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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Thursday - April 05, 2012

From: Fort Worth, TX
Region: Southwest
Topic: Pests, Cacti and Succulents
Title: Insects on yucca from Ft. Worth TX
Answered by: Barbara Medford

QUESTION:

I have a plant labeled Yucca Soft Leaf recurvifolia that I planted about 5 years ago,& was beautiful until last summer when it became infested with thousands of fast, tiny, dark insects. I sprayed with Whitney Farms Outdoor Insect Killer (pyrethins, mostly) and that seemed to kill most, but the plant is reinfested, and the leaves are either dead or pale, light green, & spotty. Can it be saved?

ANSWER:

Yucca recurvifolia does not appear in our Native Plant Database, but this article on the plant from the Master Gardeners of the University of Arizona Pima County Cooperative Extension can give you some information on it. It appears that the reason Yucca recurvifolia does not appear in our database is because it is a synonym for Yucca gloriosa (Moundlily yucca), according to the USDA Plant Profile. Both are native to North America and the southeastern states. Of course, Fort Worth is hardly in a southeastern state, but we can probably address your question by searching on pests and diseases of yuccas.

We should first mention that we are neither plant pathologists nor entomologists. Without that kind of training and without being able to see the plant, about all we can do is give you some references with pictures to help you. We also want to mention that most insects and diseases do not strike healthy plants, but plants that are weakened by improper care, watering or fertilizing.

The Horticultular Site Diseases of Sansevieria and Yuccas:

Arizona Cooperative Extension Problems and Pests of Agave, Aloe, Cactus and Yuccas

The more we read about the problems you are having with your plant, the more we are convinced that the little black bugs are not causing the problem, but that the problems with the yucca are attracting the bugs; that is to say, fungus gnats. Yuccas are desert plants, they need to be planted in a well draining soil, perhaps half of it  made up of decomposed granite. From the article from the Arizona Cooperative Extension Program, scroll down to Page 4 and read the articles under "Poorly-drained soils" and "Irrigation." At the bottom of the same page, read "Fungal diseases." On Page 5, you will find pictures of fungal lesions.

Your final question involved asking if your plant could be saved. Frankly, this is a decision only you can make. It sounds like you need to begin at Square One preparing a planting hole in full sun, amending your (probably) clay soil with a well draining cactus soil and decomposed granite, NOT sand. The gnats are not going to leave until the diseased material is gone, so removing the plant and disposing of it should discourage the gnats. Yuccas are survivors, and it could be that in your excavations, you will find some healthy root to plant a new yucca. From GardenGuides.com, here are instructions on How to Plant a Yucca Root. Just remember, good drainage, plenty of sun, no overhead watering (like a sprinkler system), no fertilizer, and minimal watering.

Pictures of Yucca recurvifolia

 

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