Establishment pests in 2020, and how to manage risks

Take home messages

• High populations of redlegged earth mite and migratory moth/caterpillar pests attacked Eyre Peninsula crops during establishment in 2020.
• Every season is different. Don’t chase this year’s pest issues next year – make decisions on merit each year.
• Assess risk first, make informed decisions, then monitor afterwards.

Background

Establishing crops are attacked by a variety of invertebrate pests that fall into two broad categories:

Resident pests

Resident pests live entirely within paddocks or paddock edges, in association with soil and their populations are present in similar locations, but different densities, from year to year. Resident pest groups are generally flightless species including earth mites, lucerne flea, weevils, other beetles, millipedes, earwigs, slaters, slugs and snails. Resident beneficial insect species that attack these pests include carabid beetles, rove beetles, spiders and predatory mites.

Transient pests

Transient pests live on short-lived host plants within or outside paddocks, either short or long distances away. They are highly mobile species and regularly move into crops from these alternative host plants. Transient pests include moth/caterpillar pests and aphids. Transient beneficial insects move into crops and attack these pests and become most abundant in spring. Transient beneficial insects include ladybirds, hoverflies, lacewings, and parasitoid wasps.

These two categories of pests require slightly different approaches to management. However, for all pests, the key to successful management is assessing risk first, and then making informed decisions. Blanket pest management approaches are not sustainable, often ineffective, and should be avoided. This paper discusses how to assess risk and manage pests strategically in future seasons.

Key Question 1 - what were the main insects causing damage in early 2020?

The crop establishment period on Eyre Peninsula in 2020 was characterised by unusually high populations of moth/caterpillar pests of several species attacking crops and pastures. Additionally, high densities of redlegged earth mite attacked emerging crops on Lower Eyre Peninsula.

The moth/caterpillar pests were: (transient species)

• Native budworm, Helicoverpa punctigera, attacked early sown crops around Upper Eyre Peninsula. This insect is a major pest of pulse and canola crops in spring but crop damage early in the season is relatively uncommon.
• Weed web moth, Achyra affinitalis, was particularly widespread and attacked medic pastures, emerging cereals and canola crop on northern Eyre Peninsula. Some pastures required treatment to control caterpillars in early May. The larvae were fast-moving and spun webs in foliage.
• Pasture webworm, Hednota sp., attacked pastures and some canola crops in lower densities than weed web moth. This pest resides in silk-lined tunnels in the soil and emerges at night to feed. They chew off leaves and drag them back to their tunnels. Typically, this pest is only problematic in paddocks following pasture phases but was more widespread this season.
• Pink/brown cutworm, Agrotis munda, attacked cereal crops from northern to central Eyre Peninsula. Larger larvae (>25 to 40 mm) chewed off leaves and stems near ground level. Damage occurred primarily in crops sown into paddock areas with heavier stubble. Paddocks grazed or windrow-burned, removing most stubble, suffered less damage. Female moths prefer to lay eggs into stubble. This species occurs relatively commonly during crop establishment on Eyre Peninsula.
• Herringbone cutworms were observed at lower densities in canola, recognisable by their variegated colour patterns. They are less damaging than pink cutworm and have only one generation per year.
• Armyworms have been observed in vegetative cereal crops on Eyre Peninsula, causing patches of missing plants. Larger larvae (>25mm to 40mm) cause defoliation and sever plant stems near ground level. Armyworm are most problematic when large larvae occur in cereal crops near harvest; in vegetative crops, they can also damage flag leaves.

These pests commonly occurred in mixed populations with two or more species. These insects feed more actively at night and are more difficult to find during the day.

The ‘common’ establishment pests were: (resident species)

• Redlegged earth mite (RLEM), Halotydeus destructor, occurred in high densities in crops and pastures around Lower Eyre Peninsula from late May onwards, but generally at lower densities further north. Some pastures that received a well-timed TimeRite® insecticide treatment in spring 2019 experienced substantially lower densities than untreated pastures and crops. Whilst a good decision on those paddocks last spring, this alone should not be used as a basis for implementing TimeRite® across the whole farm in 2020/21! In most years, crops following a relatively weed-free crop rotation will not require TimeRite®. We further discuss risk assessment below.
• Bryobia (clover) mite, Brobyia spp., occurred early in canola crops at typical densities. This mite moves from weeds onto seedlings during warmer conditions of early autumn.
• Lucerne flea, Sminthurus viridus, has reportedly been less problematic in recent seasons compared to some years.
• Vegetable weevil, Listroderes difficulus, and spotted vegetable weevil, Steriphus diversipes, damaged canola crops in patches around the west coast. Capeweed is a preferred host for vegetable weevil.
• Russian wheat aphid (RWA), Diuraphis noxia, has remained at low overall densities. A dry summer in most areas led to very low seasonal risk. Isolated patches of RWA were observed in cereals around Eyre Peninsula, even in areas that experienced a very dry summer. It is possible that some aphids were transported into cropping areas on the same winds as the moths (see following section), although this is difficult to verify. Low RWA populations on young plants will not cause significant yield loss unless there is a significant build-up of populations in spring. Economic thresholds for RWA will soon be available through the GRDC Investment “Russian Wheat Aphid Risk Assessment and Regional Thresholds”.

Key Question 2 - what conditions lead to these pests being such a problem for growers?

Moths/caterpillars (transient pests)

It is likely that unusually high moth and caterpillar activity in early 2020 resulted from a significant moth migration event/s in early autumn. This can occur when 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. Evidence for migration included the sudden appearance of several migratory moth species in crops around the same time. Large flights of weed web moth and other species were recorded on Eyre Peninsula, Adelaide and the South East region in late March and early April.

Weed web moth is a native insect and, although little is known of its biology, it is likely to feed on native plants. Large populations of weed web moth caterpillars were reported attacking crops in southern and central NSW in March 2020. This followed significant rainfall in parts of southern Queensland and NSW during February, which could have supported population growth and migration of this insect. Weed web moths observed on Eyre Peninsula in April had tattered wings, suggesting they originated from pastoral zones some distance away. These same wind systems likely transported the other moth species, including native budworm, cutworms, webworms and armyworms, and possibly some Russian wheat aphid, into cropping areas.

Redlegged earth mite (resident pest)

Large populations of RLEM in early 2020 may have resulted from suitable weather conditions in spring 2019 and autumn 2020. RLEM typically undergoes three generations per year. In the third generation (after 15th September at Cummins, based on TimeRite® spray date) mites produce over-summering diapause eggs retained in the body of the female. Mite survive summer as diapause eggs and hatch the following autumn when suitable conditions occur; over 5mm rainfall coinciding with mean daytime temperatures under 16oC for 10 days.

Parts of the Eyre Peninsula received substantial rainfall in spring 2019 (Figure 1). Moisture around the TimeRite® date can lengthen the spring growing season and lead to more over-summering mite eggs produced. In autumn 2020, high rainfall coinciding with cool temperatures occurred in late April, particularly on Lower Eyre Peninsula (Figure 1). Suitable hatching conditions over the region may have caused a synchronous hatching of RLEM coinciding with emerging crops. In years with a less synchronous autumn break, RLEM hatching can occur in a more staggered fashion, leading to lower initial densities and allowing crops to out-grow some early damage.

Figure 1. Monthly rainfall anomalies (rain compared to long term-average rain) showed above-average rainfall on parts of Eyre Peninsula during spring 2019 and April 2020, contributing to high populations of redlegged earth mite in autumn 2020. Source: Modified from the Australian Bureau of Meteorology

Key Question 3 -what can we learn from 2020 and how will this shape management decisions for 2021?

Recommended approach

The key message is not to assume pest issues in 2021 will be the same as those experienced in 2020. While it can be tempting to implement more blanket and widespread control measures following a higher pest pressure year, chasing last year’s pest problems the following season is rarely successful. Every season brings different pest issues and blanket approaches are not sustainable.

We advise making strategic decisions by assessing risk on a seasonal basis. New resources are available to assist growers assess risks in advance of sowing, and throughout the season. ‘Best Management Practice Guides’ were recently developed for redlegged earth mite, green peach aphid and diamondback moth (see Useful Resources). Each of these guides include a Risk Assessment Guide and podcast. Growers and agronomists are strongly encouraged to familiarise themselves with the guides and incorporate them into pre-season farm/paddock planning. Factors that contribute to seasonal risk of establishment pests include:

• Green bridge vegetation during February to May, driven by rainfall, supports transient pests such as aphids and diamondback moth. More green bridge increases risk, and scarce green bridge leads to low pest risk. Summer weeds also support earth mites and other resident pests and must be controlled as early as possible.
• Paddock history. For resident pests, previous pest problems, crop rotations, weed control, insecticide use and seasonal conditions, all contribute to risk.

Moth/caterpillars and aphids (transient pests)

Transient species are difficult to predict in advance but easy to manage if detected early. Standard insecticides can be applied to infested areas as warranted.

Moths/caterpillars

Moth flights indicate potential for caterpillar activity in crops several weeks later and are a prompt to start monitoring. Ensure early detection is achieved by monitoring emerging crops, and freely subscribing yourself (or ensuring your adviser is subscribed to) to regional notification services such as PestFacts South Australia newsletter. This service relies on your reports.

Will these caterpillars be a problem next year? By their very nature, issues with transient pests are often … transient. Migratory species, such as moths, tend to occur in boom/bust cycles. Typically, they arrive suddenly in large numbers, breed locally through one generation, then newly emerged adults disperse elsewhere. Immigrant populations often do not persist locally in any substantial numbers for more than a single generation, as their primary habitat occurs in source areas. It is unlikely these moth species will be problematic again in 2020 or 2021 unless further immigration events occur. Migrations of the scale observed in 2020 are relatively uncommon, but monitoring is the key.

Aphids

The abundance of green bridge vegetation is important. Dry conditions and lack of green bridge during late summer and autumn indicates low seasonal risk. Under these conditions, seed treatments for RWA control in cereals are not warranted in most instances. If build-up occurs after emergence, this pest can be easily controlled using standard registered insecticides.

Redlegged earth mite(resident pest)

By contrast, resident pests are somewhat easier to predict but more challenging to manage. The key is assessing paddock risk before sowing, monitoring crops in the first 3-5 weeks after emergence, and correct identification before deciding on control. For earth mites, correct identification is particularly important, as different mite species display differences in tolerance to insecticides. Additionally, TimeRite® is only effective on redlegged earth mite, and not effective on other mites or Lucerne flea.

For RLEM, risk of high mite populations depends on last year’s crop/pasture type, weed status, seasonal conditions and RLEM numbers, and the susceptibility of the next planned crop. A well-timed spring spray according to the TimeRite® strategy can be an effective option but should be reserved only for high-risk situations (e.g. planting canola after a legume-based pasture). Some crops, such as lentils and chickpeas, are poor RLEM host plants. In weed-free paddocks, low RLEM numbers can typically be expected the year following these rotations. In autumn, plan insecticide strategies according to paddock risk, based on the RLEM Risk Assessment Guide (Useful Resources).

Avoid pre-emergent insecticides in low risk situations. Monitor susceptible crops closely in the first 3-5 weeks after emergence. If insecticides are warranted, follow guidelines in the Resistance Management Strategy for Redlegged earth mite.

Conclusion

Every season brings different pest management challenges. The key to successful pest management is effectively assessing and managing risk. It is important to avoid chasing this year’s pest issues next year – assess each season on its own merits.

Acknowledgements

I would like to thank GRDC for their continued support, Helen Brodie for help with insect identifications, and agronomists and growers for their observations.

Useful resources

PestFacts South Australia e-newsletter, and Twitter @PestFactsSARDI

Redlegged earth mite, Best Management Practice and Risk Assessment Guide

Redlegged earth mite, Resistance Management Strategy

Green peach aphid, Best Management Practice and Risk Assessment Guide

Diamondback moth, Best Management Practice and Risk Assessment Guide

Contact details

Dr Kym Perry
SARDI, Waite Campus, Urrbrae SA 5064
088429 0738 | 0421 788 357
kym.perry@sa.gov.au