Why you need a mixture of two endoparasitic wasps including Aphidius colemani and Aphidius ervi. they can reduce the crop damage by parasitizing and killing a mixed population of a variety of host species of aphids that are responsible for the crop damage.
In the major cities of the Midwest and Northeast, they found relatively high paying jobs in meatpacking plants, shipyards, and steel mills. The second significant cause of the Great Migration was the desire of black Southerners to escape segregation, known euphemistically as Jim Crow.
The Great Migration. World War I slowed the flow of European immigrants into the United States at the same time that it increased the need for industrial workers in the Northeast and Midwest. To help fill the void, previously segregated factories began hiring African-Americans from the rural South.