I've long admired yucca (Yucca spp., family Asparagaceae) but realize many do not share my enthusiasm for these cousins to agave. Indeed, searching the web using "yucca" as the keyword yields almost as many websites offering advice on how to kill it as how to grow it.
My yuccaffection stems from growing up in Appalachia where the two most common flowering plants in farmhouse landscapes seemed to be peonies and yuccas. In my neck of the woods, the yuccas were called "ghost lilies" owing to the way the vivid white flowers borne on upright stalks resembled luminescent apparitions at night. They were a great accompaniment to late-night ghost stories on the front porch.
However, yucca plant bugs (Halticotoma valida, family Miridae) can cause serious harm to their namesake host by using their piercing-sucking mouthparts to extract the essence of yucca. Indeed, I watched a historic planting of Yucca filamentosa 'Adam's Needle' that was established in the late 1800s in Spring Grove Cemetery and Arboretum become seriously depleted due to plant bug damage. Tom Smith, a retired vice-president of Spring Grove and another yucca-lover, was the first person to alert me to yucca plant bug.
Both adult and immature (nymphs) yucca plant bugs have a somewhat oval-shaped body. Adults of this small (3/16" long) native of the southwestern U.S. have black wings and orangish‑red legs, head, thorax, and abdomen. The nymphs share this striking color scheme, but they appear more reddish in color since their black wing pads fail to cover their entire abdomen.
Yucca plant bugs may feed on several species of yucca beyond Y. filamentosa. Feeding damage by the adults and nymphs produces small, yellowish-white spots (stippling) which may coalesce causing the foliage to turn yellow then brown. The bugs further reduce the aesthetic value of yucca blades by depositing spent yucca extract in the form of black, tarry waste spots.
Feeding on yucca flower stems can reduce flower production. Intense annual feeding activity on yucca blades reduces plant vigor and can eventually cause plants to die.
The bug spends the winter as eggs inserted into the yucca leaves. Eggs hatch in early spring and there are at least three overlapping generations, so populations can build rapidly. Adults are present well into the fall.
Yucca plant bug is a common problem on yucca, particularly in the southern U.S.; however, I could find no published insecticide efficacy trials. Based on personal observations, using direct contact insecticides such as insecticidal soap is problematic. These bugs are very visual; they see you (or a spray nozzle!) coming and quickly drop from the yucca blades. Residual insecticides such as pyrethroids appear to provide limited efficacy because they work best if consumed by insects with chewing mouthparts.
Systemic insecticides have been effective. In fact, Spring Grove was the first place I ever saw imidacloprid (e.g. Merit, Xytect) effectively used to suppress an insect pest; yucca plant bugs. Later applications involved dinotefuran (e.g. Safari, Transect) which was equally effective. Impact on pollinators is minimal because yucca flowers are not visited by general pollinators. They've evolved with their own specialize mutualistic pollination system involving yucca moths (family Prodoxidae). Of course, any pollinator concerns can be addressed by delaying applications until after flowers have declined.