Insecticide, acaricide
avermectin



NOMENCLATURE
Common name abamectin (BSI, draft E-ISO, ANSI); abamectine ((f) draft F-ISO)
IUPAC name (10E,14E,16E,22Z)-(1R,4S,5'S,6S,6'R,8R,12S,13S,20R,21R,24S)-6'-[(S)-sec-butyl]-21,24-dihydroxy-5',11,13,22-tetramethyl-2-oxo-3,7,19-trioxatetracyclo[15.6.1.14,8.020,24]pentacosa-10,14,16,22-tetraene-6-spiro-2'-(5',6'-dihydro-2'H-pyran)-12-yl 2,6-dideoxy-4-O-(2,6-dideoxy-3-O-methyl-a-L-arabino-hexopyranosyl)-3-O-methyl-a-L-arabino-hexopyranoside (i) mixture with (10E,14E,16E,22Z)-(1R,4S,5'S,6S,6'R,8R,12S,13S,20R,21R,24S)-21,24-dihydroxy-6'-isopropyl-5',11,13,22-tetramethyl-2-oxo-3,7,19-trioxatetracyclo[15.6.1.14,8.020,24]pentacosa-10,14,16,22-tetraene-6-spiro-2'-(5',6'-dihydro-2'H-pyran)-12-yl 2,6-dideoxy-4-O-(2,6-dideoxy-3-O-methyl-a-L-arabino-hexopyranosyl)-3-O-methyl-a-L-arabino-hexopyranoside (ii) (4:1) 

Chemical Abstracts name 5-O-demethylavermectin A_1a (i) mixture with 5-O-demethyl-25-de(1-methylpropyl)-25-(1-methylethyl)avermectin A_1a (ii) 

Other names avermectin B1  CAS RN [71751-41-2] (abamectin); [65195-55-3] (i); [65195-56-4] (ii)  EEC no. 265-610-3 (avermectin B_1a); 265-611-9 (avermectin B_1b)  Development codes MK-0936 (Merck); C-076 (Ciba); L-676,863 

PHYSICAL CHEMISTRY
Composition A mixture containing ≥80% avermectin B1a (i) and ≤20% avermectin B_1b (ii).  Mol. wt. 873.1 (avermectin B_1a); 860.1 (avermectin B_1b)  M.f. C₄₈H₇₂O₁₄ (avermectin B_1a); C₄₇H₇₀O₁₄ (avermectin B_1b)  Form Colourless to pale yellow crystals.  M.p. 161.8-169.4 °C (decomp.)  V.p. <3.7 ×10^-3 mPa (25 °C)  K_OW logP = 4.4.3 (pH 7.2, room temperature)  Henry 2.7 × 10^-3 Pa m³ mol^-1 (25 °C)  S.g./density 1.18 (22 °C)  Solubility In water 7-10 mg/l (20 ºC). In toluene 350, acetone 100, isopropanol 70, chloroform 25, ethanol 20, methanol 19.5, n-butanol 10, cyclohexane 6 (all in g/l, 21 ºC).  Stability Stable to hydrolysis in aqueous solutions at pH 5, 7, and 9 (25 ºC). Sensitive to stronger acid and base. U.V. irradiation causes conversion first to the 8,9-Z- isomer, then to unidentified decomposition products.  Specific rotation [a]_D²² +55.7(c = 0.87 in CHCl₃)

APPLICATIONS
Biochemistry Acts by stimulating the release of g-aminobutyric acid, an inhibitory neurotransmitter, thus causing paralysis. See M. J. Turner & J. M. Schaeffer in Ivermectin and Abamectin, W. C. Cambell ed., Springer-Verlag, New York (1989) p. 73.  Mode of action Insecticide and acaricide with contact and stomach action. Has limited plant systemic activity, but exhibits translaminar movement.  Uses Control of motile stages of mites, leaf miners, suckers, Colorado beetles, etc. on ornamentals, cotton, citrus fruit, pome fruit, nut crops, vegetables, potatoes, and other crops. Application rates are 5.6 to 28 g/ha for mite control, 11 to 22 g/ha for control of leaf miners. Also used for control of fire ants.  Phytotoxicity May be phytotoxic to pome fruit when mixed with captan.  Formulation types EC.  Compatibility Not compatible with captan. 

ANALYSIS
Product analysis by hplc with u.v. detection. Residues by conversion to a fluorescent product followed by hplc with fluorescence detection. See T. Wehner et al. in Comp. Anal. Profiles, Chapt. 4.

MAMMALIAN TOXICOLOGY
Reviews FAO/WHO 74, 80, 82 (see part 2 of the Bibliography). G. Lankas & L. R. Gordon in Toxicology in Ivermectin and Abamectin, W. C. Campbell ed., Springer-Verlag (1989) pp. 89-112.  Oral Acute oral LD₅₀ for rats 10, mice 13.6 mg/kg (in sesame oil).  Skin and eye Acute percutaneous LD₅₀ for rabbits >2000 mg/kg. Mild eye irritant; non-irritating to skin (rabbits).  ADI (JMPR) 0.002 mg/kg b.w. [1997] (for sum of abamectin and 8,9-Z- isomer); 0.001 mg/kg b.w. [1995] (for residues not containing D-8,9-isomer).  Other Non-mutagenic in the Ames test.  Toxicity class EPA (formulation) IV 

ECOTOXICOLOGY
Birds Acute oral LD₅₀ for mallard ducks 84.6, bobwhite quail >2000 mg/kg.  Fish LC₅₀ (96 h) for rainbow trout 3.2, bluegill sunfish 9.6 mg/l.  Daphnia EC₅₀ (48 h) 0.34 ppb.  Algae >100 mg/l.  Other aquatic spp. LC₅₀ (96 h) for pink shrimp (Panaeus duorarum) 1.6, blue crab (Callinectes sapidus) 153 ppb.  Bees Toxic to bees.  Worms LC₅₀ (28 d) for earthworms 28 mg/kg soil.

ENVIRONMENTAL FATE
Animals Rapidly eliminated (80-100% in 96 h), mainly via faeces; urinary excretion was 0.5-1.4%.  Plants Degradation/metabolism in each of three different plants is similar and occurs predominantly by photolysis on the plant surfaces. The definition of the residues is thus expressed as the combined residues of avermectin B1 and its 8,9-Z-avermectin B1 photoisomer.  Soil/Environment Binds tightly to soil, with rapid degradation by soil micro-organisms. No bioaccumulation.