Texas - The plants growing along an increasing number of Mexican hillsides reflect trends in illegal drug use here in the United States. While marijuana fields easily outnumbered poppy plantations in prime Mexican growing regions, both government and international agency statistics show those numbers have reversed as Mexican-origin heroin use in the US has exploded.

According to a BorderlandBeat.com translation from the Mexican daily Reforma, the leading marijuana production states in Mexico are Chihuahua, Guerrero, Jalisco, Durango, Sinaloa and Oaxaca. Between 2007-2010, the Mexican army discovered 432,561 marijuana fields in these areas. But that number plummeted by 56 percent between 2011-2014, as soldiers only found 187,056 marijuana plantations during that time frame. Mexico's military department known as SEDENA indicated that opium poppies have been replacing marijuana plants in these states.

Shifts in illegal drug production by Mexican cartels tend to occur rather quickly in response to changes in demand from the US market. Unlike cocaine, which has to be brought into Mexico from other countries, heroin and marijuana can both be grown and processed internally. This allows cartels to dictate to their growers what kinds of plants they need to grow and in what quantities.

Guerrero, currently one of the most violent states in Mexico and home to the Knights Templar and Cartel de Jalisco drug organizations, remains first in poppy production. However, Chihuahua — across the border from southwest Texas — has seen the fastest increase in production, tripling its crop size in the past four years.

According to Substance Abuse and Mental Health Services Administration statistics released in May 2015 by the Drug Enforcement Administration, between 2007-2013 the number of heroin addicts in the US almost doubled from 161,000 to 289,000.

Sylvia Longmire is a border security expert and Contributing Editor for Breitbart Texas. You can read more about cross-border issues in her latest book, 'Border Insecurity: Why Big Money, Fences, and Drones Aren't Making Us Safer.'