Slugs (unsurprisingly)

There’s plenty of moisture around much of south-eastern Australia after a ripper autumn break in many regions - great for crop establishment, but also great for slug activity.

As moisture levels increase across south-eastern Australia we can also expect an increase in slug pressure in some areas. The two common perpetrators in high rainfall areas are the black keeled slug (Milax gagates) and the grey field slug (Deroceras reticulatum).

Grey field slug and black keeled slug generally become active after the autumn break, although both are opportunistic breeders at any time of the year given the right conditions. For instance, with sufficient food the grey field slug can reproduce whenever temperatures are between 4°C and 21°C and soil moisture is greater than 25%.

The usual suspects after a good autumn break, the black keeled slug (top) and grey field slug (bottom) (Source: cesar)

 

Monitoring and control

Crops (especially canola) should be monitored for slug activity in high rainfall areas. Slugs mainly feed at night so it is difficult to estimate numbers accurately. Traditionally, refuge traps such as terracotta tiles, carpet squares, flowerpot bases or even plastic drums have been used to monitor slug numbers. Traps should be placed on the soil surface when it is visibly wet, and then checked after a few days for the presence of slugs. Damp and mild conditions are optimal for slug activity; hence the mornings after these conditions are most suitable for monitoring. The feeding activity of slugs can be assessed by spreading bran based baits after rainfall, and checking the following morning for dead slugs.

Both cultural and chemical controls assist in the fight against slugs. Paddocks in which stubble has been burnt prior to sowing may have delayed breeding of grey field slugs, but not black keeled slugs which actively burrow. Seed bed consolidation (rolling) after sowing makes it harder for slugs to find seed, reduces surface clods used as moist refuges when slugs are surface active, and improves crop establishment when used in combination with cultivation.

Baits are a crop protectant and must be applied prior to crop emergence for maximum benefit. Drilling of baits will protect seed but not seedlings, hence a follow up surface application is recommended under high slug infestations or split emergence of populations due to different species co-occurring in the same paddock. Australian research suggests 30 bait pellets per m² are needed to ensure an adequate chance of encounter. Be aware that some baits are far more stable than others under adverse weather conditions including cold temperatures and significant rainfall. For more info on slugs visit the black keeled slug and grey field slug PestNotes.