Sperm Is a Limiting Resource in the Pseudogamous Bark Beetle Ips acuminatus (Scolytidae)

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Abstract
Parthenogenetic females of the bark beetle Ips acuminatus Gyll. mate with males. Evidence is presented that, in a highly female-biased population in SE Norway (Saggrenda), sperm is in short supply: 1) In 1983, 23% of femaes in one pine trunk, and 56% of females in branches, had begun egg tunnels without a male being present; 2) 43% of ovipositing females not in harems (no male present) had not been inseminated; 3) Some uninseminated females laid eggs, but none hatched; and 4) Absence of sperm in 11% of overwintering parent-generation females indicated that they had not mated during the breeding season. Despite apparent competition for sperm, there was no evidence of pre-swarming mating: 3% of 240 young (= light) females had mated, apparently with parent-generation males. Highly biased sex ratio result from high proportions of parthenogenetic females: 92% of Saggrenda females bred in the laboratory produced all-female broods. The average harem sizes for the trunks and branches from Saggrenda were 10 and 6 females per male, respectively; average harem size from a weakly biased population in W Norway was 2.4. This study thus verifies that parthenogenetic Ips acuminatus are pseudogamous (require sperm for successful reproduction), and demonstrates that sperm can be a limiting resources in highly female-biased populations.