Tropical conditions prompt webworm warning

They’ve weathered a drought and battled through last week’s cyclone…now nature is throwing pestilence at Far North farmers in the form of a pasture-devouring caterpillar.

Recent near-tropical conditions on the Aupouri Peninsula in the Far North have been very favourable for webworm caterpillars, which feed on kikuyu and can cause widespread damage.

Entomologist Jenny Dymock, who monitors tropical grass webworm numbers for the Northland Regional Council, says heavy rainfall in the last two weeks has resulted in sodden pastures which have then baked in humid, hot weather.

“Basically, these sorts of conditions are ideal for the webworm, which is a tropical insect, Dr Dymock says. “Fresh new growth of kikuyu is also providing a feast for webworm larvae which can build-up in numbers very fast.”

Cable Bay-based Dr Dymock has already observed patches of chewed pasture around Motutangi and caterpillars from the first February cycle of webworm are already forming cocoons – a couple of weeks earlier than usual.

“Moths that hatch from these cocoons will lay eggs and, if the current climate of warm/humid weather continues, large numbers of caterpillars will be munching into kikuyu by the end of February and into March.”

She says evidence of webworm feeding can be seen when kikuyu is parted at the base; caterpillars and/or bright green excreta will be present.

However, farmers can help to break the cycle by harrowing pasture where possible.

“This breaks up the mat of dry grass at the base of kikuyu, exposing the caterpillars to drying winds and lethal rays of sunlight. The action of harrowing also causes some collateral damage to caterpillars.”

Dr Dymock says farmers may also need to plan to secure alternative feed for stock if necessary.

Tropical grass webworm is a pest found in Australia, Asia and some Pacific countries and in high enough densities can literally chew out several hectares of pasture in less than 48 hours.

Webworm does not have a dormant stage and has survived in the frost-free winters of the Far North. It’s typically found in sunny, north-facing areas and forest margins on sand country of the Aupouri Peninsula.

Although the adult moth does not feed, each webworm moth can lay well over 100 eggs which hatch into the larval, or caterpillar stage, which does the pasture damage. The webworm prefers kikuyu but will eat other grasses, bamboo and some weed species.

The adult webworm is a drab, fawn-coloured moth, about 2.5cm across, sometimes with dark spots scattered over its delta shaped wings. Caterpillars are 5-10mm long and very active when disturbed. They are translucent and range from pale green to dark brown depending on what they are feeding on.

Further information and booklets on managing tropical grass webworm and other insect pests are available from the Northland Regional Council’s Kaitaia Area Manager Peter Wiessing on (0800) 002 004 or by visiting the Council’s website www.nrc.govt.nz/nasties