Insect Problems

Spider Mites

Spider Mites affecting dahlias are typically the "two spotted mite" or the "red spider mite." One indication of the presence of spider mites is seeing them.1/50th of an inch in length, these pests are barely visible as a dot on a contrasting background. Hold a piece of white paper under the leaf and shake or scrape the underside of the leaf to cause the mites to fall on to the paper. If they are present they will appear as very small dots on the paper. If you do not get those small dots, it is likely that your plant does not have spider mites. The drawing of a red spider mite on the right is several hundred times the actual size.

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Another indicator of the presence of spider mites on dahlia plants is the stippling and bronzing of the top side of the leaves. Using their needle like structures called stylets, these pests rupture leaf cells, pushes their mouth into the torn tissue, and draws up cell sap. As patches of cells are damaged, a stippling of the upper surface of leaves appears. In the picture on the right, a brownish haze shows on the leaf in the near foreground. That is the stippling effect and as the infestation grows, the leaf will become even more brown--sometimes described as a "bronzing" effect.

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A third indication of spider mites on dahlias is the presence of fine webbings--spider webs made by very small spiders. These webs collect dust and dirt making the webbing more visible. Infested plants progressively lose their color, drop their leaves and may even die if the infestations are not removed.

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Control

Spider mites are more vigorous during hot and dry periods of the year and they don't like moisture on the foliage. A strong stream of water will dislodge spider mites. However, care must be taken not to damage the plants. I have not found this control approach very practical except to remove one infestation from one plant.

Most insecticides are not effective on mites and some result in increased mite damage by killing their natural enemies.

Three insecticides known to be effective against spider mites are Kelthane, Avid, and Talstar--displayed in the picture on the right. Be sure to follow the instructions on the label.

Picture of Kelthane, Avid, and Talstar.

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