Mangold, at a time when the most exciting results were forthcoming on the control of development in amphibians. Typically, it was the larger questions and not the experimental material that challenged Dietrich's imagination. Encouraged by Mangold, he began to investigate that most challenging problem, control of molting and metamorphosis in insects. It was a study that was to occupy him to the end of his life. In his last scientific papers, published from 1978 to 1981 on work done while on an Alexander von Humboldt fellowship at the University of Marburg, Dietrich used modern chemical and immunological techniques to confirm his previous deductions on the role of ecdysone in the control of development.
In 1933, however, Dietrich's work at the Kaiser Wilhelm Institute was interrupted by the rise of national socialism in Germany. Warned by Mangold that he might be in political trouble with the Nazis, Dietrich accepted a position as research associate at the Institute of Marine Biology in Rovigno, Italy. From there he moved to a similar post at Stanford University, where he worked from 1934 to 1941. During this time he not only continued his studies on insect hormones but also collaborated with Victor Twitty in experiments on the role of ectodermal structures in the development of amphibia.
Dietrich's growing scientific reputation was recognized with a Guggenheim fellowship, which he held in the Department of Zoology at Columbia from 1941 to 1943. From there he moved briefly to the Connecticut Agricultural Experiment Station in New Haven before settling down as an insect physiologist at the Army Chemical Center at Edgewood, Maryland, a position he held from 1945 to 1958. There he met and married his lifelong companion, Jean Coon Bodenstein.