by antennal tubercles, whichmay be well pronounced or absent. These carry the antennae, which inmost aphid species are long and thin, with five to six segments. The ratioof antennae length to body length is frequently used for identification.

Wings. Wings are attached to the thorax and consist of a pair of large

forewings and a pair of smaller hindwings. Wing shape (primarily that offorewings) and vein patterns (venation) help in aphid identification.

Cornicles. Cornicles (siphuncles) are tube-like structures that projectupward in pairs from the fifth or sixth segment of the abdomen. Theirshape, length, and color vary greatly between species.

Cauda. The last abdominal segment is modified to form a tail or cauda,whose shape, length, color, and number of hairs help to distinguish thespecies.

Figure 3. In practice, only a few characteristics are important for aphididentificationthe body, head, wings, cornicles, and cauda.

4. Myzus persicae

Myzus persicae is the most widely distributed aphid, and the mostimportant vector of potato viruses (Figure 4). It usually feeds on the lowerparts of the potato plant.


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Body. Adults are 1.5 to 2.5 mm long. Wingless forms are usually lightgreen to almost translucent in color, but they may also be deep pink orpeach. The body is egg-shaped; the posterior part is rounded.

Winged forms are distinguished by a dark brown to black head andthorax. The abdomen may be green, pink, or dull red, and is marked by a

characteristic dark patch. The body outline is similar to that of winglessforms, but is modified by the presence of wing pads.

Head. The head is noticeably indented. On both wingless and wingedaphids, antennal tubercles are prominent and inward-pointing. Theantennae are almost as long as the body.

Wings. Wing characteristics are not commonly used for identifying M.persicae

Figure 4. Myzus persicae is the most widely distributed aphid, and the

most important vector of potato viruses. (Photographreproduced with permission of Her Britannic Majesty's Stationery

Office; plate British Crown copyright.)


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Cornicles. Cornicles are light-colored and slightly swollen toward theends.

Cauda. The cauda is light-colored and approximately half as long as thecornicles.

5. Study of Aphid PopulationsThe aphid is one of the most efficient insect vectors that transmit viruses.Thus, knowledge of aphid population dynamics is important for decidingwhere, when, and how to grow and protect seed potato crops. Threecommon methods to study aphid populations are:

aphid/plant counts,

aphid/leaf counts, and

yellow water traps.

Aphid/plant count. Aphid/plant counting, or the "beating method," is aquick and efficient technique to use early in the growing season whenplants are young and aphid populations are too low to be detected byother methods.

Procedure: The first winged adults arriving at a field usually establishaphid colonies on plants of the windward border rows. Begin the count byrandomly selecting plants along these rows. Although the number ofplants to be counted depends on the size of the field, most potatoworkers sample 50 plants. The frequency of countingdaily to weeklydepends on climatic conditions. Sampling should be more frequent duringwarm weather.

To collect aphids, place a light-colored board or paper under one side ofthe plant. Gently shake the plant over the board. These counts are fairly

reliable, although some winged aphids may fly away when the plant isdisturbed.

Aphid/leaf counts. Virus transmission is more closely related toprogress of aphid infestation from plant to plant than to the total numberof aphids on a particular plant. Progress of aphid infestation can beobserved by the aphid/leaf counting method. Natural enemies andparasitization of aphids can also be observed.

Procedure: At least once a week, select 50 plants randomlythroughout the crop. Examine three fully expanded leaves from eachplantone each on the top, middle, and lower parts. Record the numberof winged and wingless aphids, as well as predators, parasites, and

parasitized aphids.

Some winged aphids may fly away when the plant is disturbed.Nevertheless, aphid/leaf counting is more precise than aphid/plantcounting, although it is more laborious.

Yellow water traps. Aphids normally migrate into and within a potatocrop via the flight of winged adults. This flight can be studied by trappingaphids in a yellow water trap, which is inexpensive and simple toconstruct (Figure 5).


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Traps of various materials, sizes, and shapes can be used. The mostfrequently used traps are rectangular trays, 50 cm long, 30 cm wide, and8 cm high, with the sides sloping outward. Traps of the same type andsize should be used in a field or region to facilitate comparison of aphidcounts.

If the crops are grown under rainy conditions or irrigated by sprinklers,traps need an overflow hole. A screen or piece of muslin placed over thehole allows excess water to drain without losing the aphids. One bottomcorner of the tray should have an outlet tube of about 2-cm diameter and4-cm length, closed from the inside with a stopper.

Many aphid species are attracted by the color yellow. Therefore, thebottom and lower 2 cm of the inner sides should be painted yellow; theremaining sides and the exterior are painted dark gray. Entomologistsrecommend using British Standard prime yellow (B.S. 0-011), commonlycalled canary yellow. Although this yellow color does not attract all aphidsequally, it is effective in attracting Myzus persicae.

Procedure: Two traps per field are sufficient. Place them 5 m apart

on platforms approximately 60 cm above the ground. Eliminate any planttouching the platform to prevent aphids from crawling into the traps.

Fill the traps with fresh, clear water 2 cm above the yellow part. To breaksurface tension and to prevent aphids from escaping, add a few drops ofliquid household detergent.

Examine the traps preferably each morning. When aphid populations arelow, few adults may be caught. Identify them with a hand lens andremove them from the water. Change the water only if it is dirty.

When many aphids are present, empty the trap through the outlet tubeinto a bucket. Collect the aphids by attaching a small muslin bag to the

outlet. The water may be reused as long as it is clean. For identification,take the aphids to the laboratory in a vial containing 50 percent alcohol.

Use of aphid population data. Because of the specific advantages ofeach of the three methods, entomologists recommend using more thanone method during the cropping season.

Aphid/plant counts may help to detect the start of aphid infestations.


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Figure 5. Aphid flight can be studied by trapping aphids in inexpensive

and simple yellow water traps.

Aphid/leaf counts reveal aphid movement and population buildup within acrop. Data may be graphed over time, thus showing the progress ofaphid infestation (Figure 6). (A simple method is described by Hollings,1955).

Yellow water traps help to monitor the flight of winged aphids, which ismore directly related to virus transmission than movement of wingless

aphids.


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Figure 6. Data of aphid population studies may be graphed over time, thusshowing the progress of aphid infestation. (Above data show

comparison of two years.)

Data of aphid population studies may help in forecasting the degree ofvirus infection. However, additional variables must be considered, suchas:

varietal susceptibility

date of planting

aphid control measures

crop management practices

weather conditions

In general, data on aphid population studies are useful for:

selecting the best seed production areas

selecting the best growing seasons

scheduling aphicide application

scheduling dates for destroying the foliage and harvesting

Experiments may be designed to determine the correlation betweenaphid populations and virus spread. This is a way to determine criticallimits of vector populations. Critical limits indicate when aphid populationsbecome large enough to cause an economically significant degree ofvirus infection.

In seed areas, only a few winged M. persicaeare trapped. As the numberof winged aphids increases, virus spread increases. This correlation iscloser with PLRV than with PVY. Because the spread of PVY seemsmore erratic, determining critical limits is more difficult. Since PLRV isusually a more serious problem in the tropics, the correlation of the data


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of population studies to PLRV incidence is generally the most practicalindicator of the measures that need to be taken.

6. Control of Virus Spread

Direct control of virus spread is not feasible. Indirect control, however,helps to reduce virus transmission. This includes considering the growingarea, agricultural practices, and chemical control.

Growing area. Because virus spread is related to aphid populations,seed production areas should be selected on the basis of aphidpopulation studies. Aphid populations are generally low in areas with lowtemperature, abundant rainfall, and high wind velocity.

Within a potato-growing area, to reduce dissemination of viruses throughviruliferous aphids, seed potato fields should be located upwind fromcommercial potato fields and alternative host crops (Figure 7). Ideally,seed production areas should be completelyseparated from commercialpotato production.

Figure 7. To reduce dissemination of viruses through viruliferous aphids,

seed potato fields should be located upwind from commercialpotato fields. Ideally, seed production areas should be

completely separated from commercial potato production.

Planting time. The time of aphid migration may be more important thanthe total number of aphids trapped. Aphid flights often reach definite

peaks at certain times of the year. Virus spread is more serious earlier inthe season than later, as young plants are generally more susceptible.Additionally, plants that are infected early become more efficient sourcesfor further virus spread than plants infected later in the season.

Infection sources. Eliminate primary hosts. Avoid areas close tocommercial potato production, glasshouses, and home gardens. Within aseed field, eliminate infected potato plants as early as possible. Eliminateyellow-flowering weeds and any other host plants within and around thefield.


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Crop management. Resistance of plants to aphid infestation can beincreased by balanced fertilization and good water management.

Harvest time. After a viruliferous aphid has fed on the foliage, a virusneeds some time to reach the tubers. The highest-grade seed potatoesshould be harvested no later than 8 to 10 days after a population study

has revealed a critical aphid buildup. This is sufficient time to preventvirus infections from reaching tubers.

Tuber storage. Tuber sprouts are readily colonized by aphids. You canprevent virus transmission by storing tubers under aphid-proof conditionsand using aphicides.

Chemical control. How efficiently virus transmission is controlled withaphicides depends on the type of virus transmission involved.

In non-persistent transmission, aphids transmit viruses (e.g., PVY) fasterthan common aphicides can react to kill the vector. Within a field,aphicides may slightly reduce non-persistent virus transmission, but theycannot prevent it; neither can they control infections from outside the fieldby migrating aphids.

In persistent transmission of viruses (e.g. PLRV), the incubation period islong enough to allow aphicides to control vectors. Aphicides can greatlyreduce PLRV spread within a field, but they cannot control infections fromoutside by migrating aphids.

The most suitable aphicides are organophosphate systemic sprayformulations or carbamate granular insecticides applied to the soil.

Mineral oil sprays are effective in reducing non-persistently transmittedviruses, but fail to prevent PLRV spread. Consult local experts.

Recommended Literature

Agricultural Development and Advisory Service. 1977. Key foridentification of brassica, potato, and sugar beet aphids withphotographic illustrations. Ministry of Agriculture, Fisheries, andFood. Plant Pathology Laboratory, Hatching Green, England. 28 p.

Bokx, J.A. (ed.). 1972. Viruses of potato and seed-potato production.Centre for Agricultural Publishing and Documentation, Wageningen,Netherlands. p. 3656, 167173.

Byrne, D.N., and G.W. Bishop. 1979. Comparison of water trap pans andleaf counts as sampling techniques for green peach aphids onpotatoes. Am. Potato J. 56:237241.

Byrne, D.N. and G.W. Bishop. 1979. Relationship of green peach aphidnumbers to spread of potato leafroll virus in Southern Idaho. J.Econ. Entomol. 72:809811.

Cortbaoui, R. 1984. Roguing potatoes. Technical Information Bulletin 5.International Potato Center, Lima, Peru. 13 p.

Harten, A. Van. 1983. The relation between aphid flights and the spreadof potato virus Y

N(PVY

N) in the Netherlands. Potato Res. 26:115.


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Hollings, M. 1955. Aphid movement and virus spread in seed potatoareas of England and Wales, 19501953. Plant Pathol. 4:7382.

Hooker, W.J. 1982. Virus diseases of potato. Technical InformationBulletin 19. International Potato Center, Lima, Peru. 17 p.

Raman, K.V. 1984. Monitoring aphid populations. CIP Slide TrainingSeries IV-2. International Potato Center, Lima, Peru. 35 slides, 12 p.

Rydn, K., S. Brishammar, R. Sigvald. 1983. The infection pressure ofpotato virus Y

oand the occurrence of winged aphids in potato fields

in Sweden. Potato Res. 26:229335.

Woodford, J.A.T., B.D. Harrison, C.S. Aveyard, S.C. Gordon. 1983.Insecticidal control of aphids and the spread of potato leafroll virus inpotato crops in eastern Scotland. Ann. Appl. Biol. 103:117130.

Aphid Transmission of Potato Viruses

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