Slugs in Winter Crops

Types of Slugs
The Grey Field Slug is the most widespread and common problem in Ireland and can breed at any time of the year when conditions are favourable (mild and moist) but breeding activity peaks in spring and autumn.  Round-Backed slugs and Keeled slugs are also quite common and can be important in certain fields but most of their activity occurs below the ground e.g. seed hollowing.

Grey field slug (bottom) is more common that the likes of Round-backed slugs (top two slugs in picture)

When are winter crops vulnerable to slug damage?
Winter cereals are more vulnerable to slug attack than spring cereals, as slugs are generally present in greater numbers in the autumn, and because of their slower growth habit.  Seed hollowing by slugs is a threat to all cereals but wheat is particularly susceptible.  Some seed treatments, e.g. Redigo Deter, offer protection again this type of damage.  After germination, crops are particularly vulnerable to slug damage in the early stages of growth, with cereals being considered relatively ‘safe’ from damage after the onset of tillering and oilseed rape after the crop has reached the four (true) leaf stage. Winter oilseed rape is particularly susceptible given the crop’s palatability to slugs.

Risk Factors and Control Strategies
While slug pellets are likely to be the backbone of any slug control strategy, it is important, particularly where slug numbers are high, to employ cultural methods of prevention/control and to monitor slug numbers to ensure slug pellets are not applied unnecessarily causing environmental damage and wasting money.

Soil type-Heavier soils tend to be more open and cloddy than lighter soils and consequently slug movement is encouraged.

Weather conditions-As previously stated slugs thrive in mild, moist conditions.  Slug activity continues down to freezing but they prefer milder conditions.  Moisture is required slug activity, so they are at their most damaging in mild, wet weather.

Crop rotation-Crops that are susceptible to slug damage are likely to increase numbers in the soil.  This is particularly true for green leafy crops like oilseed rape.  Therefore, cereals grown after oilseed rape have a high risk of slug damage.  Reduction of crop thrash, such as straw, on the soil surface will help to reduce slug numbers.

Seedbed Preparation-Cultivation reduces slug numbers.  A firm, fine seedbed will result in less slug pressure because:
• There will be less slugs in the soil
• Slug movement is reduced
• Crops will germinate and grow more rapidly due to enhanced uptake of nutrients
In practise this means preparing a good seedbed and rolling where conditions permit after sowing, particularly if the seedbed is a bit cloddy.

Seeding rates and planting date-Increase seeding rates if you suspect the risk of slugs is high.  While slug numbers and activity may be lower with later plantings, slower growth of the crop makes it much more vulnerable to slug attack and therefore where possible early planting is recommended.  Deeper drilling of seeds can help reduce seed hollowing in a cloddy seedbed.

Slug pellets-Ideally bait trapping and monitoring for slug numbers should commence in the previous crop but in any event should continue until the crop has reached its ‘safe’ growth stage.  Heap about 20ml (two heaped teaspoons) of layers mash and cover with about 25cm diameter of hardboard or old tiles or plant pots.  Leave traps overnight (only put traps out if the soil surface is visibly damp) and examine as early as possible the next morning.  Position nine traps in a W pattern across the field (13 required for fields bigger than 50 acres) and apply slug pellets if traps average four or more slugs per trap.
 

Slug damage in an oilseed rape crop; bait trap and monitor crops for slug damage until the crop has reached its ‘safe’ growth stage

Slug Pellet Types and Recommendations

Draza Elite (Methiocarb) is no longer available so choice is limited to various formulations of Metaldehyde.  It should be noted that there are large differences in formulation between the Metaldehyde products and consequently their performance.

While active ingredient level is a factor, the critical issues are:
• Individual pellets (or chunks of broken pellets in the field) can deliver enough active substance to deliver a fatal dose
• Palatability
• Durability in wet weather.

On this basis, Dairygold recommends the Metaldehyde based product Metarex RG. AxCela is the next best alternative. It is very important to adhere to and record label rates and restrictions of these products.  Be particularly vigilant when it comes to the individual and total maximum dose rates.

Key point: Continue to monitor crops for slug damage until after the onset of tillering. Draza Elite is no longer available and any stocks on your farm must be used by September 2015.

 

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