Friday, 25 April 2014

Reader responds: nettle patches for butterflies

Dave Showler, ecological consultant and NWT member

I noted in the Spring 2014 edition of NWT's Tern ‘What are the best plants to grow to attract butterflies to my garden?’ that it suggests that a small patch of nettles in a sunny location may attract red admirals, commas, peacocks and small tortoiseshells (the larvae of which use nettle as a foodplant) to lay eggs. Although often stated - by many UK nature conservation organisations and in publications giving advice on wildlife-friendly gardening - that small patches of common (stinging) nettle in gardens attract these four nymphalid butterflies, there is little evidence of benefits for them as general observations and at least one  published study indicate that such patches are in fact rarely used.
 
Red Admiral, photo by Elizabeth Dack
As part of the ‘Biodiversity of Urban Gardens in Sheffield’ (BUGS) project, small patches of nettles were created in 20 urban gardens to provide breeding sites for red admiral, comma, peacock and small tortoiseshell (Gaston et al. 2005). Over the three-year study period (2000-2002) only two comma caterpillars were found on a single patch in one year (2000). Some patches were, however, used by six common specialist nettle-feeding insect species, including two micro-moths. Failure to attract butterflies was not unexpected, two main reasons for this are suggested: 
  1. Nettle patches were small and patch size is known to influence use of nettles, larger patches being preferred (at least by peacock and small tortoiseshell to support their gregarious larvae), but even larger patches in gardens examined had no caterpillars;
  2. Nettle is one of the UK’s most common and widespread plants, as such nettle is unlikely to be a limiting resource (and nettles outside of gardens are mostly used).
It would be interesting to record if NWT members have any of these butterflies using nettle patches in their gardens, and if so the approximate size and location of these patches.

I introduced wild hop to my urban Norwich garden seven years ago where it grows up a trellis on a south-facing wall open to sunlight. Two years later I had at least nine comma larvae (impressive caterpillars, flecked brown and black with a whitish ‘saddle’ giving the appearance of a bird dropping) with subsequently two chrysalises observed. In the following year two larvae were seen but I have recorded none subsequently.  

Holly blue, photo by Bob Carpenter
In truth, for most butterflies most gardens represent sub-optimal breeding habitat as they rarely provide conditions capable of supporting many larvae to maturity. One species perhaps bucking this trend is a small and fairly inconspicuous butterfly that is frequent in Norwich and other urbanised areas in the county, the holly blue. It has two broods; holly is eaten by caterpillars of the spring generation and ivy by the second in late summer. Adults, on the wing from early spring, may be seen flitting around gardens where its larval food plants are present.

For adult butterflies, growing appropriate nectar-providing flowers in gardens should certainly be encouraged as they offer forage resources for numerous more dispersive species (especially nymphalids including the migratory painted lady, and small, large and green veined whites), as well as many other insects such as bees and hoverflies. This is especially important considering the scarcity of nectar and pollen-providing flowers (other than spring-flowering, oil-seed rape) in the present-day, intensively cultivated agricultural landscape.

References

Asher J., Warren M., Fox, R., Harding P., Jeffcoate G. & Jeffcoate S. (2001) The Millenium Atlas of Butterflies in Britain and Ireland. Oxford University Press, UK.

Gaston K.J., Smith R.M., Thompson K. & Warren P.H. (2005) Urban domestic gardens (II): experimental tests of methods for increasing biodiversity. Biodiversity and Conservation, 14, 395-413. http://www.bugs.group.shef.ac.uk/BUGS1/sources/bugs-reprint2.pdf

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