Aphid Alert 1998, No. 3, August 14
Features of this issue:
- Biology of green peach aphid
- Aphid flight activity to date
- Aphid flight activity 27 July - 6 August
In the previous issue of Aphid Alert we focused on the identification of potato-colonizing aphid species with special emphasis on green peach aphid. Green peach aphid is the most efficient vector of potato leaf roll virus (PLRV) and also the most abundant potato-colonizing aphid species in the Red River Valley. The goal of this issue is to familiarize seed potato growers with the complex life cycle of the green peach aphid.
As discussed in the previous issues of Aphid Alert, control of PLRV lies in an integrated approach by planting disease-free seed stock, isolation from virus sources (table stock potatoes) and effective control of green peach aphid. Control of green peach aphid is possible by properly selected and timed insecticide applications which can reduce within-field spread of PLRV - spread that is attributed to wingless aphids walking from plant to plant. However, no insecticides can stop the spread of viruses by winged aphids. Here we must rely on other measures.
To control spread of viruses by winged aphids requires knowledge of aphid host finding behavior and the use of cultural controls such as isolation from sources of inoculum, using crop borders , and early harvest. Generally, we can expect an increase in winged aphid movement in mid-August in association with maturation of small grains, weeds and other crops throughout the Valley. Winged aphids search for new host plants by alighting randomly on any green plant, and by late August, one of the few crops that is still green in the Valley is potato. Therefore, seed potato fields that are still green in late August are at increased risk of late season spread of viruses. In order to maintain the high quality of seed potatoes produced in the Red River Valley, seed fields should be harvested as early as possible to escape late season virus infection. Foliar symptoms almost never occur if inoculated after flowering. The only "symptom" of a late season infection is infected tubers. Once a mature plant is inoculated, virus can be detected in the tubers in as few as 7 days. Within 2 weeks, nearly all the tubers can be infected.
Life cycle of the green peach aphid
The life cycle of aphids is very unusual and complicated, and includes several body forms and different modes of reproduction. Nutrition, crowding on the host plant, geographic location and the time of year are the most important factors influencing the aphid life cycle. Figure 1 shows a typical aphid life cycle.
Winged or wingless?
In most aphids, including the green peach aphid, there are at least two body forms, wingless (apterae) and winged (alatae) forms. Wingless aphids can be viewed as "residents" on plants whereas winged forms are the "explorers" responsible for dispersal of the species (Figure 2.).
Figure 1. Typical aphid life cycle (Radcliffe et al., 1993, in Potato Health Technology)
- A: wingless parthenogenic female (form 1)
- B: wingless parthenogenic female (form 2)
- C: spring migrant (parthenogenic female)
- D: wingless parthenogenic female (form 3)
- E: summer migrant (parthenogenic female)
- F: fall migrant (parthenogenic female)
- G: male
- H: sexual female
- I: egg
Figure 2. Wingless and winged forms of green peach aphid (adapted from Radtke and Rieckmann 1990).
Why are there so many aphids so quickly?
Aphid reproduction is also unique because it includes the alternation of sexual and asexual phases. Asexual reproduction, known as parthenogenesis, involves only females which give birth to live young (rather than laying eggs). With the green peach aphid, those aphids found on potatoes during the summer are all females reproducing parthenogenically. Parthenogenic reproduction allows aphids to rapidly exploit a suitable environment since they don’t have to spend energy on finding a mate. Under favorable environmental conditions, a newly born aphid becomes a reproducing adult within 7 days, and can produce up to 5 offspring per day for up to 30 days. In potatoes it is common for aphid populations to double every two days. This remarkable reproductive capacity has been observed and described in a more dramatic manner by the French naturalist, Reaumur, during the late eighteenth century. His calculations revealed that if all the descendants of a single aphid survived during the summer and were arranged into a French military formation, four abreast, their line would extend for 27,950 miles which exceeds the circumference of the earth at the equator!
As fall approaches and daylength shortens, a sexual generation of males and females are produced. For green peach aphids, the sexual forms fly to the overwintering host (peach, plum and cherry trees) where they mate and lay eggs. Eggs are the overwintering life stage of green peach aphid. However, in the Red River Valley we have not observed green peach aphids overwintering on Canada plum, black cherry, apricot, and other trees in the genus Prunus. The most preferred overwintering host, cultivated peach, is not hardy in the Valley and combined with the rather quick onset of adverse environmental conditions in the fall and the extreme cold experienced most winters, may prevent large scale overwintering of green peach aphid in the Upper Midwest.
Where green peach aphids in the spring are coming from?
In the spring, wingless parthenogenic females hatch from the eggs laid near the buds of Prunus trees the previous fall. Once these trees have fully expanded leaves, host quality begins to decline and this signals production of winged individuals known as "Spring Migrants". These winged forms fly to a wide variety of annual plants (weeds and crops). As early spring hosts begin to mature, another winged form is produced, the "Summer Migrant". It is the Summer Migrant which colonizes potato in the Upper Midwest. In a "normal" year green peach aphid can usually be found on potato by early July.
The process of colonization
Winged aphids can travel hundreds of miles with assistance from low level jet winds. As aphids begin to descend to find a host plant, they engaging in what is called "trivial flight". Trivial flight occurs at canopy levels and aphids generally move just a few feet at a time. Finding an acceptable host plant is through visual, chemical and gustatory (taste) clues. Vision is used to locate the field, not an individual plant. For example, winged aphids are especially attracted to the green-dark interfaces of field edges and tend to land in greatest numbers at the margins of fields. This is why a crop border other than potato will protect a seed lot from PVY spread by winged aphids.
Landing on any one plant appears to be a random event. Once an aphid has landed it uses chemical and gustatory clues to determine if that plant is a suitable host. The aphid "tastes" or "sap samples" the plant to make this determination. If the plant turns out to be a non-host, the aphid will continue its trivial flight and land a few feet away. It again will sap sample to discriminate between host and non-host. It is during this host finding period when some viruses like PVY are spread. With green peach aphids, once a suitable host plant is located, such as potato, the female will give birth to a few offspring. However, rather than remaining on the plant, the green peach aphid continues her search depositing a few offspring on several plant in succession.
Robert Suranyi, David Ragsdale, Ted Radcliffe
Trapping sites this week show an increase in winged aphid capture (Figure 3, Table 1). This increase is associated with the ripening and harvest of small grains and other crops throughout the Red River Valley. Green peach aphids were found in Climax, Hoople, Cando and Rolette indicating that green peach aphid flight (Summer Migrants) is now occurring throughout the Red River Valley. Scout your field frequently (at least twice a week) by selecting 25 lower leaves in each of four areas in the field and count the number of green peach aphids found on the underside of the leaf. The previous issue of Aphid Alert contains a quick key for aphid identification. Remember once the threshold of between 3 and 10 wingless aphids per 100 leaves is reached, insecticides should be applied to prevent further spread of PLRV by wingless aphids. However, the most effective way to reduce late season spread of potato viruses is to kill the vines as early as possible! Those fields that you expect to recertify should be harvested first.
Figure 3. Aphid captures on the first, second, third and fourth sampling dates, 1998.
Table 1. Aphid species (% of total capture, 07/27-08/06).
* in traps with >50 aphids / trap a sub-sample of 50 aphids
|green peach aphid||0.4||0.4||4.8||5.6|
|corn leaf aphid||32.8||7.6||13||64.5||87.4||6.8||2||33.1|
|English grain aphid||0.4||0.7|
|Identified non-vector species||0.8||0||0||0||2.5||0||0||3.4||0|
|Total # captured||319||2922||1277||389||258||1820||420||200||18|