Tropical grass webworm
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Tropical grass webworm.
Common name:
Tropical grass webworm
Scientific name:
Herpetogramma licarsisalis
Description
Webworm caterpillars are most numerous and hungry over late summer and autumn. If you have what appear to be hard grazed patches in an otherwise lush kikiuyu-dominant pasture, you could have tropical grass webworm.
The caterpillars vary from 5mm when first hatched to 20mm long when fully developed, and are very active when disturbed. They are translucent, and range from pale green to dark brown depending on what they are feeding on. Tan to dark brown pupae may also be visible.
They can completely strip grass of green tissue, often leaving only peeled white fibrous stalks. By day, the caterpillars shelter in silt tunnels which they spin in the grass. This silk webbing and abundant green fecal pellets are clear signs of webworm.
The adult webworm is a drab fawn coloured moth, sometimes with dark spots scattered over its 10-15mm wide delta shaped wings. A generation, from egg to adult takes about a month in summer.
The female lays around 250 eggs which hatch into the greenish ‘worms’ or caterpillars which give the pest its name. The caterpillars live in pasture, feeding mainly at night or on cloudy days.
It prefers kikuyu, but eats other grasses including paspalum, couch, oats, fescue, barley, perennial ryegrass, rye, wheat and bamboo as well as some weed species.
The webworm probably arrived in New Zealand naturally as a moth travelling on the trans-Tasman jet streams.
Other pasture pests
Tropical grass webworms are often pale in colour and feed at night. If you have a large number of dark brown to black caterpillars feeding on pasture during daylight hours, you probably have army caterpillar. If in doubt, take a specimen to a Northland Regional Council office for positive identification.
Where is it found?
Tropical grass webworm damage was first seen near Houhora in early March 1999. It is now found as far north as Te Paki and as far south as Taupo Bay.
The tropical grass webworm is found in tropical parts of Australia, Asia and some Pacific countries. It is a serious pest in Hawaii where natural control methods have been effectively introduced.
Why is it a problem?
Tropical grass webworm damage was first seen near Houhora in early March 1999. Apparently hard grazed patches quickly spread out, and during warm summer and autumn nights five hectare paddocks were completely chewed out in 48 hours or even less. The larval, or caterpillar stage does the pasture damage. The adult or moth stage does not feed.
Given the rate at which it spread across the Aupōuri Peninsula, the severity of the pasture damage, and the proportion of Northland with similar land form, soil, climate and pasture composition, the tropical grass webworm poses a serious threat to a large area of Northland.
The webworm has been previously recorded in New Zealand, but has failed to establish. Because it does not have a dormant stage in the winter, it must breed continuously throughout the year. All life stages are killed at temperatures below 50C. It is likely that it will survive through the frost-free winters of the Aupōuri Peninsula.
What can I do?
To check for webworm, scratch around in the base of the plants, just above ground level at the edges of the damaged patches of pasture. The caterpillars vary from 5mm when first hatched to 20mm long when fully developed, and are very active when disturbed. They are translucent, and range from pale green to dark brown depending on what they are feeding on. Tan to dark brown pupae may also be visible.
You can check for webworm by laying a sack on the ground at the edge of the suspect area. After a few days, lift up the sack. If webworms are present you will find pupae attached in webbing to the sack and live caterpillars on the grass under the sack
Recommended control method
If you think you have webworm on your property, please contact your nearest Northland Regional Council office for advice and recommended controls.
More information
For further information or control advice please contact Peter Wiessing at the Northland Regional Council Kaitaia Office on 09 408 6600.