Several different aphids can be found in potato fields. Aphids of themselves can cause wilting damage by sucking out nutrients from foliage and stem tissues and this may be a problem especially in nursery crops. But, the real problem caused by aphids in commercial fields is their ability to carry pathogenic viruses. In potato fields, the common viruses are potato leaf roll virus (PLRV), the mosaic viruses (e.g., PVA, PVY) and alfalfa mosaic virus (AMV, “calico virus”). Persistence is the key to the spread of a virus by aphids in the potato field. For this reason, the most important of the aphids in potato is the Green Peach Aphid (GPA) (Myzus persicae) which will persistently carry PLRV, usually the major viral problem on potato. Other aphids that will carry viruses non-persistently are Potato Aphids which may carry PLRV and others as the GPA, and Alfalfa Aphid which carries the non-persistent AMV/calico.
Description
GPA undergo three stages of development: adult, nymph and egg.
Adults have a tear-drop shape. Winged adults are bright green with a dark head and thorax, and a greenish abdomen with dark patches. They secrete a sticky substance called honeydew. In contrast, potato aphids are larger with more elongated bodies. The best method to obtain a positive identification is to view under a microscope at a 10X magnification and observe the head. Look at the shape and length of the converging tubercules or projections at the base of the antennae as viewed from above.
Nymphs are actually wingless female adults. They have a tear-drop shape and are about 1/10 inch long. During the early part of the season, they are light yellowish green to pinkish, and later in the season they are pink to pale orange. Nymphs move around and usually colonize on lower leaves.
[See diagrams comparing GPA with the potato aphid adults and nymphs courtesy of McCain Foods.]
Eggs are small and oval, and are shiny black.
Life Cycle
Eggs are the overwintering form of GPA. They overwinter in trees of the Prunus species such as wild plum, chokecherry, apricot. These hosts have been shown to have different preferences for GPA in different parts of the country. Harsh winters may kill some of the eggs and conversely more eggs may survive mild winters. The eggs survive in the bark of axillary buds. Hatching occurs in the spring just after full bloom (“green tip” stage) and nymphs, wingless female adults, emerge feeding for two or three generations on the leaves of the tree. In late spring, winged adults are produced and migrate to secondary hosts such as potato. But they are more likely to move to one of over 100 species of plants that can serve as secondary hosts. Some of the more common of these hosts include mustards, nightshades, ground cherries, and several ornamental or vegetable crops. During spring migration, winged adults travel many miles to many hosts depositing nymphs in colonies up to 20 on the lower third of plants. Later these nymph colonies will produce more winged adults that travel further (summer migration). During this migration cycle is when viral transmission occurs as the nymphs pick up virus from the colonized plant and the winged adults then transmit them to other plants. Many generations can be produced; the turn around rate reported is every eight to 14 days at 70oF. With the exception of the overwintering generation, the females give live birth to only females so their reproductive energy is very streamlined. With each generation, a greater proportion of winged adults, both male and female, are produced, and more are flying and spreading viruses. Toward the end of the season, the winged adults migrate to Prunus trees to lay the eggs that will overwinter.
Although the normal life cycle of the GPA involves overwintering in trees, GPA are also able to remain active all year round, and thereby overwintering, in greenhouses on bedding plants. An Idaho survey conducted in 1990 showed that over a third of the salable bedding plants were infested with GPA. These would be capable of escape either from the greenhouse or from home gardens developed with aphid-infested plants into potato fields.
Damage
GPA nymphs ingest sap from leaf veins and, if sufficiently severe, could cause some wilting damage. However, in potato fields, this is not why GPA is a concern to production. The concern dealing with all aphids that feed on potato is that they are vectors of pathogenic viruses, that is, they can carry viruses and inject them into healthy plants. The main concern with GPA over other aphids is that it can carry the PLRV persistently. Potato Aphid, for example, does not transmit PLRV persistently (non-persistent vector). Non-persistence means that the potato aphid picks up the PLRV, for instance, and injects all that if picked up on the next plants it feeds on. After that it must re-infect itself by feeding on another infected plant before it can spread the virus to another plant. GPA, on the other hand being a persistent vector, need only to pick up the viruses once and then it can infect one plant after the other without ever landing on another infected plant again.
Potato Leaf Roll (PLRV)
Note that viral infection and symptoms also occur in plants arising from seed tubers that are infected with the virus. See Biological Management -- Seed Certification.
Vine -- Early season symptoms develop mostly on young leaves. They appear upright, pale and rolled. Leaves are crisp and stiff, and give a rattling noise when shaken. Plants are usually stunted and rigid when the seed tuber or piece was infected. Later in the season, leaf rolling is increased and all leaflets are affected. The leaf rolling should be confused with wilts such as Verticillium wilt (early dying) with which leaflets are limp and browning, and one side of the plant is initially affected. Black leg will also show a leaf roll but always look for the infected stem at the base. Plants invaded with Euro. corn borer will also leaf roll so look an entry hole of the larva.
Tuber -- PLRV can affect tuber quality. This symptom is called net necrosis and appears as translucent spots from the tuber center extending ray-like. This is a concern because when fried these spots will turn brown resulting in polka-dotted french fries and potato chips. Note that this symptom can also be confused with freezing damage (see physiological disorders).
PLRV Transmission
Several species of aphids can transmit PLRV, but the GPA is by far the most efficient vector of this virus. PLRV transmission is more complicated compared to other potato viruses. PLRV is concentrated in the phloem of the plant. To pick up this virus an aphid must feed from the phloem which occurs only if the plant is a host plant. When the aphid ingests the virus, the virus will pass through the wall of the gut into the blood of the aphid. The virus will eventually move to the salivary gland where it can potentially be transmitted to the next plant the aphid injects saliva into. The virus can be picked up in less than 30 minutes of feeding, but there is a lag time of 12 to 36 hours before the virus will enter the salivary gland and be able to be passed on to the next plant. A major aspect of this type of transmission is that it is persistent and the aphid will be able to transmit the virus for the rest of its life.
Note: PLRV and other potato viruses are NOT passed from aphid to aphid. Each nymph must obtain the virus from an infected plant. These viruses also are NOT passed on from generation to generation. Each spring, the nymphs must pick up new virus fresh from emerged hosts which include many weeds such as nightshades, plants growing from infected seed tubers, plants growing in cull piles from infected discarded tubers, or from infected volunteer potatoes growing in the field of last year’s crop.
Monitoring
The inability to predict the occurrence of GPA populations causes difficulty in aphid and viral management. Several sampling tools have been used to aid in predicting GPA populations in potato. Large suction traps and smaller yellow pan traps have been used. However, current evidence indicates that these may not work best. Recommendations for GPA population monitoring rely on regular leaf sampling for aphid presence. Regular monitoring for GPA will allow the grower to identify the presence of the aphid at the earliest possible time. Beginning early in the season, monitor nearby Prunus trees and weed hosts such as mustards, nightshades and ground cherry. GPA infestations tend to begin or be higher near the edges of fields, particularly the windward side of the field and near building and shelter belts; special monitoring of these areas may provide warning on the early presence of aphids. Home gardeners and truck farmers should also be monitoring, not only their potato plants but also other garden plants and weeds in the area.
As soon as full leaves are out on the potato plant, sampling should be initiated in seed fields. GPA are more likely to be found on the lower third of the plant, so leaves should be chosen from this area of the plant. The entire field should be covered with a cris-cross or diagonal pattern when sampling. It is important to properly identify them so it’s best to collect them for later identification under magnification and note the form of the aphids, winged or wingless. The form of the aphids indicates the status of the aphid population in the field. If wingless aphids are present, colonies are likely to have been established in the field. However, if only winged aphids are present, the aphids found likely arrived only recently into the field. A minimum of 50 leaves should be sampled from across a field, but a 100-leaf sample will give an even better estimate of their potential presence. Monitoring for the aphids should continue as long as the vines are green and growing.
Leaf-sample thresholds vary a great deal depending on the type of potato being grown. For seed potato, a threshold of 10 wingless GPA per 100 leaves has been established in the upper Midwest. This threshold is used to eliminate treatments that are of little value. Some other seed producing areas use a zero-tolerance threshold. Thresholds to reduce net necrosis vary between different production regions. Idaho indicates that their threshold is 10 wingless aphids per 50 leaves, while the upper Midwest (MN, ND, WI) threshold is set at 30 wingless aphids per 100 leaves.
Management
Potato Seed Certification -- GPA and other aphids are not a major problem as long as they do not pick up potato viruses be they PLRV or mosaic viruses. A major source, though not the only source, of these pathogens is infected seed tubers that are planted. Keeping the viral load to a minimum is achieved primarily through testing of seed tubers and growing those which were found to be viral free. This job is done through the States’ Potato Certification Associations. Viral levels in nuclear and early generations of certified seed are low enough to prevent problems from developing if aphid populations are managed properly. Therefore, it is crucial to plant certified seed. This is also true home gardeners and small acreage growers.
Biological -- Several other cultural practices can be undertaken to reduce the potential for developing PLRV and other viruses. The virus pool can be reduced by eliminating cull piles and volunteer potatoes growing in the previous years’ potato fields. If possible, seed potato producing areas should be isolated from production areas. Weeds such as mustards and nightshades that may host GPA in the spring should be controlled in and around potato fields. Roguing of infected plants can help to reduce the level of viruses in the field. Proper roguing techniques should include the removal of the infected plant, plus the adjacent plants on all four sides of the infected plant. Some predation of nymphs (wingless females) by beneficial insects is possible especially during mid season. Until recent uninformed political pressure, genetically-modified (GMO) potato were being developed that were immune to infection by PLRV by not allowing the virus to replicate itself through disruption of its protein coat. Hopefully, this politicized climate will change as more people become better informed.
Varietal Sensitivity
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(sources: "North American Potato Varieties" PAA, "Characteristics of Potato Varieties" PNW #454, "San Luis Valley Production Manual" XCM-181.)
Chemical -- As with all vectored viruses, PLRV is managed by managing the vector along with minimizing the viral pool. If possible, nearby Prunus trees that may harbor overwintering eggs and be the first hosts in the spring should be treated for GPA. Optimizing GPA management requires regular monitoring, proper insecticide timing and use of the most effective insecticides. So, a major management tool for PLRV is insecticidal control of the GPA. Usually some type of systemic, broad-spectrum, soil-applied product is applied at planting protecting plants against several insects including GPA, Colorado potato beetles and potato psyllids. Recent products containing imidacloprid or thiamethoxam last up to 13 weeks after planting. Phorate, a commonly used inexpensive product, gives sporadic control of GPA and offers up to eight weeks of protection. Later season foliar treatments have been largely relied upon in the past to control GPA. Without doubt, Monitor is the best GPA control product currently registered but shows poor control of Colorado potato beetle. Multiple applications of Monitor in a year are to be avoided so selection pressure on the aphid population can be lessened and resistance to the product developed by GPA. If additional treatments are needed, Thiodan and Provado are rated as good products for both GPA and Col. potato beetle. Provado should not be used on fields where Admire was used at planting since they are both imidacloprid. Pyrethroids in general along with Sevin and methyl parathion are listed as products that will likely encourage the development of severe GPA problems. There are other products that are labeled for potato aphid but NOT for green peach aphid; they also may not very effective against either.
Resistance
A very important consideration for insecticidal control of GPA is to avoid the development of aphid resistance which has developed in several States on the East Coast and in the Great Lakes region. GPA is one of a few species of insects like the Colorado potato beetle that has developed resistance to all major insecticide classes. In some areas such as Nebraska, resistance by GPA or Col. potato beetle has not appeared yet. To delay or prevent resistance developments, it is crucial to keep the number of application of all types of insecticides to a minimum, to rotate between insecticide types within the year, and to apply products that will affect more than one pest with application. The latter requires careful monitoring and application timing as well as choosing the right product. Treatments should be chosen to minimize selection pressure.
Quick Review
Appearance:
Adult (winged) - bright green with dark head and thorax, and dark patches on abdomen; tear-drop body shape; 1/10 inch long
Nymph (wingless adult) - light yellowish green to pinkish to pale orange; like adult but wingless
Egg - small, black and oval, found only on Prunus trees
Life Cycle:
Overwinters as egg in Prunus trees
Many generations during the season starting in April/May
Damage:
Aphid - possible slight wilting
Carries viruses especially potato leaf roll (PLRV)
PLRV - stunting, leaf roll, tuber yield and quality loss
Management:
Plant certified seed
Locate away from Prunus trees
Control/treat spring hosts including trees and weeds
Eliminate cull piles and volunteer potatoes
Beneficial insects
Monitor in field, on bedding plants and in gardens
Roguing
Systemic soil and foliar insecticides
GMO potato engineered with reverse viral coat protein
Sources:
GPA - Prunus trees, bedding plants, migration, gardens, nurseries
PLRV - seed tubers, weed hosts, cull piles, volunteer plants, bedding plants, gardens, nurseries