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Getting it right from the ground up

Media Release 19 October 2001

Getting it right from the ground up

Growers of organic apples have to be very careful how they deal with the weeds under their trees, to keep their organic status. This basic fact leads to a problem: groundcovers tend to contain plants such as dock, which plays host to leafrollers. These pests, unfortunately, also attack apple trees.

HortResearch scientists got to work to find some answers. A team led by Jim Walker surveyed leafroller populations in the groundcover of a number of Hawke’s Bay organic apple orchards to find out why some had much worse levels of leafroller damage. They found higher levels of leafrollers in orchards with weedy ground cover rather than mainly grasses. Some broad leaf weed species such as dock are very significant hosts of leafrollers, so the suggestion is that growers try to see that grasses, or other non- host plants, form most of what is technically called ‘the understory vegetation’.

Many orchardists would like to establish flowering plants within the understory to attract parasitoids of the leafrollers. This sits comfortably with ideas of biological control. But the research also found that some leafroller parasitoid species behave quite differently from others, either attacking larvae on weeds or leafrollers on apple trees, but usually not both.

For the home gardener with five apple trees an answer may be to go traditional and keep a couple of hens, letting them roam for an hour a day to eat anything that moves but keeping them shut in at harvest time so they don’t eat the crop as well. For orchardists with seriously large acreages this becomes more difficult, and as more New Zealand growers try to go cleaner and greener, other options have to be found for dealing with the problem.

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Future work in this area may provide answers to parts of this problem, but meanwhile introducing flowering plants in organic orchards may be of little or no benefit. It’s even possible it would result in greater leafroller damage. Spot treatment with flame weeders is an option, and steam weeders are beginning to be used. But with research so strongly suggesting a need to eliminate host weeds, another traditional remedy - get out and grub it out – may be back in favour in New Zealand’s organic orchards.

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For further information please contact Dr Jim Walker, HortResearch, Hawke’s Bay Private Bag 1401 Havelock North, New Zealand JWalker@hortresearch.co.nz Tel +64-6-877 8196 ext 576

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