Does too little pollination reduce crop yields?

Summary

  • Pollinator numbers have been falling in recent decades, and crops that depend on them are thought to be at risk of lower yields.
  • This study compared hand-pollinated areas of four crops with areas pollinated by insects.
  • Inadequate pollination reduced crop yield moderately across the study (2.8%), with sunflowers affected the most.
  • Sunflower yields were 8% lower than their potential because of limited pollination, despite many visits by honeybees.
  • The level of pollination was not related to the number of visits from pollinators.
  • Other factors, as well as the availability of pollinating insects, are important in whether crops reach their full potential.

Background

Bumblebee on oilseed rapeIn recent years, declining insect populations have been reported in both the scientific literature and the mainstream media. Falling numbers of pollinators have been a central focus, because of their important role in crop production as well as them playing a vital part in natural ecosystems, and their intrinsic value. It has been estimated that 85% of European food crop species are dependent to some extent on pollination by insects.

We know that wildflowers can be limited by a lack of pollination, but the effect of inadequate pollination may have on crops is less well understood. This study looked at whether pollination limited crop yield of four insect-pollinated crops across Europe, and whether that was correlated to the number of insect visits.

What they did

The study compared the yield from flowers naturally pollinated by insects with those pollinated by hand using a soft brush. The hand pollination gave a maximum yield from fully pollinated flowers to compare with the rest of the crop.

Four insect-pollinated crops were studied: sunflowers, oilseed rape (OSR), pears and pumpkins, across six countries.

The scientists also surveyed insect visits to the crops at each site when the crop was in flower, between April and July. Using different techniques for the different crop species, they estimated the number of visits that each receptive flower received from insects including bees, hoverflies and other flies, butterflies and moths.

What they found

Yield reduction

Overall, there was a moderate yield reduction of 2.8% across all the sites through lack of pollination. However, some species and sites were more heavily affected. The yield from sunflowers was found to be 8% lower when naturally pollinated than when hand-pollinated. Yield from the insect-pollinated area of the OSR site in Switzerland was 6% lower than the hand-pollinated area.

Pollinator visits

There was enormous variation in the number of visits to crop flowers between crop species and sites, with sunflowers receiving over 100 visits per day, but less than 10% of OSR flowers receiving a visit. The variation in visit numbers did not correlate with the yield reduction that was seen. Most pollinators seen were honeybees, followed by flies.

What does this mean?

Oilseed rapeThe yield returned from some crops was in this study was limited to a degree by inadequate pollination. The reduction was smaller than may have been expected given pollinator population declines and previous papers reporting a larger effect, but the moderate reduction found is likely to be economically important at some sites. The study also found that the number of visits from pollinators did not correspond to the amount of pollination achieved.

As well as the availability of pollinators, the authors suggest that at least two other factors affected pollination in this study:

  • Firstly, alternative pollination mechanisms mean that some crops are not entirely reliant on insects, for example OSR can be cross-pollinated by wind, through flower collisions when it is planted densely enough, but flowers can also pollinate themselves. These are the main methods by which winter-planted OSR crops in the UK are pollinated in the early spring, before pollinating insects become active, so this alternative method of pollination may explain the good OSR yields in this study despite the low number of visits from pollinators. Pear yields were also not limited by pollination levels in this study, perhaps because, although the flowers are insect-pollinated, pears are also capable of producing fruit in the absence of pollination.
  • Secondly, different pollinating species may vary in how effectively they pollinate a crop. For example, honeybees were the predominant species seen to visit sunflowers in Italy, but despite many visits the maximum yield was not achieved, and lack of pollination seems to have limited this. Honeybees may be relatively inefficient pollinators of sunflowers, either through visiting only a few florets on each flower head (of which there can be thousands), or through not making robust enough contact with the correct areas of the floret to effectively pollinate each one on each visit. On the other hand, pumpkin crops in this study were not limited by pollination. Pollination in the German pumpkin fields was mostly by honey and bumblebees, which are effective pollinators of pumpkin flowers. Conserving these species should prevent losses in crop yield because of insufficient pollination.

Read the original abstract

John M. Holland, JM, et al, 2020. Moderate pollination limitation in some entomophilous crops of Europe. Agriculture, Ecosystems & Environment, Volume 302, 107002.

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