Control of insect pests in sorghum has been one of the real success stories of Integrated Pest Management over the last 10 years, with the standout being the widespread adoption of midge resistant hybrids.

However, with big sorghum plantings expected across northern New South Wales and Queensland this summer, crop entomologists are warning that, while the flow-on benefits from the IPM approach have been amazing, not all developments are positive.

They say Rutherglen Bug has become more prevalent, requiring control in some sorghum crops, and they note “a general complacency” among growers, who are putting too much faith in the midge resistant ratings of their hybrid varieties and not carefully monitoring insect thresholds. 

Queensland Department of Primary Industries and Fisheries entomologists David Murray, Adam Hardy and Brad Scholz have put together a paper suggesting that there are “good, bad and ugly” pest insects, and the trick is to find the balance between them.

The paper assesses the current pest situation and potential of midge, Rutherglen bug, heliothis, sorghum head caterpillar and aphids.

Dr Murray says that, in the past, when hybrids were susceptible, growers routinely treated sorghum crops with one or more sprays of organophosphates or pyrethroids to control midge.

“Often only 80 per cent effective, these products are also extremely disruptive to natural predators and parasites and, when applied at anthesis, often exacerbate problems with other pests, particularly heliothis and aphids,” Dr Murray said.

“The combined efforts of public and private sector research have seen the average level of midge resistance increase in all commercial hybrids released. In 2003, an expanded midge tested scheme was established to deliver commercial hybrids that have levels of midge resistance approaching practical immunity.

“If researchers working on a potentially novel source of midge resistance from India can isolate the genes responsible, this resistance may be selected and deployed into current commercial hybrids, possibly doubling their resistance rating.

“Meantime, growers should have a midge management plan.”

Other pest insect considerations raised by the QDPI&F entomology team included:

• Rutherglen bug incidences increasing over recent seasons, possibly because fewer broad spectrum sprays were used against midge and heliothis; researchers were developing economic thresholds for Rutherglen bug and working to determine the effect of low rates of a range of currently available insecticides and biological products;

• Heliothis spray thresholds had been revised downwards recently by Dr Bernie Franzmann; nucleopolyhedrovirus products remained the preferred tool for heliothis management in sorghum but timing was important for good results; 

• sorghum head caterpillar was mainly a pest of sorghum in humid coastal areas, and a succession of crops could favour a build-up of numbers; chemical control was of limited value, and

• high corn aphid numbers building up on plant panicles, with the resulting honeydew causing sticky grain and harvesting and handling difficulties.