PestFacts October 2022
Armyworm activity
In the past few weeks armyworm caterpillars have been reported in the Murray Plains and west of Gladstone in the mid north.
How to tell if it’s an armyworm?
Armyworms are distinguishable from other noctuid caterpillar pests by the three distinct white parallel stripes on their cervical shield (neck), which typically run to the end of their body.
There are three commonly found species of armyworm in southern Australia, but they are difficult to distinguish in field and control methods are similar.
For more information, see armyworm ().
Assessing current armyworm risk
Armyworms feed mostly on leaves, but in crops close to maturity they will feed on the last remaining green section of the stem just below the head and resulting in head loss. The most critical risk period is 3 to 4 weeks prior to harvest. When determining the risk, consider the:
- timing of harvest
- green matter available in the crop
- expected return on the crop
- larval development stage.
Like most caterpillar species, about 80% of the feeding is done in the last instar (large larvae, 30–40 mm) over a period of 3 to 7 days. The economic threshold for armyworms during crop ripening is 1 to 3 larvae per m2.
Assessing the numbers of armyworms can be difficult because their behaviour can vary with weather conditions and feeding preferences. Armyworms are most active at night, although can sometimes be seen during the day feeding on leaves and stems of cereal crops. Look for caterpillars on plants and on the ground, especially under leaf litter between the rows, and on damaged plant heads. Caterpillars have also been found sheltering inside stubble. Use of a sweep net in a crop in the early evening provides an indication of the relative densities and stages of armyworm. Check frequently for signs of head-lopping.
Fall armyworm update
To date, fall armyworm (FAW, Spodoptera frugiperda) has not been detected in South Australia. If you suspect FAW is present in your region, report it immediately to the Exotic Plant Pest Hotline on 1800 084 881. If you are experiencing atypical control failures or are seeing unusual caterpillars in grains broadacre crops they can be reported to PestFacts SA. For more information on FAW see Fall armyworm movement update.
How to report
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 – and even the usual pests.
Please send your reports or identification requests via the PestFacts map.
Alternatively, please contact:
Rebecca Hamdorf
Email: rebecca.hamdorf@sa.gov.au
Mobile: 0429 547 413
Maarten van Helden:
Email: maarten.vanhelden@sa.gov.au
Mobile: 0481 544 429
Springtime slug monitoring
Slugs are typically a pest in high-rainfall zones where annual rainfall exceeds 500 mm. However, black keeled slug can be found in medium rainfall areas, especially after a couple of wetter seasons where rainfall exceeds 450 mm. Crops are particularly vulnerable to feeding by slugs at establishment. However, monitoring in spring provides a good opportunity to assess slug numbers and distribution in paddocks to assist preparation for management next year.
Another wet outlook for spring
The Bureau of Meteorology has declared a La Niña in 2022 for the third year in a row. As a result, much of eastern Australia is expected to receive above average rainfall this spring, which is likely to create favourable conditions for slug breeding in the coming months.
The wet outlook for spring in 2022 is a contributing factor for a high-risk scenario for slugs in establishing crops in 2023.
Monitor this spring for understanding your 2023 establishment risk
This season in South Australia slug numbers were low at establishment, and current monitoring by Dr Michael Nash at Finniss, Yallunda Flat and Wirrabara suggests low numbers. Traditionally slug prone areas in the southeast (Millicent and Francis) have experienced greater slug numbers with some crop losses. What will a wet spring mean for slug breeding and numbers? Spring monitoring at these sites is continuing and we will provide updates on numbers soon.
Slug activity can be monitored with refuges (for example, carpet mat or tiles) placed out in paddocks and checked in the morning. As slugs often have a patchy distribution in paddocks it’s important to use multiple refuges. Fifty monitoring refuges are recommended when paddock size is 40 hectares due to variability of Australian conditions. Despite this recommendation, most will use less labour-intensive monitoring such as direct searching early in the morning when wet, or fewer refuges. Monitoring is encouraged in wet conditions when slugs are active.
Keep in mind that if you do detect high numbers of slugs in your maturing crops, control options are limited. Current research does not recommend springtime baiting intended to reduce slug numbers for next season. Long harvest withholding periods, up to 8 weeks, further limit when baits can be applied to maturing crops. Monitoring during spring provides an understanding of population dynamics for estimating the risk of possible establishment issues next season. Baiting should be done in autumn, after sowing, to protect seed and emerging crops.
New slug resources available
Best practice management of slugs requires an integrated approach. Rainfall estimates and spring monitoring provide key information to assess risk for 2023 and helps to make informed management choices.
New resources are available from GRDC outlining understanding of the risk to crops from slugs, including information on slug species and on monitoring and control:
- Slug control fact sheet: Successful crop protection from slugs (PDF 3.5 MB)
- Slugs in crop: The back pocket guide
More information on slugs in broadacre agriculture:
The latest information for growers and advisors on the activity and management of pests in all broadacre crops during the winter growing season.
