Huatokitoki Deep Burrowing Worms

April 2012

The importance of worms in a pastoral farming system

Deep burrowing worms are uncommon in New Zealand but very beneficial and a new trial has explored the first commercial introduction in Central Hawke’s Bay. In other work, three high school students carried out a Royal Society project to find if deep burrowing worms could be introduced to the Central Plateau area.

Earthworms have an important role in New Zealand pastoral systems, but all the earthworms in these systems are exotic. Although there are 170 native species of earthworms, these developed in forest ecosystems. All the introduced earthworms were unintentionally introduced.

Across the country there are only 17 exotic species of earthworms, and their distribution is patchy.

Earthworms are split into three functional groups: the epigenic which live in the topsoil and litter layer on the soil surface where they feed on dung and plant litter; the endogenic which build complex lateral burrows through all the upper layers of the soil surface to about 20cm deep, and deep burrowing worms or anecic earthworms. These can go down to 40cm or so, and they build deep permanent vertical burrows from an opening on the soil surface down through the soil profile. They are generally large worms that feed on decaying litter on the soil surface.

Evidence is mounting of the benefits of introducing deep burrowing earthworms to New Zealand soils. Earthworms are important because they increase pasture production, increase water infiltration and increase resilience around the margins of the season.

But there is a lack of deep burrowing worms in some areas of New Zealand, which was reinforced by surveys on the West Coast and on the Central Plateau in 2009. In the Central North Island only 14% of paddocks sampled contained species from all three earthworm functional groups.

The three students who have been looking at deep burrowing earthworms first worked on a Silver Crest project for the Royal Society, surveying worm populations on the West Coast, and then in the last year on a Gold Crest project looking at worms in the Central North Island. The three were all students at Palmerston North Girls High School, and all three are heading into medicine this year. These Royal Society projects are a way to encourage more young people into science.

Deep burrowing earthworms (Aporrectodea longa) were released on this Porangahau farm in 1981 by a MAF soil biologist Jo Springett. This farm only had two of the three functional groups of worms before this release. A similar release was carried out in Taihape too.

In winter last year good numbers of the worm were found close to the release point and 130 m up the hill from the release. The worms were active 13-18cm deep in the soil. They had spread 5-10 metres a year over the 30 year period.

Work back in the early 1980’s found a 10-20% increase in pasture production where deep burrowing worms were present. This has been the only piece of work looking at the impact of worms on pasture production, and it showed improving water infiltration of the soil and improved aeration.

As there have been no commercial attempts to introduce A. longa, the scientists wanted to carry out a trial to see how well they could be introduced.

The scientists wanted to see if they could use pasture turfs to introduce the species on a 9ha paddock on the farm which had been recently drained. They designed a trial with four different treatments to introduce the deep burrowing worms last August. After a year and then again in two years they will check to see how the worm populations are going.

They had to dig out 150 turfs and replace them with the new turfs with about 1000 worms going into the paddock. This has added more “bioengineering capability” to the soil, and will fundamentally change the character of the soil.

They found the worm Amynthas diffringens, a worm from Asia, which behaves like a tiny snake, during their West Coast survey. They felt if this species could be introduced to the Central Plateau, where there are no deep burrowing worms, then there would be benefits of better organic matter cycling, better plant rooting, improved soil aggregate stability and structure.

First they carried out a pot trial in a greenhouse to see if they could survive in pumice soil, and to find out what they preferred to eat. They coped just as well with these harsh conditions compared to living in a silt loam soil.

The worms also preferred dung and pasture litter rather than tree litter. The students looked at whether the worms could live with other worm species, finding they co-existed well with two other species, Aporrectodea caliginosa, a surface living worm, and A. longa.

This information is a world first.

In September 2010 the students successfully introduced the worms at three sites in the Central Plateau, but A. diffringens wasn’t found. This could be due to the driest spring for many years, with rainfall half the normal level. Pumice soils dry out fast and they are poorly developed.

They also found A. longa was more active than A. caliginosa at the three sites, which was unexpected. They did better together than apart.

Their message to farmers is that where no earthworms are present, A. calignosa, should be introduced as well as A. longa, because together there will be a better success rate.