Question
New York.  I have had a grapefruit in a pot for over 40 years which has been cut back every year and goes from inside to outside on a deck yearly.  This year it grew up to 6 feet. For years now, after a while be it inside or outside, there are tiny webs all along the trunk and branches and a sticky oil residue falls all over the leaves from above and on the floor or the deck around the plant making the floor or deck sticky and slippery.  

I cut the plant back drastically every time.  I wipe off the webs, spray everything with a soapy liquid.  BUT this condition always comes back. The leaves remain beautiful - although there may be an occasional tiny brown spot behind the leaves.  I suspect that whatever it is, is on the trunk and I cannot see it?

Answer
Hi Liliane,
  What you are describing sounds like spider mites. You don't need to cut the plant back drastically unless you are removing severely damaged foliage. Spraying everything with soapy water like you have been doing is really one of the best things to do.

Spider Mites are tiny, spider like mites that can barely been seen with the naked eye. The most common spider mite is the Two-spotted Spider Mite, they have soft pear shaped bodies with 2 dark spots on their backs. The most visible sign of a spider mite presence is webbing on the undersides of leaves and between stems. Webbing usually indicates a high population of spider mites. Although adult spider mites are very hard to see with the naked eye, their eggs can usually been seen easily, grouped together on the undersides of leaves. Looking at the underside of a leaf at an angle is the best way to see them.

Spider Mites are very difficult to get rid of or control. Often they are resistant to chemical pesticides and many insecticides do not work for Spider Mites.
The first step in getting rid of spider mites (and most crawling insects) is to wash the plant thoroughly by spraying the leaves, especially the undersides, with a strong spray of water. This will wash off as many of the spider mites and their eggs as possible.
Spray the plant thoroughly with soapy water as you have been doing. Be sure to cover all leaf surfaces completely. Repeat treatment every 3-5 days until there is no sign of any spider mite presence.

This is likely something you are going to have to deal with every year when the plant goes outside. Washing the plant with a strong spray of water then spraying it with soapy water about once a week while it is outside will help keep the population of spider mites down and make it a bit easier to get rid of them when it is time for the plant to come back inside for the winter. It will also keep down the amount of damage the spider mites do to the plant during the summer. It will grow a lot better without the stress and damage of a high population of spider mites.

I hope this helps. If you have any questions or need additional information please don't hesitate to ask.
          Thanks
              Tracy