Chimneys in your soil?

Pasture tunnel moths might be on the way if there are chimneys in your soil

Cropping
Pasture tunnel moth larvae are very slender. Image: cesar

Pasture tunnel moth larvae are very slender. Image: cesar

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If you see little 'chimneys' appearing in your soil during these colder winter months, you might unknowingly be playing host to pasture tunnel moths.

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If you see little 'chimneys' appearing in your soil during these colder winter months, you might unknowingly be playing host to pasture tunnel moths.

Pasture tunnel moths (Philobota spp.) are native to Australia and are sporadic pests when their larvae (caterpillars) feed on certain pastures and crops, particularly in high rainfall zones.

Damage is often only noticeable from early winter, with the worst damage occurring between July and September.

Pasture tunnel moth larvae are night-time feeders, emerging from vertical silk-lined tunnels they construct in the soil that can extend up to 75 mm deep.

While the larvae prefer to feed on annual and perennial grasses, and sometimes clovers, they can cause damage to certain broadacre crops including wheat and barley, especially after a pasture rotation.

They tend to tear at the leaves of plants and drag them back down into their tunnels.

Their night-time feasts are often evidenced the next morning by the chewed-off leaves that remain strewn across the ground.

A number of other insect pests are known to make soil tunnels, including the blackheaded pasture cockchafer (Acrossidius tasmaniae), alongside whom the larvae of pasture tunnel moth are often found.

Winter corbie (Oncopera rufobrunnea), underground grass grub (Oncopera fasiciculata), Oxycanus grass grub (Oxycanus antipoda), and pasture webworms (Hednota spp.), also construct and live in vertical soil tunnels.

However, pasture tunnel moth soil tunnels are unique due to their protrusion from the soil surface to about 10 - 20 mm in height.

These tunnel protrusions often have the appearance of tiny chimneys with collars of silk and grass fragments.

Feeding by pasture tunnel moth larvae sometimes results in bare patches occurring within pastures, or occasionally establishing cereal crops, when the pest occurs in high numbers.

However, due to the small and sporadic spread of the pest, damage is often limited and infestations greater than a hectare are considered rare.

Compared to other moth larvae, the pasture tunnel moth larvae are unusually slender in appearance (generally 2-3 mm wide) and grow up to about 35 mm in length with bodies a grey-brown colour.

The dull and almost translucent surface of their bodies allow them to camouflage well within the soil.

Their body sections have several rounded, dark and shiny lumps with a long hair protruding from each.

The heads of the larvae are black and shiny in contrast to their bodies and covered in stout hairs.

There are several species of pasture tunnel moth, many of which have not been subject to extensive research.

The most well-known is Philobota productella, whose adult moth is white to cream in colour and approximately 20 mm long.

Its forewings are sometimes marked by longitudinal dark streaks and span around 25 mm.

The transformation from larva to moth occurs in the burrows with adult moths emerging in late spring or early summer (generally November through to January).

Timing of pupation tends to be influenced by rainfall with heavy rains followed by the emergence of high moth numbers.

During these summer months the adult moths are found among the grasses throughout the day, moving to tall objects such as tree trunks or fence posts to mate as darkness falls.

The eggs are laid towards the end of summer and early autumn, deposited in small crevices such as cracks in wood.

The larvae of pasture tunnel moth can sometimes be mistaken for pasture webworms, which also sporadically feed on grass pastures and cereals during winter.

However, there are some key differences to look out for when distinguishing between the species; pasture webworm larvae are not as slender as pasture tunnel moth, and the former also leaves behind webbing on foliage.

Growers that find pasture tunnel moth specimens in their pastures and cereal crops should remember that damage is rarely widespread and often only occurring under high densities of the pest.

For more information on pasture tunnel moths visit PestNotes Southern: http://cesaraustralia.com/sustainable-agriculture/pestnotes/insect/Pasture-tunnel-moth

About the author: Francesca Noakes is a research and extension scientist with cesar.

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