Slugs to decimate canola and new pastures

Malacologist Michael Nash said it was critical farmers check their paddocks every three days with baits and refuges before crops were decimated.

Ironically, excellent seasonal conditions since spring had created the ideal breeding ground for two species: the Grey Field and Black Keeled slugs.

This season, Dr Nash said, it was "not unusual" to discover 300 to 400 slugs per square metre.

"We're starting to associate wet, cool springs - particularly when farmers get good, dense crops and high yields - with slug outbreaks," Dr Nash said.

"The Grey Field slug, which is the most common, damaging slug in southwest Victoria, actually got a second generation of breeding in.

"They then survived the reasonably good summer this year and the early break has favoured them.

"All that in combination means the slugs are active earlier and their numbers are a lot higher."

Slugs could wreak devastating damage in days but often escaped undetected.

"Overnight, they can wipe out areas of crop so, in three days, they can wipe out a couple of hectares very easily," Dr Nash said.

Using tiles as refuges and bait lines was key to identifying an infestation early - both in the fortnight leading up to sowing and during the establishment period.

Dr Nash said a large tile or canola mat placed on moist paddocks overnight and checked before it was warmed by the mid-morning sun would reveal Grey Field slugs.

Black Keeled slugs, however, burrowed into soil for protection, so had to be baited.

He recommended farmers growing canola took action if just one Black Keeled slug or one to two Grey Field slugs were found per square metre.

For wheat crops, the threshold was 5/m2 for Black Keeled slugs and 8-10/m2 for Grey Field slugs.

"In reality, if growers see one Black Keeled slug under any of the monitoring, they'll be baiting because they are so damaging," Dr Nash said.

Bait should be applied to the soil surface after the crop or newly-sown pasture had been rolled so it was easy for the slugs to locate.

And canola growers were not the only farmers who should also be on the lookout.

"I think it's been a silent problem with dairy farmers," Dr Nash said.

"That's why I've been doing more work in southwest Victoria to try and quantify it and the numbers that that we're getting in some of the pastures is quite high."

"I would say that farmers, even if they're sowing now when they would traditionally think slugs aren't that active, should be putting some bait out to protect those ryegrass seedlings."

Not a great deal of research had been done with irrigators but Dr Nash said field work had revealed some extremely high populations.

"I've been doing a little bit of work with the maize growers and, by the third year of maize, they had horrendous slug populations as well," he said.

"Because they're putting water on all summer, there's food for the slugs when they come out in April, May and the numbers are just huge: 50 to 70 per mat, which is 200 to 300 per square metre."

And Dr Nash urged farmers to check for other slimy pests while looking for slugs.

"The other mollusc we're seeing in more and more pastures, all the way round to places like Eden and Bega in the south coast of New South Wales, is small conical snails," he said.

"Small conical snails are a huge problem in southeast SA, some parts of southwest Victoria, the isolated pocket in south Gippsland and in dairy pastures in southern NSW."

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