Quoting from the research link above, this was tested in summer: "This research shows that summer sprays of soybean oil effectively controlled populations of two-spotted spider mites with minimum phytotoxicity to burning bush plants (Euonymus alatus Thunb. Sieb.‘Compactus’). A single spray of 0.75 or 1.0% or two sprays of 0.25 or 0.5% soybean oil gave effective mite control."
Soaps can be damaging in hot weather too and some plants are particularly sensitive to soap sprays. We don't know what plants have the problem or where foussi is, might be winter there
With both oil and soap sprays it's a good idea to test treat a plant it hasn't been used on before.
In the link to an article on spider mites by entomologists that I gave in another thread the other day, it was noted that horticultural oils used at the “summer oil” rate are "perhaps the most effective miticide available for home use." For insecticidal soap they noted "marginally effective against twospotted spider mite and where webbing prevents penetration."
http://extension.colostate.edu...
This article on insecticidal soap lists some of the sensitive plants.
http://www.clemson.edu/extensi...
