Karl was born on May 26, 1912, in Buffalo, New York. His father was a physician and surgeon who had excellent training in biology and paleontology in high school and collected butterflies as a hobby. As a youngster Karl learned from him, collecting and rearing butterflies from the meadows and woods near his home.

He started his college training in metallurgical engineering at Carnegie Tech. Discovering soon that he would be much happier as an entomologist, he transferred to Cornell. At first he was attracted to medical entomology, but subsequently a course in taxonomy of Hymenoptera led to a lifelong career devoted to systematic and behavioral studies of solitary wasps and bees. Karl received BS, AM, and PhD degrees from Cornell University. After completing residency at Cornell in 1939, he worked briefly for an insecticide company in western New York.

In March 1941, he was appointed associate entomologist, Division of Insect Identification, U.S. Department of Agriculture with an office in what is now the National Museum of Natural History. Karl was to provide identifications of wasps and bees, to prepare revisionary studies, and to care for the national collection in those groups. After Pearl Harbor he was commissioned a first lieutenant in the Army. Following training and several state-side assignments, he was appointed commanding officer of one of the Army Malaria Survey Units. His unit served with Fifth Air Force in New Guinea, Leyte, Luzon, and Okinawa. While in the field, six days a week were spent in malaria survey work, but Karl reserved the seventh, Sunday, for collecting wasps and bees. He remained active in the Air Force Reserve, retired as a full colonel in 1972, and then was appointed the first national consultant in Entomology to the Air Force Surgeon General.

He returned to the Museum on February 1, 1946. In 1951, Karl was appointed leader of taxonomic investigations of Hymenoptera with a staff of four specialists. Early in the post-war years he began an active program of field work for collection and behavioral study of wasps and bees, collecting locally in areas that are now lost to suburbia’s increasing spread. Family vacations alternated between coastal North Carolina and West Virginia, providing him with the opportunity for field work in two diverse ecological areas.

In 1954, he started a trap-nesting program in the Washington area. Bundles of wooden traps with borings of four diameters were set out in areas likely to have numbers of twig-nesting wasps and bees. In 1956, Jack Clarke invited him to set out traps at Plummers Island. The program continued at the Island through 1964 and also included traps set out by Karl or collaborators near Buffalo, New York, coastal North Carolina, Archbold Biological Station in Florida, Southwestern Research Station, and other localities in Arizona. Karl’s book, Trap-nesting Wasps and Bees--Life Histories, Nests and Associates was published by the Smithsonian Press in 1967 and was supported in part by a grant from Washington Biologists’ Field Club. He had an active research program at the Island through the 1960s and published five numbers in the Natural History of Plummers Island Maryland series including an annotated list of the 274 species of wasps that had been collected on the island through 1963.

After almost 25 years with the U.S. Department of Agriculture, he transferred to the Smithsonian in 1965 to become the second chairman of the Department of Entomology, replacing Washington Biologists’ Field Club Member, Jack Clarke. The most important accomplishment during Karl’s chairmanship was the negotiation of the PL 480 contract between the Smithsonian and the National Museum of Colombo establishing the “Ceylon Insect Project” with himself as principal investigator. He was appointed a senior scientist when his term as chairman expired in mid-1971.

The Ceylon Insect Project added numerous species to the national collection not represented previously, and many newly identified species to the Colombo Museum collection. More than a dozen specialists in different orders of insects visited Ceylon during the 1970s with an assistant hired a couple of native technicians to assist in the field and collected many specimens using a variety of techniques. Over 100 specialists all over the world were project collaborators; they studied and identified the specimens collected in their specialties, describing many of them as new. Karl published a series of 21 revisionary and behavioral studies, Biosystematic Studies of Ceylonese Wasps, totaling more than 800 pages, including descriptions of many new species and genera as well as a wealth of behavioral observations. In 1980, Karl was honored with a second doctorate from Peradeniya, Sri Lanka, for his extensive work on the Ceylon Insect Project.

He was co-editor and co-author of a catalog of Hymenoptera in America North of Mexico in 1951 and served subsequently as chairman of the editorial board and co-author of the revised, computerized three-volume edition of the catalog in 1979 listing almost 17,500 species.

Karl retired in 1993 at 81, but continued to be active in the field and, when in Washington, was at his office in the Natural History Building five days a week. In August 2000, Karl was forced by declining health to finally close shop and move into an assisted living facility in Lorton, Virginia.

Among his honors and awards are the Legion of Merit and Air Force Commendation Medals. He was an honorary member of the Entomological Society of Washington and the Societe Entomologique d’Egypte, a fellow of the American Association for the Advancement of Science and of the Entomological Society of America, a corresponding member of the American Entomological Society, and a research associate of the Archbold Biological Station. He was on the editorial board of Journal of South Asian Natural History. Ninety-three taxa of arthropods were named for him, several having Plummers Island as the type locality. He was honored in 1996 with a Festschrift volume of the Memoirs of the Entomological Society of Washington: Contributions on Hymenoptera and Associated Insects, which was dedicated to Karl V. Krombein.

He was elected to Washington Biologists’ Field Club in 1957, served as president from 1970 to 1973, and was awarded an honorary membership in 1995. He enjoyed greatly the camaraderie of the field days and in his later years shopped for the gourmet cheddar and Swiss cheeses that accompanied our pre-dinner happy hour and the inevitable dessert pie.

Karl married fellow Cornellian, Dottie Buckingham, in 1942 and they had three daughters: Kristen, Kyra, and Karlissa. The family made frequent weekend trips to Plummers Island during the late 1950s and early 1960s.

Karl died on September 6, 2005, of cardiac arrest at the age of 93. His memorial plaque was the last installed on the Island.