Integrated slug control in arable crops: Risk assessment, trapping, agronomy and chemical control

Abstract

Slugs are major pests and often cause serious damage to winter wheat and oilseed rape at establishment. Because molluscicides are a limited market, products based on new chemistry are unlikely to be available for some years. This project was targeted by industry to improve pest management strategies using existing crop protection chemicals, based on a system of risk assessment and integrated control as summarized below.

Slug damage risk.
Slugs (especially Deroceras reticulatum, but also Arion, Milax and Tandonia spp.) are most damaging when they feed on wheat seeds because each slug can kill up to about 50 seeds in the first week after sowing. Seed kill increases at a decreasing rate with slug body weight, thus, weight-for-weight, smaller slugs kill more seeds than larger slugs. Feeding on shoots and leaves of wheat can also be important. Oilseed rape seedlings are highly vulnerable to feeding by slugs but seeds are not attacked.

Assessing slug risk.
Slug populations in autumn can only be predicted for one or two weeks at most, as slugs respond rapidly to weather changes. Put slug traps out before cultivation, when the soil surface is visibly moist and the weather mild (5-25oC). Traps consist of a cover about 25cm across, with a small heap (20ml or 2 heaped teaspoonfuls) of chicken layers' mash (NOT slug pellets) beneath. In each field nine traps (13 in fields larger than 20ha) should be set out in a 'W' pattern. Concentrate on areas known to suffer damage. Leave traps overnight and examine early next morning. Trapping between drilling and emergence can be worthwhile in wet weather. Threshold trap catches and decision trees for control are described in more detail in HGCA Topic sheets 84 (winter wheat) and 85 (oilseed rape).

Reducing risk.
A fine and consolidated seedbed protects seeds and germinating seedlings. Shallow cultivation to incorporate crop residues after harvest reduces slug numbers especially in dry conditions. Slugs are denied access to cereal seeds if seeds are drilled at 3cm depth in a fine consolidated seedbed. In cloddy seedbeds, increase sowing depth to 4-5cm. Monitoring crops regularly for slug damage from sowing to first tillering (cereals) or the four-true-leaf stage (oilseed rape) is worthwhile. Damage after this stage is less likely to result in further plant loss, but monitoring should continue through the winter.

Applying slug pellets.
The greatest benefit is generally achieved from an application just after drilling (and after rolling if this is done). Soil from rain splash on pellets does not significantly affect the time taken for slugs to feed, so do not wait until after rain; slugs start to kill wheat seeds almost immediately after drilling. Slugs are killed more quickly by broadcast pellets compared to pellets drilled with seeds. Broadcasting gives more consistent slug control, particularly in combination with fine, firm seedbeds that help protect seeds and seedlings. Admixed pellets will be ineffective in fine seedbeds because both seeds and pellets are unavailable to slugs, which survive to attack seedlings. Pellet admixtures with wheat seeds can be effective when direct-drilling, or in open cloddy seedbed. Earlier treatments (on stubble) or later treatments (at emergence) are likely to be less effective, especially when conditions favour slug activity. Further treatment is justified where slug activity is high and crop growth is slow. See also HGCA Topic Sheet 88.