Pestfacts Issue 2, 2020
In this issue
On the wind: weed web moth
Large numbers of small brown moths have appeared suddenly overnight in several regions; what are they, and how much of a risk will they be?
Very high numbers of adult weed web moths have been reported in Bordertown and Naracoorte. SARDI Entomologists have also observed them at Waite and Maslin Beach. PestFcts south-eastern recently reported high numbers of weed web moth caterpillars in broadleaf plants including canola, lucerne and broadleaf weeds. Current reports from South Australia only indicate large swarms of moths. We do not know where these have come from but weed web moths can develop on native vegetation like saltbush.
While detecting moths provides a useful warning, there is a poor relationship between moths caught and number of caterpillars, so it should always be considered a prompt to monitor.
The biology of the weed web moth is poorly understood, but it is thought to be a long-distance migrator, suddenly appearing in large numbers as seen here. This abundance is thought to occur with seasons with early autumn rainfall and warm weather. The moths already seem to have disappeared in some areas, so it is unlikely that they will be a risk for the crops to be sown soon. Broadleaf weeds and volunteers may host these caterpillars, so if you’re worried about possible weed web moth, you can monitor for them.
Identifying weed web moth
Weed web moth caterpillars are up to 15mm long. They generally found between leaves of the plant they are feeding on, held together with some webbing, skeletonising foliage. They 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.
When disturbed, weed web moth caterpillars wriggle violently. They can also be very active and crawl rapidly. Weed web moth caterpillars are known to feed on canola, soybeans, lupins, lucerne, and a wide variety of broad-leafed plants.
As adults, weed web moth have a 20 mm wingspan and are buff coloured with darker brown or reddish flecks, mainly on the forewings.
For more information on weed web moth management see cesar Australia’s recent article.
Source of reports: Adam Hancock (Elders Naracoorte), Ted Langley.