Bait for slugs and snails before egg lay

Slug populations

There are two main species of slugs which are recorded as agriculturally significant pests in SA and cause damage by eating seed and new seedlings: the black keeled slug, Milax gagates, and the grey field slug, Deroceras reticulatum. Other pest species possibly present in paddocks include the brown field slug, Deroceras invadens or Deroceras leave, and the striped field slug, Ambigolimax species.

Black keeled slugs will be emerging after the season break seeking suitable food sources – seeds or seedlings. Be on the lookout for signs of slug damage in your high value crops such as canola. Young seedlings are highly susceptible so you should familiarise yourself with management options. During warm dry days, these slugs will be hiding under refuges such as rocks, stubble, logs, summer weeds or within the soil. Soaking rain and cooler temperatures in autumn and winter increase slug activity on the surface.

A joint SARDI and University of Adelaide project, funded by GRDC, aims to improve the management of slugs by investigating spring baiting options and population monitoring across multiple sites. Sites for this project include the Eyre peninsula, the Mid North, the Fleurieu Peninsula and the South East, as well as sites in Victoria, NSW and WA. Information on the project will be provided through PestFacts and GRDC updates.

Snail populations

Conical snail (cernuella virgata) numbers have significantly reduced but whether that is due to dry conditions or pre-emptive baiting has not been determined.

Drier conditions prevailed over 2023 with rainfall deciles below 3 recorded for July to October and higher than average rainfall seen in the late spring and summer months.

Snails can breed from autumn to spring, but most eggs are typically laid early in the breeding cycle. The late wet spring and summer may have allowed breeding to extend later into 2024 than usual, hence activity in the field is currently still low.

SARDI's observations in February and March 2023 suggest that it is important to monitor refuges bordering paddocks, such as roadsides, pastures, or headlands, and to bait fence lines to prevent snails reinvading paddock edges when rainfall increases.

Tips for effective snail baiting

Round and conical snail activity is currently low. However, first rainfall and increased humidity is the perfect time to kill the adults by baiting snails before they breed.

The most efficient time to bait snails is at the end of their summer dormancy as soon as feeding activity begins. This is typically in late summer or early autumn, depending on weather. At this time, snails feed voraciously as they prepare to breed. They quickly metabolise bait toxins as there is limited alternative food and so their encountering pellets instead of another food source (for example, crop seedlings) is more likely.

The timing of bait application is critical to success. Snails must be moving and actively feeding to encounter pellets and ingest a lethal dose of toxin. Key points for successful baiting are:

1. Monitor snail activity.
   Snails move when moisture is present. In early autumn, relative humidity above 80–90% encourages movement. Overnight dew or light rainfall (less than 2 mm) increases relative humidity to 100%. Most movement occurs overnight between midnight to just after sunrise. To monitor movement, place bait pellets in a snail-infested area and check daily. The presence of dead snails around baits indicates snails are feeding, warranting widespread application.
2. Apply bait at the right time.
   Baiting before egg lay is critical. Apply bait when moisture is present (see above), and as soon as feeding commences at the end of summer dormancy. Snails breed from autumn to spring, but most eggs are laid by early winter. If mice are present, bait mice before baiting snails to avoid mice consumption of snail bait. Mice have been reported across the Upper Eyre Peninsula. If mixed populations of round and conical snails are present, consider a second bait application around sowing time to target conical snails. The onset of peak activity can be several weeks later for conical snails than round snails.
3. Monitor and re-apply bait as necessary.
   Continue to monitor and re-apply bait as needed until around early winter. Baiting efficacy declines after this time. Baiting must cease at least 8 weeks before harvest, as there is a zero tolerance for bait contamination of grain.
4. Select an appropriate bait product.
   Three active ingredients (and several formulation types) are registered for snail control in Australian crops and pastures:
   • metaldehyde (15 to 50 g per kg)
   • chelated iron (60 g per kg)
   • iron phosphate (9 g per kg iron).

Choose a product according to your preferences. Consider the number of pellets applied per square metre at the registered application rate. Always adhere to directions on the product label. As a guide, use the baiting guidelines from Nail the snails.

5. Apply an adequate rate of bait.
   Pellet densities of at least 30 pellet points per square metre (up to 60 per square metre where snail densities exceed approximately 120 snails per square metre) are required to ensure an adequate chance of snails encountering the bait. Re-apply bait as needed according to product label directions. High rates or re-application may be needed around fence lines.
6. Broadcast pellets evenly using a calibrated spreader.
   Always calibrate the spreader for the selected bait product to ensure an even spread. Uneven spread can result in poor efficacy. Manually check the spread width and drive at pass widths no larger than the effective spread. The SnapBait app can assist with estimating bait pellets applied per square metre.

More information

• Movement, breeding, baiting and biocontrol of Mediterranean snails
• Snails
• Snail and slug baiting guidelines ( PDF 494.6 KB )

Reduce pest movement by destroying the green bridge

Managing the green bridge pre-season helps reduce the risk of early season pest and disease outbreaks.

What is the green bridge?

Weeds and self-sown crops are often host plants for various diseases and insect pests – particularly aphids and diamondback moth (DBM, Plutella xylostella). These plants provide refuge for pests to survive between growing seasons and this area is known as the 'green bridge'.

The green bridge commonly grows along roadsides, in water courses, paddock perimeters, headlands and any other non-cropped areas of land. Early rainfall will increase the amount of green bridge. The presence and timing of green bridge is often critical in determining risks of early season pest and disease outbreaks in broadacre crops.

Green bridge management

For effective green bridge management in paddocks, control weeds early and, ideally, well before sowing (at least 4 weeks prior, and for disease management purposes include the time taken for herbicides to achieve complete plant death). Relying solely on herbicides applied at or after sowing will often be too late to prevent movement of pests and diseases into emerging crops. Insecticides are not required for control as insects feeding on herbicide-treated green bridge will starve as the plants die.

Green bridge can also harbour a range of beneficial insects, including ladybird beetles, hoverflies and parasitoid wasps that build up on pest populations. Consider the number of beneficial insects when deciding to control green bridge. Insecticide application on green bridge is not recommended and beneficials can often survive for a while without prey or hosts. Much cooler weather later in autumn can also dramatically reduce aphid populations.

More information

• Green bridge factsheet
• Reducing aphid and virus risk
• Diamondback moth best management practice guide – southern
• Green peach aphid best management practice guide – southern
• Resistance management strategy for the green peach aphid in Australian grains

Emerging crop pests – redlegged earth mite

Redlegged earth mite (RLEM, Halotydeus destructor) is a common resident pest. RLEM are active from autumn to late spring, when the third generation of mites produce over-summering eggs that will hatch in the following autumn. This behaviour provides the potential to limit populations using the TimeRite strategy if necessary, before eggs are laid.

To help you decide if a TimeRite application will be beneficial, the GRDC Redlegged earth mite best management practice guide will allow you to evaluate the risk of RLEM on your property based on:

• previous crops
• observation of mites this month
• sensitivity of the future crops.

You can also use the RLEM severity risk assessment developed by Cesar Australia. A typical high-risk situation would be a crop like canola in a paddock that was used for grazing the year before.

RLEM is becoming increasingly resistant to many pesticides. It is important that you only spray if:

• RLEM numbers are currently high
• a sensitive crop is being sown next year.

Ensure you correctly identify the mites you find, since the TimeRite strategy only works on RLEM. The GRDC Crop Mites Back Pocket Guide is a useful tool to help distinguish between these mites. As a PestFacts SA subscriber, you can always send photos (using GoMicro or a similar macro phone lens works best) and specimens to us for review.

With rain and cool temperatures arriving, the summer eggs will now hatch very rapidly. Keep an eye on populations in your paddock, and make sure that they are RLEM and not a different species of red spider mites. Estimate your risk level and apply a Timerite application where needed. Spraying for RLEM in autumn is rarely needed if the paddocks have been monitored and managed prior to the winter growing season.

If you have any capeweed growing on the crop edge, look under the leaves on the ground, as they usually harbour RLEM.

More information

• Resistance management strategy for the Redlegged Earth Mite in Australian grains and pastures
• Simple in-field test to detect pyrethroid resistance in redlegged earth mites

Look out for invasive pests like fall armyworm

Fall armyworm (FAW, Spodoptera frugiperda) is a moth from the tropical and sub-tropical areas of the Americas, known for long flights and voracious feeding by the larvae. FAW eats many things but has a preference for wild and cultivated grasses including corn, rice sorghum and sugarcane.

FAW has been observed infesting forage oats on the Darling Downs in Queensland. Modelling for FAW in SA shows that the climate here is only suitable for low population growth. FAW doesn't do well in cooler temperatures and is unlikely to be a pest for winter cropping. Although there have been no detections of FAW in SA, it pays to be vigilant about their presence.

Identification

FAW looks like other armyworm species and caterpillars grow to a size of 30 to 40 mm, similar to budworms and cutworms.

Large FAW can be identified by an inverted 'Y' shape on the head capsule and 4 raised spots in a square at the rear.

Reporting fall armyworm

If you suspect FAW in your field, report it immediately to the Exotic Plant Pest Hotline on 1800 084 881.

Unusual caterpillars can be reported to PestFacts SA.

More information

• GRDC Grains Research Update, online – Fall armyworm
• Fall armyworm: what threat does it pose, and what tools do we have to manage it?
• Fall armyworm
• The Beatsheet: Fall armyworm
• Regional and seasonal activity predictions for fall armyworm in Australia
• Fall Armyworm Continuity Plan for the Australian Grains Industry (PDF)

Report to PestFacts

The PestFacts SA team always wants to know what invertebrates you find in your crops and pastures, whether it is a pest, beneficial or unknown – even the usual pests.

Please send your reports or identification requests via the PestFacts map.

Alternatively, please contact:

Maarten van Helden:
Phone: 0481 544 429
Email: maarten.vanhelden@sa.gov.au