Abamectin (avermectin B1) and ivermectin (22,23-dihydroavermectin B1)
are high molecular weight hydrophobic compounds, active against a vari
ety of animal parasites and insects. Numerous environmental fate and e
ffects studies have been carried out in the development of these two c
ompounds as antiparasitic agents and for abamectin as a crop protectio
n chemical. They were found to be immobile in soil (K(oc) greater-than
-or-equal-to 4000), rapidly photodegraded in water (degradation half-l
ife (t1/2) in the summer 0.5 days or less) and as thin films on surfac
es (t1/2 < 1 day), and aerobically degraded in soil (ivermectin in soi
l/feces mixtures (t1/2)=7-14 days; avermectin B1a in soils, t1/2=2-8 w
eeks) to less bioactive compounds. Abamectin is not taken up from the
soil by plants, nor is it bioconcentrated by fish (calculated steady-s
tate bioconcentration factor of 52, with rapid depuration). Daphnia ma
gna is the fresh water species found to be most sensitive to ivermecti
n and abamectin (LC50 values of 0.025 and 0.34 ppb respectively); fish
(e.g. rainbow trout) are much less sensitive to these compounds (LC50
values of 3.0 ppb and 3.2 ppb, respectively). In the presence of sedi
ment, toxicity toward Daphnia is significantly reduced. The metabolism
and degradation of ivermectin and abamectin result in reduced toxicit
y to Daphnia. Abamectin and ivermectin possess no significant antibact
erial and antifungal activity. They display little toxicity to earthwo
rms (LC50 values of 315 ppm and 28 ppm in soil for ivermectin and abam
ectin, respectively) or avians (abamectin dietary LC50 values for bobw
hite quail and mallard duck of 3102 ppm and 383 ppm, respectively), an
d no phytotoxicity. Residues of the avermectins in feces of livestock
affect some dung-associated insects, especially their larval forms. Th
is does not delay degradation of naturally formed cattle pats under fi
eld conditions; however, in some cases, delays have been observed with
artificially formed pats. Based on usage patterns, the availability o
f residue-free dung and insect mobility, overall effects on dung-assoc
iated insects will be limited. As abamectin and ivermectin undergo rap
id degradation in light and soil, and bind tightly to soil and sedimen
t, they will not accumulate and will not undergo translocation in the
environment, minimizing any environmental impact on non-target organis
ms resulting from their use.