Slugs ready to spring into battle
Spring crops should be the priority in the battle against slugs with growers facing the possibility of another season of high populations.
David Glen of Styloma consulting warns growers to be on the look out this spring, but believes winter crops should be able to cope.
“Although growers will still need to be wary of their winter crops, as soon as the temperatures warm up they should grow fairly quickly so the focus should be on spring crops,” he says.
This increased risk is due to the knock-on effects from last year’s horrendous slug problems, caused by the ideal wet conditions during the April-May breeding cycle, which saw record slug populations.
Bayer’s molluscicide manager Peter Stacey highlights that the problem was reflected in increased pellet sales. “Across all slug products, sales were 400-450% up on 2011. We actually sold three times more methiocarb than the previous year as well,” he says.
Seed-beds will be one of the first lines of defence and can make a big difference in determining how bad individual slug problems will be, according to Dr Glen.
“If you’ve got a good seed-bed then you probably don’t have to worry as much, but if you’ve got a cloddy seed-bed, then this is where the problems can occur.”
Slugs on farms – the facts
- Slugs have been found to go up to 2m deep in the ground
- House prices in Norway have dropped in areas where “Spanish slug” infestations have been reported
- Key actives – Ferric phosphate, metaldehyde and methiocarb
Mr Stacey says slug control stays relatively similar each year. However, the key lesson from 2012 is that some seasons offer no margin for error.
“In the years where slug conditions are perfect, it’s critical you get it right. Nothing really changes in how you deal with them, it just shows if you don’t”.
Selecting the right pellets is critical for slug control. Last year was a very difficult year due to harsh weather, leading to many of the lower quality pellets struggling to remain intact.
Bait-shy slugs is another consideration for growers when making their spring control plans and selecting which pellets to use.
“Slugs do learn and can become bait shy. So if you fail to kill them with the first pellet, they may not eat another pellet,” says Mr Stacey.
“We certainly saw this with sequenced applications. When you go in with a low-quality slug pellet first, you can actually get less control from the better-quality pellet used second time around because they are less likely to go back,” he adds.
According to slug expert Gordon Port at Newcastle University, the density of slug pellets doesn’t make a difference in how quickly a slug will find a pellet.
“However if you have a large population of slugs obviously a higher density is probably advisable,” he says.
Assessing numbers
Slug traps are still one of the best ways to assess how many slugs are in the field, but getting a chance to do so this year may be a problem due to the cold weather. This means relying on past experiences and knowing what crops will be at greatest risk.
“You need to put bait traps out before cultivation and when it’s not so cold. After cultivation you’ve disrupted the surface activity patterns so you could find no slugs and yet, there’s still a high risk of slug damage,” notes Dr Glen.
The thresholds on slugs in traps depends on the crop. Mr Stacey says sugar beet and vegetable crops need only one slug before the threshold is breached and treatment is required.
This low threshold also applies to potatoes due to the high value of the crops and the low market tolerance of slug damage to tubers – often a 5% maximum for all defects including slug damage is accepted.
Traps should be checked early in the morning. In fields destined for winter wheat, a catch of four or more slugs per trap indicates a possible risk if soil and weather conditions favour slug activity. For oilseed rape, the threshold is lower, at four slugs per trap if the traps are in preceding standing cereals and one slug per trap in cereal stubble.
Dr Glen warned potatoes were one crop that baiting was not straightforward and probably not worthwhile unless done in the autumn before.
Independent potato specialist John Sarup, says realistically potato growers assessing their slug problem will be unable to do anything until canopy closure.
“Potatoes essentially grow towards the area of highest risk in between rows, so as the season progresses, the more at risk they become,” he says.
Potato slug damage mainly depends on market rejection rather than the amount they eat, so even small numbers of slugs can cause problems.
Potato growers are urged to make the habitat as unfavourable as possible to slugs, getting rid of weeds and any possible food sources prior to planting and making the most of cultivation techniques.
| The Spanish slug |
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Last year saw the discovery of the Spanish slug (Arion vulgaris). It is still not clear when it first appeared in the UK or the scale of the problem. Accurate identification of the invasive species is hard and requires looking at their internal anatomy, something few in this country have the skills to do. Dr Gordon Port of Newcastle University admits no one is sure how serious this problem could be. “The situation as I see it, is we know this species is present in East Anglia, but whether they are going to reach plague-like proportions as seen in mainland Europe, we don’t yet know. “The message for this season will be to keep an eye for any large-scale infestations of slugs which could be Spanish slugs, but focusing on what growers have already got will be key this year,” he says. |
