The influence of tannins on the extrafloral nectar characteristics and insect mutualists of Vicia faba L.

The main objective of this research project was to investigate the influence of tannins on the extrafloral nectar characteristics and insect mutualists of Vicia faba L. Tannin-free cultivars of V. faba have become increasingly popular in Western Canada due to the greater digestibility of their protein by monogastrics; however, the effect of their lack of tannins on mutualistic insects is unknown. Tannin-rich cultivars of V. faba produce characteristic dark spots on the flowers’ wing petals, and on the stipular extrafloral nectaries (EFNs), which are often used by insects to help locate nectaries. Tannin-free V. faba cultivars lack these nectar guides and spots though, and may be unable to attract as many beneficial insects to the EFNs for herbivore-control purposes, and to the flowers for pollination. Accordingly, this study investigated two tannin-rich (Fatima, SSNS-1) and two tannin-free cultivars (Snowbird, Snowdrop). Extrafloral nectar characteristics were also examined, as the production of tannins can be metabolically expensive, and could come at the cost of extrafloral nectar secretion. Tannin-free cultivars are therefore expected to attract fewer beneficial insects due to their unmarked flowers and EFNs, unless the lack of tannins corresponds with an increased production of nectar or nectar sugars. To examine the effects of tannins on the insect mutualists of V. faba, surveys of insect visitors to the EFNs and flowers were conducted throughout the summers of 2013 and 2014. The vast majority of insect visitors to EFNs were ants (Formicidae), followed by ladybird beetles (Coccinellidae), flies of Camptoprosopella borealis Shewell (Lauxaniidae), and predatory (Vespidae) and parasitoid (Ichneumonidae) wasps, whereas the bees Apis mellifera L. and Bombus nevadensis Cresson were the most common visitors to the flowers. The cultivars which those species were present or absent at during surveys were analyzed using generalized mixed models. The results did not support any consistent differences in insect visitors to plants with tannins, compared to those without, suggesting that the marked difference in the visibility of EFNs on tannin-rich cultivars is not essential for EFN recognition by many insect species. Furthermore, insect visits to EFNs occurred at a highly conserved relative location along the stem, due to a probable increase in nectar production a short distance from the shoot apex. For future reference, stipules at this node were termed the Primary Active EFNs. Extrafloral nectar characteristics were studied in a growth chamber through a combination of nectar sampling by microcapillaries and refractometer measurements to examine nectar volume and sugar concentration, respectively, as well as high performance liquid chromatography to measure the proportion of each of the nectar sugars present. On average, the extrafloral nectar per stipule ± S.E.M had a volume of 0.363 ± 0.021 µL, a nectar sugar concentration (by weight) of 32.5 ± 1.3 %, a nectar sugar mass of 137.6 ± 10.0 µg, and a sugar composition of 54.4 ± 1.0% glucose, 31.1 ± 1.0% fructose, and 14.5 ± 1.0% sucrose. Although extrafloral nectar characteristics varied between cultivars, the variability did not appear dependent on tannin presence or absence, nor did it appear to influence the presence or absence of the abundant ant species Formica neoclara Emery, F. podzolica Francoeur, and Lasius pallitarsis (Provancher), at different cultivars in the field. The increased digestibility provided by tannin-free cultivars of V. faba to monogastrics such as chickens, therefore, does not appear to come at the cost of reduced visitation to the plants by ants and other beneficial insects