South Australian grain growers are being urged to eliminate the “green bridge” of weeds and volunteer crops that harbour invertebrate pests which could pose a threat to this year’s cereal, pulse and oilseed crops.
Decent autumn rains in many parts of the State have promoted substantial weed growth, and the emergence of volunteer canola has been widespread.
Not only does a green bridge of vegetation act as a host to invertebrate pests – it also robs cropping soils of valuable stored moisture and nutrients needed for healthy crop establishment and growth.
Following rains, entomologists with the South Australian Research and Development Institute (SARDI*) have been surveying green bridge growth and conducting field sampling in several cropping regions to assess potential abundance levels of pests, particularly aphids.
SARDI entomologist Bill Kimber says surveys showed a proliferation of Lincoln weed on Eyre Peninsula in early March 2016, with later reports indicating widespread volunteer canola.
“After late rains in the Mid North, green vegetation is also becoming prolific in this region,” said Mr Kimber, who along with his SARDI colleagues, co-ordinates the PestFacts South Australia electronic news service, which is supported by the Grains Research and Development Corporation (GRDC).
“Despite weed abundance on Eyre Peninsula, aphids were not detected in extensive plant sampling in early March. In late March, some volunteer canola plants on roadsides in the Lower North and Mid North were found to be colonised with cabbage aphid.
“Very low numbers of green peach aphid (GPA) and turnip aphid were also present, and cabbage aphid and GPA were identified from a sample of aphids from the Cummins area on Eyre Peninsula at the end of March.”
Mr Kimber said virus testing was conducted on five volunteer canola samples from the Mid North, and beet western yellows virus (BWYV) presence was found in one plant collected at Riverton. BWYV is vectored into canola by a number of aphid species (including cabbage and turnip aphid), with GPA being the most efficient.
Removing weeds that host a range of insect pests that may invade paddocks is being encouraged and already many SA growers have treated paddocks with herbicide or are in the process of doing so.
“Ideally, herbicide application should be undertaken far enough out from planned sowing dates to remove all green growth from the paddock for at least 14 days before sowing,” Mr Kimber said. “If spraying is undertaken closer to sowing, aphids that are present on the weeds and volunteers are likely to move from dying plants onto emerging canola.”
Following emergence of canola crops, it is advisable to monitor for the presence of aphids regularly.
“If aphids are found in very low numbers on only a few plants in the crop, we advise treatment should not be immediately undertaken, but instead further regular monitoring conducted to assess whether the aphid numbers increase to a level that warrants spraying.
“Growers should monitor for aphids in canola paddocks from emergence forward, and make treatment decisions based on careful consideration of the level of aphid activity.”
When making decisions to treat crops for aphids, monitoring for beneficial insects that predate on aphids should also be undertaken.
Mr Kimber said a range of beneficials including ladybird beetles, hoverflies and Braconid wasps that parasitise aphids have been observed during March in close proximity to volunteer canola heavily infested with cabbage aphids.
“These beneficials and others that prey on aphids, once established can greatly reduce aphid populations,” he said. “It should also be remembered that the advent of much cooler weather later in autumn can also dramatically reduce aphid populations.”
To assist growers and their advisers in remaining up-to-date with advice on these problematic pests and others, PestFacts SA is a valuable source of timely information being made available again in 2016 through the GRDC-funded National Pest Information Service (NPIS).