Aphids are threatening canola crops just as the plants are reaching a crucial grain-fill phase.

The incidence of aphids around Young and Cowra and the Riverina has prompted some farmers to areal spray.

NSW Department of Primary Industries (DPI) district agronomist at Cowra, Jan Edwards, said for some reason the aphids were particularly attracted to drought-stressed crops.

She said typically the aphids could strike crops at any time during the growing season but this year they had made their presence when canola was at the grain-fill stage.

"There seems to be more aphids about this year; and we don’t see them every year," she said.

"We have definitely got some aphid numbers in this district and right across the region."

Ms Edwards said the aphids could wreak havoc on moisture-stressed crops by sucking the sap from the stems.

"It results in a fair bit of damage," she said.

Landmark Temora, agronomist Craig Warren said it was the worst outbreak of aphids he had witnessed in his 13-year career.