Insecticides in the Urban Environment: Mode of Action
Insecticides in the Urban Environment: Mode of Action
Insecticides in the Urban Environment: Mode of Action
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<strong>Insecticides</strong> Used <strong>in</strong> <strong>the</strong> <strong>Urban</strong> <strong>Environment</strong>: <strong>Mode</strong> <strong>of</strong> <strong>Action</strong> Page 3<br />
toxicity, are nonpersistent <strong>in</strong> <strong>the</strong> environment, and<br />
cause death slowly. Most <strong>of</strong> <strong>the</strong> currently registered<br />
IGRs mimic <strong>the</strong> juvenile hormone produced <strong>in</strong> <strong>the</strong><br />
<strong>in</strong>sect bra<strong>in</strong>. Juvenile hormone tells <strong>the</strong> <strong>in</strong>sect to<br />
rema<strong>in</strong> <strong>in</strong> <strong>the</strong> immature state. When sufficient growth<br />
has occurred, <strong>the</strong> juvenile hormone production ceases<br />
trigger<strong>in</strong>g <strong>the</strong> molt to <strong>the</strong> adult stage. IGR chemicals,<br />
such as hydroprene, methoprene, pyriproxyfen, and<br />
fenoxycarb, mimic <strong>the</strong> action <strong>of</strong> juvenile hormone and<br />
keep <strong>the</strong> <strong>in</strong>sect <strong>in</strong> <strong>the</strong> immature state. Insects treated<br />
with <strong>the</strong>se chemicals are unable to molt successfully to<br />
<strong>the</strong> adult stage, and cannot reproduce normally.<br />
<strong>Insecticides</strong> that Inhibit Cuticle<br />
Production<br />
These chemicals are known as chit<strong>in</strong> syn<strong>the</strong>sis<br />
<strong>in</strong>hibitors or CSIs. They are <strong>of</strong>ten grouped with <strong>the</strong><br />
IGRs. The most notable chemical be<strong>in</strong>g used as a CSI<br />
is <strong>the</strong> benzoyphenyl ureas. This class <strong>of</strong> <strong>in</strong>secticides<br />
<strong>in</strong>cludes lufenuron (Program®) which is a systemic<br />
<strong>in</strong>secticide used for flea control (fed to your pet),<br />
diflubenzuron (Dimil<strong>in</strong>®) used aga<strong>in</strong>st fly larvae <strong>in</strong><br />
manure, and hexaflumuron (Sentricon®) used <strong>in</strong> a<br />
termite bait station. These chemicals <strong>in</strong>hibit <strong>the</strong><br />
production <strong>of</strong> chit<strong>in</strong>. Chit<strong>in</strong> is a major component <strong>of</strong><br />
<strong>the</strong> <strong>in</strong>sect exoskeleton. Insects poisoned with CSIs are<br />
unable to syn<strong>the</strong>size new cuticle, <strong>the</strong>reby prevent<strong>in</strong>g<br />
<strong>the</strong>m from molt<strong>in</strong>g successfully to <strong>the</strong> next stage.<br />
<strong>Insecticides</strong> Affect<strong>in</strong>g Water Balance<br />
<strong>Insecticides</strong> with this mode <strong>of</strong> action <strong>in</strong>clude boric<br />
acid, diatomaceous earth, and sorptive dusts. Insects<br />
have a th<strong>in</strong> cover<strong>in</strong>g <strong>of</strong> wax on <strong>the</strong>ir body that helps to<br />
prevent water loss from <strong>the</strong> cuticular surface. Silica<br />
aerogels (sorptive dusts) and diatomaceous earth are<br />
very effective at absorb<strong>in</strong>g oils. Therefore, when an<br />
<strong>in</strong>sect contacts one <strong>of</strong> <strong>the</strong>se chemicals it absorbs <strong>the</strong><br />
protective waxy cover<strong>in</strong>g on <strong>the</strong> <strong>in</strong>sect result<strong>in</strong>g <strong>in</strong><br />
rapid water loss from <strong>the</strong> cuticle and eventually death<br />
from dessication. Unfortunately, <strong>in</strong>sects that live <strong>in</strong><br />
environments with high relative humidities, or that<br />
have ready access to a water source, show an <strong>in</strong>creased<br />
tolerance to silica aerogels and diatomaceous earth.<br />
This is because water loss can be m<strong>in</strong>imized by ei<strong>the</strong>r<br />
<strong>of</strong> <strong>the</strong>se conditions and <strong>the</strong> <strong>in</strong>sect may survive despite<br />
<strong>the</strong> absence <strong>of</strong> a wax layer.<br />
Borate conta<strong>in</strong><strong>in</strong>g <strong>in</strong>secticides also disrupt water<br />
balance <strong>in</strong> <strong>in</strong>sects. The exact mode <strong>of</strong> action (more<br />
specifically <strong>the</strong> target site) <strong>of</strong> borate conta<strong>in</strong><strong>in</strong>g<br />
<strong>in</strong>secticides is not currently known.<br />
September 1998