Pollination of tomatoes by the bees Bombus terrestris and Lasioglossum spp.

Three treatment combinations of bee pollinators foraging on tomatoes were compared:
1) B. terrestris alone (a single 30-40 worker colony);
2) Lasioglossum spp. alone, predominantly malachurum but with some pauxillum);
3) a combination of B. terrestris (a single 30-40 worker colony) and Lasioglossum spp..

All experiments were undertaken under field conditions within three, 2 x 3 m net cages designed to hold free flying bees. B. terrestris colonies were obtained as commercially available stock (Biobest Ltd) but there is no equivalent for Lasioglossum spp.. To address this resident populations of L. malachurum and L. pauxillum were identified. These were found on bare and compacted soil oil of arable field corners. Over populations of Lasioglossum spp. two net cages were established, with a third in close proximity although not covering populations of the Lasioglossum spp.. Into this latter cage a B. terrestris colony (30-40 workers) was established. A further B. terrestris colony (30-40 workers) was allowed to forage in one of the cages covering Lasioglossum nests (Treatment 3), while the other was left without such a colony (Treatment 2). Population density of Lasioglossum in both cages was broadly equivalent, with c. 4.2 (± 0.8) nest borrows m-2. Treatment combinations were additive in terms of bee density reflecting likely practice under greenhouse growing conditions. Nets were only positioned over supporting cages on days when the experiment was being undertaken, to prevent starving of wild Lasioglossum populations.

As tomato plants produce no nectar it was necessary to cage a small quantity of Asteraceae garden plants to provide a nectar source for the bees. Experimental tomato plants (var. 'Moneymaker') were established under controlled greenhouse conditions where pollinators were excluded and were translocated to the experimental site when they had multiple flowering stems. Assessments were restricted to the first five mature flowers on each stem; subsequent flowers were removed. On each plant two flowering stems were selected, one was left open to pollination in the cages with treatments 1-3, the other was treated as a control and covered in a small net bag to prevent access by pollinators. Batches of five plants were placed in each of the three cages for a four-day period, with this repeated four times. After each period of exposure all plants were returned to greenhouse conditions and left to mature. When tomatoes had reached the same point of maturation (deep red colour) the number of fruits produced was counted (out of a maximum of five per stem) and the individual mass of each fruit was determined.