Oil prices fell on Friday posting their biggest monthly drop in six months, after President Donald Trump threatened tariffs on imports from Mexico. Unless the Mexican government stops people from illegally crossing into the US, he would impose a 5 percent tariff on imports starting on June 10th that would increase monthly, up to 25 percent on Oct. 1. Following the threat Brent crude futures fell $2.38, or 3.6 percent, to settle at $64.49 a barrel and New York futures fell $3.09 to $53.50 a barrel, a 5.5 percent loss. Brent touched a session low of $64.37 a barrel, lowest since March 8. WTI hit $53.41 a barrel, weakest since Feb. 14. During May Brent futures posted an 11 percent slide and WTI 1 percent, their largest monthly losses since November.