Wildflower strips must be in Goldilocks zone

The success of wildflower strips to attract pollinators depends on having the right mix of natural habitat and farmland

Forty percent of the Earth’s land surface is used for agricultural production resulting in natural habitat transformed into croplands, a primary driver of biodiversity loss.

Yet many biological communities such as bees (for pollination) and wasps (that kill crop pests) provide important ecosystem services to agriculture.

Finding ways to conserve that biodiversity and balance it with crop production has become a great challenge, especially given the increasing demand to feed a growing global population and the challenge to conserve bee populations.

One strategy lies in planting native perennial wildflower strips bordering crop fields to attract pollinators and pest predators. But a recent study showed that the success of wildflower strips depends on having the right mix of natural habitat and agricultural land.

That right mix is the so-called Goldilocks zone.

For the first time, a study by researchers at Cornell University of Ithaca, New York, tested this not-too-big, not-too-small approach and found that the perfect place for the Goldilocks zone was where 25 to 55 percent of the surrounding land was natural habitat.

Heather Grab, post-doctoral researcher in Cornell’s Department of Entomology, led a three-year study on 12 farms within the Finger Lakes region of central New York state. Each farm represented different landscapes, some with natural habitat and others in dedicated agricultural areas. Each farm had two plots of strawberries. One was edged by a wildflower strip four metres wide by 10 metres long. The other was a control plot edged by mowed grass.

Nine native perennials were planted in the wildflower strips including golden alexander (Zizia aurea), foxglove beardtongue (Penstemon digitalis), lance-leaved coreopsis (Coreopsis lanceolate), shrubby cinquefoil (Potentilla fruticosa), black root (Veronicastrum virginicum), yellow giant hyssop (Agastache nepetoides), cup plant (Silphium perfoliatum), blue cardinal flower (Lobelia siphilitica), and Canada goldenrod (Solidago canadensis). They were chosen for their attractiveness to bees and natural enemies of pests and because their blooming times overlapped so that flowers bloomed throughout the growing season from April to November. Other visitors included snakes, butterflies, toads, frogs and spiders.

Over three years, the researchers monitored pollinators, pests, wasps that parasitize pests, fruit yield and fruit damage. A total of 5,684 bee visits to the strawberries were recorded and 1,307 bee specimens collected. Wild bees were dominant (95.8 percent) while honeybees represented 4.2 percent of recorded visits. In total, 99 species were recorded.

As the wildflower strips became established, they were increasingly effective at attracting pollinators and they were the most effective at attracting bees when the surrounding natural landscape was 25 to 55 percent of the landscape range.

“If you put a wildflower strip in a landscape that has no natural habitat, totally agricultural, all you have is a tiny wildflower strip and it’s going to have no effect whether you are talking about biodiversity or crop pollination because with not enough flowers, there is nothing for pollinators to recruit to,” said Grab.

“If you put it in a landscape with all kinds of environmental habitat with hedge rows and wind buffers and other vegetation, then there are so many beneficial resources that the wildflower strip is a drop in a bucket. There is so much biodiversity that adding a tiny wildflower strip won’t change things much. It was theorized that the benefit would be in this Goldilocks zone that will provide the greatest contribution and you will get the most benefit for conservation, pollination services, pest control and the environmental services you want.”

Grab said that wildflower strips outside that zone might result in a pest increase. An excessive amount of habitat will allow pests to thrive and become invasive. In those landscapes with both the least and the greatest natural habitat, the abundance of pests was greater, especially generalist pests like the tarnished plant bug (Lygus lineolaris), which feeds on the seeds of developing strawberries. The natural enemies of the bug are parasitoid wasps. However, they found that wildflowers outside the Goldilocks zone did not add more wasps, so their parasitism rates were actually reduced. Further studies will help understand why.

“Ultimately,” Grab wrote in their report published in Proceedings of the Royal Society B: Biological Sciences, “the benefits of ecological intensification practices like flowering crop borders can be measured in terms of increases in crop yields. When comparing border treatments, wildflower borders reduced fruit damage and increased yield most strongly in landscapes with intermediate natural habitat cover. However, in landscapes with either high or low natural habitat cover (outside the Goldilocks zone), wildflower borders tended to increase fruit damage and reduce yield.”

Bee populations are under threat from habitat loss and infectious diseases. While concentrations of flower species in strips attract bees, they may also be carrying higher loads of pathogens but doing so successfully because abundant nutritional resources keep them healthy. During the study period, Grab added sunflowers to the floral mix and sunflower pollen has been proven in independent studies to lower infection rates in bees and improve colony health.

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