Insect Quiz January, 2005

January Quiz 2005

You were given an English ivy for a house-warming present. You set it in the living room next to a window so it would receive plenty of light. After a couple of months, you notice that the leaves have lost the lush green color they had when you first received it. When you look more closely, you notice there is webbing covering some of the foliage. When you turn the leaves over and use a hand lens you see some very small creatures crawling around.

1. What are these pests?

They are spider mites. These pests are not insects but are more closely related to spiders and ticks. Spider mites are about 1/50th inch long and yellowish or greenish in color. They have eight legs and lack antennae or wings. They are a common pest on many houseplants, including ivy. They particularly thrive in warm, dry conditions.

2. Where did they come from?

There are a couple of possibilities. Spider mites may have already been infesting your ivy when it was first brought into your home but the numbers were low enough that you didn’t notice their damage. It is also possible that other houseplants in your home, especially if they were outside during summer, became infested with spider mites which eventually spread to your ivy. You could have even moved the spider mites yourself as they are easily transported on your hands or clothing from one houseplant to another. If you brought a live evergreen tree or branches (e.g. in a wreath) into your home during the holidays, any mites hibernating on this greenery could become active and move to nearby plants. A minor infestation would be difficult to detect until the spider mites became more numerous and their damage more severe.

3. How injurious are these pests to houseplants?

Spider mites use piercing-sucking mouthparts to feed on the sap of the leaves of houseplants. At every site where they inject their needle-like mouthparts, they cause tiny, discolored spots. As the mites continue to feed, this damage gives plants an off colored appearance. When this injury becomes more severe, leaves can yellow or bronze and even drop. Spider mites create webbing as they feed which becomes conspicuous when they are numerous. Spider mite injury can reduce the vigor of plants and can seriously damage or even kill plants.

4. What is the best control?

If a plant is severely infested, the best control is to throw it away. Better to lose one plant than to risk infesting others. It’s a good idea to thoroughly inspect your plants routinely with a hand lens to detect problems before they become too severe. If you find a mite infestation, use the shower or a kitchen sprayer to knock off the spider mites and their eggs from the plant. Spray the plant periodically over several weeks. You can also treat infested plants with an insecticide, although chemical control is usually challenging. Products containing bifenthrin or a horticultural oil are reasonably effective while insecticidal soap will marginally kill mites.