The grey field slug can breed at any time, with a pair able to produce up to 1000 eggs a year.
PHOTO: Michael Nash
Slug control rules of thumb
- A combination of cultural and chemical control is required to control slugs.
- Research has found burning, light cultivation and rolling improve slug control.
- Control measures must be carried out before slug damage is observed.
- Paddocks with a previous history of slug damage are always a good place to start monitoring in a susceptible crop such as canola.
- Slug bait should be applied at a rate to provide sufficient bait points per square metre relative to slug populations in the paddock.
- Identify slug species present in a paddock for most effective control. Different species demonstrate different behaviours.
New GRDC-funded research has found that high slug populations can be reduced through correct bait rate and timing and by using a range of cultural measures.
Slugs have been an increasing problem in recent years in the high-rainfall zone, and slug populations have been difficult to control.
Two slug species responsible for most crop damage are the grey field slug (Deroceras reticulatum) and the black keeled slug (Milax gagates).
In response to the challenge, the GRDC funded a one-year ‘fast-track' project by Southern Farming Systems (SFS) and IPM Technologies to quickly evaluate a range of slug control practices.
The trial examined three Victorian sites at Hamilton, Inverleigh and Skipton, chosen after significant populations were observed from paddock monitoring in the late spring in 2012. Strategies tested included varying bait doses, type and timing, as well as the effect of rolling post-sowing.
SFS chief executive officer Jon Midwood says the project reaffirmed the key message that if a paddock has a high slug population, using only one technique will not guarantee control.
“Growers should be using a number of techniques at the same time – chemical and cultural – that when combined provide sufficient control to reduce numbers to levels where a susceptible crop, particularly canola, is able to withstand the low numbers that remain,” Mr Midwood says.
Due to the unique conditions experienced in the summer of the trial, several cultural controls were not able to be tested.
The plan had been to test control methods, including light cultivation, in the previous year’s heavy stubbles; however, the growers in the trial burnt their paddocks so no comparison was possible.
“Evidence would suggest that burning the stubble gives you a better level of control of slugs because you’re removing the habitat,” Mr Midwood says. “Light cultivation improves control by disturbing the area where the slugs lay their eggs.”
However, rolling was trialled, even though many growers did not think this necessary because they were using press wheels on seeders.
“Press wheels tick the box from the crop emergence point of view, but if you’re sowing on a 200 to 300-millimetre row spacing and you’re using a narrow press wheel, there’s all that area in between the rows where there’s no consolidation of soil, so it’s easy for slugs to move between the rows,” he says.
“Then they just drop into a row where they’ve got a nice little channel to crawl along to go and find where the emerging canola is.
“If rolling is done properly it can probably save at least one slug bait application or maybe two. If you cost that out with the cost of slug bait that’s a pretty cheap application to be able to go roll a paddock. Plus you have the added bonus of a better soil surface for herbicides.”
Dr Paul Horne from IPM Technologies hopes the trial’s results will change the perception that once populations are out of control and too high, nothing can be done.
“The study showed it was quite possible to significantly reduce slug numbers if growers get the timing and the application of baits right,” Dr Horne says.
At all trial sites, the most promising results were from bait applications where a split dose was applied. While timing was based on the conditions, the first was as soon as possible after seeding and then eight to 10 days later.
“The right time and the right rate are the critical things,” Dr Horne says. “Baits need to be applied before you see damage. If you know there are slugs there and you wait to see damage, it’s too late.”
The project also shows that using baits below label rate for some products did not control slugs because there were not enough baits per square metre.
“It’s a false economy basically. Re-sowing would cost more than baiting at the label rate,” Dr Horne says “If you’ve got high pest pressure, why on earth would you use below-label rate? It doesn’t make sense.”
Mr Midwood says it is important that growers and advisers identify the species of slugs they are dealing with.
“The grey field slug lives around the surface and juveniles live right at the top of the soil profile. These are the ones that cause the greatest level of damage, but they’re also the ones for which burning and light cultivation achieve the best level of control.
“The black keeled slug lives deep in the soil when it’s dry, so a burn and light cultivation wouldn’t affect them. They wait until the profile is wet enough and then come to the surface and that affects bait timing.”
This means if the main slug species is the black keeled slug, a baiting application at or around seeding, without a follow-up, may not be effective, even if cultural controls are in place.
While the fast-track project is complete, further research on slug control will continue. SFS is part of the GRDC’s five-year stubble initiative project, in which there will be further studies on slugs, including cultural control.
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A fact sheet on Slug Control is available at: www.grdc.com.au/GRDC-FS-SlugControl
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