I've long admired yucca (Yucca spp., family Asparagaceae). It was one of the most common flowering plants in farmhouse landscapes where I grew up in West Virginia.
Indeed, it was common for yuccas to mark the site of long-abandoned homesteads reduced to nothing more than foundation stones. Channeling Hoyt Axton: if those yuccas could talk, what tales they’d tell about dogs, and cats, and the people as well, but the yuccas were cool, and they never said a mumblin word.
Yuccas also taught me that common names for plants are commonly provincial. In my neck of the West Virginian woods, yuccas were called "ghost lilies."
To understand this regional common name, you must view blooming yuccas at night in the stygian darkness far from the light of urban centers. The sword-like leaves and lower flower stalks merge with the shadows making the vivid white flowers appear as luminescent spectral apparitions floating just above the ground. They’re a great backdrop for telling late-night ghost stories on a farmhouse porch.
It wasn’t until I left my native state that I discovered others did not share my yuccaffection. Two frequent questions during the early years of my Extension career were how to control yucca and what was the best dinosaur repellent. I was appalled. Jurassic Park aside, what did people have against dinosaurs?
Thankfully, there seems to be a growing appreciation for these agave cousins. With their southwestern look, yuccas provide a point of interest in Ohio landscapes and require little watering. In fact, yuccas can be killed by over-watering. Of course, I’m not giving a tip on how to control yucca.
I’m now getting questions about how to control various pests of yucca, rather than how to control yucca. The most common yucca-razer is the Yucca Plant Bug (Halticotoma valida, family Miridae).
Both adult and immature (nymphs) yucca plant bugs have an oval-shaped body plan. 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 appear more reddish since their black wing pads do not cover their entire abdomen.
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 in Ohio, so populations can build rapidly. Adults are present well into the fall.
Yucca plant bugs can cause serious harm to their namesake host. As with other plant bugs, both the adults and nymphs use their piercing-sucking mouthparts to inject a toxic brew of digestive enzymes into the yucca plants and then they extract the essence of yucca.
Feeding damage produces small, yellowish-white spots (stippling) which may coalesce causing the foliage to turn yellow and 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.
Indeed, I watched a historic planting of Y. filamentosa 'Adam's Needle' that was planted in the late 1800s in Spring Grove Cemetery and Arboretum in Cincinnati become significantly depleted due to plant bug damage. And yucca plant bugs may feed on several species of yucca beyond Y. filamentosa.
Yucca plant bugs have excellent eyesight. If they see you coming, which is highly likely, they will zip to the base of the yucca blades to conceal themselves. This makes discovering the bugs (and taking pictures!) a serious challenge.
The image below illustrates the challenge. The blade originally had dozens of bugs; however, by the time I snapped the shot, the vast majority were ensconced in their hiding place ... no doubt waving their middle tarsi at me.
Despite yucca plant bugs being a widespread problem on yucca; particularly in the southern U.S., I could find no published insecticide efficacy trials. However, based on personal observations, using direct contact insecticides such as insecticidal soap can be effective, but the applications are also problematic. They can see a spray nozzle coming and scurry to their hideouts. So, you need to be sneaky.
Insecticides with the active ingredient spinosad are also effective against plant bugs. Spinosad has translaminar activity meaning it penetrates the leaf surface which provides longer residual action.
The systemic neonicotinoid insecticides imidacloprid (e.g., Merit, Xytect) and dinotefuran (e.g., Safari, Transect) are also effective in suppressing yucca plant bugs. However, applications should be delayed until after yucca plants have dropped their flowers.
Although the flowers are not visited by our usual cast of pollinators, there may be a negative impact on yucca moths (Tegeticula spp., family Prodoxidae). This group of moths has a mutualistic relationship with their namesake host that’s so co-dependent one cannot live without the other. Delaying a systemic application until after the flowers are shed would preserve the moths.