Monday, 13 August 2007

Red Spider Mite

What is your first thought if a fine pale mottling develops on the upper leaf surface of your Cannas? Of course, the first thought is the dreaded Canna Virus. However, it pays to first think of Red Spider Mite, especially if the affected plant is grown indoors or your Cannas are enjoying a long hot summer outdoors.

Damage caused by these mites can be significant when spider mites occur in high numbers. They are plant sap feeders. The first sign of damage is a speckling on leaves, eventually turning them yellow. Young plants' growth is distorted and older plants loose their leaves and can be killed. When mite levels build up a fine webbing is formed, which is used to transport very high numbers of red spider mites about, either by wind or on animals or humans!

They are usually too small to view with the naked eye, and a magnifying glass becomes useful to identify them as the cause. They are only red for a period in their life cycle, and can be either white or green at other times. When young they have six legs, when mature they have eight legs. It lays its eggs on the leaves, and it poses a threat to host Cannas by sucking cell contents from the leaves cell by cell, leaving very tiny, pale spots or scars where the green epidermal cells have been destroyed. Although the individual lesions are very small, commensurate with the small size of the mites, the frequently-observed attack of hundreds or thousands of spider mites can cause thousands of lesions and thus can significantly reduce the photosynthetic capability of plants, greatly reducing their production of nutrients, sometimes even killing the plants. Although this way of feeding could spread plant viruses, this is considered of secondary importance.

Treatment can be either chemical or by using their natural predators to control and eliminate them. Insecticides containing bifenthrin (Bayer Sprayday Greenfly Killer Plus, Scotts Bug Clear or Doff All-In-One Garden Pest Killer) will control red spider mite, provided it has not gained resistance to this substance. Alternatives are sprays containing plant oils, plant extracts or fatty acids (see above). The latter pesticides may require more frequent applications.

A predatory mite (Phytoseiulus persimilis) feeds on the eggs and active stages of glasshouse red spider mite. It needs good light and daytime temperatures of 21oC (70oF) or more on a regular basis if it is to breed faster than the pest. Its effective period of use is normally April to October in glasshouses or June to September out of doors.

As the predator is susceptible to insecticides, biological control cannot be used in conjunction with most chemical controls. The exceptions are those with very short persistence, such as plant oils or extracts (Vitax Organic 2 in 1 Pest and Disease Control, Growing Success Fruit & Veg Bug Killer) or fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer or Fito Get Off Insect) or mineral lattice (SB Plant Invigorator), which can be used to keep mite numbers down before it is time to introduce the predator.

So, before destroying your plants, why not think about getting out that magnifying glass and go looking under the foliage for the red spider mite?

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