Use all the tools in the toolkit to stay ahead of slugs this autumn, advises Dr David Ellerton, technical director at Hutchinsons. Even though this year’s crops are still to be harvested, now is the time to turn thoughts to assessing slug risk prior to drilling crops.
“Identifying risk is an essential part of protecting susceptible crops against slug damage. Although weather conditions over the next few months will play an important role, there's no substitute for putting traps out in fields before any cultivations and monitor for slug activity to get an idea of what's happening on a field by field basis," says Dr Ellerton.
Slug traps, containing a handful of layers mash to attract slugs, can be put out in either the standing crop or in stubbles after harvest. Although the official recommendation is to place 9 traps in a W-formation across the field, Dr Ellerton believes that it's practical and just as useful to target traps at heavier, wetter areas of the field or areas that have had historical slug problems.
"It's important to get some idea of slug activity before any cultivations have been made. Threshold numbers that will trigger a treatment in the growing crop are 1 slug per trap prior to oilseed rape and 4 slugs per trap for cereal crops," he says. "Cultivations will disrupt slugs and some will die, but once cultivations are underway it's difficult to get a true picture of likely slug populations in the next crop."
Each cultivation technique has its good and bad points, points out Dr Ellerton. "It's not as simple as it may first seem to say one method reduces the risk of slug damage more than another. Ploughing probably kills more slugs but is often associated with cloddy seed beds, something to be avoided in slug prone situations.
"Even when direct drilling, where you may expect minimal slug damage because the soil hasn't been disturbed, slugs can move along the drill slots and attack seeds and plants," he says. "Min-till can be a good alternative, as long as the seedbed is properly consolidated and seed is drilled deep enough to mitigate any problems with trash, which can be attractive to slugs."
Hollowing of seed can be a big problem, especially when soils are heavy, wet and cloddy. "As well as doing all you can culturally, treating seed with insecticidal seed dressings containing clothianidin can help reduce grain hollowing," suggests Dr Ellerton.
Signal seed treatment (cypermethrin) has also recently been observed as having a useful effect on small slugs in trials work.
Dr Ellerton doesn't believe that treating fields with slug pellets, before crops are planted, is particularly effective, preferring to apply as soon as possible after drilling. "It's important to treat crops only where the risk is high enough," he says.
"Regularly monitoring fields by having a look for slugs under clods of soil, in addition to trapping, can give an idea of slug pressure. You may also be able to see clusters of translucent slug eggs when lifting clods or under surface trash, indicating slugs are breeding."
Traps placed in fields need to be monitored during early establishment until the risk of economic damage from slugs has passed. In oilseed rape this is when the crop has reached the four leaf stage and in cereals, early tillering.
"There's no doubt that ferric phosphate is just as effective as metaldehyde. We know it works, but in a very different way to metaldehyde," says Dr Ellerton. "If there is any risk to water, growers should automatically be using ferric phosphate.
"You can assess your own risk by looking at the Environment agency's 'What's In Your Back Yard' website (www.wiyby.co.uk) and by considering the three S's - soil type, slope and proximity to a stream in each field. If you can get away with applying less than 210g/ha metaldehyde then use the 160g rate," he recommends.
"We have to use metaldehyde responsibly to minimise peaks in drinking water catchments. We can’t afford to lose yet another molluscicide and the bottom line is that, if everyone doesn't stick to using metaldehyde within the guidelines, it will go."