Pesticides and Agro-chemicals
Classification of pesticides
By target: insecticides, fungicides, herbicides, nematicides, rodenticides, acaricides. By chemistry: organochlorines (DDT, endosulfan - persistent, mostly banned or restricted), organophosphates (malathion, chlorpyrifos - cholinesterase inhibitors), carbamates (carbaryl), pyrethroids (cypermethrin), neonicotinoids (imidacloprid - act on nicotinic acetylcholine receptors). Biopesticides include Bacillus thuringiensis and neem (azadirachtin).
Herbicides are classified as selective (2,4-D kills broadleaf weeds in wheat) or non-selective (glyphosate, paraquat); contact or systemic; and by timing - pre-plant, pre-emergence (atrazine in maize) or post-emergence. Fungicides are protectant (mancozeb, copper oxychloride) or systemic (carbendazim). Formulations include EC, WP, SC, granule and dust.
Toxicity and residues
LD50 is the dose killing 50% of test animals, in mg/kg body weight; a lower LD50 means a more toxic compound. MRL is the maximum residue limit in produce, and the waiting period (pre-harvest interval) is the minimum gap between the last spray and harvest.
Worked example
A wheat crop at 30 days is infested with Chenopodium and Vicia. Reasoning: the crop is a grass and the weeds are broadleaf, so a selective translocated herbicide is needed - 2,4-D amine as post-emergence. Atrazine is for maize, and glyphosate or paraquat would injure the wheat because they are non-selective.
Traps
Assuming a higher LD50 means a more toxic pesticide; treating all insecticides as broad-spectrum; confusing pre-emergence with post-emergence; ignoring the waiting period; assuming botanical pesticides are harmless to all non-target organisms; ignoring resistance and resurgence from repeated use of one mode of action.