Currently, flies are used to pollinate Welsh onion for seed production. However, the use of flies for pollination has caused problems in Welsh onion seed production due to sanitation issues, smell, and the difficulty of feeding the flies. Therefore, we investigated the effects of pollination with bumblebees (Bombus terrestris), flies (e.g., houseflies, flesh flies, blue bottle flies), mason bees (Osmia cornifrons), honeybees (Apis mellifera) and blowflies (Lucilia illustris) in a small net screen house (2.2m×1.7m×2.5m) for onion seed production. Although the number of flies was about three times than that of other insect pollinators, fies and bumblebees showed equivalent pollination ratios, but the remaining three insect species showed lower values. For the percentage of seed set per umbel, the number of seeds per flower and umbel, bumblebees and flies showed similar results. These results showed that pollination by bumblebees can be used as an alternative approach in breeding Welsh onion. An analysis of the most appropriate colony of bumblebees for use as pollinators for breeding Welsh onion in a small net screen house showed that colony of fifty to eighty bees was the most efficient level. The visiting time on the flower and spending time from a flower to another flower of B. terrestris worker on small net screen house was 2.2 seconds and 1.8 seconds, respectively.