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High numbers of web-spinning moth larvae are feeding on broadleaf plants

By Julia Severi, Extension Scientist
Last updated: March 31st 2020


Reports to PestFacts south-eastern indicate that very high numbers of web-spinning moth larvae are currently present in some paddocks in southern and central New South Wales. Based on their behaviour and general appearance, the weed web moth, Achyra affinitalis, is the prime suspect. 

Larvae have been seen feeding on establishing canola, lucerne and broadleaf weeds in the Riverina, South West Slopes, and Central West Slopes and Plains.

The weed web moth is a native moth species and like many Australian native insects, has not been subject to much research, locally or internationally.

Nevertheless, here we outline the best available information on the weed web moth:


Key identification features

Weed web moth larvae are slender, hairy, and can vary in colour between grey-green, dark-green or pale brown. They have a black head and older larvae have a dark stripe down the middle of their back with three rows of small dark spots on each side.

Weed web moth larvae generally don’t grow to be very large. Past records document weed web moth larvae as growing to 15 mm. However, this season, several larvae have been measured up to 20 mm, and even 35 mm long in some instances.

The weed web moth spins a web on plant foliage that can bind leaves together. This webbing is often noticed before the larvae themselves.

Weed web moth larva. Image: cesar

Weed web moth were thought to grow to 15 mm, however larvae have been growing larger than 20 mm this season. Image: Greg Parker


Weed web moth larvae wriggle violently when disturbed. Larvae can also be very active and crawl rapidly.

As adults, weed web moth have a 20 mm wingspan and are buff coloured with darker brown or reddish flecks, mainly on the forewings.


Cotton Web Spinner, Achyra affinitalis

 Weed web moth adult. Image: Victor Fazio, Flickr (CC BY-NC-SA 2.0)

Biology and population dynamics

The biology of the weed web moth is not well understood.

Reports of larvae to PestFacts south-eastern have been made over autumn and winter over previous years.

The weed web moth is thought to be abundant and problematic in seasons with early autumn rainfall and warm weather. The welcome above average rainfall throughout much of New South Wales and southern Queensland in February (and subsequent growth of vegetation), is like to be a contributing factor to the outbreak this season.

The weed web moth is also thought to be a long-distance migrator. It can suddenly appear in very large numbers and overwhelm establishing plants, as observed by several agronomists recently.


Host range

Weed web moth larvae is known to feed on lucerne, canola, medic, soybean, cotton and lupins. They have been seen moving off pasture paddocks, particularly those with a broadleaf weed component.

The larvae seem to enjoy eating a wide range of broadleaf weeds. Reported weed hosts include mintweed (Salvia reflexa), khaki weed (Alternanthera pungens), Bathurst Burr (Xanthium spinosum), roly-poly (Salsola tragus), boxthorn (Lycium ferocissimum), and goosefoot (Chenopodium sp.)


Feeding symptoms

In vegetative crops, weed web moth larvae shred foliage, creating a skeletonised appearance. Feeding can be severe enough that leaves are defoliated and the plant dies.

Weed web moth larvae defoliating young canola. Image: Andrew Daley


A lucerne stand recently damaged by weed web moth larvae. Image: Tim Condon



Weed web moth has long been considered a sporadic and minor pest; the numbers and incidences recently reported by agronomists is unusual.

Nevertheless, chemical control can be warranted when numbers are high.

There are registered chemistries available. Keep in mind that that weed web moth has another common name, cotton webspinner and may listed under ‘webspinner caterpillar’ on labels. Weed web moth larvae are reportedly stubborn to kill at times; do not cut label rates.

If spraying weed web moth this season, we encourage spot or border spraying where possible to help preserve beneficial, natural enemies in paddocks.

As an insect without much research behind it, we know very little about what biological controls help supress weed web moth. Nevertheless, it likely does have natural enemies.

For example, ants have been spotted attacking the larvae as shown below (Video: Andrew Daley):


A recent study on the diet composition of seven species of insectivorous bats in northern NSW showed that weed web moth was present in 76% of tested faecal samples, demonstrating that some vertebrates can too be considered as predators in integrated pest management.


For crops that will be sown in April and May, control broadleaf weeds and crop volunteers in and around paddocks prior to sowing. It is important that there is a break of at least 2 weeks before sowing, in which there is no green material remaining to harbour pests. In practise, this means controlling weeds 4 – 6 weeks before sowing. This will help to prevent weed web moth larvae moving off weeds onto vulnerable seedlings.



Thanks to the following people for providing field observations: Chris Turner (Nutrien Ag Solutions), Phil Bowden (Bowden Rural Services), Greg Parker (AGnVET Services), the Grassroots Agronomy team, Barry Haskins (Ag Grow Agronomy and Research), Tim Condon (Delta Agribusiness), Andrew Daley (Premier Advisory), and Bob Ronald (Nutrien Ag Solutions).

Thanks to Jessica Lye (cesar) for edits.

Header image: Greg Parker

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