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

Underwood, DLAS, A. M.,  Evolutionary Ecology Research,  1:703-717. 1999.

We investigated the sex ratio and sex-biased mortality in the Mexican pierid butterfly, Eucheira socialis westwoodi. We studied two populations between 1990 and 1997 along Mexico Highway 40, which runs from Mazatlan, Sinaloa to Durango, Durango, Populations occurring between km 64 and 101 and between km 163 and 213 were designated ‘eastern’ and ‘western’, respectively We determined the primary sex ratio of egg masses from these populations in 1992, 1995, 1996 and 1997, The primary sex ratio varied from 68.0 to 75.5% male and from 63.7 to 79.3% male in the eastern and western populations, respectively. The frequency of significantly male-biased full sib groups was between 55 and 100% in the east and between 67 and 94% in the west. For all years, 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 across 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 was consistently disproportionately male, varying from 83 to 100% male. Pupal mortality tended to be female-biased. The operational sex ratio varied from 65 to 71% male in the east and from 70 to 76% male in the west.