Occasionally
a sap sucking insect known as Brown or Black Olive Scale will
be seen on olive trees. It is rarely a problem if the trees
are in good health. We usually only spray our mature trees
for scale every two to three years and only then if they
need it. However, certain areas of Australia are more prone to
the scale than where we are.
The
adult females are very easy to recognise on the olive tree stems.
They are dome shaped, dark brown to black in colour, and about
the size of a match head.
The
tiny eggs laid under the female look like piles of very fine sand.
Mainly during the summer, these eggs hatch into tiny, six-legged,
cream coloured 'crawlers'. The crawlers move up the stems and
usually settle along the veins of young leaves. At this
stage they don't have the impervious shell of the adult and can
usually be killed with one or two applications of white oil about
two weeks apart. White oil should be used only as directed
on the label by the manufacturers (and by your agricultural department)
and never during the hot part of the day. It puts an oil film
over the young 'crawler' and suffocates it. If applied in the
hot part of the day it also stops the leaves from breathing
properly and can be detrimental to the tree. The white oil application
will also tend to rid the tree of 'sooty mould' as discussed
soon.
If
the crawlers are allowed to live, they will moult after about
one month and then migrate to the young stems and twigs of the
tree. Here they will mature and lay more eggs and their protective
brown shells will be impervious to white oil. Squash the scale
between your fingers to see if it is alive. If it is alive,
then your fingers will be wet from the juices squeezed out. If
it is dead then your fingers will be dry and dusty.
Bad
infestations of live mature scale may need spraying with an insecticide
such as Supracide or Lebaycid. (Important: See note on
page 2 regarding "Treatment") In Greece, Supracide
is the main spray used for most olive problems. Once again, check
with your local agricultural chemical supplier and the product
label, for directions.
Probably
the damage done by the scale itself to the tough olive tree is
negligible compared with what happens next.
As
the scale feeds, the 'manure' they excrete is a sweet, sticky,
'honeydew'. This excreted sticky liquid can finally cover
the leaves of the entire tree. A fungus known as sooty mould
feeds on this food and multiplies until the entire tree may be
covered with the black sooty mould. This is where the real problem
lies.
The
leaves are coated with the black deposit so the sun's light can't
penetrate the leaves properly. Therefore photosynthesis can't
take place efficiently. Therefore, 'root producing' food
is not manufactured in the leaf. Therefore roots don't develop
properly. Therefore the poor root system can't collect enough
food and water from the soil to send up to produce more leaves
which in turn will produce more root. Once the vicious cycle begins,
a stunted and unhealthy tree with poor crops is the result.
To
make the problem worse, sweet 'honeydew' on the leaves also attracts
large numbers of ants. It appears that as the ants
constantly move over the scale, they frighten away the small wasp
parasites which in normal cases would keep the scale under
control.
The
good news is that healthy olive trees don't get the scale, sooty
mould and ant infestation to any great extent. More good news
is that heavily infested trees are easily fixed.
Normally,
one thorough spraying of the entire tree and soil below with a
systemic insecticide will be adequate. Nevertheless, to be sure,
a second spray about two weeks later may be worthwhile.
Now,
if there is no more live scale, there is no more eating, therefore
no more 'honeydew' excreta, therefore no more sooty mould and
ants. Over a period of time the dead sooty mould deposit will
peel off the leaves from exposure to the rain, wind and
sun. The green leaf surface will be exposed and growth will continue
as normal. Treat the tree to an occasional feeding of manure and
some water and watch its health come back.
The
following excerpt comes from "Olives - Pest Management
Guidelines" (UCPMG Publication 8, 1994). These guidelines
cover the major olive problems found in Australia and California
and are available for free from their website. (The information
comes from California so all references to places, seasons, months
and treatments are Californian).
SCIENTIFIC
NAME: Saissetia oleae
DESCRIPTION
OF THE PEST: Black scale adult females are about 0.20 inch
(about the size of a match head) in diameter. They are dark brown
or black with a prominent H-shaped ridge on the back. Young
scales are yellow to orange crawlers and are found on leaves
and twigs of the tree. Often, a hand lens is needed to detect
the crawlers. Black scale usually has one generation per year
in interior valley olive growing districts. In cooler, coastal
regions multiple generations occur. Black scale prefers
dense unpruned portions of trees. Open, airy trees rarely support
populations of black scale.
DAMAGE:
Young black scale excretes a sticky, shiny honeydew on leaves
of infested trees. At first, affected trees and leaves glisten
and then become sooty and black in appearance as sooty mould fungus
grows on the honeydew. Infestations reduce vigour and productivity
of the tree. Continued feeding causes defoliation that reduces
the bloom in the following year. Olive pickers are reluctant
to pick olive fruits covered with honeydew and sooty mould.
CULTURAL
CONTROL: Pruning to provide open, airy trees discourages black
scale infestation and is preferred to chemical treatment.
BIOLOGICAL
CONTROL: A number of parasites attack black scale, the most
common are Metaphycus helvolus, Metaphycus bartletti, and Scutellista
cyanea. These parasites, combined with proper pruning, provide
sufficient control in northern and coastal orchards. In other
regions, biological control is often ineffective because
the black scale's development pattern hampers parasite establishment.
ORGANICALLY
ACCEPTABLE METHODS: Cultural and biological control and
oil sprays.
WHEN
TO TREAT: If infestations are resulting in honeydew, treat
the crawlers. In interior valleys, delay treatment until hatching
is complete and crawlers have left protection of the old
female body. Once crawlers have completely emerged, a treatment
can be effectively made in summer, fall or winter provided
the scales have not developed into the rubber stage (later
second instar, which are dark, mottled grey, and leathery, with
a clear H-shaped ridge on the back).
TREATMENT:
[Due to the chemical nature of the treatments, Olives
Australia is unable to recommend dosages or chemicals to be used.
Please check with your agricultural chemical supplier as
to the suitability for olives, method of application and safety
precautions needed for the following: Summer or Petroleum Oil,
Supracide or Lebaycid. Californian olive growers use Oil
Emulsions, Diazinon 50WP, Methidathion and Carbaryl. Greek olive
growers use Supracide as an all-rounder for many olive problems.]
