ENVIRONMENTAL-EFFECTS OF THE USAGE OF AVERMECTINS IN LIVESTOCK

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.