Influence of Tunnel Size and Nesting Medium on Sex Ratios in a Leaf-Cutter Bee, Megachile rotundata12

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Abstract
In the leaf-cutter bee Megachile rotundata (F.), sex ratios of 3 ♂:l ♀ and 2 ♂:l ♀ can be consistently obtained from media with nesting-tunnel diameters of 5.5 and 6.0 mm, respectively. Smaller tunnels with inside diameters of 4.0 mm yielded bees with sex ratios of 5–11 ♂:l ♀. The type of medium of which the tunnels are composed appears to exert little effect on the sex ratio. Tunnels from 2 to 4 in. long are considered optimal for leaf-cutter bee propagation. Unfavorable sex ratios result from tunnels less than 2 in. long, regardless of their diameter. Space is inefficiently utilized in tunnels more than 4 in. long, and viable adults in the lower part of these cell series are often unable to chew their way out. Bees reared in large-diameter straws are 2 to 4 times the size of those reared from small tunnels, collect more pollen per foraging trip, and provide each cell with a greater quantity of provisions. As a result they pollinate more alfalfa florets to secure the greater quantity of pollen, and bee-for-bee are economically more advantageous. The innermost cells of each series usually contain females and the outermost contain males. Females, which may construct several cell series during their lifetime, repeat this pattern consistently and are therefore believed capable of controlling the sex of each egg deposited. Exceptions to the general pattern of sex location in each series of cells are not uncommon. It is suggested that consecutive occupation of 1 tunnel by several females or the construction of the series by a senile female may account for these deviations.

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