The influence of pollinator diversity and behaviour on pollen movement in Brassica rapa chinensis (Pak-Choi) crops, and its significance for gene escape
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The overall aim of the study was to assess the risk of gene flow from Brassica crops by insectmediated pollen transport. I measured the viability of pollen in Brassica flowers throughout crop development and compared this with the viability of pollen transported by insects inside and outside one early- and one late-season crop. In order to evaluate the relative importance of different species in pollen transport, I measured abundance of flower visitors during crop development, and measured the foraging behaviour of five key pollinator species throughout the growing season, in relation to variation in microclimate, crop phenology and the relative abundance of other pollinator species competing for flower resources. Flower visiting insects of Brassica rapa crops were highly diverse, and their abundance and diversity changed with crop phenology. I found similar abundances at the family level for both crops studied, although capture rates were greater in the early- than in the late-season crop. Across flowering development, the greatest numbers of insects were captured at the peak of flowering for both crops. During the flowering period, Diptera was the most abundant order collected, followed by Hymenoptera. The most abundant family in Hymenoptera was Apidae which tracked crop development in both fields, with greater numbers of insects captured inside than outside the field. Standardized-count pollen loads were smaller in Diptera than in Hymenoptera. Of the five key pollinator species sampled, Lasioglossum sordidum (Hymenoptera: Halictidae), Apis mellifera (Hymenoptera: Apidae) and Bombus terrestris (Hymenoptera: Apidae) transported similar pollen loads, which were much greater than those carried by Eristalis tenax (Diptera: Syrphidae) and Melangyna novae-zealandiae (Diptera: Syrphidae). The numbers of insects captured outside of the crop were 10% and 33% of the totals captured inside for the early- and the late-season crop, respectively. The proportion of insects entering versus leaving the crop varied considerably across species, crops and trap location (i.e., whether traps were inside or 50 m outside the border of the crop). However, it is worth noting that not uncommonly more insects were attracted into the crop early in the season, staying there rather than leaving, and then when flowers started to disappear there was a massive escape of insects leaving. This research provides evidence for the influence of crop age on the foraging behaviour of key pollinators and for species-specific variation in the foraging behaviour of Brassica visitors with crop development. Temporal variation in the rate and variability of movement between flowers, and the duration and variability in time spent on each flower, throughout the growing season differed markedly between pollinator species. Flower density, plant density, and the abundance of other insects contributed to the observed variation in pollinator behavioural activity for A. mellifera, E. tenax, M. novae-zelandiae and L. sordidum. Bombus terrestris had the greatest rates and variability of movement, and the greatest rates of flower visitation among all key pollinators studied. Therefore B. terrestris might contribute to gene flow to a greater extent than other key pollinators. Additionally B. terrestris had the greatest variability in the rate of movement, increasing the risk of pollen movement over long distances. In summary, I found that (i) insect abundance and diversity changed with crop phenology and Diptera was the most abundant order collected, (ii) flower density, plant density, and the abundance of other insect pollinators were important factors explaining pollinator behaviour for all key pollinators, except B. terrestris, (iii) B. terrestris might contribute to gene flow to a greater extent than other key pollinators, because it has a greater rate of flower visitation and a greater flight distance between flowers than other pollinators, and (iv) pollen viability tended to decrease with crop development and declined sharply even just 50 m outside the edge of the crop.