Factors Affecting the Ratio of Female to Male of Pinewood Nematodes Bursaphelenchus xylophilus (Steiner & Buhrer) Nickle

The pinewood nematode Bursaphelenchus xylophilus (B. xylophilus) is responsible for a devastating disease of pine forests. Its pathogenicity is closely related to the numbers of individual B. xylophilus. The ratio of female to male (sex ratio) is key to understanding population change in this species. The sex ratio of B. xylophilus varies widely, but it is unclear how it is affected by environmental changes. The sex ratios of nematodes, isolated from different samples in the wild, varied between 0.93 and 2.20. Under laboratory conditions, maternal age and the population did not affect the sex ratio of progeny. A change from good to poor nutritional status was associated with a reduction of the sex ratio of progeny from 1.85 to 1.41, which was speculated to result from a change in the primary sex ratio. Thus, B. xylophilus effectively maintains the sex ratio with maternal age and population changes but adjusts the sex ratio of progeny on the basis of the changes in nutrition.