Chemical structures by Mode of Action (MOA)

This post discusses variation in insecticide chemical structures that share the same MOA, including natural and synthetic products, ordered roughly by weight of pesticide use in California during 2016 (O’Malley M. “Pesticides”, in Current Occupational and Environmental Medicine, 6^th Edition. Edited by J. LaDou Robert Hearrison, McGraw-Hill, 2021).

Individual MOAs fall into the following groups:

Multi-site activity compounds, IRC -8 – Halogentated Fumigants, MITC releasers, elemental sulfur, Arsenic compounds

Disruptors of nervous system function

Cholinesterase (ChE) inhibitors, affecting Central Nervous System (CNS) and autonomic nervous system

nicotine and synthetic modulators of nicotinic receptors

Sodium channel modulators and blockers

GABA-gated chloride channel antagonists,

Ryanodine receptor modulators, IRC 28

Octopamine receptor agonists, IRC 19

Chordotonal TRPV channel modulators

DISRUPTORS OF ENERGY METABOLISM

Electron transport chain disruptors; ATP-synthase, uncouplers of oxidative-phosphorylations

MOA’s interfering with insect development

Inhibitors of lipid,, chitin synthesis

Ecdysone receptor agonists, IRC 18 , triazine moulting disruptors, IRC 17, Juvenile hormone mimics IRC 7

Pheromone compounds – interfering with population at highairbone concentrations, used as lure together with insecticides (pyrethroids or ChE inhibitors)

Microbial agents

B thuringiensis and other entomopathogens, targeting lepidopterea, flying insects, nematodes

Granulosis virus – affecting lepidoptera

Details of Chemical Structures by MOA

Compounds with Multi-site Activity IRC 8

Fumigants typify compounds with multi-site activity, with effects on insects, nematodes, and plants, used to sterilize soil prior to planting, and as post-harvest commodity treatments.

Halogentated Fumigants

Many halogenated fumigants had 19th century trials as general anesthetics following the discovery of chloroform and di-ethyl ether, with little or no pre-use safety evaluation. Discarded compounds found multiple other uses., typified by methyl bromide, used as a refrigerant, then as a fire retardant before its general use as a fumigant

Methyl bromide CAS # 101-26-8; PCID -6323

20th century discoveries included ethyline dibromide, initially used as an additive to preventing lead engine build-up in gasoline the containing the anti-knock compound tetraethyl lead.

Neifert described the insecticidal properties of halogenated fumigants in a report for USDA in 1924. Their systematic application as fumigants began in the 1940’s, as indicated by a feature article in a Salinas, Ca newspaper in 1947, describing increased crop yields in experimental studies with ethylene dibromide and D-D mixture (DPR chemcode 185 1,2-dichloropropane, CAS # 78-87-5; PCID), 1,3-dichloropropene and related c3 compounds)

Salinas Californian article demonstrating results of fumigation with EDB on production of carrots and beans, in collaboration with Allied Chemical – retrieved through Newspapers.com archive

(Z)-1,3-dichloroprop-1-ene pcid 24726 CAS # 10061-01-5

Halide fumigants used in 1974 and not used in 2016 included 1,2 dibromochloropropane (DBCP), ethylene dibromide, ethylene dichloride, and the D-D mixture (chemcode 185 1,2-dichloropropane (CID6564). Most have a high degree of reactivity and present safety issues because of their flammability, irritant properties (excerpts from pubchem warning information shown below), and toxic effects on insects, plants, fungi, nematodes, and vertebrates.

1,2 dibromo-3-chloropropane (DBCP) CAS # 96-12-8 PCID – 7280

Dibromochloroprone (DBCP) had extensive use because of its limited phytotoxicity and its water solubility, allowing post-plant treatments to control nematodes.

Methylisothiocyanate (MITC Releasers)