The role of insect visitors of Glechoma hederacea (Lamiaceae) and Anthriscus sylvestris (Umbelliferae) as pollinators

Glechoma hederacea (Lamiaceae) and Anthriscus sylvestris (Umbelliferae) are common species in the Netherlands. Both species can be pollinated by insects. G. hederacea has zygomorphic flowers. Nectar is easily accessible. The visiting group of insect species is called the visitor guild. Each insect species has his own pollination effectiveness. This effectiveness depends on (behavioural) characteristics: pollen load, pollen deposition, foraging speed, and flight distance. The composition of the visitor guild and the frequency of the visits of the vanous insect species combined with the effectiveness leads to the relative importance of visitors. Research was done in road verges in the surroundings of Assen in the Netherlands. There were three areas with differing agricultural activity. The visitor guilds were investigated by plot observations and transect observations. Glechoma hederacea was split into plant types with female or hermaphroditic flowers. Both G. hederacea and A. sylvestns tumed out to be self-compatible by a breeding system experiment. Glechoma hederacea produced nectar faster during the day than during the night. Lamium album, another common species, produces four times as much nectar, and is thus more attractive for pollinators. The most common visitor on G. hederacea (both plant types) was Apis melifera. Rhingia campestns was most abundant species on hermaphroditic flowers during transect walks. The number of visits per plant did not differ in three areas with increasing agricultural activity. Extra hand pollination in one verge showed that at least two visits are necessary for maximum seed set in that area. Effectiveness was mostly investigated by following individual flower visiting insects. For each trait, different insect species had the most favourable value. A fluorescent dye powder (= pollen analogue) experiment showed, that pollen grains can be transported over 64 meters in two days. For A. sylvestris, the same methods were used. The main visitors (walking transect observations) were Enstalis tenax/pertinax and Empis tessellata. Plot observations said that E. tessellata was the main visitor. For each trait, other insect species had the most favourable values. Fluorescent dye powder showed that pollen grains could be transferred over 100 meters in a day. Overall, insects seemed not to have preferences for fluorescent coloured umbels. On another common umbellifer, Aegopodium podagraria, the main visitor was Helophilus sp.. Anthnscus sylvestns and hermaphroditic G. hederacea had halfspecialised, and female G. hederacea had mostly specialised visitors. Results show that each insect species has its own contribution to cross-pollination: either it deposits a lot of conspecific pollen or flies large distances between two consecutive visits, etc. When relative effectiveness is used and combined with relative abundance of the visitors, Syrphidae rest and Bombus pascuorum were the most important pollinators for female G. hederacea. Rhingia campestris was the most important pollinator for hermaphroditic G. hederacea. Eristalis tenaxipertinax was the most important pollinator for A. sy!vestns.