Is Tripping Necessary for Seed Setting in Alfalfa?¹

Pollinating Bees (Hymenoptera: Apiformes) of U.S. Alfalfa Compared for Rates of Pod and Seed Set

“…The typically poor pollination performance of honey bees in commercial alfalfa seed Þelds has been known for decades; they often trip Ͻ5% of the ßowers that they visit (Tysdal 1940, Bohart 1957, Dylewska et al 1970), a Þgure afÞrmed by the 2.4% tripping rate reported here. Rather than be struck by a tripped sexual column while probing for nectar, a honey bee forager works ßowers from the side, a behavior that fails to trip the sexual column and fails to pollinate the ßower.…”

“…The overall average of 2.7 seeds per pod resulting from bee visits to ßowers exceeds that known for autopollination of alfalfa (e.g., less than 1 seed per pod) (Free 1993, Strickler 1999. Cross-pollination of alfalfa has been shown to yield more seeds per pod than geitonogamous pollination (Tysdal 1940, Lesins 1950, Pankiw et al 1956, Bohart 1957, Strickler 1999, with honey bee pollination yielding an intermediate value (Pankiw et al 1956). However, the reported differences and maximum values for seeds per pod are highly variable, probably due to a combination of factors that includes growing conditions, weather, cultivar, available bloom, and the health, age and genotype of individual plants (Pedersen andNye 1962, Strickler 1999).…”

“…The irrigation methods used there, plus the intense summer heat, allows growers to subject plants to repeated bouts of water stress, which reportedly diminishes the force of the ßoral tripping mechanism. Elsewhere, the honey bee has been shown to rarely trip the alfalfa ßowers that it visits (Tysdal 1940, Reinhardt 1952, Bohart 1957, Dylewska et al 1970. In North America, most seed alfalfa is pollinated by one of two managed nonsocial bee species.…”

Bees and pollination in our changing environment

“…Because they were owned by beekeepers and thus already had value in the economic system (in economic terms, were 'internalized') and could be managed, honey bees (Meyer and Turner, 1992;Vitousek, 1994 (Müller, 1873;Brand and Westgate, 1909;Westgate, 1911;Piper et al, 1914;Hadfield and Calder, 1936;Tysdal, 1940;Knowles, 1943;Linsley, 1946;Akerberg and Lesins, 1949;Stephen, 1955;Semmens et al, 1993; many more recent authors). Other species of bees (especially Megachile) tripped the flowers and were soon found by astute observers to be efficient pollinators (Brand and Westgate, 1909;Piper et al, 1914;Archer, 1917;Sladen, 1918;Helmbold, 1929;Hadfield and Calder, 1936;Salt, 1940;Tysdal, 1940;Knowles, 1943;Stapel, 1943;Linsley, 1946;Akerberg and Lesins, 1949). Recommendations for conservation of habitats where bees could nest (Vansell, 1951;Franklin, 1952;Stephen, 1955), and the rearing or management of selected bee species (Knowles, 1943;Peck and Bolton, 1946;Bohart, 1972) (Olmstead and Wooten, 1987;Peterson et al, 1992;Wichelns et al, 1992).…”

Inbreeding alfalfa, Medicago sativa L., by sib-mating

“…Hay (1925), Carlson (1928), Kirk and White (1933), and Brink and Cooper (1936) maintained that a considerable propor tion of flowers set seed without tripping. However, Southworth (1928), Engelbert (1932), Knowles (1943), and Tysdal (1940Tysdal ( , 1946 have concluded that at most only a very small percentage of flowers set seed without first tripping, and that ordinarily there is not sufficient automatic tripping to produce satisfactory seed crops. Additional support for the idea that tripping is necessary for seed set has been reported by Armstrong and White (1935), They found that in the act of tripping, a stigmatic surface was ruptured, and that the release of a stigmatic material initiated pollen germination.…”