F. Thomsen was assistant chief locust officer in the Division of Entomology, Department of Agriculture, of the Transvaal Colony in 1906. During the next few years he contributed 13 notes and articles to the Transvaal Agricultural Journal (1906-1910, Vols 5-8). These dealt with the fumigation of citrus trees with hydrocyanic acid, fumigation of buildings against insect pests, the fumigation of nursery stock and greenhouses, an army worm in the Transvaal, notes on termites, recent insect pests, the brown locust and redwing locusts campaigns of 1908/9, insect pests of fruit trees, and insects new to the Transvaal. His "Notes on termites" were published also in the Department of Agriculture's Farmers' Bulletin No. 89 (1909).
At the annual congress of the South African Association for the Advancement of Science in 1906 Thomsen read a paper on "Locust birds of the Transvaal" (though he does not seem to have been a member). Only an abstract was published in the Association's Report for that year (p. 331). Also in 1906 he became a member of the South African Ornithologists' Union and the next year his paper was published in full in the Union's Journal (Vol. 3(1), pp. 56-76). In May 1911 he demonstrated a cocoon of a gregarious native silk spinning moth of the genus Anaphe (Family Thaumetopoeidae) at a meeting of the Transvaal Biological Society. Seven years later he was still (or again) employed in the Division of Entomology, Department of Agriculture (now of the Union of South Africa), and published an article "On a visit to Namaqualand" in the Bulletin of the South African Biological Society (1918, Series 1, No. 2, pp. 20-25). In this article he provided a general description of the topography of the region, its natural vegetation and crops.