▁effective ▁in ▁many ▁situations ▁but , ▁with ▁regard ▁to ▁the ▁direct ▁costs , ▁the ▁relative ▁advantage ▁of ▁D DT ▁vis - à - vis ▁alternative ▁insect ic ides ▁seems ▁to ▁be ▁dimin ishing . ▁The ▁context ual ▁evidence ▁base ▁on ▁cost - effect iveness ▁needs ▁strength ening , ▁and ▁the ▁external ▁costs ▁of ▁D DT ▁use ▁vis - à - vis ▁alternative ▁insect ic ides ▁require ▁a ▁careful ▁assess ment ." ▁ ▁Res idents ' ▁opposition ▁to ▁I RS ▁ ▁For ▁I RS ▁to ▁be ▁effective , ▁at ▁least ▁ 8 0 % ▁of ▁homes ▁and ▁bar ns ▁in ▁an ▁area ▁must ▁be ▁sp rayed , ▁and ▁if ▁enough ▁residents ▁refuse ▁sp ray ing , ▁the ▁effect iveness ▁of ▁the ▁whole ▁program ▁can ▁be ▁je op ard ized . ▁Many ▁residents ▁resist ▁sp ray ing ▁of ▁D DT ▁in ▁particular . ▁This ▁is ▁due ▁to ▁a ▁variety ▁of ▁factors , ▁including ▁its ▁sm ell ▁and ▁the ▁st ains ▁it ▁leaves ▁on ▁the ▁walls . ▁While ▁that ▁st ain ▁makes ▁it ▁easier ▁to ▁check ▁whether ▁the ▁room ▁has ▁been ▁sp rayed , ▁it ▁causes ▁some ▁vill agers ▁to ▁resist ▁the ▁sp ray ing ▁of ▁their ▁homes ▁or ▁to ▁res ur face ▁the ▁wall , ▁which ▁elimin ates ▁the ▁resid ual ▁insect ic idal ▁effect . ▁Py re th roid ▁insect ic ides ▁are ▁reported ly ▁more ▁acceptable ▁since ▁they ▁do ▁not ▁leave ▁visible ▁resid ues ▁on ▁the ▁walls . ▁ ▁In ▁addition , ▁D DT ▁is ▁not ▁suitable ▁for ▁this ▁type ▁of ▁sp ray