Scale
insects are bugs that belong to the order homoptera/hemiptera, and they belong
to the family coccidae. These insects get their common name from the covers
(scales) which they produce as juveniles and used to protect themselves. Scale
insects are basically small round insects which are parasitic to plants, and
young scale bugs appear to be light brown in colour (beige)and the colour
becomes dark (black)as they get older .There are two types of scales that these
juveniles produce and these are soft and armoured. the soft scale is formed
from honeydew which is the insects faeces
excreted by the insect, and are larger and more convex than armoured scales and
because they have a soft cover, they lack the detachable protective cover,
while the armoured are formed from the waxy filament that the insects secrets
as it rotates its body in a circular pattern and this waxy coating is used for defence
and makes them quite resistant to pesticides, which are only effective against
the first-instar nymph crawler stage.
However,
scales are often controlled with horticultural oils, which suffocate them or
through biological control. A black fungus called sooty mold may grow on the
honeydew which reduces the photosynthetic capacity of leaves and additionally
they attract wasps and ants.interms of appearance and size there actually
variations as others are hemispherical in shape and while others are oval and
flat and the size vary depending on the type of species. Scale insects feed by
sucking sap from plant tissue and this is achieved by the long, threadlike mouthparts
(stylets) that it has which is actually six to eight times longer than the
insect itself, hence being considered as pests. These pests affect the host
plant by reducing its food supply and may result in poor, stunted growth due to
heavy infestation and this becomes the main source of its food which they
demand on for their whole life.
Scale bugs in Zambia
In
Zambia, scale bugs have been observed on senna spectabilis. Senna spectabilis
is an exotic tree species found in Zambia belongs to the kingdom plantae,
subkingdom; tracheobionta which are vascular plants, super division
spermatophyta which are seed plants, division magnoliophyta which are flowering
plants, class magnoliopsida which are the dicotyledons order fabales, family
fabaceae which is the pea family genus senna mill and species senna
spectabilis. Senna spectabilis is a medium to large tree from tropical America
and it grows to as high as 60feet, but most of them are usually small and range
between 15 to 20 feet.it has pinnately compound leaflets that are about 3
inches long. The tree has bright yellow flowers that are 1.5 inches wide. It produces
seedpods that are about 12 inches long.
Senna
spectabilis is mainly used for aesthetic purpose, it gives the public a viewing
pleasure and it generally adds beauty to an environment because of the bright
yellow flowers which in Zambia occur in January and February and it has the ability
to stay evergreen throughout its growing season, this makes this tree suitable
to be grown along streets. Senna spectabilis forms a uniform crown that is
symmetrical with a round shape spreading about 15 to 20 feet. This enables the
tree to give sufficient shade for pedestrians and parking space for vehicles in
a city.
Scale
bugs observed on senna spectabilis are wingless bugs with soft scales which are
formed from the honey dew excreted by the insect and because of the soft scale,
they lack the detachable protective cover.
The
bugs vary in size from young ones which are small in size ranging from 1.4cm to
3.1cm in length and 1.1cm to 2.3cm in width, and the older ones which are bigger
in size ranging from 3.1cm to 3.8cm in length and 2.3cm to 3.2cm in width. They
also possess numerous minute feet like structures about 52 feet equivalent to
26pairs of feet. The feet are responsible for the strong adhesive force between
the bug and the bark of the tree were they are found hence making it very
difficult for the bugs to be removed from the tree trunk and when removing the
bugs they release a yellow fluid which is believed to be the honeydew. These
bugs appear to have threadlike mouthparts (stylets) which are used to draw the
sap from the plants which are their source of food and are constantly inserted
in the plant tissue. This is possible because the bugs are observed to be
attached on the soft bark making it easy for the threadlike mouthparts (stylets)
to penetrate to the vascular system for the consumption of sap. Scale insects
on a branch appear to be clustered while others are scattered and on a single
unit branch, 211 bugs were taken note of and majority of the scales are located
on the lower end of a branch due to the soft bark. After observation, scale
bugs were not found at the tip of the branch it could be due to the fact that there’s
less amount of sap as compared to the other parts of the branch. In most species the female will
live within the scale for the entire life cycle, often remaining both legless
and wingless. Males live under their scale cover until maturity, when they
emerge as winged adults. Mating or fertilisation takes place through the scale
cover of the female. In some species of soft scale insects, fertilisation is
not necessary as the female is capable of reproducing parthenogenetically (i.e.
reproducing without requiring a mate).most soft scales overwinter as immature,
fertilized females. In spring they resume feeding, mature and lay eggs which
hatch into tiny crawlers. After locating suitable feeding sites, crawlers
settle and begin feeding. Additionally some species lose their legs once
they’ve settled, others retain legs and are able to crawl short distances to
find suitable overwintering sites in the fall. Except for those soft scales
which infest indoor plants, most have only a single generation per year at our
latitudes. While scale insects of armoured species, mostly overwinter as eggs
beneath the female cover. In spring, these hatch into tiny mobile crawlers
which migrate to new feeding sites. The crawlers settler after a few days, then
insert their mouthparts in the plant and begin to feed. Soon they secret a
protective cover and lose their legs. Large populations can build up before
plants begin to show visible symptoms.
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