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Magnolia Scale Is Now A Serious Pest!
by Art Drysdale
by Art Drysdale

email: art@artdrysdale.com

Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at http://www.artdrysdale.com


August 16, 1998

Only as far back as 1993, if anyone had suggested to me that our magnificent magnolias could be in danger of going into demise because of a scale insect, I would have said he or she was crazy.

But that's exactly what has happened. Magnolia scale (Neolecanium cornuparvum) [and its close cousin Tuliptree scale (Toumeyuella liriodendri)] has suddenly spread through much of Ontario where the trees are hardy, and likely it may attack in other provinces (if it hasn't already) with growing areas of zone 5b and upward.

It is known in other countries where magnolias are popular, but some prominent gardens such as Callaway Gardens at Pine Mountain (near Atlanta) Georgia appear not to know anything about it! We certainly do here in southern Ontario. If you have a magnolia tree or shrub, and you find it infected with this scale insect, you are in danger of losing your tree within three or four years if you do not control the insect.

Three years ago, a neighbour called and asked me to come look at her lovely large saucer magnolia tree. She said it had "black goo and mildew on the leaves and a peeling substance on all but the major trunk and new growth." That was in July, and when I looked at it, I realized the "peeling substance" to which she referred was actually thousands of scale insects, many over a centimetre (up to 1/2 in.) in diameter. The black goo and mildew was actually the tree's sap being exuded as a result of the feeding the scale insects. I had never seen scale insects anywhere near that large!

Upon checking, I found that Ken Duncan, with the Rhododendron Society of Canada, and responsible for the planting and care of a large collection of magnolias at Toronto's Edwards Gardens, had noted the culprits on just one tree the previous year. He had asked the parks department to spray the tree, but it was not done, and the following year (the same year I saw them at my neighbour's), they had spread to almost every tree in the collection.

According to Pascal P. Pirone, author of "Diseases & Pests of Ornamental Plants" (now in its fifth edition), this highly unusual insect can be controlled in two ways. First, by applying a dormant spray in early spring just before the foliage appears, and second, with an insecticide such as Sevin when the insects are in the tiny crawler (caterpillar) stage, usually about the end of August.

This year, with everything occurring two or three weeks early, the crawlers are out now, and if your tree has shown signs of decline (reduced foliage and/or dying branches), you likely have the pest. Search on the undersides of the leaves for the crawlers, and soak those undersides with Sevin. You may also note some smaller, pinkish scales forming. These are the new insects and it is these that will over-winter to do their harm next season. These pink scales may also be able to be killed by using what is referred to as a summer oil spray--a half-strength solution of Volck oil spray. But you have to hit the insects on the base of the leaves.

A further suggested solution is Cygon systemic insecticide. Though magnolias are not listed on the product labels, it can be used on the trees. There could be damage to weak trees. In any case I would not suggest spraying Cygon directly on magnolia trees, but rather using it either as a soil drench, or placing it in small augured holes (1 cm diameter) about 90 cm (3') apart in the drip spread area of the surrounding ground. I would regard Cygon as the last resort chemical treatment.

Last week, I decided to call one of North America's top experts about this problem. Alex Shigo is numero uno when it comes to tree matters. He has written the top books on tree matters, and was the first to come up with some of the newer ideas in tree care. For example, 35 years ago, the standard tree care practice in cutting limbs from trees was to cut it off perfectly flush with the larger branch. It was Alex Shigo who discovered that it was far superior to leave a slight "shoulder" of the removed limb on the larger branch, instead of cutting it flush.

It took years for the entire tree-care profession to realize Alex Shigo was correct. When I mentioned magnolia scale, Alex immediately said, "Are they using nitrogen on a lawn near the specimens you're talking about?" The answer was, "probably the parks department does." Little did I know, and only found out later while discussing the problem with Ken Duncan on my TALK 640 radio programme last Saturday, annually the Rhododendron Society adds urea (45-0-0--a fast acting form of nitrogen to the beds where the rhodos, azaleas and magnolias grow.

This is done because the beds are mulched with wood chips and chips have a reputation for robbing the soil of nitrogen in their rush to decompose. The same can sometimes also be said of bark mulch. I'm not certain what the answer is, but obviously to lessen the threat from magnolia scale, it will be necessary to reduce the presence of nitrogen. If you use a mulch anywhere near a magnolia, and you add nitrogen to supplant what is used up in the decomposition process, then you might consider changing the mulch; say to pine needles. These make an excellent mulch and do not use up nitrogen in their desire to decompose. They are not as easy to obtain, but are worth searching out. If your magnolia is sited in a lawn area, and you regularly fertilize the lawn, I suggest you use only the highest quality long-lasting lawn fertilizer. For example, use Golden Vigoro Lawn Food (24-4-8) in the spring, and/or Golden Vigoro Fall Fertilizer (12-3-12) in the autumn, both of which are made with the best source of synthetic organic nitrogen--IBDU.

For those who have a chip or bark mulch around magnolias and who must add nitrogen, try using a similar long-lasting turf fertilizer until the mulch can be changed so that the nitrogen fertilizer is no longer needed.

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