Horticultural oil controls spotted spider mites

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Q. I have a parlor palm that I have had for about 10 years. It has always been healthy and grown very well. I recently noticed that the leaves do not look as green and healthy as they always have. They have a bleached-out appearance. Do you think it is getting too much sun? Is there anything I can do to help it recover its rich, dark green color?

A. Two-spotted spider mites (Tetranychus urticae) are one of the most common problems we see on palms. The symptoms you describe are consistent with their damage. Although parlor palms (Chamaedorea elegans) are one of the easiest, least problematic houseplants to grow, they are as susceptible to spider mites as any other palm.

Two-spotted spider mites are just that, tiny little spiders that cannot be seen clearly with the naked eye. Take a sheet of white paper and tap the affected fronds sharply over it, then observe what lands on it for a few minutes. There will be some dirt from the plant, but if two-spotted spider mites are present, you will see small creatures resembling specks of pepper moving around on the paper. You can see them more clearly with a magnifying glass or hand lens. With enough magnification, you may even be able to see the clear to translucent eggs.

Although their color can vary through the year, actively feeding two-spotted spider mites are usually greenish-yellow, with a dark spot on either side of the abdomen. Those spots are actually an accumulation of bodily wastes and may not be visible on immature life stages. They are generally found on the undersides of affected fronds. You might even be able to see fine webs covering severely infested areas of the palm.

Their development is accelerated by a warm, dry environment. Under optimum conditions, the time from egg to adult can take as little as five days. Adult females can live for two to four weeks and may lay several hundred eggs. There are many overlapping generations through the year, especially under indoor conditions.

Two-spotted spider mites are common pests of outdoor trees, shrubs and flowering plants during hot, dry summer weather. Indoor heating systems create warm, dry conditions through the winter that can create the same conditions for our houseplants, especially those that may acquire these pests from spending summer months outdoors.

Two-spotted spider mites feed with piercing-sucking mouthparts, which means that they insert their mouthparts into the frond much like a very small hypodermic needle. As they feed, they literally suck the chlorophyll out of the leaf, which is why the fronds have such a bleached-out appearance.

Because two-spotted spider mites are arachnids, not true insects, regular insecticides do not control them very well and often create an outbreak by killing natural predators (outdoors) and perhaps making host plants more nutritious for these pests, depending on the insecticide applied to them.

Their rapid reproduction rate can make two-spotted spider mites difficult to control -- Large populations can build up before you realize it. Repeated applications of ultra-fine horticultural oil will get them under control. Although insecticidal soap is also labeled to control spider mites, many palms are so sensitive to the soap that the damage from the soap application is worse than the damage from the mites.

The bleached-out appearance of damaged fronds will not go away until those fronds are replaced with new ones. You can prune out the fronds with the worst damage, or those that show webbing, to reduce the overall mite population before you start making horticultural oil applications.

Be sure to purchase ultra-fine horticultural oil or summer-weight horticultural oil rather than dormant oil. Horticultural oil is lighter and more refined than dormant oil, and less likely to burn tender plant tissues.


Send questions to Sandy Feather by email at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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