Other high slug risk factors include prolonged wet weather, trash blankets, weedy fallows and a previous slug history. Slugs are best controlled before the crop is planted. 

Below are some management recommendations following consultation with consultants in badly affected areas and colleagues in southern states, where slugs and snails (molluscs) are more prevalent.

 Monitor populations

Determine the slug risk in your paddocks prior to planting. Monitor regularly so slug numbers can be detected early, ideally before seeding as there are more control options at this time. To get an estimate of the numbers of slugs present; place wet carpet squares, hessian sacks or tiles on the soil surface.  They should at least be 32 cm x 32 cm (10% of a square metre). Place slug pellets under them and check after a few days. Count the number of slugs under and around each square. Multiply the numbers by 10 to get an estimate of slugs per m²

 If an average of more that one (1) slug per trap is found, the slug problem is significant. If more than eight (8) slugs are found per trap the problem is severe.

Ideally, fallows should be bare so the only food source for slugs is the baits. For this reason, baits applied post-emergence are less effective than pre-emergent baits, as slugs often prefer the emerging seedlings.

Chemical control

Take action if there is significant slug activity in the pre-crop fallow, 2 weeks before planting. Two equally effective bait types are registered for slug control in field crops – those based on metaldehyde, e.g. SlugOut and those based on an iron chelates (EDTA complex), e.g. Multiguard.

Metaldehyde based baits are highly toxic to mammals and birds (Schedule 5 poisons) and must be spread evenly to avoid heaping which might attract non target animals. Metaldehyde based products are registered in pulses for use prior to and up to the 4 leaf stage.

Iron chelate based baits are specific to slugs and snails (molluscs) and slaters (crustaceans) and have low toxicity to mammals and birds (no poison schedule). They have no impact on carab beetles which are key snail predators and hence are the preferred IPM option. Iron chelate based compounds are registered for use in the bare fallow prior to planting, and also in crop boundaries. While of low toxicity, iron chelate baits are attractive to some animals and birds. The bait’s mild alkalinity may cause certain animals to vomit, especially dogs. For this reason, spread the bait evenly to avoid heaping which might attract dogs and birds.

Insecticide sprays targeting other soil pests, such as armyworms and cutworms, are ineffective against slugs. Where there is extreme slug pressure, baits alone will not bring slugs under control.

Cultural control

Cultural practices which discourage slugs and snails include cultivation (2 shallow discings) to bury trash and levelling the seedbed with a roller to crush clods, but not the use of press wheels which creates a humid furrow. These strategies are at odds with zero/minimum till and stubble retention practices aiming to conserve soil moisture.

Cultural practices to reduce high slug numbers may have to be employed periodically, as chemical control alone is unlikely to eliminate slugs in farming systems that retain stubble blankets.

Slug samples

To help build up a slug incidence data base, and to determine which species are causing problems in NE Australia, please collect and forward slugs to Australian slug expert Michael Nash at CESAR, Bio21 Institute, Melbourne University, 30 Flemington Rd., Parkville, Victoria 3010.  Ph (03) 83 442 521.  Mob 0417 992 097

Post/courier slugs in a jar with moist paper and record the location (including GPS coordinates). Also record the soil type, paddock history (e.g. zero or minimal till or regular cultivation) and the paddock’s cropping history.

Article by Hugh Brier and Kate Charleston

The castor oil looper is a minor and sporadic pest but larger outbreaks are possible in some seasons. Large flights of castor oil looper moths have been reported over Southern Queensland during March.

Identification of castor oil looper

The mature castor oil looper is purple-brown, has both light and dark forms and has two conspicuous black spots on the back behind the front legs. Loopers are up to 60 mm long and in addition to 3 pairs of true legs, also have four pairs of prolegs towards the rear of the body. Their body tapers noticeably towards the head. Loopers, as their name indicates, move with a distinctive looping action.

The adult moths are a mottled dark brown with darker lines. The hindwings have a white band across the wing and three evenly spaced white blotches along the outer margin of the wing. The wings are usually folded roof-like at rest. Moths are 25 mm long and 55 mm across the out-stretched wings.

 Lifecycle

Little is currently known of the lifecycle of castor oil loopers. The egg stage is estimated to last about 5 days, with the caterpillar (looper) stage taking about four weeks and the pupal stage lasting about 10 days. Unlike Helicoverpa which pupate in the soil, looper larvae usually pupate on the plant under leaves in a thin silken cocoon.  

Damage

Loopers are mainly leaf feeders but can occasionally feed on seed. Larger caterpillars can consume whole leaves and large infestations can cause severe defoliation.

Management

Loopers are easily controlled with biopesticides such as Bt (e.g. Dipel) when they are small.  Regular inspection, at all crop stages, is necessary to detect infestations early so as to enable control measures to be applied before serious damage is done.

Loopers are attacked by numerous predators and parasites.  Many of these also attack Helicoverpa (e.g. predatory bugs, tachinid flies, braconid wasps and ichneumonid wasps).  Loopers are frequently parasitised by small wasps (Apantales sp.) with scores of parasite larva developing per looper host.  The use of Bt for looper control will help preserve beneficial insects and also reduce the risk of subsequent whitefly and mite attack. 

Outbreaks of looper viruses are frequently observed in crops with high looper populations.  However, larvae are usually not killed by virus until they are a medium-large stage (instars 4-5).  Looper virus is not the same as Helicoverpa nuclear polyhedrosis virus (NPV). Application of NPV (e.g. VivusMax)  will not control loopers.

Other caterpillar pests

We have received a few reports of caterpillar infestations in pastures. Without specimens or good quality photos the entomology team is not able to identify these pests.  If you have any concerns about pests in crops or pastures please forward photos or samples of the pests to Zara Ludgate at DEEDI, PO Box 102, Toowoomba 4350 Qld or send images to Zara.Ludgate@deedi.qld.gov.au

Article by Kate Charleston