Seed production and predispersal seed predation in the biennial composite species, Arctium minus (Hill) Bernh. and A. lappa L.

Summary

Comparison was made of the seed production of two species of burdock, Arctium minus (Hill) Bernh. and A. lappa L., which are found in a variety of recently disturbed habitats throughout Waterloo County, Ontario. The mean number of seeds per plant of A. minus was significantly greater, 13,400 compared with 8,200 seeds in A. lappa. The mean number of heads per plant was also greater for A. minus than for A. lappa, 338 versus 112 heads respectively. Above-ground biomass allocated to reproductive heads was similar in A. minus (44.2%) and A. lappa (43.7%). Significant variation in the mean number of seeds per head was observed in A. minus, ranging from 29.0 to 47.1. The between-plant variation in this character is significantly greater than the variation between populations from various sites.

Predispersal seed predation by the microlepidopteran Metzneria lappella (Fam. Gelechiidae) resulted in 28% to 71% loss of seeds of A. minus at different sites. The observed loss in A. lappa was dramatically lower at 15.5%. The number of seeds attacked by one larval predator varied from 2 to 15 with means from 7 to 9. In addition, great variation in the number of seeds and the number of predators per head was observed. Larvae were randomly distributed among the heads at 4 of 7 sites and aggregated at the other sites. Seed predation per head greater than 60% was found with the aggregated larval distributions. Only rarely was the number of seeds attacked by a larva associated with the number of larvae in the head. The winter dispersal of heads of A. minus was much lower than that for A. lappa, 62.2% and 21.0% respectively. Also, the dispersal of seeds from heads differed between the two species. Only 21.8% of the seeds of A. minus were lost compared with a seed dispersal rate of 56.5% for A. lappa.