Pre-dispersal seed predation of Sidalcea malviflora subsp sparsifolia by Macrorhoptus weevils

Gynodioecious plant species may pose unique interactions with pre-dispersal seed predators because flowers are not identical between females and hermaphrodites.  Previous studies on gynodioecious Sidalcea (Checkerbloom) species have found that pre-dispersal seed predation by Anthonomus and Macrorhoptus weevils can severely limit seed production and fitness. One study discovered the occurrence of sex-biased seed predation of S. hendersonii by Anthonomus weevils. The objectives of this study were to determine the impact of pre-dispersal seed predation on Sidalcea malviflora subsp. sparsifolia (Dwarf Checkerbloom) by Macrorhoptus weevils by asking the following questions: 1. Are weevils present in higher abundance on hermaphrodite flowers than on female flowers? 2. Does sex-biased seed predation by weevils occur in Dwarf Checkerbloom populations? 3. What are the rates of seed predation in Dwarf Checkerbloom populations? Populations of S. malviflora sparsifolia were studied at the Santa Rosa Plateau Ecological Reserve in Riverside County, California. Flowering racemes were systematically sampled along linear transects.  The presence and number of weevils were observed within open flowers of tagged racemes for two minutes.  Tagged racemes were collected and mature fruits were dissected in the lab.  Seeds from dissected fruits were categorized as predated, viable or nonviable/undeveloped. Weevil presence within dissected fruits was noted. Fruit dissection results showed that the number of weevils within fruits, the percentage of predated fruits/raceme, and the percent of predated seeds/fruit were significantly higher for hermaphrodites than for females suggesting that sex-biased seed predation of S. malviflora sparsifolia by Macrorhoptus weevils does occur.  The percentage of predated fruits/raceme ranged from 0 – 75%.  The percent of predated seeds/fruit averaged over a raceme ranged from 0 – 35%. Weevil census data indicate that weevils occurred twice as frequently in hermaphrodite flowers than in female flowers on the six days of data collection.