Are native cultivars good for insects?

I had a question from a garden designer that I thought would be useful to share..

Question:

Are non-native plants that have a UK native counterpart (e.g. Sorbus alnifolia, which is from Korea) any good for wildlife? Or does it have to be the UK native for insects to benefit?

Answer:

Sorbus alnifolia doesn't appear on any of the databases that show plant / insect interactions in the UK so I can't find any data.

However, the larvae of many insects, including butterflies and moths which we have the most accessible information on, more often than not, require a specific host plant that they evolved with to feed.

With Sorbus alnifolia having evolved with a completely different insect ecology there is less likelihood of it sharing a similar chemical composition to our native Sorbus and so may not be compatible with specialist feeding insect larvae but would likely act in a similar way as a nectar food source for pollinating insects that are less fussy generalist feeders. The native Sorbus aucuparia (rowan / mountain ash tree) supports 20 butterfly and moth caterpillars, 15 other herbivorous insects (beetles, true bugs and sawflies) and has 35 identified pollinator interactions.

Many insects go through a full metamorphosis in their life cycle, some of which we are familiar with, such as Lepidoptera (butterflies, moths and skippers) involving a larval stage as a caterpillar; but also others, such as, Coleoptera (beetles) with grubs; Diptera (flies) with maggots and Hymenoptera (sawflies, wasps, ants and bees) all of which involve an egg, larval, pupa and adult stage.

The feeding stages of the larva and adult of these insects are quite different. The larva is basically an eating machine, it has no sexual organs and just needs to gather enough resources to survive the pupa stage from which it emerges as the adult insect. While the feeding and raising of larva are different amongst the insect groups (orders) and individual species, 99% of butterfly and moth caterpillars are herbivorous.

The adult is a sex machine that just needs nectar (sugary water) to fuel its job to mate. While the adult can eat any of this nectar, providing the flower shape allows access to it (pollinating insect), the larva is very often limited to a very few plants whose chemicals it can ingest, so is known as a specialist feeder (monophagous insect).

A plant that has evolved with a different set of insects trying to eat it will almost certainly have produced a different chemical make up to deter them. The closer the plant relative the more likely they are as a suitable host to specialist insects.

With this in mind, if the native Sorbus aucuparia doesn’t fit the designers brief, would it be better to look for a cultivar from Sorbus aucuparia before specifying another species, Sorbus alnifolia in this case, from a distant geographic location?

This may well be better, because although the native cultivar has been adapted for alternative qualities such as the colour of the berries, it’s size, leaf shape or colour, it is still closer to the species (wild) specimen than an alternative species. There are just a couple of things to watch out for though, in breeding plant adaptations into the cultivar, these can affect its physical and chemical make up, such as a different shape leaf, a thicker or tougher leaf, leaf colour or a compact habit which may not be recognisable to the adult needing to lay their eggs, deter the larval dependant from recognising it as the food source, or being able to physically cope with the new physiology or chemistry.

So with all that in mind let’s have another look at the Sorbus alnifolia in question as an example, the attributes of this plant that the designer was looking for is that it would be suitable for the design of a very small back garden. It is actually a similar size to the native Sorbus aucuparia at 10 to 15m but does have a different appearance with the leaves being oval and entire rather than pinnate and that may well be another reason why this plant was being considered.

Interestingly, Sorbus aucuparia is also native to Northern China, so will a Chinese variant support the same specialist feeding insects as the British species? Well maybe but as discussed above, there is a strong likelihood that it would have distinct genetic variability based on its geographic origin.

There is a cultivar, just to confuse the issue, that is called Sorbus aucuparia ‘Chinese lace’ that is actually bred from the British / European species, not the Northern Chinese one. This grows to approximately 6m at maturity so would be a good contender for a small garden. It does have deeply lobed leaves that could affect its attractiveness to larvae but on balance is almost certainly more attractive than its Korean counterpart.

I’m just going to throw something else into the mix before I sign off. We might consider a completely different species if it fit the bill without being an alternative species or a species cultivar..

Crataegus monogyna (hawthorne) is native to the UK, is similar to the desired Sorbus, in that it has white clustered flowers in spring, and red berries in autumn / winter but is actually much smaller than the rowan at approx 4 to 8m in its species form.. Another benefit to the hawthorn is that it supports 115 butterfly and moth caterpillars as opposed to the 20 of the rowan.

Or just go with the tree that you want, in this case Sorbus alnifolia but try to make sure it is propagated and grown in the uk to stop the risk of importing harmful pathogens or insects, and is raised without the use of pesticides which will kill the insects that eat it anyway.. Other decisions can always be made around the rest of the plants in the garden that can all have a great beneficial effect.

Summary

Species (wild) type plants that evolved where you garden are best for specialist insects, such as most caterpillars, to feed on. Other plants including native cultivars, if not contaminated with pesticides or imported with harmful insects or pathogens, are mostly fine for generalist feeding insects and pollinating insects.

Have fun and happy gardening!!

Photo by Alicja Trepka