What does it look like?
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 the frass (excreta) of the larvae is bright green – this is a distinguishing feature.
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.
Scientific trials soon after the Tropical grass webworm’s arrival in Northland – backed up by more recent trials near Awanui – had indicated that harrowing (which will break up the kikuyu mat) showed promise as a potential control by exposing Tropical grass webworm caterpillars to light and desiccation. More than one pass with the harrows may be necessary.
If you think you have Tropical grass webworm on your property, please contact Dr jenny Dymock, Entomologist at 09 4060 033 or your nearest Northland Regional Council office for advice and recommended controls.
Monitoring updates in Northland
There has been an increase in the area affected by Tropical Grass Worm (TGW) across the Aupōuri Peninsula since last week (6 March 2018). See below monitoring results for affected areas.
UPDATE: 15 March 2018
- Paparore – increased areas damaged. Late instar larvae and pupae with lots of moths sheltering in the rank kikuyu
- Waiharara – start of Turk Valley Road – kikuyu is recovering from last week’s damage and there were 8 larvae +4 pupae /square metre (25 x 0.25m2 quadrats)
- Waiharara – end of Turk Valley Road – damage has appeared at the site. Numbers up from 19 larvae/m2 to 80 larvae + 4P/m2
- Motutangi – population has pupated compared to last week (37 larvae + 38 pupae/m2) with kikuyu damage and 2larvae + 23 pupae/m2
- Pukenui – damage reported
- Ngataki – damage reported
The Tropical grass webwormj populations are currently at various stages of their life cycle. Larvae will move into pupae and then large numbers of moths will emerge over the next few weeks and if the weather is still warm enough, these will lay a vast number of eggs.
UPDATE: 21 March 2018
- Paparore – kikuyu pasture recovering from earlier damage (down to 2 pupae and no larvae/m2)
- Waiharara – kikuyu pasture recovering from earlier damage at Turk Valley Rd (7 larvae and 9pupae;/m2)
- Wiharara - new patches of TGW damage north and south of Wiharara store area and also at the end of Wireless Road
- Motutangi – extended patches of TGW patches at Motutangi north and south around 3320 SH1
- Ngataki – damaged patches still present
UPDATE: 04 April 2018
Area monitored: Aupouri Peninsula as far north as Ngataki
No larvae or pupae of TGW were found at the 3 intensive monitoring sites. Quadrats were sampled at 25 x 0.04m2. However, there were a large number of TGW moths present in rank kikuyu in Ahipara.
Three moths were also seen at the window at night at Cable Bay and one moth at Taipa (Doubtless Bay) on 2 April 2018.
One new damaged area of kikuyu pasture was recorded 10km south of Waiharara. All other previous damaged pasture patches were recovering with new kikuyu regrowth.
With the current cooler night temperatures (below 15C) TGW moths, which are nocturnal, are not laying eggs at present. Last significant rain was on 24 March 2018 with no rain predicted for the next 8 days so pasture on the Aupouri Peninsula is drying out. If cooler, dryer conditions persist then TGW damage will be low but if weather turns warm and wet again then TGW moths may start laying eggs on kikuyu again.