A male-biased primary sex ratio and larval mortality in Eucheira socialis (Lepidoptera : Pieridae)

We investigated the sex ratio and sex-biased mortality in the Mexican pieri d butterfly, Eucheira socialis westwoodi. We studied two populations betwee n 1990 and 1997 along Mexico Highway 40, which runs from Mazatlan, Sinaloa to Durango, Durango, Populations occurring between km 64 and 101 and betwee n km 163 and 213 were designated 'eastern' and 'western', respectively We d etermined the primary sex ratio of egg masses from these populations in 199 2, 1995, 1996 and 1997, The primary sex ratio varied from 68.0 to 75.5% mal e and from 63.7 to 79.3% male in the eastern and western populations, respe ctively. The frequency of significantly male-biased full sib groups was bet ween 55 and 100% in the east and between 67 and 94% in the west. For all ye ars, in both populations, the primary sex ratio was significantly different from 1:1. Lf the probability of a given zygote being male is the same acro ss sibships, the distribution of the primary sex ratio should be binomial. For years where at least 10 egg masses were collected, the distribution of primary sex ratio was significantly different from a binomial in all years in the east and two of three years in the west. We studied larval and pupal mortality in these populations in 1990, 1991 and 1997. Larval mortality wa s consistently disproportionately male, varying from 83 to 100% male. Pupal mortality tended to be female-biased. The operational sex ratio varied fro m 65 to 71% male in the east and from 70 to 76% male in the west.