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’).
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.
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.
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
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 candidates 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), 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), and
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.