Since Foster's background was Irish, and he was writing just after the recurrent Irish famines of the 1840s, when blight and colorado beetle repeatedly destroyed the food crop eaten by tenants in Ireland whose other crops went to pay their rent (*pace* Hobsbaum), I assume that this is the source of Foster's imagery - America was flooded with the desperate, starving people of Ireland, the survivors exported by brutal landlords like Lord Leitrim at the time.