| | Potato aphids | Green peach aphids | Buckthorn aphid | Scouting and control of aphids
Return to the Guide to Commercial Potato Production on the Canadian Prairies.
Aphids are small, soft-bodied insects that originate mostly from local populations, but some arise as migrants blowing in from the United States
Courtesy of Agriculture and Agri-Food Canada.
After appearing in fields in mid-June to early July the population can increase rapidly to peak in the second or third week of August then decrease by early September. Although aphids are normally wingless, winged forms may appear when environmental conditions in the field deteriorate or as a response to overpopulation.
Aphids transmit viruses and in rare circumstances reduce marketable yield. Identification and control of aphids is important in potato seed production, where the tolerance for total virus levels is extremely low. Aphids control is less important in processing and table production where only the potato leaf roll virus (PLRV) causes an economic loss. PLRV, which is transmitted by the green peach aphid, causes net necrosis in the tuber and reduces market quality. In rare circumstances, high populations of feeding aphids will remove enough plant sap to damage the crop and reduce yield. The plant damage resulting from feeding aphids occurs in small-localized areas of the field referred to as “aphid holes”.
Potato Aphids
The potato aphid is the largest and most numerous of the aphid species found in potato fields. Potato aphids (PA) are up to 1/10th of an inch (3.5 mm) long, and are usually green; however individuals may be red, brown, yellow, orange, or even purple. The body of the potato aphid is elongate and pointed at the hind end. These aphids are found on the underside of potato leaves. Potato aphids over-winter on the Canadian Prairies in the egg stage on rose bushes. Wingless females hatch from these and produce live young females, some of which develop wings and fly to new hosts. There are several generations of winged and wingless forms during the summer. Winged males and females fly to rose bushes during late summer and early fall where the females give birth to wingless females that mate with the winged males and produce the winter eggs.
Potato aphids are efficient vectors of mild and rugose mosaic (PVY) viruses. The potato aphid is a poor vector of potato leaf roll virus when compared to the green peach aphid (discussed below). In spite of being a poor vector of PLRV, extremely high populations of potato aphids may be as much of a factor in PLRV transmission as a small population of green peach aphids. In addition to virus spread, a toxin produced by the potato aphid is injected during the feeding process and may cause mottling, curling or premature death of the leaflets. Large quantities of honeydew excreted by the insects may cause the tops of the plants to become sticky if populations are extreme.
Green Peach Aphids
Green peach aphids (GPA) are smaller than the potato aphid (not more than 1/8" or 2.3 mm long), and the body is egg-shaped and lacks a pointed hind end. GPA varies in colour from creamy white to peach and even pinkish, but is most often pale green. In the green peach aphid, the antennal tubercles are prominent and inward pointing (appearing like a bottle cap opener). The GPA does not over winter outdoors in Manitoba. Populations may be maintained from year-to-year by over wintering in greenhouses, but it is likely that many populations migrate from the southern United States. In the spring, winged females are carried outside with bedding plants and then fly to weeds. They multiply rapidly for several weeks and by July they move to potatoes and fly from field to field, transmitting viruses to healthy plants. Winged females usually appear in the fall and some of these are successful in becoming established in greenhouses or root cellars.
Green peach aphids are highly efficient vectors of the potato leaf roll and rugose mosaic (PVY) viruses. Like potato aphids, they inject a toxin during the feeding, which causes leaf deformities, streaking, and even leaflet death.
Buckthorn Aphid
The buckthorn aphid (BA) is smaller than the potato aphid (1/16-1/8" or 1.2 – 2.0 mm) and the same shape as the green peach aphid. They are relatively easy to tell apart based on the antennal tubercles, which are small and hardly noticeable in the buckthorn aphid. In the green peach aphid, the antennal tubercles are prominent and inward pointing (appearing like a bottle cap opener). BA over-winter as eggs on buckthorn plants. In the spring, the eggs hatch and the larvae move from host plant to weeds, and by mid-July to potato plants. Late in the summer, the winged males and females give birth to wingless forms that mate and lay eggs for over-wintering. Summer populations are usually localized but, in those fields, this may be the most abundant species of aphid and, in a dry year, it can spread throughout the field. Buckthorn aphids are not as efficient at vectoring PLRV, but are a very effective vector of PVY.
Scouting and Control of Aphids
Seed potato growers must control problem populations of aphids early to maintain seed quality. The amount of potato leaf roll virus (PLRV) that spreads in a field is influenced by the population of aphid vectors in the field, the incidence of PLRV in the aphid vector and the incidence of PLRV in the field. To determine the potential for leaf roll virus spread, aphids must be identified and counted starting in early July when aphids begin to occur in potato fields. Divide fields into four plots, 25 leaves are taken from each of the four plots, and potato aphids and green peach aphids are counted on each compound leaf sampled.
Courtesy of Agvise Laboratories
A ten-powered magnifying lens is required to identify young aphids. The large body size and elongated shape of the potato aphid distinguish it from the smaller oval-bodied green peach aphid. Green peach aphids are typically found on the lower 1/3 of the canopy, nearer the ground. BA and PA are typically found in the upper 2/3 of the canopy. It should be noted that depending on how quickly the aphids are located after colonizing a plant, green peach aphids might still be found in the upper 2/3 of the canopy if insufficient time has passed for it to move to the lower canopy.
The number of aphids detected in a count is totalled and compared with the economic threshold for that field. Thresholds for green peach aphid are based on sampling 100 lower leaves (25 lower leaves in each of 4 areas of a field). The economic threshold is 3-10 aphids per 100 lower compound leaves for seed and 30-100 aphids per 100 lower leaves for processing. There currently are no established economic thresholds for buckthorn and potato aphids.
In order to accurately identify aphid species, consult with an agrologist. Check with your provincial specialist for recommendations on which registered products are most effective for controlling aphid populations. Repeated applications of other insecticides may select for resistance very rapidly, and the second and subsequent uses of the compound may not be effective.
Written by B. Elliot |
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