Migrant workers can feed themselves most of the time.
Well ... if You are Mexican and sneaks over the US/Mexican border, you're not entitled to US welfare. Neither did the US and Canada open their border for all Mexicans, when NAFTA was signed.
In Europe however, it's different. Bar a few restrictions, which are about to expire, all EU citizens can move to any EU country and is thereby entitled to same welfare package as everyone else living in that EU country.
Not surprisingly, Rumanian families with their 10 children are now flocking to Germany. Several German cities have seen such large inflow of new east families, they've had to ask the Federal government for assistance.
In the UK, they're trying to discourage Bulgarians and Rumanians to move to the UK with different campaigns. One telling those new EU citizens how poor the weather is .. (quite amusing actually).
When new EU citizens earn a fraction in their own country compared to welfare in say Germany and the UK, it doesn't take a PhD in economics to figure out in which direction the immigration will flow. As those behind the EU expanding towards East aren't imbeciles, one can only conclude, this was the desired effect all along.
The effect already has been rising unemployment in old EU countries and pressure on wages. Pure supply side economics. Again, you don't need a PhD to have been able to predict this development.
West Germany managed to incorporate their poor cousins from East Germany during a decade. An amazing effort and no doubt no other country in Europe would be able and willing to go through those sacrifices needed to do this.
Now EU has been expanded with 5 times East Germany.
Personally, I fear we'll see social unrest and political polarization in Europe in ways we've not seen in the last 60 odd years.