Five considerations when finding cutworm in crops

Have you noticed chewing damage in your young canola and cereals?

We’ve received reports of cutworm damaging crops in southern NSW this year.

While finding cutworm or suspected cutworm damage in young crops can be a concern, there are several points to consider before reaching for insecticides.

1. Multiple species, one endangered

The term cutworm refers to the caterpillars of several species of night-flying moths, which have the habit of feeding at ground level and chewing through, or cutting off the stems of young plants.

In south-eastern Australia, cutworm often refers to the Agrotis genus, particularly three species, some of which have more than one common name.

One species Agrotis ipsilon, the black cutworm, has an uncertain origin and is regarded as a cosmopolitan agricultural pest in several countries. Another species is the native Agrotis munda, known as the pink cutworm or brown cutworm.

A third species, Agrotis infusa, is also a native and is known as the common cutworm or the Bogong moth. The Bogong moth are known for their mass annual migration to the Australian Alps where they provide a vital source of nutrients in the alpine food web. In recent years, there has been a dramatic decline in Bogong moth numbers undertaking their annual migration, leading to their listing as endangered under the IUCN Red List of Threatened Species.

While these three species are unique in appearance as adult moths, telling the caterpillars apart in the field is not possible. As caterpillars, all three species grow to around 40-50 mm long and have a dull, hairless, plump, and greasy appearance. Black cutworm and Bogong moth caterpillars are usually darker in colour than the pink/brown cutworm, however this feature alone is normally not enough to distinguish them from one another or from other Agrotis species.

The Bogong moth (*Agrotis infusa*), also known as the common cutworm. Photo by CSIRO entomology, CC BY 3.0

2. Not to be confused with armyworm

At this time of year, cutworm can also be confused with native species of armyworm such as the common armyworm (Mythimna convecta) or the southern armyworm (Persectania ewingii).

Cutworm and armyworm caterpillars are predominately night-feeding insects. During the day they take shelter at the base of plants and are found curled up on or just below the soil or stubble surface.

There are two features that are useful in distinguishing cutworm from armyworm.

Firstly, armyworm caterpillars have three white stripes on a ‘collar’ just behind their head. These three stripes often continue all the way down the length of their body. Cutworm lack these three stripes.

Secondly, armyworm species that occur during winter and autumn preferentially feed on grasses and cereals, whereas cutworm can feed on all field crops and pastures. If you are finding caterpillars eating broadleaf plants such as canola at this time of year in south-eastern Australia, it could be cutworm or another pest species, but not armyworm.

3. Feeding symptoms like other establishment pests

Cutworm caterpillars have chewing mouthparts and sometimes their feeding symptoms can look like those of other chewing pests such as the European earwig and even slugs.

If chewing damage is evident and you suspect cutworm, search the base of the plant. Cutworm caterpillars are usually found curled up under soil clods during the day.

4. Mostly an establishment pest

As cutworm is mostly an establishment pest of young seedlings, once the crop is advancing, the risk of damage to the crop lessens.

An exception to this is that cutworm can occasionally cause damage at tillering and early stem elongation in winter cereals.

5. Spot spraying

Given that the Bogong moth is an endangered native insect and plays an important role in the Australian ecosystem and can’t be readily distinguished from other Agrotis sp. in the field, spraying should be avoided if possible.

Cutworm damage is usually patchy in distribution. As cutworm are known to move off weeds, monitoring efforts should be focused there.

If chemical intervention is necessary, spot spray or perimeter spray according to thresholds.