Looks like it is time to do an assessment on the impact illegal border crossers are having on the disturbance of these butterflies. And, based on my own experiences, butterflies learned to adjust to walls in areas I have lived in throughout my life.
If used correctly they could leave areas like this open to create a bottleneck area to capture them as they cross. Sentence them to community labor (building the wall) and a pat on the head and back home they go.
I'm an environmentalist, but i'm not worried. The environment impact will be minimal. It will be probably less than a dirt road. At least with a wall, there is no road kill. When you factor in the decrease supply for drugs, some habitats will gain protection. Less people will be clearing the forest to grow marijuana.
You could also make a deal with environmentalist. Use some of the money that would be saved from welfare, and put it towards retrofitting ageing nuclear power plants. Use the savings to help deal with real environmental threats.