The effect of avermectins on feeding, salivary fluid secretion, and fecundity in some ixodid ticks.

We tested the effects of the potent acaricides, avermectin B1a (AVM) and 22,23-dihydroavermectin B1 (ivermectin; IVM) when injected directly into partially fed and fully engorged female ticks. When injected into small ticks (Amblyomma hebraeum Koch), neither drug (up to 100 micrograms/kg b.w.) inhibited subsequent engorgement nor affected oviposition latency, weight of total egg mass laid nor viability of laid eggs. At higher concentrations (1000 and 5000 micrograms/kg b.w.), AVM and IVM were markedly toxic. When injected into engorged ticks, both drugs increased oviposition latency, and reduced fecundity at about 75-100 micrograms/kg b.w. Vitellogenesis, as assessed by a spectrophotometric assay of the ovaries, was not inhibited. Also at 50-100 micrograms/kg b.w., AVM and IVM caused paralysis of the abdominal dorso-ventral muscles and the leg muscles. Both drugs, at 7 days post-injection, proved detrimental to salivary gland function in both small and large ticks, but had little effect on salivary gland weight. At concentrations which did not inhibit oviposition (20-50 micrograms/kg b.w.) many of the eggs dried out even though they were kept at high RH. We then demonstrated in Amblyomma americanum, Dermacentor andersoni and D. albipictus that removal of egg wax (by extraction with hexane) induced a marked increase in water permeability. IVM neither increased water permeability of D. andersoni eggs nor diminished the amount of egg wax deposited on the surface of the eggs, when injected posteriorly through the alloscutum. However, injection of IVM, dimethylsulphoxide (vehicle for IVM) or distilled water through the articulation between the capitulum and scutum ('anterior injection'), did markedly reduce the wax coating and increased egg permeability. We suggest that anterior injection damages Gené's organ and thus causes the latter effects.