The relationships between host selection and subsequent larval performance in three free-living graminivorous sawflies

1. Selection of host plants for oviposition by females of three graminivorous sawfly species, Dolerus puncticollis C. G. Thomson, Dolerus picipes Klug. and Pachynematus clitellatus Lepeletier, was investigated experimentally using three grass species, Lolium perenne L., Festuca rubra L. and wheat, Triticum aestivum L. cv. Hunter. 2. Both species of Dolerus showed significant preferences for one grass over the other two, D. puncticollis preferring L. perenne and D. picipes selecting F. rubra. P. clitellatus females used L. perenne and F. rubra equally for oviposition, but avoided wheat. 3. The results correlated closely with larval performance trials, with D. puncticollis larvae having maximal survival and growth rates on L. perenne, and D. picipes larval performance being maximized on F. rubra. P. clitellatus larvae survived well on both grasses, but had a high level of mortality on wheat. 4. P. clitellatus larval growth rates were significantly higher on L. perenne than on F. rubra. Individual P. clitellatus adult females developing from larvae reared on L. perenne reached a significantly greater dry weight than those reared on F. rubra, but there was no significant difference in weight between males reared on the two hosts. There was some evidence that this disproportionate effect of host plant quality on the weight gain of the two sexes was translated into a bias in sex ratio of the eggs laid towards a greater proportion of females on L. perenne than on F. rubra. This manipulation of sex ratio would have a potential benefit for ovipositing females through disproportionate fecundity gains for female offspring.