Abstract
The tendency of the two-spotted spider mite, Tetranychus urticae, to lay eggs on the abaxial (lower) side of host plant leaves has been explained as an adaptation to avoid rainfall and solar ultraviolet radiation. However, differences in the nutrition and effects of gravity between the abaxial and adaxial (upper) leaf surface could affect mite fitness. We investigated the fecundity of mites using kidney bean leaf discs with their adaxial or abaxial sides facing upward or downward. Fecundity was greater on adaxial leaf surfaces and on leaf discs facing downward but did not differ between adaxial surfaces facing upward and abaxial surfaces facing downward, suggesting that the effects of gravity and nutrition compensated for each other. Consequently, although leaf surface nutrition and gravity direction affected fecundity, these factors do not explain the biased distribution of T. urticae.
