Q: I have a plum tree that was diagnosed with lecanium scale last year. It was treated twice to suppress the insects, but I am not sure how successful the treatments have been. Now I have been advised to prune the dead and damaged branches out of the tree. Since it is winter, how will I know if branches are dead or alive? Isn’t it too stressful to prune now since it is cold? Should I apply dormant oil in the spring to suffocate the overwintering insects? Should I make any other treatment later in the season? Finally, if I need to replace this tree, can you suggest some alternatives? I am looking for something hardy and pest-resistant, no more than 25 feet tall – the primary goal is to provide some shade.
A: Although you do not specify what kind of plum you are growing, a lecanium scale known as globose scale (Sphaerolecanium prunastri) is a common pest of purpleleaf plum trees (Prunus cerasifera ‘Atropurpurea’ and ‘Thundercloud’).
Globose scale insects overwinter as juvenile adults that mature in early May. After mating, adult females can produce as many as 3000 eggs apiece. The eggs hatch in a matter of hours into the immature phase of their life cycle known as crawlers. That is the only time in their life cycle they move about freely and when they are most susceptible to control.
Once they settle on a branch and insert their mouthparts into the tree’s vascular system, they stay there for the rest of their lives, feeding on the carbohydrate-rich sap. As a result of this carbohydrate-rich diet, their excrement is sugary and sticky – creating a mess for anything unlucky enough to be under a tree infested with this pest.
As they mature, they begin to secrete a waxy covering that makes the adult scale insects very resistant to insecticide applications. There is one generation of crawlers a year.
To manage globose scale infestations, start with a dormant application of horticultural oil. This application should take place in late March or early April, when the buds are swelling, but before the leaves unfurl. Make sure you get thorough coverage of the trunk and branches. Horticultural oil will suffocate many of the immature insects, but many will survive. You will need to make additional applications when the crawlers are active in June.
To time your applications properly, examine the tree carefully for the tiny, reddish-pink crawlers. Although they are very small, crawlers are visible, especially with the aid of a magnifying glass. Crawlers are very susceptible to control with environmentally friendly products such as summer rate applications of horticultural oil, insecticidal soap and neem (azadirachtin). Repeat application of these materials according to label directions will be required for good control because they have limited or no residual activity once the spray has dried.
Conventional insecticides that are labeled to control globose scale include Bayer Multi-Force Insect Killer (cyfluthrin). Fewer applications are necessary because it has longer residual activity. Another option is to use Bayer Advanced Tree & Shrub Insect Control (imidacloprid).
This material is applied as soil drench in early spring so that it has time to be taken up by the roots and taken up into the crown of the tree where globose scale is actively feeding. One application provides season-long control. Be sure pull back any mulch and apply the product to bare ground within 18 inches of the trunk. Water it in after application and replace the mulch.
The recommendation to prune the tree to remove dead or heavily infested branches is a good one. Such pruning is best accomplished while the tree is still dormant, although you can wait for a warmer day. Dead wood is obvious, even in winter, once you get out there and start pruning. It has a different feel and appearance on close examination. By removing heavily infested branches, you reduce the number of insects left to control. Obviously, once you prune them off the tree, you need to get rid of them by burning or sending them out with the trash.
While you can get good control of globose scale by monitoring the tree frequently in spring and early summer and making applications as necessary, this is an ongoing problem that will require your attention every year. It is not unreasonable to replace this tree with one that is not as problem-ridden.
Some good trees for you include:
Paperbark maple (Acer griseum)
Japanese maple (Acer palmatum – not cut-leaf or dissectum varieties because they generally are not tall enough)
Serviceberry (Amelanchier arborea)
Eastern redbud (Cercis canadensis)
Fringetree (Chionanthus virginicus) -Photo below-
Kousa dogwood (Cornus kousa)
Corneliancherry dogwood (Cornus mas)
Washington hawthorn (Crataegus phaenopyrum – yes, it has thorns)
Sweetbay magnolia (Magnolia virginiana)
Crabapple (Malus spp. – especially disease-resistant cultivars), sourwood (Oxydendrum arboreum)
Japanese stewartia (Stewartia pseudocamellia)
Blackhaw viburnum (Viburnum prunifolium – a large shrub easily trained as a small tree)
I have included the botanical names so you can look up good information about them on the Internet. Websites from land grant universities such as Penn State, Cornell, Ohio State, University of Maryland, University of Delaware and Rutgers are a good source of research-based information.
Also, websites from public gardens and arboretums such as Longwood Gardens, Morris Arboretum, and the Missouri Botanical Garden are good sources of information and pictures of the plants listed above. Other references that you might find helpful include Dirr’s Hardy Trees and the Manual of Woody Landscape Plants, both by Michael A. Dirr.