Agriculture Reference
In-Depth Information
Chemical control is obtained by using insecticides. Insecticides act by
poisoning the insects that ingest the sprayed plant. The ingestion of the
insecticide can occur by direct contact, inhalation of the toxic vapours or
when systemic products are used that get into the plant sap through either the
leaves or the roots, and as the insect sucks this poisoned sap, it is killed. There
are also repellent substances that keep the insects from attacking the plants;
in some cases they are plants. For chewing insects, products that act through
ingestion or contact are used. For insects in older stems, trunks, roots and
seeds, control is obtained either by fumigation or with a contact insecticide.
For insects sucking aerial or root parts, a contact and a systemic insecticide
can be used.
Most newer insecticides are less harmful to the environment, the farmers
and the consumers. The new products are very specifi c and short-lived in the
fi eld, so that their poisoning ef ect is not as long-lasting as it used to be with
the old insecticide compounds. The grower should always make sure what
the best product is for controlling an insect and if it has been approved by the
government or the importer for that species, especially in case of an export
product. For fi eld safety, the requirements as to how soon after spraying
people can re-enter a fi eld and what is the minimum time before harvest that
the insecticide can be applied should be determined. Some friendlier products
include pure soap, oils and plant extracts, which can be very ef ective for
some insects.
Mites
Mites belong to a dif erent group and are related to spiders. They normally
rasp the underside of the leaves and feed on the sap, damaging the plant and
its ability to photosynthesize. Mites can become more of a problem when
the plants have a shortage of water or fertilizer. Normally they are on the
underside of the leaves, making it more dii cult for sprays to reach them.
Their control should start with feeding and watering the plants. Biological
control measures include the use of predatory mites. The chemical approach
includes the use of some fungicides such as sulfur and some dithiocarbamate
fungicides. Specifi c acaricides are based on pyrethroids, and in recent years
many new compounds have been developed, most of them interfering with
mitochondrial activity. For best control, it is often necessary to apply another
product 5 days later to kill the new adults that emerge from the eggs. Rotating
products avoids the build-up of resistance. During spraying it is essential to
cover the underside of the leaf where mites are found.
 
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