I have this on a grapefruit tree that is 30 years old and I can not figure out how to treat it. I thought it was some sort of insect so I have tried Bonide Systemic House plant insect control and a house plant and garden insect spray and stylet oil at 1.5 % Concentration. My wife has tried wiping the leaves with citrus oil. I am not sure if our efforts have been too sporadic or if we are dealing with a more unusual pest. I see the white stuff on the trunk in addition to on the back of the leaves and at the point where the leaves and stem meet Thanks for any insight Roy
From looking at your picture, I would say that it is a mealy bug infestation. Mealy bug is rather easy to control, if you spray properly. You need to kill both the live insect, and the eggs. If you do not kill the eggs the infestation will return again and again. The use of a Horticultural oil called Volk Oil, or Ultra Fine Horticultural oil, works very well. Of the two oils I prefer Ultra Fine, as it is has been refined to a higher level. Ultra Fine should be rather easy to locate, at your garden center, or in the garden sections of Home Depot or Lowe's. Make a 1 percent oil solution, and spray the upper and lowers leaf surfaces, plus the branches and trunk. Be sure you completely cover the tree. Ultra Fine is completely non-toxic, and totally safe to use. Horticultural oil works by smothering the insect. You can also use the oil to control spider mites, scale and aphids. Because oils reduce leaf transpiration, be sure to spray ONLY in the EARLY morning or at dusk in the evening. Below is a link that will provide you with some very good information. - Millet http://floriculture.osu.edu/archive/oct97/mealybug.html
