Prevalent invertebrates in the 2020 winter cropping season in South Australia and new tools available for management

Take home messages

• An unusually high diversity of migratory moth/caterpillar pests was reported in the 2020 winter cropping season, particularly earlier in the season due to weather events.
• Don’t chase this year’s pest issues next year – make decisions on the merits of each year. Every season is different and large regional differences occur.
• Key to successful invertebrate pest management is assessing the risk, carrying out observations and making informed decisions.

Background

In 2020 unusually high populations of a very broad variety of migratory moth/caterpillar pests were reported to PestFacts SA during the winter cropping season. Notably, early moth flights brought in multiple species during April and May, and caterpillar feeding started much earlier than typical. Several unusual and uncommon pests were also reported, adding to an already unusual year. This paper discusses what pests were reported and what factors contributed to their prevalence.

New resources for management of invertebrate pests are now available to help growers and agronomists in pre-season paddock planning, risk assessment during the winter cropping season and future management. A brief overview of each new resource is provided.

The key to all successful invertebrate pest management is assessing the risk first, conducting appropriate observations and making informed decisions. Blanket pest management approaches are unsustainable, costly, often ineffective and not recommended.

What insects were prevalent in 2020?

The 2020 winter cropping season was typified by early moth/caterpillar pests and continual pressure throughout the season. Several species of moths were seen in very high numbers after migrations and were widespread across South Australia (SA). An unusually wide variety of moths and caterpillars were reported overall. Regional high numbers for a few pests occurred throughout the season and several unusual invertebrate species were also reported.

Common moth/caterpillar pests observed in the 2020 season

The common moth/caterpillar pests were:

• Native budworm, Helicoverpa punctigera, fed on early sown crops on the Eyre Peninsula. This insect is a major pest in spring but crop damage early in the season is relatively uncommon. The spring moth trap network detected high numbers in the Mid North during peak flights (late September), but spring rains limited flights.
• Weed web moth, Achyra affinitalis, was particularly widespread early in season after a large migration of the moths during April. They fed on medic pastures, emerging cereals and canola crops. Larvae are fast-moving and produce large amounts of webbing which they feed under, skeletonising foliage and webbing leaves together.
• Cutworms, Agrotis sp., were reported steadily across the season, often seen in mixed populations alongside armyworms and herringbone caterpillars. Larger larvae (>25 to 40 mm) chewed off leaves and stems near ground level, primarily from crops sown into paddock areas with heavier stubble. Female moths prefer to lay eggs into stubble.
• Herringbone caterpillars, Proteuxoa sp., were observed at low densities in cereals and some mixed pastures, often in mixed populations with cutworms. Visually similar to cutworms but they have a herringbone pattern on the back. Reported more widely than usual this season.
• Armyworms, Persectania ewingii and Leucania convecta, damaged some vegetative cereal crops during winter, causing patches of missing plants. Reports were widespread and common throughout the season, in part in response to concern about fall armyworm and requests for reports. To date (06/01/2021), only native species of armyworm have been reported in SA.

These species commonly occurred in mixed populations with two or more other species. The larvae feed mostly at night and are more difficult to find during the day.

Several other caterpillar species were also reported, but there were only a few reports of each. What is notable is the range of species reported.

• Pasture webworm, Hednota sp., fed on cereal paddocks that had recently come out of long-term pasture. This pest resides in silk-lined tunnels in the soil and emerges at night to feed when they chew off leaves and drag them back to their tunnels.
• Pasture day moth, Apina callisto, reported in cereals in very low numbers with no feeding symptoms evident. Caterpillars can grow up to 60mm, and they are distinctive with dull dark brown with reddish-orange and yellow markings and very prominent bristles. They prefer broadleaf plants but will feed on all crops, particularly if favoured hosts have been removed through herbicide usage.
• Pasture tunnel moth, Philobota productella, were observed early in the season on cereals. Caterpillars construct vertical, silk-lined tunnels that protrude above the soil surface forming ‘chimneys’. More prevalent on cereals after a pasture phase.
• Brown pasture looper, Ciampa arietaria, were reported a few times on pasture, often in mixed populations. The caterpillars feed on broadleaf plants, have two wavy yellow lines running down their brown body and move in a characteristic looping motion.
• Diamondback moth(DBM), Plutella xylostella, while total reports of DBM were low, when reports were received the numbers were often high/near threshold. Several agronomists noted in discussions that DBM numbers were higher than average in 2020. Low reporting of DBM might be a function of reporting bias - growers and agronomists know and deal with DBM regularly so don’t report it, especially compared to rare or unusual pets.
• Cabbage centre grub, Hellula hydralis. Moths were reported in large numbers several times during 2020, but not many caterpillars were found after searching.
• Woolly bear caterpillars, Family: Arctiidae, were reported feeding in high numbers along the fence line of a canola crop coming out of the scrub. Woolly bear caterpillars are opportunistic and are a very rare pest, more likely to feed on weeds.
• Caper white butterfly, Belenois java teutonia, had a large-scale migration in mid-late spring, and huge masses of them could be seen along roadsides fluttering around. They are highly migratory but only feed on several species in the Capparace family, most notably caper bushes (Capparis spp.). They are not an issue for broadacre agriculture.

Other common pests observed at higher numbers in the 2020 season

Other common pests seen at higher numbers were:

• Redlegged earth mite (RLEM), Halotydeus destructor, occurred in unusually high densities in and around Lower Eyre Peninsula from late May onwards. Pastures that received a well-timed TimeRite® insecticide treatment in spring 2019 experienced substantially lower densities than untreated pastures and crops.
• Russian wheat aphid (RWA), Diuraphis noxia, remained at low overall densities. A dry summer in most areas led to very low seasonal risk. Areas in the mid North and upper Eyre Peninsula, where high February rainfall caused barley grass to start growing early followed by a dry June and July where the cereals were particularly drought stressed and had stunted growth, saw higher RWA populations.
• Rutherglen bug, Nysius vinitor, was reported in high numbers in medic pastures that also contained a lot of capeweed. Feeding symptoms were limited to capeweed and medic. Numbers were increasing in several areas closer to harvest. This highly migratory bug most likely developed on native plants growing in early season rainfall areas.

Other unusual/rare invertebrates observed in the 2020 season

Other unusual/rare invertebrates were:

• Ryegrass mealybug, Phenacoccus graminicola, was reported on barley and volunteer cereals in two locations but in low numbers and no noticeable feeding symptoms. They were also reported, unusually, on lentils in low numbers. A continuous green bridge of grasses over summer and into autumn, and favourable summer conditions, is likely to have been the cause of these small populations.
• Leafhopper, (Cicadellidae) species unknown, was reported on canola early in the season in high numbers, but only minimal feeding symptoms were seen. Leafhopper migration and activity is favoured by warming conditions, so populations did not persist for long.
• Bean root aphid, Smynthurodes betae, was reported on the roots of some poor- looking faba bean. These root aphids are globular and white. They have only been recorded in SA twice before and are unlikely to be an issue in future. Their primary host is pistachio, but secondary hosts include beets, potatoes, some pulses and a range of asteracea weeds.
• Purple scum collembola, Hypogastura vernalis, was seen in large numbers multiple times across the season after large rains. The large numbers (thousands of individuals, often seen floating in rafts on puddles) seem concerning, but there are no known records of plant damage caused by the purple scum collembolan. They are beneficial to the soil, playing an important ecological role in decomposition.

What conditions led to these pests being an issue this season?

Moths/caterpillars

Unusually high moth and caterpillar activity early in the season is thought to have resulted from significant moth migration events in early autumn. Rainfall in inland source areas causes growth of insect host plants, warm temperatures support population build-up and flight, and suitable wind systems transport the insects into cropping zones.

The sudden appearance of several migratory moth species in crops around the same time provides strong evidence for large migration events. Large flights of weed web moth and at least three other species were recorded on first at the SA-Victorian border, then 10 hours later further down in the South East, several areas in metropolitan Adelaide and then moving across the Eyre Peninsula region in late March and early April. Wings of many of the moths reported were tatted and broken, suggesting they originated farther afield.

This major migration event followed significant rainfall in parts of inland and eastern Australia during February and March (Bureau of Meteorology 2020), which likely supported population growth.

Redlegged earth mite

Redlegged earth mite (RLEM) typically undergoes three generations during the winter cropping season. In the third generation, mites produce over-summering diapause eggs, which are retained in the body of the female. Over-summering diapause eggs hatch the following autumn when suitable conditions occur; at least 5mm rainfall coinciding with mean daytime temperatures under 16oC for 10 days.

During 2019, parts of the Eyre Peninsula received substantial rainfall in spring (Bureau of Meteorology 2019). Moisture around the TimeRite® date can lengthen the spring growing season and lead to production of more over-summering mite eggs. In autumn 2020, high rainfall coinciding with cool temperatures occurred in late April across parts of Lower Eyre Peninsula, which is expected to have caused a synchronous hatching of RLEM during crop emergence. In years with a less synchronous autumn break, RLEM hatching occurs in a more staggered fashion, leading to lower initial densities and allowing crops to out-grow some early damage.

Other pest invertebrates

The variety of rare or unusual pest invertebrates seen in 2020 is likely due to favourable climatic conditions, and in some cases the availability of a green bridge over summer. A lot of these unusual broadacre pests aren’t well understood, and as they are so infrequent and aren’t typically a concern, very little research is done into them. All rare and unusual pest invertebrates seen this season did not warrant control.

New tools available for pest invertebrate management

Several new tools have been created and updated for integrated pest management (IPM) practices.

Lucerne seed web moth degree-day model web application

The SARDI Etiella degree-day model is now available as a web-based interface (Useful Resources).

Users input GPS coordinates to obtain the predicted Etiella degree-day accumulations for their location in real time and can compare with previous years of interest. This tool, developed as part of a SAGIT-funded Etiella project, replaces the need to manually download and input daily maximum and minimum data for your nearest weather station from 21 June onwards.

The date when the model reaches 351 degree-day accumulations corresponds to the predicted date of 10% onset of peak moth flight activity. New findings of the Etiella project recommend commencing in-crop monitoring of lentil crops around 7-10 days earlier than this date, at 300 degree-day accumulations.

Best practice management guides

New best practice management guides have been developed for RLEM, green peach aphid and DBM. Each guide includes a Risk Assessment Guide for assisting with planning decisions and key IPM decision points and timing (Useful Resources). Podcasts have also been developed to support this work (Useful Resources).

Russian wheat aphid economic threshold calculator

Through the GRDC investment UOA1805-018RTX 'Russian wheat aphid risk assessment and regional thresholds’, investigated regional risk and management tactics for Russian wheat aphid (RWA)’ an economic action threshold calculator has been developed to help growers and agronomists determine if they should treat for Russian wheat aphid (RWA); based on the impact to crop yield and cost.

The calculator indicates if the economic injury level is likely to be exceeded between GS30 (start of stem elongation) and GS50 (start of head emergence), and therefore, if control of RWA should be actioned. This action threshold can be applied to winter and spring cereal varieties.

Recommended approach

While it is tempting to implement blanket approaches to pest invertebrate control based on the pests from last season, it is rarely a successful approach. Climatic conditions play an important role in the development of invertebrate pests and as this differs widely, season to season, so do the pests. Do not assume pest issues in 2021 will be the same as those experienced in 2020. Each season is different and brings different pests, so blanket approaches are not sustainable.

Strategic decisions are advised by assessing risk on a seasonal basis, complete with observations of pest occurrence. Growers and agronomists are encouraged to familiarise themselves with the best practice management guides and resources (Useful resources) and incorporate them into pre-season farm/paddock planning.

Some general factors that contribute to seasonal risk:

• Green bridge vegetation during summer and early autumn, particularly February to May, which can support aphids, DBM, resident pests such as many of the beetles and weevils. In general, more green bridge increases the risk of these pests and possible virus transmission (in some aphids).
• Paddock history provides useful hints about possible risk and should be considered during pre-season planning. Factors to consider include previous (resident, not migratory) pest problems, crop rotations, weed control, insecticide use and seasonal conditions.

Moth/caterpillars

Migratory species like moths are difficult to predict in advance but easy to manage if detected early. Moth numbers are poorly correlated with numbers of caterpillars, so flights should be used as a prompt to monitor for caterpillars several weeks later. Early detection can be achieved by keeping an eye out for moth flights, monitoring emerging crops and subscribing to free regional notification services such as PestFacts SA newsletter. This service relies on your reports.

Will the moth/caterpillar pests be a problem again in 2021?

By their very nature, issues with migratory pests are transient and tend to occur in boom/bust cycles. Typically, they arrive suddenly in large numbers, breed locally through a generation, then new adults disperse elsewhere. Immigrant moth populations generally do not persist locally in any substantial numbers beyond a single generation, as their primary habitat occurs in inland source areas.

Migrations would need to occur again in 2021 for these pests to be problematic. While migrations of the scale observed in 2020 are relatively uncommon, monitoring and early detection is the key to successful management.

Redlegged earth mite

The risk of high RLEM populations this season depends on last year’s crop/pasture type, weed status, seasonal conditions, RLEM numbers and the susceptibility of the next planned crop.

Large numbers of RLEM in spring can be indicative of a high-risk situation the following autumn. If a well-timed TimeRite® spray was implemented, it should be effective at reducing populations. Only use the TimeRite® strategy in high-risk situations. Some crops, such as lentil and chickpea, are poor RLEM host plants and in weed-free paddocks, low RLEM numbers can typically be expected the year following these rotations.

Plan autumn insecticide strategies according to paddock risk, using the RLEM Risk Assessment Guide (Useful Resources). Avoid pre-emergent insecticides in low-risk situations as this will suppress beneficials and may encourage pesticide resistance. Monitor susceptible crops closely in the first three to five weeks after emergence. If insecticides are warranted, follow guidelines in the Resistance Management Strategy for Redlegged earth mite (Useful Resources).

Conclusion

Every season brings different pest invertebrate management challenges. Avoid chasing last season’s pest problems and assess each season on its own merits. The key to successful pest invertebrate pest management is assessing and managing risk.

Acknowledgements

The work undertaken as part of this project is made possible by the significant contributions of growers and agronomists through both reporting invertebrates and the support of the GRDC, the author would like to thank them for their continued support.

Useful resources

PestFacts South Australia e-newsletter, PestFacts Map and Twitter @PestFactsSARDI

PestNotes Factsheets

Economic threshold calculator for Russian wheat aphid

Redlegged earth mite, Best Management Practice and Risk Assessment Guide and Podcast

Redlegged earth mite, Resistance Management Strategy

Green peach aphid, Best Management Practice and Risk Assessment Guide and podcast

Diamondback moth, Best Management Practice and Risk Assessment Guide and podcast

Broadacre grains pest knowledge cards

TimeRite®

References

Bureau of Meteorology (2020) ‘Recent and historical rainfall maps 2020’,[Verified 6 January 2021].

Bureau of Meteorology (2019) ‘Recent and historical rainfall maps 2019’, [Verified 6 January 2021].

Contact details

Rebecca Hamdorf
SARDI, Waite Campus, Urrbrae SA 5064
(08) 8429 0682 | 0429 547 413
Rebecca.Hamdorf@sa.gov.au