Submitted to: United States Department of Agricultural Interagency Gypsy Moth Research Forum
Publication Type: Proceedings
Publication Acceptance Date: 10/20/2002
Publication Date: 1/11/2003
Citation: Fuester, R.W., Swan, K.S., Taylor, P.B., Hopper, K.R., Ode, P.J. 2004. Male-biased sex ratios in laboratory rearings of gypsy moth parasitoids. pp. 24-27. In S. Fosbroke and K. Gottshalk (eds.), Proceedings, United States Department of Agricultural Interagency Research Forum on gypsy moths and other invasive species 2002: 2002 January 15-18; Annapolis, MD. Gen. Tech. Rep. NE-300 Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern Research Station. 116p.
Interpretive Summary: The gypsy moth is the most important forest and shade tree pest in the northeastern U.S. About 20 species of parasitic wasps have been imported and released against gypsy moth, but some of these had male-biased sex ratios in culture. Male-biased sex ratios in laboratory colonies of parasitic wasps used in biological control are harmful, because they can prevent the establishment of introduced species or hinder commercial production of species used for augmentative control. One of the natural enemies imported for study as a potential agent for its biological control is GLYPTAPANTELES FLAVICOXIS, a parasitic wasp that attacks the caterpillar stage of the Indian gypsy moth. Earlier studies by ARS scientists indicated that it readily attacked and successfully developed within caterpillars of the Indian gypsy moth, but that male-biased sex ratios in laboratory rearings hindered its use as a biological control agent (only females can attack and lay eggs on the caterpillar). Sex determination in this wasp is conditional whereby fertilized (diploid) eggs give rise to female progeny and unfertilized (haploid) eggs, male progeny. In this study, we monitored the sex ratios for five generations in a newly founded laboratory colony using the gypsy moth as a host. Sex ratios were male-biased in all five generations, but differed significantly being highest in the 2nd and lowest in the 3rd. Sex ratios did not differ among progenies of females that had been outcrossed and those mated with siblings nor were they correlated with those of the parental generation. Host switching (from Indian gypsy moth to gypsy moth), inbreeding depression, or failure to transfer sperm could not be ruled out as contributory factors to the male-biased sex ratios.
Technical Abstract: Male-biased sex ratios in populations of parasitic wasps used in biological control are undesirable, because a low ratio of females can prevent the establishment of introduced species or hinder commercial production of species used for augmentative control. This problem has arisen in the culture of at least four species of parasitic wasps imported for biological control of the gypsy moth, Lymantria dispar. Studies were conducted on potential factors contributing to male-biased sex ratios that have been encountered in laboratory rearings of the braconid endoparasitoid Glyptapanteles flavicoxis (Marsh) using the gypsy moth, Lymantria dispar (L.), as a host. Sex determination in this wasp is arrhenotokous, a system in which fertilized (diploid) eggs produce female progeny, and unfertilized (haploid) eggs, male progeny. In the first experiment, we stored adults at 13 or 16 oC and allowed them to mate at 20 or 25oC and found that sex ratios did not differ among progenies of parents; many females produced all male progeny, indicating that they had not actually mated. In the second experiment, females were exposed to hosts soon (0-60 min) after mating or 23-25 h later. Sex ratios were higher in progenies of females provided with the rest period, than in those which were not. In a third experiment, females were allowed to mate from one to four times with a given male. Although differences between these groupings were not statistically significant, the data suggested that more than two matings might depress sex ratios of progeny. An alternative analysis with only two groupings (1-2 matings and 3-4 matings) suggested that more than two matings might be detrimental. Therefore, it is suggested that matings of this species be controlled (limited to 1 or 2) and that females should be provided with a period of repose after mating before they are offered hosts for parasitization.