Sap-sucking Rutherglen bugs that have invaded South Queensland grain sorghum crops should only be sprayed with insecticide as a last resort.

Queensland's Department of Primary Industries and Fisheries (DPI) entomologist Adam Hardy said with high soil moisture conditions, grain sorghum plants could withstand relatively large numbers of Rutherglen bugs before they affected grain quality and caused economic loss.

Mr Hardy said while spraying was sometimes necessary, it was an added cost and would kill the natural enemies of Rutherglen bugs, heliothis and aphids.

"This is why unnecessary spraying worsens insect pest problems. Another issue is that spraying is often ineffective because nymphs are well hidden within sorghum heads."

He said spraying was necessary to prevent economic damage if there were more than 30 adults or nymphs on flowering heads or developing grain.

While 30 bugs per head caused minor damage, this number could lead to 100 or more per head, resulting in severe pinching of developing grain and possible downgrading.

He said the critical period was during flowering and for a month after flowering, so farmers should check their crops twice a week during this time.

"One way to do this is to shake a panicle (developing flower cluster) into a bucket of water and count the bugs."

He said Rutherglen bugs were only 4-6mm long. Farmers would find wingless nymphs that had rounded bodies, and winged adults.

The nymphs developed during two to four weeks while the adults could live for two months, with each female laying 400 or 500 eggs over that period.

Rutherglen bugs could move to adjacent sorghum or sunflower crops, resulting in a continuing population buildup.

If spraying were necessary synthetic pyrethroid insecticides would control Rutherglen bugs. Insecticide retailers could advise on products and application rates.

He said DPI researchers were investigating the biocontrol of bugs in crops, which if successful, would have economic and environmental advantages.