In late July, I heard from someone here in the Napa Valley very concerned about a small tree in their front yard being attacked by something strange. And it was spreading to the neighbor’s plants.
The small tree, as it turned out, is a shiny Xylosma (Xylosma congestum), a broadleaf evergreen shrub, one of the first plants I learned by name back in my landscape labor days. It’s marketed as a medium to large shrub but often ends up getting out of hand and becoming a tree.
Nursery people and gardeners in our region know it as a virtually problem-free plant, except for its thorns. It is a broadleaf evergreen, tolerant of a wide range of climate and garden conditions. Back in the 1970s, it was one of the go-to plants in landscape design. Now it seems to be out of favor, probably as a matter of style and taste.
Nevertheless, many have thrived in landscapes of that era, mostly undeterred by pests, diseases, droughts and freezes.
A few years ago, I got a call about a small tree, a Xylosma, in a Napa front yard, alarmingly covered with whitish wooly material. After looking at it, and digging a bit into references, I found it was the “giant whitefly.” And now, years later, it turned up again, only the second time in my years of experience here in the Bay Area.
Whiteflies are well known to gardeners. Most people who have grown tomatoes and various other vegetables have had to deal with them. They are smaller than houseflies, have a white covering on their wings, and clouds of them flutter about when you disturb infested plants.
Whiteflies feed by sucking plant fluids from the undersides of leaves. And some species of them have a quick life cycle, breeding new generations in just a few days in warm weather.
Hatching from eggs on the undersides of leaves, the immature crawlers and nymphs suck plant juices then morph into pupae. In some whitefly species, the pupae exude wax and/or white filaments. Then they hatch out as young adults.
Local native oaks are sometimes infested with a species called “crown whitefly,” which can breed in alarming numbers, but infestations typically subside because of naturally occurring predators or egg parasites.
In the late ‘80s and early ‘90s in Southern California, there was a horrible outbreak of “ash whitefly,” which I recall personally as a nightmare for those of us in the wholesale nursery trade. The things wreaked havoc in landscape and urban settings and interrupted nursery trade in ash trees, which were a staple crop in landscaping.
Insecticide sprays seemed only to piss them off. They could breed so quickly, clouds of them would be back within a few days.
Then, plant scientists found and released Encarsia inaron a tiny egg-parasitic wasp, native to the eastern Mediterranean area. It turned out to be an iconic example of successful biocontrol of a plant pest. Now, about 25-years later, ash whitefly is pretty much off the radar.
“Giant whitefly” (Aleurodicus dugesii) is native to Mexico and known as a significant pest in Southern California, Arizona, Texas, Louisiana, Florida and Hawaii. It has made only two appearances (in my experience) here in the valley. The first, in Napa, was brought under control with a single systemic insecticide application. At least, I have never heard otherwise from the property owner.
The other infestation, in Yountville, spread from a large Xylosma specimen to a dwarf Xylosma shrub in the adjacent front yard. Looking closely, I saw no signs of it on other plants.
According to the University of California IPM article (ipm.ucanr.edu/PMG/PESTNOTES/pn7400.html ):
About 60 species of trees and shrubs are susceptible but most of those are too frost tender to thrive here, except as houseplants.
Aside from Xylosmas, the more common susceptible plant species in our local gardens would be: Avocado, Bamboos, Begonia, Bougainvillea, Japanese boxwood, Cannas, Eucalyptus, English ivy, Lantana, Liquidambar, fruitless mulberry, geraniums, and willows.
Even though it is typically a warmer climate pest, consider what is happening to climates and be on the alert.
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