Aphids are small (one to four millimetres), soft-bodied sucking insects. Their life cycle is complex and includes both winged and wingless forms. Wingless aphids are by far the most common form seen on potatoes during the growing season. Aphids reproduce very quickly, especially during warm weather. Their numbers can go from a low level to above the economic threshold in just a few days.
Aphids affect the potato crop directly by feeding and indirectly by transmitting viruses that cause disease. Aphids are particularly serious in seed-potato fields because seed-borne viruses severely limit potato yields.
The three most common species of aphids found on potatoes in Ontario are:
- green peach aphid
- potato aphid
- buckthorn aphid
There are several other species of aphids that do not colonize the crop, but may introduce viruses as they probe the plants. To ensure the production of high quality, virus-free seed tubers, special monitoring techniques are used in seed fields
Monitoring Aphids in Commercial Fields
Direct counts: Aphids are carried into potato fields by the wind, so they are first found on plants along the windward edge of the field. This is usually the north or west side. Scout border rows separately as is done for Colorado Potato Beetle. Also, winged aphids tend to land on diseased, yellowing plants growing at the edge of fields.
For the rest of the field, monitor aphids at 10 sites. You need to check 15 compound leaves per site (3 leaves of each of 5 plants).
Count the aphids on three compound leaves taken from the top, middle and lower canopy. Different species of aphids prefer different parts of the plant.
The first species to infest potatoes in Ontario is the potato aphid. The potato aphid prefers to colonize the upper and middle leaves. The green peach aphid begins to infest the crop as early as beginning of July. It is first found on lower leaves. When green peach aphid numbers are very high, this aphid species will also be found on middle and top leaves.
If wingless aphids are present, colonies are likely to have been established in the field some time before, and they are reproducing. If only winged aphids are present, however, the aphids likely arrived only recently in the field
Report the average number of aphids per leaf.
Economic Thresholds for Aphids in Commercial Fields
Crop Age Average Number Per Leaf
Tuber Initiation to 2 weeks before vine kill……………………………4
Within 2 weeks of vine-kill………………………………………………10