Visiting different properties around the Keys, I’ve noticed an increased number of Mexican petunia plants, Ruellia simplex, covered in white patches. These aren’t mealybugs or whiteflies, it is caused by gall mites and it’s the plants response to the feeding. This is known as erinea, excessive development of leaf trichomes, or surface hairs in this case. The tiny, microscopic mites called eriophyid mites, cannot be seen with the naked eye. The mites cause the plant to produce the white, velvety masses on the leaves and stems. This distorted tissue provides shelter to the mites as they continue to feed. Because of this, the mites can be very difficult to control since they are essentially protected from any contact pesticides.
These gall mites are not going to kill the plants, but it is not going to look attractive either. Since mites prefer warm, dry conditions, populations may be greater in the winter. During periods of new growth, applications of a horticultural oil can be used as a preventative since these tender plant parts are preferred by the mites. However, damage can also appear on the older leaves, stems, and flower petioles. For existing infestations, a heavy pruning followed by repeated spray applications will protect the new foliage. Irrigation and fertilization should be reduced or eliminated at this time because that will promote more growth and therefore more infestations. Caution should be used when using horticultural oils since phytotoxicity can occur in high temperatures, especially applications made during midday. For professionals, when oils are not an option, miticides may be used.
Removal of Ruellia
This could also be an excellent opportunity to remove Mexican petunia from our landscapes. Ruellia simplex, is listed as a category 1 invasive plant by the Florida Invasive Species Council. It rapidly spreads via numerous seeds and lateral roots. If it is in your yard, you know the dense mat it forms, crowding out anything else in its path. Unfortunately, this also happens when it ends up in protected habitat, displacing native plants and quickly overtaking large areas. Even when the plant is removed, regrowth may occur for years. Plants should be dug up with a shovel to remove all vegetative parts including the entire root mass.
Whenever invasive plants are removed from yards and gardens its a good idea to quickly replace them with native or other Florida Friendly plants, so the invasive plants do not reestablish. Choosing the right plant to replace Ruellia will depend on the area they are planted, since it can handle a wide range of light and soil conditions. In areas of deep shade, ferns might be a better option. Southern shield fern, Thelypteris kunthii, is a wonderful native fern that will grow well in shady to partial sun environments; also, wild bamboo, Lasiacis divaricatus. Wild coffee, Psychotria nervosa and Psychotria ligustrifolia, also will flower and thrive in shadier areas of the landscape and provide food and habitat for birds. For sunnier locations, ornamental grasses like muhlygrass, Muhlenbergia capillaris, or saltmeadow cordgrass, Spartina patens, make beautiful additions and are salt tolerant. To add a bit of color, East coast dune sunflower, Helianthus debilis subsp. debilis, blue porterweed, Stachytarpheta jamaicensis, tickseed, Coreopsis leavenworthii, tropical sage, Salvia coccinea, or firecracker plant, Russelia equisetiformis can be used.
For more recommendations on possible replacements, contact our Master Gardener Volunteers at firstname.lastname@example.org. Out with the old invasive bad guys and in with the new, native good guys!
Control recommendations for Ruellia: https://plants.ifas.ufl.edu/plant-directory/ruellia-simplex/
Florida Invasive Species Council plant list: https://floridainvasivespecies.org/plantlist.cfm
This article has been reviewed by Dr. Alexandra Revynthi, Extension Entomologist