Aphids in Canola
Aphids that infest canola are the Green Peach aphid, Cabbage aphid, and the Turnip aphid. The aphid colonies cluster at he tops of the stems causing wilting and cessation of flowering. In most cases, individual or small groups of plants are infested in maturing fields and the damage is rarely significant since the bulk of pod formation has been completed. In those cases, spraying is not economical. Several kinds of beneficial insects, like the ladybird beetle and lacewing, feed primarily on aphids. Their populations increase as aphid populations increase, usually in numbers sufficient to control the aphids.
The big concern, however, is in later planted fields. Green peach aphid are very difficult to control because they tolerate most of the insecticides currently available. Insecticides labeled in canola are: methyl parathion, 6,3-ethyl,methyl parathion, endosulfan (Thiodan and Endocide), and bifenthrin (Capture). Methyl parathion has been used in Georgia on winter seeded canola to control aphids: endosulfan has not been a strong product on them in local potatoes. If treatments are made, labels require that honeybees be protected by limiting treatments to evening hours, 3 hours before sunset, a time when bees should no longer be foraging in the fields.
There are no established thresholds for this problem. Researchers from Georgia indicate that aphids can cause a 10 to 30% yield reduction when infestations reach populations averaging 1 to 2 aphids per leaf. This threshold guideline does not fit circumstances observed in our fields.
If aphid control is attempted, methyl parathion use is suggested at this time. Follow-up treatments with a field scouting to evaluate success and to determine if/when aphid populations increase again. Treatments will eliminate the abundant numbers of predators that are moving in to feed on the aphids. Without the predators and the natural control they offer, aphid populations could increase rapidly after an insecticide is applied.
These treatments will be risky from the standpoint that population decrease of the aphids may not reach levels we desire because of the poor control expected for green peach aphid; with few predators, the aphid population could rebound quickly without any natural controls in the field; and, impact on yield is not well understood. We know there is going to be control difficulties. If this problem persists, the outcome is unknown.