Environmental Engineering Reference
In-Depth Information
Many different pesticides (fungicides, herbicides, and insecti-
cides) are found in water, sediments, and aquatic organisms of
estuaries in agricultural areas.
What are “third-generation” pesticides?
As people have realized the widespread effects of both first-
and second-generation pesticides, attempts have been made
to develop new pesticides that are more specialized in their
toxicity to insects. Insect growth regulators are much less
toxic to birds, mammals, and fishes. Some of these newer
pesticides target the molt cycle of insect larvae by mimick-
ing their specific biology or hormones. Larvicides target the
insect's larval stages and are less harmful to nontarget organ-
isms, and generally more effective and specific than chemicals
that focus on adults. Larvicides target the limited breeding
habitat before adults have had a chance to disperse widely.
One larvicide in use is methoprene, an insect growth regu-
lator that mimics the insect's juvenile hormone (JH), which
normally prevents larvae from metamorphosing prematurely
into adults. When the insect stops secreting JH during the
pupal stage, it is then ready to develop into an adult. If metho-
prene is present in the insect's system when it begins the
pupal stage, the triggering of adult development is prevented
and it dies as a pupa. While few impacts have been observed
in nontarget aquatic organisms, there is concern that these
chemicals might have harmful effects on crustaceans, which
are closely related to insects. Another larvicide is Dimilin
®
(diflubenzuron), a chitin synthesis inhibitor. Chitin is a major
constituent of the outer exoskeleton of arthropods (including
both target insects and nontarget crustaceans). A chitin syn-
thesis inhibitor prevents the larvae from molting, resulting
in their death. Unfortunately for crustaceans, they have the
same chitin in their exoskeletons and also need to molt, so
they can be severely harmed when diflubenzuron is sprayed.
Crustaceans make up the majority of small animals in the
Search WWH ::
Custom Search