Cane farmers rely on the insecticide Imidacloprid to control cane grubs. But after several years of sharp increases in Imidacloprid concentrations in water samples from the Tully, Murray and Herbert River catchments, the cane industry rolled out a program and it’s better protecting local waterways.

Richard Hunt, Science Officer at Wet Tropics Waterways, says a recent drop in pesticide detection levels in the three southern catchments is a standout result from the 2023 Waterway Health Report Card.

“Following several years of increases – which spiked in the 2016-2017 wet season – there is clear evidence of a downward trend.

“Imidacloprid is a really important chemical for the agricultural industry, but it is also highly soluble and mobile, with a high risk of ending up in waterways where it is also lethal for aquatic bugs and crustaceans.”

A new agricultural extension program – and strong support from growers – has made the difference.

Greg Shannon, Cane Productivity and Development Manager at Tully Sugar, says Imidacloprid is invaluable to cane growers. Cane grubs are beetle larvae that feed on sugar cane roots and cause plants to die.

“Imidacloprid is currently the only solution available for cane grub outbreaks. It is essential that the industry keeps its social license to use it. So we got together with industry partners to develop and deliver a program that would increase growers’ awareness and understanding.”

The program began as part of the Tully Variety Management Group (TVMG), an industry initiative led by Tully Sugar with several partners that determines which sugar cane varieties are recommended to the district’s growers. One of the key issues addressed by the group was the management of Pachymetra, a soil disease with similar symptoms to a cane grub outbreak.

“We conducted surveys across the district in 2013 and 2018 (with a follow up one in 2023) and found that Pachymetra levels were very high in some of our sub-districts at the same time as there was a cane grub outbreak.

“We trialled several new sugar cane varieties bred by Sugar Research Australian (SRA) that were Pachymetra-resistant, and they have been very successful.

“During this process we also quickly discovered that cane grubs were not always the issue, so the widespread use of Imidacloprid could be cut back to a more strategic approach with only high-risk areas and paddocks with a history of grub damage being treated.”

After the programs were rolled out, Imidacloprid levels in water samples started to drop. About 30 per cent of water samples collected in the 2016-2017 wet season exceeded Imidacloprid guidelines – but by 2019-20 it was less than five per cent and the toxicity risk level was the lowest recorded since monitoring began in 2016.

Greg says the TVMG is still going strong and while their initial objective was to promote cane varieties that are resistant to Pachymetra, having a more sustainable approach to Imidacloprid use is a positive spin-off.

“There was very strong buy-in from growers because it’s a win-win. But we have to remain vigilant. The cane grub pressure is still there, so we need to continue to use Imidacloprid wisely in the best, most sustainable way while also managing Pachymetra through resistant varieties.”

Lawrence DiBella, manager at Herbert Cane Productivity Services, says the reductions in Imidacloprid levels are also due to changes in how and when the chemical is applied.

“Growers have been modifying equipment to improve placement depth and soil coverage, which makes the chemical less prone to losses to the environment.”

Hotspot for Reef Runoff

The Wet Tropics is a hotspot for nutrient and pesticide runoff into the Great Barrier Reef lagoon for several reasons:

• Heavy rainfall
• Several short sharp rivers
• Intensive agriculture along the coast
• The reef is close to the coast.