Insect visitors and their relation to canola yield components in Australia

Canola is a major oilseed crop and benefits from insect pollination. The relative role of insect abundance, diversity, and roles of particular taxonomic groups however have been little explored. In Australia, it has been assumed that the introduced honey bee Apis mellifera is the pollinators of canola, which has lead to concerns about pollination in the wake of incursion of the parasitic mite Varroa destructor in Australia. Wild insects are also facing declines, and therefore landscape management such as semi-natural vegetation retention near crops, may help increase pollinator populations. To disentangle the contribution of insects to canola yield, and the role of semi-natural vegetation in the landscape we conducted caged experiments and pollinator observations over two years across 44 farms in New South Wales, Western Australia and South Australia. We compared canola yield components (seed weight, pod number, seeds per pod, and 1000-grain weight) from five plants harvested from quadrats with and without pollination exclusion cages along three transects each 20 m and 100 m into the canola crop. During canola flowering along these transects within a quadrat, timed observations were made of insect visitors. Our experiments were conducted across We found that insects significantly improved canola yield components. Whilst honey bees were the most abundant visitors, the dominance of honey bees did not drive canola yield, and instead only total insect abundance did. Wild insect abundance was reduced at greater distances into canola fields, whilst insect abundance was enhanced with greater forest cover connectivity in the landscape. Overall our results show that rather than honey bees, promoting greater overall insect abundance in the landscape has the greatest benefit to canola field.