Pesticides and Agrochemicals: Chemistry and Safe Use
The exam tests chemical groups and their modes of action, formulation arithmetic, and the safety framework of LD50, PHI and MRL.
Core rules
- Organochlorines (DDT, BHC, endosulfan) are persistent and lipophilic and are largely banned in India. Organophosphates and carbamates inhibit acetylcholinesterase. Pyrethroids act on sodium channels; neonicotinoids act on nicotinic acetylcholine receptors.
- Herbicide modes of action: triazines and ureas block photosystem II, glyphosate inhibits EPSP synthase, sulfonylureas inhibit acetolactate synthase.
- Fungicides include dithiocarbamates (mancozeb) and copper preparations; Bordeaux mixture is copper sulphate plus lime, alkaline and incompatible with many other products.
- Formulation codes matter: EC, WP, SP, SC and granule differ in active ingredient (a.i.) content and in how you calculate dose.
- LD50 is a toxicity value (lower means more toxic); WHO classes Ia, Ib, II, III and U run from extremely to unlikely hazardous.
- PHI is the pre-harvest interval set for each crop–pesticide pair; MRL is the maximum residue limit in produce. Both must be respected independently.
Worked example
Prepare 200 L of spray at 0.05% a.i. from a 50% WP. Active ingredient needed = 200 × 1000 g × 0.05/100 = 100 g. Because the product is only 50% a.i., product required = 100/0.50 = 200 g. If you had calculated 100 g of product, you would under-dose by half and promote resistance.
Traps that cost marks
- Confusing product dose with active-ingredient dose.
- Assuming PHI compliance guarantees MRL compliance; application rate and number of sprays also count.
- Mixing alkaline copper formulations with incompatible partners and losing efficacy.