Warning: Increased Slug Risk This Autumn
Wet and cool summers have brought perfect living and breeding conditions for the Grey Field and Black Keeled slugs. With these conditions there has been an increase in slug numbers especially where there are dense stubbles as the results of high yielding crops. With crop establishment on the horizon, farmers need to be taking extra care to ensure their crops are baited and checked every 3-4 days to ensure there is not an outbreak that could result in reduced yield potential.
The Black Keeled Slug are a burrowing species, allowing them to attack germinating seeds underground. These slugs attack both cereals and canola as well as clovers and can result in seedling death. Damp and mild conditions are optimal for slug activity, hence mornings after these conditions are the best time to monitor.
Figure 1. Black Keeled slug adult (left) and eggs of the black keeled slug (right) (Source: MA Nash)
The Grey Field Slug loves wet environments and is an opportunistic breeder, therefor can go through two breeding cycles in one season. The Grey Field Slug attack cereals, canola, clover, with Canola being very susceptible at the young seedling age. Typical symptoms are irregular chewing marks and shredded leaf edges. Monitoring these slugs using terracotta tiles, carpet squares or something similar, placed on the soil surface and returning after a few days to check the slug presence.
Figure 2. Grey Field slug adult (Left) and eggs of the Grey Field slug (Right) (Source: Cesar)
Studies have shown that there is an increase in slug activity when there is soil moisture at 50-60cm in depth, with an increase in the black keeled slug activity with soil moisture deeper in the profile. Burning off has been used not only to benefit cleaning up stubbles but also destroys the habitat for these slugs. Through burning stubbles, working and rolling paddocks, we reduce the risk of high slug populations and allow for a smooth and level ground for the baits to be spread onto which allows them to be located easier by the slugs. Slug activity is higher through heavy clay soils with moisture easily accessible for slug reproduction, allowing crops in these heavy soils to be more impacted by slugs.
There are 2 main bait types used; Metaldehyde and Iron Phosphate. The most common being a Metaldehyde based bait. The higher rate of Metaldehyde provides best protection against slugs as has a more direct effect on the slug. Metarex and Metakill are very similar products with both consisting of 50g/kg of Metaldehyde and both having a recommended spreading rate of 5-8kg/ha, aiming for 35 pellets per m2 (Metarex) and 76 pellets per m2 (Metakill). Meta is a cheaper option, only consisting of 15g/kg of Metaldehyde and aiming for 25 pellets per m2. Metarex and Metakill will generally last longer in the paddock as they are more waterproof than other baits. This can allow them to last up to 2-3 weeks depending on environmental conditions and slug pressures, while more commonly used baits tend to last less time through the same pressures.
Some bait products are more stable under adverse weather conditions, such as cold temperatures and rainfall. Significant rainfall (+35mm) can degrade bran-based pellets, and particularly decrease the efficiency in iron chelate products. UV exposure did not decrease the efficiency of the baits, while high temperatures degrade the active ingredient in Metaldehyde baits.
For the upcoming season with the current climate conditions, it is highly advised to check crops regularly and bait when necessary. Baiting should be applied to the soil surface immediately after the crop or newly sown pasture has been sown.
For further information on the best baits to protect your crops and pastures, please get in touch with your local Western AG agronomist.
Article produced by - Damien Goodman, Western AG Derrinallum