Native vs immigrant trees: what's best for attracting wildlife? If you are choosing a tree for your garden, several criteria are likely to be important. Size, for one. Maybe attractive flowers and autumn colour, too. You might, if you care about value for wildlife, consider whether the flowers are useful for pollinators or the fruit for birds. A cherry, a hawthorn or a rowan might appeal to you on both counts. I suspect you're less likely to consider how many plant-eating insects find your tree's leaves tasty, partly because the question may not have occurred to you, and partly because it's not clear where you would find such information anyway. But all the evidence shows that trees are crucial for garden wildlife, mostly as sources of food for herbivorous insects. These insects are not only an important part of your garden's wildlife, they are in turn food for many other creatures, especially birds. So which trees are best for this? And specifically, is nativeness a good guide to wildlife value? Do native trees support more species and individuals of insects than exotic trees? Lots of evidence says they do, but much of this evidence consists of the accumulated lists of insects that have ever been recorded on a particular tree. Which may or may not be a good guide to what you find if you go out and look at actual trees growing in towns and cities. Simon Leather and colleagues at Imperial College decided to do just that, using trees in Bracknell, and their findings are reported in the journal Urban Ecosystems. They looked at bugs (insects with piercing mouthparts), because they're both extremely abundant and a favoured food of insect-eating birds. Specifically Heteroptera (shield bugs and their numerous less conspicuous relatives), Auchenorrhyncha (froghoppers and leafhoppers) and Psylloidea (plant lice). Some of the trees examined were in parks, but most were on roundabouts. Bracknell is famous for roundabouts and some uncharitable folk apparently consider that the entire place is simply an overgrown roundabout. So what did the researchers find? Well, huge variation between different species of trees; so huge in fact that it overwhelmed any general distinction between natives and aliens. If you look at the data, it's clear that the best trees, in terms of numbers of species and individuals of bugs, were natives. The top five trees on both counts were native, with hawthorn, silver birch, ash, oak and goat willow all supporting many different species. On the other hand, the worst trees also tended to be natives; in terms of numbers of species of bugs supported, four of the bottom five were natives. The worst tree on both counts was holly, and beech and lime weren't much better. Exotic trees weren't exactly bad, they were just middling, with apple and Swedish whitebeam among the best of the bunch. Which prompts all kinds of questions. Not many exotic trees turn up on Bracknell roundabouts, so are those that do representative of the huge diversity of trees in gardens? We don't know. Although silver birch and hawthorn are outstanding, we don't know how well foreign species of birch or hawthorn would do. The omens are mixed; our native oak is better than American red oak, and field maple is better than sycamore, but there isn't a clear winner between rowan and whitebeam. And we don't know anything about aphids, the commonest bugs of all (as anyone who has parked under a sycamore knows, they're not short of aphids). The bottom line is that nativeness alone is a poor guide to value for native wildlife. In any case, you may not want to grow the best natives, because they're too large (ash, oak) or not very attractive (goat willow). And - perish the thought - growing trees that are magnets for every herbivorous insect in the neighbourhood may not be your highest priority. Anyway, in terms of providing services like shade, cooling and flood protection, all trees are good. Ken Thompson is a plant biologist with a keen interest in the science of gardening. His latest book is 'Do We Need Pandas? The Uncomfortable Truth About Biodiversity