Aphids: They cluster like crazy but is spraying economical?

To answer the question, not likely. In most cases, only individual or small groups of plants are infested.

Noticeable numbers usually appear at the tops of plants in late July to early August. The aphids frequently cover the entire top 10 to 15 cm (4 to 6″) of plants. Damage is rarely significant since the bulk of pod formation has been completed, and the damaged top few small pods contribute little to the overall yield.

The nominal thresholds for turnip aphid or cabbage aphid in canola is when 10-20% of stems have aphid clusters.

Several 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.

Different for pulses. This same approach not be true for pulses or other crops where controlling aphids may have some value. Consult a local agrologist or your provincial entomologist for more details.