An Outbreak of the Beet Webworm, Loxostege sticticalis L., in Western Kansas in 1934

Abstract

One of the most widespread insect outbreaks experienced in Kansas in recent years occurred over the western two thirds of the state during the late summer and fall of 1934. It was caused by Loxostege sticticalis L., commonly known as the beet webworm. The damage from this outbreak was comparatively slight because the chief host plant was the Russian thistle (Salsola pestifer A. Nels.), which was totally destroyed over large areas. The extent of the webworm outbreak was limited only by the distribution of the Russian thistle, the only vegetation to flourish on cultivated lands over much of the western two thirds of the state during 1934. The wheat stubble, abandoned lands, allotment acres, and roadsides carried unusually heavy growths of this plant. It was also present on much of the land planted to corn and sorgo. By the middle of September the thistles were generally populated by webworms. Lamb's quarters and pigweed, though present only in limited numbers, were also hosts; the former appeared to have been the first species attacked and to have suffered the most serious injury. Occasional minor infestations also occurred in fields of sugar beets and in alfalfa that was excessively weedy with Russian thistles. The beet webworm is a Lepidopteron of European origin which reached this section by way of the Pacific coast. It has been in this country and in Canada for many years and is now firmly established in the Rocky Mountain and plateau states, where its frequent and serious attacks upon sugar beets have earned its common name, beet webworm. In addition to the plants mentioned, numerous truck and garden crops are also listed as its hosts.2 The 1934 outbreak came to the attention of the wheat growers at seeding time and caused them considerable worry. As the horses and machinery moved through the Russian thistles while disking or drilling stubble lands and allotment acres, they jarred to the ground thousands of green and black stripped caterpillars. These worms, though smaller, resembled and acted much like the destructive army cutworms that have frequently destroyed wheat in various parts of the state. Fortunately wheat is not a host plant for the beet webworm. The writer became aware of an impending outbreak of the webworms during the first week of September. At this time in the Arkansas river valley between Larned and Syracuse, from early evening until midnight the moths swarmed in countless numbers about every available light. The heat and drought of the summer had not restricted the webworms as it did the grasshoppers and many other insects, because the Russian thistles gave them a food supply upon which to complete their development. From the thistles the worms moved to the soil and constructed silken tubes, which were very resistant to desiccation. Within these tubes the larvae pupated and awaited rains. The general rains of late August and early September over much of western Kansas not only made conditions ideal for the emergence of the webworm moths, but also stimulated the Russian thistles to vigorous growth. Through-