a) What is the special property of a 'persistent' insecticide?
b) In what ways is 'persistence' (i) a useful property, (ii) a harmful property?
(a) A persistent insecticide is chemically stable, i.e. it takes a long time to break down.
(b) (i) This property is useful because the substance will continue to kill insects long after it has been applied.
(ii) 'Persistence' is damaging due to the insecticide lasts long enough to pass up a food chain and become concentrated in the bodies of secondary or tertiary consumers.