sustainability through science & innovation

Cutworms

Cutworms continue to cause widespread damage across most cropping regions of southern NSW and Victoria, but why are they so widespread?

 

Where have they been reported?

Further to issues reported in PestFacts No. 3, we have continued to receive increasing numbers of cutworm reports, particularly from the Victorian Wimmera. For example, numerous paddocks (vetch, wheat, barley and canola) around Donald have experienced severe damage; large areas with seedlings completely removed. These infestations included various sizes of caterpillars from small (10-20 mm) to larger grubs (30-45 mm). Most paddocks have required chemical control, often with alpha-cypermethrin or chlorpyrifos. The level of control has been satisfactory, although there were initial concerns about the speed of kill, particularly of the larger larvae. Caterpillars have been found alive during the day several days after spraying, but had apparently stopped feeding. It is possible these caterpillars received an initial sub-lethal (anti-feeding) dose because they were hidden under the soil surface at the time of chemical application.

Also in the Wimmera, at Gooroc, cutworms have severely damaged a canola crop and moved into a neighbouring faba bean crop, damaging the edge before they were controlled. Growers were also concerned about cutworm control in cereal crops near St Arnaud with 10-20% of larvae surviving alpha-cypermethrin. At Warracknabeal, cutworms have eaten out large patches in a number of wheat crops (2-4 leaf), particularly crops that had been sown into canola stubble. In the worst patches, up to 10 larvae per square metre were found. Some areas may need to be re-sown. In barley, west of Birchip, cutworms are also suspected of causing crop losses.

Near Elmore in Victoria’s Northern Country, large populations of cutworms have been responsible for variable levels of damage in barley, lupins, vetch, canola and oat crops. Most required control. In Henty in southern NSW, farmers and agronomists have reported to SARDI researcher Michael Nash that caterpillars were taking out whole areas of wheat crops, and then marching into others. Although not confirmed, it is likely that these were cutworm caterpillars.

Can we explain this cutworm outbreak?

The vastness of this outbreak of cutworms is startling, but perhaps not surprising. The species is almost certainly Agrotis infusa, the common cutworm, or the larvae of the Bogong moth. This moth is regarded as one of Australia’s most iconic insect migrants – it is known to breed in autumn/winter in Australia’s eastern ‘grasslands’, and in spring, moths emerge and migrate to the Australian alpine region where they seek refuge from the summer heat. It is believed that the moth then returns (westwards) to ‘the grasslands’ to restart the cycle. Read more about Bogong moths here. It is however, probable that while still migrating, many moths find safe and cool refuges over summer in closer habitats across their grassland range.

The moths can lay eggs from early summer, but generally wait until conditions are more favourable in autumn. Each female can lay hundreds or thousands of eggs. They almost certainly choose to lay these following autumn rains, probably on broad-leafed plants and weeds. The survival of these young caterpillars (1-2 mm long) can be highly variable as they are often exposed to predators (beetles and spiders), hot dry conditions, and their host plants often die out through moisture stress or herbicide applications.

So, what has happened in 2014? We believe this year’s widespread outbreak has been the result of a combination of conditions triggered by an early break: (i) a larger number of moths than usual laying eggs following the unseasonably early rains, and (ii) conditions (timing of egg-lay, mild weather and food availability) that have favoured greater than average survival of young larvae. After all, larval survival only needs to increase from 0.1-0.5% (probably typical) to 10% to get a 200% increase in full sized cutworms seeking food from crops and pastures in May and June. In most regions, the conditions this year have provided extensive green hosts much earlier in the season than normally experienced. We predict that, in late spring 2014, many towns and cities of south-eastern Australia will be ‘invaded’ by countless numbers of migrating, light-seeking moths for several weeks. Wait for it!

Our advice

Crop damage by cutworms can often present as mouse damage - seedlings completely cut off and consumed. When looking for cutworms, look under clods or within the top few millimetres of soil in the drill row where they often hide during the day. Searching at night is recommended. Where necessary, we strongly recommend spraying for cutworms at dusk. Cutworms are most active just after dusk and will therefore get maximum exposure before the chemical residual declines. This is particularly important when using alpha-cypermethrin, which has little residual activity. For further information on managing cutworms, see PestFacts Issue No. 3.

 

* Sources of field reports of cutworms

Matt Bissett - Agronomist, AgriVision Consultants (Victorian Mallee)

Jim Cronin – Agronomist, Landmark (NSW Central Western Slopes & Plains)

Chris Dunn – Agronomist, Landmark (Victorian Northern Country)

Heidi Gooden – Agronomist, Delta Ag (NSW Riverina)

Frank Henry – Grains Pathology Team, Vic DEPI (Victorian Wimmera)

Rob Launder – Consultant, AgriTech Rural (Victorian Wimmera)

Stuart McColl – Research Trials Manager, BCG (Victorian Wimmera)

Rik Maatman – Agronomist, Landmark (Victorian Wimmera)

Michael Nash – Researcher, SARDI, Adelaide

Mary Raynes – Pulse Australia (Victorian Wimmera)

Glen Smith – Consultant, Riverina Co-op (NSW Riverina)

Greg Toomey – Agronomist, Landmark (Victorian Northern Country)

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