Organic NZ – September 2019

Advocate • Connect September/October 2019 29

Gardening

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Call Maria on 09 432 2722

or 021 170 4881

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'ĞƚƚŚĞŝŶĨŽƌŵĂƟŽŶzKhE͊

at http://www.denz.co.nz

Pure Diatomaceous Earth

Food Grade Fossil Shell Flour

In recent world-first trials carried out by the BHU, the mesh was

shown to even control potato blight. Later experiments in Israel

confirmed these results.

The mesh is very light and normally laid directly onto crops as

soon as they are sown or planted. This prevents insects from landing

on them in the first place. Once the mesh is in place, cover the edges

with soil or stones to prevent insects from getting underneath.

When growing potatoes, it is important to keep the mesh off

the leaves by making a cloche with it or supporting it in some way.

This is to prevent another pest (the potato aphid – Macrosiphum

euphorbiae) from getting through the mesh when it hatches.

If some crops need pollinators, the mesh can be taken off for a

few days to allow bees or other insects to do their work, or to hand

pollinate. Field trials have shown that tomatoes are still successfully

wind-pollinated, without having to remove the mesh.

If a few tomato-potato psyllids do manage to get into the crop

under this mesh, the level of UV light penetrating it is at a level they

don’t like, so they won’t cause much damage.

You can buy crop mesh from good garden centres or online.

Biological control

Encourage predators and parasitoids into your garden to help

reduce pest populations of all insect pests (see sidebar). They won’t

give 100% control but will help lessen their impact.

Also now released in New Zealand, but not yet available

to home gardeners, is a tiny wasp (Tamarixia triozae) that is a

parasitoid specific to the psyllid – its sole host. The wasp is found

mainly in North America and Mexico, where the tomato-potato

psyllid originated.

In October 2017 these wasps were released in Hawke’s Bay and

Canterbury onto African boxthorn – a TPP overwintering host. It

will hopefully be self-sustaining, begin to spread and reproduce

without any further human intervention.

Insecticides

No insecticides are recommended to control TPP here. According

to the literature, the use of organic sprays to control them has not

proved to be very useful anyway; these include soft soaps, plant and

herbal extracts, light mineral oils, pyrethrum, ryania and neem.

Non-organic insecticides, and those permitted by organic

certifiers, are broad-spectrum, killing not only your pests but also

the good guys in your garden, i.e. the predators and parasites that

help you. So you need to be very careful if using them.

Happy gardening!

Duncan Smith, MSc, is a retired plant pathologist and research

agronomist. He and his wife Judy live in Whakatane.

Above: Psyllid nymphs (the roundish ones) and newly emerged adults

(with wings) on a tomato leaf. Note also the white psyllid sugars dotted

about, and a few tiny yellow eggs. Photos: Duncan Smith

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P lants t o attract t he good bugs

Above: Symptoms of psyllid yellows on a tomato leaf. If you look

closely you may be able to see the orangey-yellow psyllid eggs on the

leaf edges.