Friday, August 6, 2010

Plant pest Mites

Witches' Broom on Hornbeamdistorted growth caused by mites
Plant Mites are arachnids and thus related to spiders, ticks and scorpions. Most gardeners are familiar with the damage some of them can cause to foliage though they are barely visible to the naked eye and have often departed before their 'work' becomes noticeable. At the moment, I have a batch of the shrub Brugmansia ,the new growth of which displays stiff downward cupping on the top leaves and twisted older leaves. This is caused by the Broad mite (Polyphagotarsonemus latus) which is most active during warm humid summer/ autumn weather.The damage to the "Brug" was probably done months ago when the new leaves were quite tiny. The other mite of concern to gardeners is the 2-spotted mite (Tetranychus urticae). The warm dry weather of early spring suits this pest best. The first sign of an infestation is a speckling or mottling visible on the upper surface of the leaves. This is caused by the mites feeding on the underside of the leaves, where silver and rusty marking can be seen as well as some fine webbing. This mite has also been given the name of red spider mite as the adult females develop a red orange colour during winter. Plants which are stressed due to lack of water or low nutrient levels in the soil or pot are often more susceptible to infestation from this mite. So how to deal with them?...... I use the excellent Neem oil or a wettable Sulphur spray or dry Sulphur powder. You can also use biological control by releasing predatory mites (Phytoseiulus persimilis, Amblyseius womersleyi and Typphlodromus occidental) or ladybirds (Stethorus sp). These are readily available from specialist suppliers.
It is important also to remove mite host plants especially low growing herbaceous weeds. Mites are distributed around by wind, birds or man and can travel on clothing, garden tools and farm machinery. Keeping your plants healthy and having a bio-diverse garden with a range of plants to attract beneficial insects goes a long way to keeping the pesky little mites from doing much damage.
Two Spotted Mite 0.5mm

The red-legged earth mite

Broad mite egg 0.1mm

Brugmansia showing Broad mite damage

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