Dry weather and an invasion of aphids are troubling winter cereal crops in parts of the Queensland grain belt.

Department of Primary Industries and Fisheries principal entomologist Bernie Franzmann said aphids had been reported in cereal crops in the Maranoa and Central Queensland.

He said the impact of the sap-sucking aphid was worse where crops were suffering the effects of moisture stress because of dry weather since planting.

Mr Franzmann said moisture stress was also thought to be behind the poor results from insecticide applications to control the pests.

"The recommended insecticide is a systemic, which needs sap movement to spread the chemical throughout plants where it can kill the aphids.

"This doesn't happen when plants are stressed and has caused disappointing results in some sprayed crops.

"Rain now would really help by reducing aphid damage and improving the efficacy of the recommended insecticide," he said.

Mr Franzmann said four different species of aphid commonly attacked winter cereals and all preferred barley to wheat.

"Winged or wingless adults and immature stages that are smaller wingless versions of the adults suck sap from the plants.

"Under heavy infestations plants may turn yellow, be stunted and appear generally unthrifty.

"The aphids are small, only about 1.5 to 2.5mm long," Mr Franzmann said.

He said the oat or wheat aphid (Rhopalosiphum padi) was causing most of the problems and was the most common aphid attacking winter cereals now.

"The oat aphid is brown to muddy green with rusty red patches at the base of the tubes at the rear end of the body and normally occupies the base and lower parts of the plant."

Mr Franzmann said the corn aphid (Rhopalosiphum maidis) was also found in Queensland. and was a green to dark olive-green with a purplish area at the base of the tubes at the rear end of the body.

It normally lived on the tops of the plants particularly within the rolled up terminal leaf.

The rose-grain aphid (Metopolophium dirhodum) was pale green with a darker green stripe along the middle of its back and normally lived on the undersides of the leaves, colonised the lower leaves and moved up the plant as leaves matured.

He said the rice root aphid (Rhopalosiphum rufiabdominalis) was a honey-brown colour with a rusty red area at the base of the tubes at the rear end of the body and lived on the roots of the plants under the soil surface.

Mr Franzmann said deciding whether controlling aphids on winter cereals would provide an economic return was often complex and not just dependent on the size of the aphid population.

"Other factors that influence the decision are, the age and state of the crop (for example moisture stressed), the value of the grain, the expected yield, cost of control and the abundance of natural enemies.

"As a general recommendation check for aphids by choosing six widely-spaced positions in the crop and at each position examine five consecutive plants in a row.

"If 27 out of 30 plants are covered with aphids and there are less than two ladybird beetles, ladybird larvae, hoverfly larvae or lacewing larvae per plant on each of the infested plants, then spray.

"The exception is if about 5 per cent of the aphids are parasitised (mummified), indicating the presence of beneficial insects.

"The rice root aphid can't be controlled by spraying," Mr Franzmann said.

"The recommended chemical control is to spray dimethoate which has various trade names. It is applied at 500 mL/ha of 400 g/L of product and has a withholding period of 28 days for harvest and one day for grazing.

"Dimethoate has a contact action but is also a systemic insecticide taken up through the leaf and then translocated through the upper portion of the plant.

"Aphids are subsequently controlled when they feed on the plant. Because of this systemic mode of action, control may not be fully evident until four days after treatment.

"Dimethoate may be tank-mixed with certain broadleaf herbicides but check the label before use. Good quality water should be used to avoid problems caused by a high pH," Mr Franzmann said.