FYI
Insect pharmacology: Channel, Receptors, Toxins and Enzymes
Edited by: Lawrence Gilbert, Sarjeet Gill (2010)
"3. The Insecticidal Macrocyclic Lactones
…The most dramatic differences between application methods are seen with certain insect ectoparasites. While a number of blood-sucking parasites, such as sucking lice of cattle, are controlled by therapeutic doses of macrocyclic lactones administered orally or by injection, chewing lice are generally not susceptible. Topical applications (generally at about a 2.5-fold higher dose rate) are usually highly effective against chewing lice (Strong and Brown, 1987; Vercruysse and Rew, 2002b). Similar results have also been observed in lice on sheep (Coop et al., 2002). While the efficacy and spectrum of these topical formulations against internal parasites is similar to the oral or injectable treatments, this greater efficacy against chewing lice is thought to be a result of a high concentration of the active ingredient on the surface of the skin where these parasites feed (Titchener et al. 1994). Similarly, most of the macrocyclic lactone pour-on formulations for cattle have demonstrated a persistent efficacy against the blood-feeding horn fly and buffalo fly that is not seen with oral or injectable formulations (Vercruysse and Rew, 202b).
This apparent greater efficacy of pour-on formulations against external parasites is presumably the result of deposition of the active compound on, and in, the skin. The high lipophilicity of macrocyclic lactones is likely to result in the formation of depots of active ingredient in skin lipids and oil and fat secretions. This characteristic has been exploited in special formulations of ivermectin that provide long-term residual control of blowfly and lice on sheep (Eagleson et al., 1993a, 1993b; Thompson et al., 1994), and selamectin which controls fleas on dogs and cats for at least 1 month (Bhaoui et al., 2000).
In sheep, topically applied ivermectin is thought to bind to skin lipids and secretions and may be passively distributed around the sheep’s body in this medium following application to a discrete site ((Rugg and Thompson, 1997). Selamectin is thought to selectively partition into sebaceous glands of dogs and cats from where it is slowly released to the skin over an extended period (Hennessy and Alvinerie, 2002).
…Toxicity is generally greater through ingestion by residual contact and compared with other neurotoxic agents, macrocyclic lactones are considered to be relatively slow acting. Intoxicated organisms usually die slowly, often over a period of days, and there is no quick knockdown effect. Death usually follows paralysis and immobility. The comparatively poor contact activity of these compound plus their short environmental persistence generally results in good compatibility with beneficial and nontarget organisms.
…Despite the widespread, intensive use of avermectins against a number of pests with a high propensity to develop resistance, development of resistance has been relatively slow and rare. The rapid reversion of resistance in the absence of selection pressure and the lack of any confirmed cases of cross-resistance make them amenable for use in resistance management programs. However, in order to prolong the longevity of these compounds in the market place, ite will be necessary to minimize selection pressure in practice, especially in high use situations, through proper rotation, good pest management practices, and proper resistance management strategies.”
YMMV (but it wouldn't hurt to look at Draftiemama's thread:
https://www.backyardchickens.com/t/457471/a-picture-with-bugs-new-pic-p3 ).
