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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.
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.
If you think you have webworm on your property, please contact your nearest Northland Regional Council office for advice and recommended controls.