Tiny wasps could control boxtree moth

Boxtree moth caterpillar green and black
Boxtree moth caterpillar - pyrale de buis - leaves box hedges a mass of web threads and they can hang down from trees in woods

Millions released in tests to find way to protect one of the most common garden and woodland plants in France

Millions of tiny parasitic wasps have been released in eastern France as part of large-scale tests to find ways of getting rid of boxtree moths that have stripped leaves on woodland and hedges across the country.

The pyrale de buis moth (Cydalima perspectalis) and its very hungry green and black caterpillar have spread rapidly after the invasive Asian species first appeared in France in 2008.

Dozens of Connexion readers on Facebook have told us of the damage the caterpillars have caused: stripping hedges and coming back after repeated treatments with organic products and insecticides.

Boxtree hedge attacked by pyrale de buis caterpillars
Boxtree hedge attacked by pyrale de buis caterpillars

Now the communes of Dieulefit and Marsanne in Drôme, Rhône-Alpes, have released millions of the wasps in test areas after studies showed that the tiny parasitic wasps – they are only about a millimetre long – can kill the box moth eggs.

Each moth can lay from 800 to 1,200 eggs and there are four laying periods over the year meaning that the insect can spread enormously very quickly.

Trichograma wasp attacking moth egg
Trichograma wasp attacking moth egg

The tests come after researchers at the Institut National de la Recherche Agronomique (INRA) carried out studies on various controls as part of the SaveBuxus programme and identified six varieties of the trichogramme wasp that killed most of the eggs in samples. Once tests are complete work will start to produce commercial products.

With about two million hectares of box woodland and hedges plus topiary in thousands of ornamental gardens, box is a major part of France’s natural heritage and the researchers are due to give results later this year on the best ways to beat the box moth and its caterpillars.

Pheromone traps and the organic product bacillus thuringirensis, which paralyses the caterpillar, are known to work well but need regular work.

Traps need to be cleaned and refilled regularly to avoid putrefaction and the bacillus only works for about 7-10 days – but also because the moth’s short life cycle means several infestations over the middle part of the year.

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INRA has already noted that eggs can be very difficult to attack with chemical or organic treatments as they may be protected by web ‘cocoons’ that can stick leaves together. Spray treatments need to be ‘very forceful’ to reach them.

Other chemical products can also be used but may harm beneficial insects including bees.

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