Alfalfa tripping by insects

The pollinating insects of alfalfa in Utah were studied during 1943 and 1944 to determine (a) the species of economic importance, (b) their abundance and habits, and (c) the influence of ecological factors on their pollinating activities. In Utah bees play an essential role in the commercial production of alfalfa seed. Tripping of blossoms occurred, almost without exception, only during the working hours of bees, and as a rule the amount of tripping in the fields was proportional to the number of pollen-collecting visitors. The flower structure of alfalfa is adapted to pollination by insects, and practically no tripping occurred on plants from which bees were excluded. The production of a 500-pound seed crop per acre, it is estimated, would necessitate the tripping of at least 38 million flowers. In most of the fields observed there was an insufficient number of tripping insects to accomplish that amount of tripping. A scarcity of tripping insects may therefore be an important factor contributing to the present low seed yields. The flowers producing the alfalfa seed crop in Utah are tripped mainly by the following kinds of bees listed in order of their importance: Pollen-collecting honeybees, Nomia or the alkali bee, and Megachile or leaf-cutting bees. Honeybees are most valuable to alfalfa tripping in areas where they collect alfalfa pollen; Nomia is the leading tripping insect only near its isolated nesting sites; while Megachile populations are so widely dispersed that nowhere were they the leading tripping agent, but they aid substantially in the tripping in many localities. Competitive plants in the environment of alfalfa fields probably determine in large measure where and when bees will work alfalfa for pollen. Nomia appeared to prefer white sweetclover; Megachile the gumweeds; and honeybees preferred mustards, clovers, thistle, and chicory. Cultural practices may influence the pollen-collecting activities of honeybees on alfalfa. Honeybees were collecting pollen from fields of nonsucculent, slow-growing alfalfa, while little or no pollen collection was in progress on more succulent fields nearby. Honeybees were working the succulent fields for nectar, but nectar-collecting bees usually trip few alfalfa blossoms. On plots from which DDT had eliminated Lygus and thrips, however, alfalfa flowered profusely on very long racemes, and many blossoms were being tripped by nectar-collecting honeybees. Applications of sulfur-pyrethrum appeared to repel bees for 2 or 3 days. Although honeybees are now the predominant trippers of alfalfa in Utah, seed yields are much lower than when wild bees were depended upon for tripping. Honeybees did not collect pollen from all fields, but only from those growing under conditions where competing pollen sources were at a minimum. Even in favorable areas they appeared to select fields for pollen collection which were in an attractive state of growth, collecting only nectar from others.