Black keeled slugs

Agronomist, Craig Drum (Tatyoon Rural), reports finding significant numbers of black keeled slugs (Milax gagates) in several paddocks around Willaura and Lake Bolac, in the Western district of Victoria. The slugs appear quite patchily distributed within paddocks, with higher numbers observed around grassy fencelines. Craig says damage has been observed in newly emerging canola and pasture crops. Agronomist, Steve Dickson (CropPlus Agronomy), reports a high number of slugs found across numerous paddocks near Inverleigh, also in the Western district of Victoria. Both Craig and Steve suspect the early soil moisture has favoured slug survival this year. Farmer, Michael Nash, says slug numbers have also been high around Mortlake, and that several canola paddocks have required baiting to protect emerging seedlings from attack.

Black keeled slugs usually emerge in the first two to three weeks after the break of season, coinciding with seedling emergence. They are more likely to be found in paddocks where they have previously been a problem. Heavier soils are preferred, typically heavy red loams, gravelly loams and grey clays. Slugs damage plant seeds and seedlings, often removing irregular shaped areas from leaves due to their rasping action of feeding. Although all seedling crops and pastures may be attacked, canola is particularly susceptible.

Because slugs are usually more active at night, it can be difficult to estimate numbers accurately without monitoring. Using terracotta tiles, or another type of ‘trap’ such as carpet squares or flowerpot bases is the best way 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 underneath. Alternatively, inspect paddocks at night using a torch to look closely at individual plants. Check susceptible crops at least twice a week for the first four weeks after seedlings emerge.

The most common control method for slugs is to apply baits, which should be applied early in the season after good germinating rains. This is when slugs are emerging from their hiding places and actively looking for food. This means there is little alternative food to compete with the bait pellets and adults are likely to be killed before they have laid eggs. Baiting will generally only kill 50% of the slug population at any one time, mainly the larger ones. Iron-chelate based products rather than traditional chemicals, should be considered in order to help maintain a healthy population of beneficial species, such as carabid beetles - which are voracious predators of slugs. Good crop hygiene, weed control and removal of potential refuges can also help to reduce slug problems over time.

 Click here for images of black keeled slugs.