Study recommends large buffer zones for sensitive habitats

Buffer zones of 100 metres or more are needed around sensitive habitats to protect butterflies from Bt maize pollen, according to a new study by German and Swiss researchers.

The researchers explored the potential exposure of butterfly larvae to Bt maize pollen drift, taking the region of Reusstal in Switzerland as a case example. The region is characterised by a patchwork of intensive arable land interspersed with nature reserves.

The researchers measured the amount of maize pollen deposited on a favourite butterfly host plant, the nettle, at varying distances from maize cultivations.

They found that many butterfly larvae would overlap with periods of pollen dispersal from maize crops. Most of the pollen is deposited near the maize field, but some is deposited at a distance of 500m.

Using a simulation model, the researchers examined the effect of different pollen dispersal ranges and Bt maize adoption rates on the exposure of protected habitats, and explored the consequences of different buffer zones around protected habitats.

They found that if Bt maize were grown in a setting like the Reusstal, where arable fields exist in close proximity to nature reserves, every protected habitat would receive some Bt maize pollen.

In such situations, the authors concluded, a case-specific assessment should be mandatory to estimate if a predicted Bt maize cultivation could pose a serious risk for protected habitats and species, even for distances above 100m.

The researchers recommend buffer zones of at least 50–100m around Bt maize fields. They add that where highly exposed and Bt-sensitive butterflies must be protected, 800m buffer zones might be justified, which would mean an almost complete ban on Bt maize cropping.

Other scientists have found that maize pollen can drift over several kilometers.

In contrast, the European Food Safety Authority (EFSA) recommends buffer zones of only 30m to protect butterflies in protected habitats from Bt maize pollen.

Potential exposure of butterflies in protected habitats by Bt maize cultivation: A case study in Switzerland

Andreas Lang, Bernadette Oehen, Jan-Henning Ross, Katharina Bieri, Andreas Steinbrich
Biological Conservation
Volume 192, December 2015, Pages 369–377 (open access)


• Possible exposure of butterfly larvae to Bt maize pollen drift was explored.
• Many butterfly larvae would overlap temporally with the period of maize anthesis.
• Most of the maize pollen was deposited near the maize field.
• However, maize pollen was also dispersed to a distance of 500 m.
• Thus, buffer zones without Bt maize farming are required around protected habitats.


Transgenic Bt maize can produce insecticidal Cry proteins toxic to butterflies and moths (Lepidoptera). In protected habitats near maize fields, Bt maize pollen containing the toxin can be drifted by wind onto host plants of Lepidoptera, and inadvertently harm lepidopteran larvae feeding on these host plants. For a heterogeneous, agricultural landscape in Switzerland, we investigated the butterfly community of protected habitats and their potential exposure to possible cultivation of Bt maize, recorded the densities of maize pollen deposited on a butterfly host plant, simulated the effect of different pollen dispersal ranges and Bt maize adoption rates on the exposure of protected habitats, and explored the consequences of different buffer zones around protected habitats. On average, the 49 recorded butterfly species showed a temporal overlap of larvae of 50.10% ± 30.09% with the maize pollen shedding period. Mean maize pollen density on nettles (Urtica dioica) was 6.49 ± 13.58 pollen/cm2 (range: 0–100). Most of the pollen was deposited close to maize fields less than 30 m distance, but pollen also drifted onto host plants as far as 500 m away. In simulations, protected habitats were highly exposed to Bt maize pollen deposition even at low adoption rates of Bt maize, given that maize pollen is distributed to larger distances. The conflict between species conservation and Bt maize cultivation could be minimised by establishing buffer zones around protected habitats, where non-Bt maize is grown. The results and the known sensitivities of lepidopteran larvae to Bt suggest at least 50 m–100 m broad buffer zones, and case-specific risk assessments for distances above 100 m.