Insect Growth Regulators
The insect growth regulators (IGR) or juvenile hormone (JH) analogs target insect growth and development at an early stage; by contrast, most ectoparasiticides used are adulticides. The IGRs specifically target insects and not mammalians because mammalians do not have a JH system. The effect of IGRs and JH analogs is to prevent reproduction by preventing metamorphosis of insect larvae into viable adults when such insecticides are applied to the larvae.
Methoprene
Methoprene (1-methylethyl (E,E)-11- methoxy-3,7,11-trimethyl- 2,4-dodecadienoate) has a chemical formula of C19H34O3, with a molecular weight of 310.48.
Courtesy of Dr. Ramesh Gupta and Dr. Robin Doss.
Methoprene is a racemic mixture of the R- and S-enantiomers in a 1:1 ratio, with its activity as a JH analog (growth regulator) being restricted to the S-enantiomer. Methoprene does not kill insects. Instead, it interferes with an insect’s life cycle and prevents it from reaching maturity and reproducing. S-methoprene is commonly used in ectoparasiticide products.
The acute oral LD50 of racemate and S-methoprene in rats is >5,000 mg/kg and is between 5,000 and 10,000 mg/kg in dogs. The acute dermal LD50 for both racemate and S-methoprene in rabbits is >2,000 mg/kg. These acute toxicity data suggest that S-methoprene is very safe to animals and humans. Methoprene can cause irritation to the mucous membranes and upper respiratory tract. It can be harmful by inhalation, ingestion, or skin absorption. Experimentally, dogs treated topically with ectoparasiticide products containing S-methoprene did not show any signs of toxicosis or reaction at the site of application.
Hydropene
Courtesy of Dr. Ramesh Gupta and Dr. Robin Doss.
Hydroprene (ethyl (2E,4E)-3,7,11-trimethyl-2,4-dodecadienoate) has a chemical formula of C17H30O2 with a molecular weight of 266.425.
Like methoprene, hydroprene is a racemic mixture of R- and S-enantiomers, and its activity as a JH analog is attributed to the S-enantiomer. The oral LD50 of racemate and s-hydroprene in rat is >5,000 mg/kg. Toxicity and safety profiles of hydroprene are similar to that of methoprene.
Pyriproxyfen
Courtesy of Dr. Ramesh Gupta and Dr. Robin Doss.
Pyriproxyfen (4-Phenoxyphenyl (R/S)-2-(2-pyridyloxy)propyl ether 2-[1-(4-phenoxyphenoxy)propan-2-yloxy]pyridine), has a chemical formula of C20H19NO3, with a molecular weight of 321.376.
Pyriproxyfen is a JH analog and an IGR affecting the embryonic, larval, and reproductive stages of insects. It prevents larvae from developing into adulthood, thus rendering them unable to reproduce. In veterinary medicine, pyriproxyfen is commonly used for flea control on household pets and for killing indoor and outdoor ants and roaches.
Pyriproxyfen has a low mammalian toxicity. At higher doses (>5,000 mg/kg), pyriproxyfen may affect the liver, alter cholesterol concentration, and cause anemia in animals.
Chitin Synthesis Inhibitors
Chitin is the main component of the outer shell (exoskeleton) of arthropods, such as insects and crustaceans. Insects grow by repeatedly molting or hatching and creating a new exoskeleton, and this requires the synthesis of chitin. Inhibitors of chitin synthesis can prevent this process. A few chitin synthesis inhibitors are diflubenzuron, flufenoxuron, and lufenuron.
