Slugs

It reminds growers to monitor for slugs at this time of year when establishing autumn sown crops, particularly when direct drilling as surface trash provides an ideal habitat. Crops are most vulnerable during establishment because slugs attack and hollow out seeds and bite young seedling plants off at ground level. Monitoring for slugs should continue until the crop is well established (6-8 weeks).

FAR says that the best way to establish if there is a slug problem is to put out wet sacks or tiles.

Slugs require moisture for reproduction, survival and activity, resulting in greatest crop damage during wet weather. Burning and cultivation reduce slug pressure, however this does not prevent ingress from paddock edges.

There are several types of baits available; although they all kill slugs they differ in cost, ease of application, longevity and impacts on non-target organisms. Secondary poisoning of generalist beetle predators occurs when the beetles, which eat a slug that has ingested methiocarb baits, so those wishing to protect these beneficial insects it is not advisable to use methiocarb based baits.

Timing of bait application is critical and applying baits for peace of mind when slugs are not active is a waste of money. In the right conditions slug populations will recover rapidly from baiting, cultivation, or declines due to a period of dry weather. Slugs are most active in spring and autumn, so this is when slug baits should be applied. Baiting in the previous crop may need to be considered if monitoring shows high numbers and a slug sensitive crop is being planned.

Work in Oregon has found that baiting under the following conditions increases efficacy:

• Applying the bait at dusk;
• Not applying the bait if windy or too cold (below 8C);
• Not applying the bait if it is raining or too wet.

Another thing to consider is the trade-off between how rainâ€�fast the baits are and the concentration of baiting points. The more rain-fast baits have the active embedded (i.e. Metarex®) but if there are very high slug numbers then the number of baiting points may be a more important consideration.

Grass grub

Monitoring for grass grub should be carried out now by digging spade squares and counting the number of larvae present. Grass grub is most destructive during the larval period from February to September. The use of seed treatments and a granular insecticide at establishment of autumn sown crops or pasture is an effective control measure. Seed treatments and granular diazinon offer short-term protection (perhaps up to eight weeks) compared with more sustained protection from controlled release granules.

Note: Follow up applications of liquid diazinon (e.g. Dew 600) must be applied as per label conditions.

Information on grass grub biology and chemical control can be found on the FAR website:

https://www.far.org.nz/assets/files/uploads/X106_Grass_grub_biology_and_nonchemical_control3.pdf

Further information on grass grub chemical control can be found here:

https://www.far.org.nz/assets/files/uploads/X111_Chemical_control_of_grass_grub1.pdf 

White butterfly

Monitor autumn sown brassica crops for white butterfly. Check seedlings for 1 mm long, creamy coloured cigar shaped eggs hanging off the underside of the leaves.

Argentine stem weevil

The parasitoid introduced to control Argentine stem weevil (ASW) is not performing as well as it used too. Research is underway to understand why this is occurring, but in the meantime, be aware that ASW damage levels will be increasing in autumn sown crops. These crops should be monitored for ASW damage.

A current research project has found that ASW damage is higher in some cereals species than others.

Damage was highest in ryecorn, followed by wheat, barley and ryegrass. In this research the insecticide seed treatments were having limited impact on the amount of damage. More research is needed to understand why this is the case.

Pre-emergence herbicides

Pre-emergence herbicides (e.g. Firebird) taken up by the emerging shoots of weeds require adequate soil moisture to be effective. If the current dry weather continues, it may be worth considering an application of 5-10 mm of irrigation if possible to activate the herbicide. These products require a fine seedbed to work effectively and will only control weeds germinating from seed; any that have survived cultivation are not usually controlled.