PestFacts 31 July 2025
Watch for slugs post-rain
We've seen slug activity in the south-east despite this year's drier weather and slower start to rainfall across South Australia, though without reported crop damage.
Slugs tend to thrive in paddocks with high moisture retention, particularly those with clay-rich soils, high organic matter, and retained stubble. These conditions provide ideal shelter and food sources, allowing slugs to remain active near the soil surface.
Research suggests the risk of slugs reaching damaging levels increases significantly when subsoil moisture at depths of 500–600mm is high and when rainfall exceeds 70–80mm over 3 weeks.
Even in drier years, moist microhabitats (under stubble, rocks, or clods, for example) can allow slugs to persist. Paddocks with a known history of slug activity are at greater risk, especially in reduced-tillage or no-till systems. These systems retain more surface residue, which helps conserve soil moisture and provides ideal shelter for slugs to remain active near the surface. As a result, slugs may still pose a threat to establishing crops even when surface conditions appear dry.
Black keeled slugs (Milax gagates) grow to 60mm in length and are usually dark brown-black in colour, with a prominent ridge (keel) running along the back. They are the dominant species in South Australia and well suited to the environment thanks to their ability to burrow. Because of this ability, black keeled slugs tend to emerge from the soil later in the season than grey field slugs, when soil moisture reaches the depth where they are aestivating. They're more difficult to detect compared to surface-feeding species like the grey field slug, striped field slug, and the brown field slug.
Also known as the reticulated slug, the grey field slug (D. reticulatum) can grow up to 50mm long and vary in colour from brown to grey with netted dark-brown markings. They produce a milky coloured mucous when irritated to deter or hinder predators. Grey field slugs are mainly surface active, using moist refuges at the soil surface, seeking volunteer crop plants and broadleaf weeds. Populations as low as one per square metre can cause severe damage to canola seedlings during establishment.
Monitor
Monitoring is essential in high-rainfall areas (great than 500mm), particularly in paddocks intended for canola. The most effective approach is to start monitoring in the previous spring to identify high-risk areas, followed by assessments after autumn rainfall and prior to seeding.
Slugs are most active under damp, mild conditions, so early morning inspections following such weather are ideal. Slug activity can persist in moist microhabitats (under stubble or soil clods) and, if left unmanaged, their feeding can cause significant damage to emerging seedlings.
Traditional monitoring tools like refuge traps (terracotta tiles or carpet squares, for example) work by attracting slugs that seek shelter under surface covers. To use them effectively, place traps on the soil surface when it's visibly moist and inspect them after a few days. Adding slug bait under each trap can increase detection rates by drawing slugs to the area. However, relying solely on surface refuge traps can lead to an underestimation of black keeled slug populations due to their burrowing habit.
When the crop's sown and germination starts, you should visually examine crops for damage and apply sufficient rates of baits at sowing to protect seedlings. It’s essential during early crop development to continue monitoring for slug damage, particularly in canola and cereals.
Dr Michael Nash covers the common slugs found in broadacre farming
Manage
Repeat baiting is often needed where slug pressure remains high during establishment. Slugs may not consume all baits in a single application, especially under patchy populations or if environmental conditions extend their activity. Monitoring between baiting rounds helps assess effectiveness and determines if you need more applications.
Bait lines provide a simple and effective monitoring alternative to traditional surface refuges. This technique involves applying a 1-metre strip of bait (either on bare soil or alongside crop rows) and checking for slug presence the following morning. Bait lines are especially useful when conditions are insignificant and surface-active slugs are otherwise hard to detect.
Key baiting recommendations:
- Use baits with high pellet density, even size, and durability under wet conditions.
- Apply at label-recommended rates (usually aiming for 30 to 40 pellets per square metre for uniform coverage).
- Avoid applying bait when the surface is dry and slugs are inactive. Ideal timing is after rain or in the evening when slugs are most active.
- Spot baiting in known hotspots or paddock edges can improve efficiency and reduce costs.
Bait options include:
- iron phosphate-based baits, which are low-toxicity and effective under cool, damp conditions
- metaldehyde-based baits, which may offer faster knockdown but degrade more quickly in rain and can be more hazardous to non-targets.
Led by SARDI research scientist Dr Kate Muirhead, a GRDC-invested research project to optimise slug management is monitoring slug populations intensively over 3 years and across 37 Australian sites. These sites span mid-to-high rainfall regions with the highest slug threats.
The project includes 11 sites in South Australia, 13 in Victoria, 7 in New South Wales, and 6 in Western Australia. Slug population and climate data collected from these locations will support the development of a predictive model and decision-support tool. This tool aims to help growers assess seasonal and paddock-level risks, allowing for more targeted and cost-effective slug management.
For more information, read Slugs In Crops: The Back Pocket Guide (GRDC).
Acknowledgements
This information has been extracted from:
Keep an eye out for green peach aphid and turnip yellows virus
We received a recent report of turnip yellows virus (TuYV) from irrigated canola in the south-east. While this is the only known detection in the state so far, it highlights the importance of ongoing monitoring of canola paddocks where green peach aphids (Myzus persicae, GPA) may be active.
In neighbouring regions, green peach aphids have been seen in young canola crops across Victoria’s northern country and southern New South Wales. Positive TuYV test results and visible symptoms followed these sightings, particularly in irrigated crops.
GPA are the primary vector of TuYV. They get the virus by feeding on an infected plant and transmit it to healthy plants during subsequent feeding. Once infected, the aphid carries TuYV for life. Radish, capeweed, volunteer canola, mallow, and turnipweed are common hosts for both GPA and TuYV.
Even though TuYV is a serious disease for canola, it can also infect several pulse crops, including chickpeas, lentils, field peas, lupins, and faba beans. It can reduce crop yields anywhere from 10% to 80%. The earlier a plant is infected, the greater the potential yield loss.
Read more about current green peach aphid (GPA) and turnip yellows virus (TuYV) reports interstate.
Monitor
Closely monitor aphid activity, especially along field edges where infestations often begin. Confirm the presence of GPA through proper identification before making any spray decisions.
Keep an eye out for symptoms of TuYV, which includes red, yellow, or purple discolouration. These symptoms are most noticeable along the edges of older leaves, and colours are usually more intense between the veins and on the leaf's upper surface.
Manage
See the following resources for GPA management:
- Green peach aphid best management practice guide – southern (GRDC)
- Neonicotinoid resistance in the green peach aphid (Cesar Australia)
- Beneficial chemical toxicity table (Cesar Australia)
Cool and wet conditions can reduce aphid activity. Aphid flight activity generally drops below minimum thresholds at temperatures under 16 to 17°C, resulting in fewer winged adults.
TuYV survives over summer on green-bridge hosts and later spreads into crops by aphids. Once infected, plants cannot be cured. Unlike fungal diseases, there are no effective chemical treatments for viruses.
A GRDC-funded national project to advance Australian virus management is underway to improve virus management in Australian grains, including research on aphid vectors.
Please contact us if you spot any TuYV-like symptoms.
On the radar: Herringbone caterpillars, weevils, and cowpea aphid activity
Herringbone caterpillar activity
Herringbone caterpillars (Proteuxoa spp.) have recently been reported in wheat and lupin crops around Tumby Bay, Cummins, and Port Lincoln on the Eyre Peninsula.
Herringbone caterpillar larvae typically have a light or dark central band along the upper abdomen, with distinctive v-shaped or herringbone-patterned pale markings on both sides and across several abdominal segments. Colouration patterns are more obvious in larger larvae, while smaller larvae are more uniformly dark.
Herringbone caterpillar feeding typically causes low-to-moderate economic impact and is most common in late autumn and winter. Larvae mainly feed on the lower parts of plants, with damage potentially more significant in later-sown, less advanced crops. They don't cause the head-lopping damage sometimes associated with armyworm but both species often coexist.
More weevils
In recent weeks, there have been reports of:
- mandalotus weevils in barley, wheat, and oat crops across the Eyre Peninsula and the south-east
- vegetable weevils affecting lentils in the northern regions.
Mandalotus weevils (Mandalotus spp.) appear mainly in areas with lighter soil types. Adults are 3-to-5mm long and emerge onto the soil surface in late autumn. They're often difficult to find, so search at night using a torch or during the day, checking the top 1cm of soil at the base of weeds or damaged seedlings.
Adult vegetable weevils (Listroderes difficilis) are 8mm long with grey-brown bodies, a prominent weevil snout, and a distinctive pale-coloured 'v' on their back. Larvae are legless, yellow or green in colour, but with an orange-brown head.
Both feed on foliage at night in early winter. Most of the damage caused by vegetable weevils usually happens around crop edges or where host weeds are present.
Check The Back Pocket Guide- Crop Weevils by GRDC for more detailed identification and management advice.
Cowpea aphid activity
We recently received reports of cowpea aphid (Aphis craccivora) activity in lucerne around the upper south-east.
Cowpea aphids are glossy black insects characterised by distinctive black and white legs. They usually infest a range of legume crops, including lentils, faba beans, field peas, lupins, lucerne, clover, and medics. They form dense colonies on plant tips, sucking sap and injecting toxins, which leads to yellowing veins, leaf wilting, bunching, and even plant death.
Cowpea aphids spread several plant viruses, like cucumber mosaic virus, bean yellow mosaic virus, alfalfa mosaic virus, and pea seed-borne mosaic virus.
Cowpea aphids can also indirectly affect livestock health when grazing on infested crops and pastures. These aphids are suspected to cause photosensitisation through the photodynamic pigments they carry, though the exact mechanism remains unclear. Symptoms in livestock may include sunburn-like skin irritation and eye inflammation. If you see signs, seek veterinary advice for accurate diagnosis and management.
Regular monitoring should start at early-season, with particular attention to crop edges, weedy borders, and irrigated areas.
Check GRDC's Crop Aphids Back Pocket Guide for more detailed identification and management advice.
Report to PestFacts
The PestFacts SA team always wants to know what invertebrates you find in your crops and pastures, whether it's a pest, beneficial, or unknown species. We even want to know about the usual pests.
Please send your reports or identification requests via the PestFacts map.
Alternatively, please contact:
Maryam Ehsangar
Phone: 0448 010 339
Email: maryam.ehsangar@sa.gov.au
Maarten van Helden:
Phone: 0481 544 429
Email: maarten.vanhelden@sa.gov.au