FLEA EGGS: TARGET OF THE NEW IGR ON-ANIMAL TREATMENTS

Abstract:
Currently 3 insect growth regulators either are or soon will be labelled as on-animal treatments to control fleas on pets and livestock. These chemicals, fenoxycarb, methoprene and pyriproxyfen are juvenile hormone mimics that disrupt embryonic and post-embryonic development when present during certain critical phases of cellular transformation. When applied to the host animal, these compounds either act internally in the adult flea to prevent egg development or they act systemically in the flea eggs to prevent development of the subsequent adult stage. Recently completed studies showed that pyriproxyfen has more than one effect on adult fleas. Adults exposed to pyriproxyfen in glass vials prior to feeding on a cat, deposited little or no yolk in their eggs over the next three days. Yolk was deposited in eggs produced during days 3 and 4 but the embryos failed to grow because no organized cleavage center developed. Thus, normal egg production was disrupted for at least 4 days after pyriproxyfen exposure. Similar studies with methoprene treated fleas showed no effect on yolk deposition. However, eggs laid during the first 3 days after fleas exposed to methoprene were placed on a cat, failed to survive because the larvae either died as fully formed embryos or died soon after hatching. In addition to IGR uptake during ovarian development, flea eggs are also affected by these insecticides. Exposure of eggs to treated pet fur for just 1 minute can disrupt embryonic or larval development. Microscopic examination of eggs revealed that this rapid effect is due to the highly permeable chorion or shell of these eggs. Flea eggs were found to have a unique chorion consisting of only a monolayer of gelatinous material overlying the vitelline membrane. This non-sclerotized chorion evidently allows rapid penetration of insecticides before the egg falls from the host.