Hibiscus Plant Care
...Is This Quiet Killer Stalking Your Hibiscus?
If you live in Florida, the Gulf Coast states, or bring plants from these locations into your garden, the answer is likely to be, unfortunately, YES!
Pinnaspis strachani is otherwise known as snow scale and it is nearly ubiquitous in the southeastern states of the United States. Let me tell you my own snow scale story...
Charles Black's Misbegotten Adventure with Snow Scale
I was warned many years ago when a well known Texas hibiscus enthusiast told me that I should dunk a toothbrush in alcohol and scrub every piece of hibiscus wood that made its way to me from Florida. A few years later this same warning would come to apply to Texas, Louisiana, and just about anywhere warm and humid enough for this insect to survive. That includes Hawaii and French Polynesia (Tahiti) which have been infested for decades with snow scale.
Unnaturally Bright White Specks on the Bark
I mean, the hibiscus were all sprayed for mites, and systemic imidacloprid protected all the plants from white fly and aphids! Surely that was good enough to get rid of the lazy snow scale???
Another year passed, and I admit that more plants showed up with these white specks all over the trunk and on many stems. But I had many thousands of hibiscus by this time and just did not have the time to deal with the few dozen that seemed to have this seemingly harmless insect.
Perhaps it was another year, or maybe less, when I came face to face with the first plant that was close to death. The snow scale were sucking the life right out of it and spreading to cover more and more of it. Yikes! What to do now - the toothbrush method seemed to be too little, too late.
Well, before I could figure out what to do, I had the whole matter taken out of my hands. A sharp-eyed Agricultural Inspector spotted a plant with the scale and erupted. "Hey, this insect does not exist in California!" Uh-oh, I thought, life is about to get more complicated. And sure enough the next day the place was crawling with inspectors who all wanted a look at snow scale in real life. Sheesh....They were all nice enough, but I was not fooled. The proverbial $% was about to hit the fan.
Anyway, to cut to the chase, the inspectors conferred and came to a conclusion: I had to locate and then incinerate every hibiscus that had snow scale. After that, we had to treat the remaining plants with a heavy duty organophosphate pesticide. Sadly, but happy to be rid of the insects, we gathered up about 100 hibiscus (yes! 100 of my precious cultivars!) and tossed them into the coal-fired incinerator. I don't think the pesticide treatment did much but the word was now out. Whenever anyone who worked at HVH found a plant with snow scale, we destroyed it and the scale living on it.
That pretty much got rid of my snow scale collection, but not quite. They are very persistent pests - hard to kill, and even harder to find every last one of them. The inspectors were satisfied and disappeared after the first encounter, but I did not want to have another such experience, and so made it my mission to find and deal with every instance of snow scale. I quickly tired of destroying valuable hibiscus and began to experiment with other ways. Most did not work well, or took too much time and effort, but in the end I did find one method that works and have continued to use it on any and all plants that make their way here from Florida or the Gulf Coast. Now I keep imports to a minimum due to a history of snow scale infestion in about one out of every three shipments from warm, humid places.
Why Don't Pesticides Work on Snow Scale?
OK, maybe I got your attention. Let's take a quick look at Pinnaspis strachani so we can understand how it operates and how to keep it out of our gardens.
The key problem with these insects is that they live most of their lives underneath a hard protective shield. This is the white armor that is seen when snow scale are spotted on hibiscus. That is why pesticides that are sprayed on them are largely ineffective. The armor isn't alive and the pesticide doesn't penetrate it to where the live insects are.
The reason the bugs don't seem to move is that what we mostly see are females, who settle in one place and attach themselves to the plant with penetrating mouth parts. They just sit there, suck plant juices, and lay eggs that hatch, and develop under their armor. It takes about 30 to 45 days from egg to reproductive adult, which means that many generations will be produced in one year's time.
The only life stage at which snow scale have legs is when they are ready to leave the protective cover of their mother and find a feeding place of their own. They are called crawlers at this stage as they venture out to other locations on the same plant or to nearby host plants. This usually takes just a few hours, then the females settle in to make their own armor and start the next generation. Males live only long enough to mate and die, generally only a few hours after they reach adulthood. During the crawler stage, snow scale are briefly vulnerable to pesticides at the moment while they are crawling out from their mother's protective armor. But since they quickly settle into a new location and make their own armor immediately, it just isn't impossible to control this pest by using sprays or pesticides in the hope of getting all the crawlers.
What I learned by experience is that snow scale are inexorable. If left to their own devices, they WILL take over your hibiscus garden. Along the way they can distort leaves and flowers and create a real mess on hibiscus. If you see even one snow scale, take it from me that you will be doing yourself and your hibiscus a huge favor if you get rid of it right that minute! Don't put it off!
Then How do You Get Rid of Snow Scale?
Horticultural Oil Smothers Snow Scale
First obtain some horticultural oil. This type of oil is made for use on plants and won't stain the wood or damage the plant the way other oils or WD-40 can. Yes, you can use these other things, but why should you? Hort oil is the best way to go and it is not expensive.
Next isolate the hibiscus that are infested. Bring them into a shady area away from other potential host plants (hibiscus, orchids, palms, anything tropical). If your hibiscus are in the ground and can't be moved, wait until late afternoon or evening to treat them. The oil can burn the plants if applied in direct, hot sunshine. But hort oil is very lightweight and will soon evaporate off the plant in evening shade, posing no hazard by the heat of the next day.
There are two ways to apply the oil. You can pour some of it in a container and, using a small paint brush, dip it in the oil and then paint the undiluted oil directly over the snow scale that you see. This way gives the very best control of where the oil goes, and is great for using on just a few plants or for just a few insects. If you have a bigger problem, fill an empty hand-held spray bottle with the horticultural oil. Get up close and spray it directly over the insects, avoiding the leaves unless the scale have gotten on the leaves too.
The key is to carefully inspect every infested plant and find every one of the bright white scale. By leaving your plant(s) in an isolated area after they are treated, you can return daily to inspect them for a week or so to make sure no other scale show up that you missed the first time around. Usually you will find a few more insects after the first treatment, so this follow up is important. After about a week of finding no new scale, you can return the hibiscus to where you want it, feeling confident that there are no more live snow scale on that plant.
Isolate, Examine, Treat - The Key to Prevention
It's been quite a while since I saw any snow scale, and I hope to keep it that way. I always isolate and examine closely any plants that have spent any time in known areas of infestation before arriving here, and about a third of the time I do find snow scale. So isolate, inspect, then treat with hort oil if you find any of the telltale bright white specks. That's the way to keep your garden safe from Pinnaspis strachani!