K wrote:
They have those plants in their gardens, but they have natives as well
(usually known as 'weeds')

)

When
did the insects started to distinguish natives from non natives?

Look at the statistics for numbers of species supported by native oaks
compared with non-native trees.

That's due to pollution and the loss of habitats not the planting of a
Verbena officinalis Kay!!

Compare the speed with which rowans and
elder berries are taken with, for example, skimmia, pernettya. Look at
the species, eg himalayan balsam, which grow unchecked to pest
proportions here when they are not troublesome in their own habitat.

Sure. I understand and know what you mean. But my question was
regarding the insects distinguishing between vervain b. compared with
vervain o. Most of our wild flowers are from Europe and not 'British'
as one would like to beleive.

You may be too young to be able to compare the number of butterflies
around now with what was around in the 1950s. But as I said, gardens
aren't 100% non-native, and gardens do not make up the total of UK land.

Sure. I, as a child, remember far more butterflies and I understand
that the loss of butterflies is due to chemicals used in
gardens/agriculture, loss of edgerows and habitats and not the import
of rhodos or skimias in the 16th Century! The forestry commission in
20002 found SOD mainly on viburnums and rhodos in around 6% of
nurseries and garden centres surveyed by Defra. The spread of the
disease is due, I beleive, to garden centres bad health.

Plenty of examples, including dutch elm disease and the current thing
that is spreading on rhodos and other trees/bushes - forget its name.

Phytophthora (p.ramorum) was found on diseased rhodos. But oak decline
also involves recurrent episodes of drought, other root infecting
fungi, repeated insect defoliation and scale insect attack. P.Ramorum
wasn't found in such a large proportion that people are lead to
beleive. See the forestry commission website.

http://www.forestry.gov.uk/pramorum