It seems to me employers would have to pay higher wages without migrant labour.

Some things we might never have again. For instance you might not get sufficient utility out of strawberries at $30/kg. The market for people who do might be very small. Then the guy who wants to grow strawberries at cost X, the guy who wants to get strawberries at cost Y, and the guy who wants to be paid to pick strawberries at cost Z all want to indulge in the activity but because one part of the chain falls below a legislated minimum, an economically efficient action does not occur.

That doesn't mean it's bad that it doesn't occur. Just that if the equilibrium price has any link of the chain be legally unacceptable you might literally kill a market. There is no new equilibrium. There just isn't product.

The fruit picker's wages aren't necessarily that large a component of the consumer price for the fruit. This economist argues that in the US: "For a typical household, a 40 percent increase in farm labor costs translates into a 3.6 percent increase in retail prices. If farm wages rose 40 percent, and this wage increase were passed on to consumers, average spending on fresh fruits and vegetables would rise about $15 a year..."

https://www.nytimes.com/roomfordebate/2011/08/17/could-farms...

That just means strawberries become a luxury fruit. That’s fine. Nobody needs strawberries. And it provides an incentive for automation.

I’m not sold on expansive UBI. (The EITC seems like a better middle step.) But I don’t buy this argument.

Yes, but then expand the strawberries issue to virtually all vegetables and fruit (except those harvested outside the country).

Making fruit and vegetables a luxury doesn't fit onto anyone's list of good public health outcomes.

> expand the strawberries issue to virtually all vegetables and fruit

Lots of produce production is already 90% automated. (Corn, for instance.) Strawberries are a particularly manually-intensive fruit.

Or leave the ground fallow if it could not be farmed economically.

The obvious solution to that is to import labour intensive goods.

> The obvious solution to that is to import labour intensive goods.

This is known as colonialism. It comes with a set of extremely ugly incentives. For example, keeping the population of the source country as poor as possible, at all costs, so that they accept to work for you for even less money.

If that even worked. I suspect that a lot of people would just give up fruit and eat more bread rather than pay what it would cost to hire locals to pick strawberries.

Alternatively, we might see a rise in pick your own. I've picked many pounds of strawberries for freezing (smoothies). It's better quality than buying from the store, but not particularly cheaper. If the price of strawberries went up significantly, I'd absolutely do it more often.

Spending on food as a fraction of income is at historic lows throughout the developed countries. People used to pay more for food, why can't they do it again?

Because the money's gone elsewhere.