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Journal American Rhododendron Society

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Volume 55, Number 2
Spring 2001

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What Are Those Spots?, Part II
Bill Steele
West Chester, Pennsylvania

This is a follow-up article to "What Are Those Spots?" published in the summer 2000 issue of the Journal (Vol. 54,No. 3) and the March 2000 issue of The Azalean (Vol. 22, No. 1). The spots were described as irregular, randomly spaced yellow dots from the size of a pinhead to 1/16 inch (0.2 cm) in diameter. Spots occurred on the new growth of azaleas, lepidotes, and elepidotes around the beginning of June. Later new growth was unaffected. The result of a virus, fungus, or pollution has been eliminated. It was speculated that there was some correlation between earlier undetermined egg sacs that appeared on the undersides of the leaves in May and the spots. The spots have been seen on azaleas in Long Island, New York, northern and central New Jersey, Wilton, Connecticut, the mainline area of Philadelphia, and Swarthmore, Pennsylvania. The spots did not seem to harm the plants. Although the spots have been around for twenty-five years, no one knew what was causing them. My concern was that if I ever got these spots in my nursery the plants would be difficult to sell no matter what I told the customer, such as, "Those spots don't really affect the well-being of the plant." I felt it was necessary to find the cause and control before my plants develop spots. These were the steps that lead to the solution.

The following entomologists agreed to work with me to help identify the probable cause: Greg Hoover, Department of Entomology, Penn State University; Jim Stimmel, Pennsylvania Department of Agriculture; Margery Daughtrey and Dan Gilrein, Long Island Horticultural Research Laboratory, Cornell University; and John W. Neal (retired), Agricultural Research Service, US Department of Agriculture. Letters were sent in early April 2000 outlining procedures I intended to use to identify the critter that was laying the egg sacs. The entomologists were all very helpful, offering suggestions and in one case telling me what had not worked for them, so that I did not have to duplicate their efforts.

May 1, 2000, I went to my friend's garden that had the spots in previous years. She allowed me to place many yellow insect-monitoring cards throughout her garden. I thought whatever was laying the egg sac was a crawling insect, not flying, so I attempted to wrap flypaper strips around the branches of her plants. I knew she entered many of her azalea sprays in the pending shows. Explaining flypaper on flower sprays could be difficult. I again asked if she minded. She said, "No." I still tried to be careful not to ruin all the showable sprays. After I had completely decorated her garden, I drove home. I discovered it was almost impossible to get flypaper glue off your hands, except when you touch something. I think there will be glue on the steering wheel as long as I own the car. I went back a week later. The yellow cards held nothing but gnats. The flypaper seemed to stick mainly to hands and steering wheels but not to azalea branches. Many had fallen off the branches and I discovered flypaper does stick to the bottom of shoes. The flypaper strips were a brownish color and almost impossible to locate on the plants. I replaced all the yellow monitoring cards. This time to locate the new pieces of flypaper strips around the branches I marked the branches with the only thing I had available, "slip-on" tags from my nursery. She now not only had flypaper all over her plants, she had my nursery labels as well. She usually has many people visit her garden at this time of the year. Again I asked, "Are you sure this is all right?" She assured me that it was fine. More glue on the steering wheel.

May 11, 2000, the egg sacs had started to appear. I went to my friend's garden only to find nothing! The egg sacs were there, but there was nothing unusual on the yellow cards or the flypaper. I was really discouraged with the thought of having to wait another year to find what was laying the egg sacs. Jenkins Arboretum also had the spots and bloomed a few days later than my friend's garden. I stopped there on the way home.

Dr. Harold Sweetman, director of the Jenkins Arboretum, said the egg sacs were just forming. He had noticed that the egg sacs were only on plants where a scale was present. Last year the egg sacs and spotting were limited to two areas at Jenkins. We went around and checked many plants carefully. The plants that had the spots last year now had egg sacs on their leaves. Clustered at the base of the leaves of these plants was what appeared to be the crawler stage of azalea bark scale. They were pale pink in color, but they did not have the cottony covering. When the crawlers were crushed, they produced the same purplish pink juice that azalea bark scale does. We checked other plants that did not have the egg sacs. The scale-like crawlers were only on the azaleas with egg sacs, and only on plants that had spots last year. Dr. Sweetman had noticed the scale-like crawlers moving fairly rapidly up and down the stems of azaleas on a few warm days back in April. I observed some moving and they were fairly mobile. Then I got lucky. Upon close examination of the egg sacs, I noticed that the females were at the end of the egg sacs and many were in the process of laying the eggs. I collected samples, placed them in Ziploc plastic bags and mailed them Priority Mail to the four entomologists listed above.

The pink scale-like crawlers at the base of the leaves soon disappeared after the egg sacs were laid. It appeared that the female encased herself at the end of the egg sac and then died. Jim Stimmel identified the insect laying the eggs sacs as maple mealybug (Phenacoccus acericola). This was the same as the one Longwoood Gardens, Kennett Square, Pennsylvania, had sent him a couple of years ago. He had taken it to the US Department of Agriculture Systemic Entomology Laboratory at Beltsville, Maryland, for identification. Maple mealybugs mostly inhabit maples, but they will get on other trees and shrubs. They feed in the vascular tissue of twigs and stems. He said he would be surprised if this is what was causing the spotting, but he would not completely rule it out.

Margery Daughtrey and Dan Gilrein had not positively identified the female at the end of the egg sac, but said, "If anyone would know, Jim Stimmel would know." They put the egg sacs that I sent them on an azalea to see if they got spotting when the eggs hatched.

John Neal was pleased to learn that it had been identified but still could not understand the reluctance to accept that this was causing the spots.

Greg Hoover had spoken with Jim Stimmel and was surprised to learn they were maple mealybugs. When I asked what the difference is between mealybugs and scale insects, Greg said that mealybug life stages are more mobile.

Frank Furman was the gentleman in central New Jersey who had the egg sacs and the spotting limited to two azaleas. Last year, just as the egg sacs were hatching, he sprayed with Malathion. I visited Frank and his wife Jean's lovely garden this spring. The two azaleas were clean. There was no sign of maple mealybugs or the egg sacs.

Crawlers at base of leaves and
female at bottom laying eggs.    Female at end of egg sac.
Crawlers at base of leaves and female at bottom laying eggs.
Photo by Bill Steele
   Female at end of egg sac.
Photo by Bill Steele

Waiting for the Eggs to Hatch
The weather was cool, damp, and cloudy. I thought the eggs would hatch in three weeks - not this year! I had discovered the quick way to see if the eggs had hatched was to crush the egg sac. If you had a moist mess, they had not hatched. I showed this to Harold Sweetman. He tested so many I was concerned there would not be any left to hatch. It was noticed that something was parasitizing some of the egg sacs. Some were torn open and the eggs were eaten.

The eggs finally started to hatch on June 6, 2000. The crawlers were pale green and about the size of a period on this paper. On June 12 they were still hatching, and the spots had started to appear. The spots did not appear on the leaves with the egg sacs. The crawlers seemed to migrate to the newest, most tender, uppermost leaves of the plant, and that was where the spots started to appear faintly, growing more distinct with age.

On May 25 I placed an azalea from my nursery (I don't have the spots) in a plastic bag. I put a lot of leaves, with the egg sacs on them, on the leaves of my azalea and on the medium in the pot. I closed the bag and put it on one of my propagating shelves under lights. The eggs started to hatch June 6, and faint spots appeared on a few of the leaves.

I called Frank Furman to see if he had spots on his two azaleas this year. He did not! This indicates that there is a relationship between the egg sacs and the spots. He did not have egg sacs this year and also did not have spots.

On June 12 I sent samples of the leaves with the hatching eggs, the crawlers, and the spots to the following entomologists: Greg Hoover, Jim Stimmel, Dan Gilrein, and John Neal. John Neal responded, "You now have enough indirect evidence to come to a reasonable conclusion that the stipples are probably from feeding probes of a scale crawler. Since this is a short-term phenomenon and results in very little, if any, significant damage to the plant or the leaves, spraying is not required. Spotting of this type we can live with. Spotting by the azalea lace bug is from feeding, increases as well as accumulates, and can cause weakening of the plant, with reduced flower vigor."

Dan Gilrein wrote: "I received your recent azalea sample and can see the yellow spots as you described. Several were associated with the crawlers. Your close observations seem to be paying off and I agree that the crawlers seem to be the cause of the yellow spotting. It is interesting that the species was identified as maple mealybug. At least one reference I have reports this species on various maples and 'possibly on other trees,' but this literature is not necessarily complete and new information is added all the time. I suspect that many of the crawlers die, lost to predators, blown off or other causes, which may explain why you aren't seeing enormous population increases and subsequent plant stress. This is normal and part of nature's way of regulating populations. Here on Long Island we have noticed natural (and artificially induced) fluctuations in pest populations. For example, in some years we have numerous reports of European fruit lecanium and cottony maple leaf scale infestations, while in others the insects are almost impossible to find. Such fluctuations aren't easily explained although they have been studied in some cases (e.g., gypsy moth). There may also be maples or other hosts nearby which are maintaining a low-level maple mealybug population; crawlers can blow from these trees to the azaleas below, maintaining a low level infestation and the annual symptoms on the plants you observe."

Jim Stimmel responded by phone. He was surprised that the maple mealybug would cause the spotting. When I asked why there was no apparent stress related to the mealybug, he said it is like a person donating blood. One has an excess, and giving a little does not cause any harm. Apparently the mealybug population does not reach the point where they cause damage.

I asked Jim if there was any sooty mold associated with the mealybug. He said mealybugs secrete honeydew so there should be some sooty mold associated with them. When sooty mold is observed, care should be taken to make sure that azalea bark scale is not present (cottony masses on the stems).

Greg Hoover left a message on my answering machine stating that the crawler stage is doing something to cause that spotting to take place at the point where the crawlers are feeding. This was a surprise to him. He felt if we could find what cultivars the maple mealybug is attacking, he might be able to make some suggestions to control this aesthetic problem.

Jim Stimmel suggested spraying with a contact material (Malathion, Decathlon, Orthene, horticultural oil, or insecticidal soap) when the crawlers first hatch, before they get a waxy coating. Timing and good coverage are important! I have had a number of reports that oil and soap were not effective. Perhaps the timing was off, or the coverage was not adequate. Good coverage with these materials is important, especially horticultural oil and insecticidal soap.

Frank Furman successfully controlled the mealybugs by spraying with Malathion just as the eggs were hatching. I am sure he soaked the plant, top and underside of the leaves, stems, and branches. One must watch carefully, around the end of May to mid June, with a magnifying lens to see the crawlers. The spots don't appear until about a week later, and at first are difficult to see on the new growth. Timing is important!

The scale-like pink crawlers, without a cottony covering, and the egg sacs have been identified by Jim Stimmel, entomologist at the Pennsylvania Department of Agriculture, as maple mealybug (Phenacoccus acericola). The crawlers that hatch from the egg sacs in June do cause the spotting on the current new growth after they hatch. Later new growth is not affected.

Right now, other than spots and the egg sacs, the mealybug does not seem to cause significant harm to azaleas, lepidotes, or elepidotes. However, I have received reports from some people who have the spots, who say now that they know the spots are caused by maple mealybug, have looked at their maples and they appear to be showing signs of stress.

Although the crawlers are fairly mobile, they seem to stay on the same plant, or any that overlap. However, a person or animal brushing against an infested plant could easily spread them.

It is possible to control and eliminate the maple mealybug if it is discovered early and treated at the proper time. Control may be difficult, if not impossible, for those who live in a community that is infested with them.

I have discovered new yellow spots. They are not really spots but irregular marks. They differ from the spots caused by the maple mealybug in that they are not on the current new growth but on last year's growth. The irregular marks are very yellow, not pale yellow. When the underside of the leaf is examined, occasional brown spots are associated with the marks, and an oystershell-like scale may be observed. They are on Gable's rhododendron 'Atroflo' at Longwood Gardens. Casey Sclar said the yellow marks were a result of kalmia scale. He said he has been unable to control the scale with horticultural oil because the oil could not reach the scale due to the indumentum (the felt-like covering on the leaf underside).

Bill Steele is a member of the Valley Forge Chapter and owner of Steele's Nursery.

Volume 55, Number 2
Spring 2001

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