David Norman said that a Horticultural Development Company project currently under way has so far found that root lesion, stubby root and needle nematodes do not affect the dry weight of the onion crop.
"Onions can tolerate much higher populations of some species than previously thought," he said. "That suggests our crop damage thresholds for these are far too conservative and could be raised five or even ten times."
This contrasts with the impact of the stem nematode, which can be "devastating" to onion crops, he added.
Accurate sampling leading to targeted treatment would be more economical, said Norman.
"Sampling costs around £20 per hectare, compared with £150 a hectare for a nematicide treatment."