Indigenous plants for reducing insect decline

A steep drop in insect numbers since the 1970s has environmentalists very concerned. While we might not miss some species - mosquitoes and termites spring to mind – it would be wonderful if there was a way of reducing insect decline. Without insects our world will slowly grind to a halt1. Dave Goulson2 lists some of the vital work insects do: crop pollination, recycling dung, leaves and dead bodies, keeping the soil healthy, and controlling pests. They are also food for many other creatures including fish, reptiles, some birds and animals; and they pollinate native plants. In fact, they form a critical middle link in the ecology, between the plants they pollinate and the many creatures higher up the food chain which live on them.

Causes of decline in insect populations

One cause of the decline seems to be the use of pesticides (i.e. insecticides and herbicides) and fertilisers, especially in agriculture. Monoculture crop raising means an almost total absence of any other plants over huge areas. Crops are no longer grown on small areas surrounded by borders like the old hedgerows in the UK, with so many different plants to sustain insects. Habitat loss is another factor: clearing of land for agriculture as well as for housing and industry. The green spaces left are often too few, too small, and too far apart to be useful for insects – they are “green deserts”.

Climate change is a further factor: many plants now flower around a month earlier than 40 years ago, due to increasing warmth3. But many insect species haven’t been able to adjust: they’re looking for nectar and pollen too late, when the flowering’s over. They go short of food, and since there’s less pollination, some plants may die out. Beneficial insects like ladybirds and hoverflies which keep many insect pests in check, don’t adapt to change as fast as others, so are more vulnerable. The implications for some native orchids in Australia are serious, since some only have one pollinator. There are also implications for human food security, since many crops are pollinated by insects (mostly bees).

Other likely reasons for insect declines are the over-abundance of artificial light in cities, road traffic4, and in Australia, large numbers of introduced insects such as European honeybees and wasps and Asian paper wasps, which out-compete the native ones for resources and habitat.

A “green” solution for reducing insect decline

Bulbine Lilies and Chocolate Lilies. Image by Gill Read

How can we make our existing green spaces more suitable for insects? Pointers are emerging. A group of researchers chose a small 200 m2 plot surrounded by buildings, with just two gum trees and a kikuyu lawn5. They planted 12 indigenous species. A year later, there were 5 times as many insect species as there had been at the start; three years later, there were seven times as many, with 94 species recorded in total.

In a larger study, research examined looked at numbers of bats, birds, bees, beetles and/or bugs visiting various kinds of urban parks and reserves in Melbourne6. They found the more species and numbers of understorey plants there were in a park - and the more often they were native plants - the more visits there were by the nominated insects and other creatures. It was clear that a wide variety of understorey plants, especially native plants, attracted many more creatures than simple grass or lawn – and more than our usual simple landscaping using a limited number of species. It was also clear that plants local to the immediate area - indigenous plant species - were best at attracting indigenous insects.

So we can start reducing insect decline by adopting the twin principles of biodiversity, a rich variety – and the use of indigenous plants. We can do this on a small scale, in the home garden; on a medium scale in local parks and reserves; and over large areas in state forests, national parks or on farms.

A vegetable farm in Victoria has been trialling the use of rows of flowering plants between blocks of leeks or lettuce7. The extra plants attract and shelter beneficial insects which eat the pests. Pesticide use has been cut by 50 – 75%, so costs are lower, and yields are higher. This method is already in use in several other countries.

We can encourage our local councils to choose more indigenous plants for streets and parks. As Gio Fitzpatrick points out, many species used in Melbourne streets, like flowering gums, lemon-scented gums and spotted gums, are beautiful – but being natives of other states or regions, they don’t offer much to local wildlife8,9.

What can gardeners do?

Brachyscome multifida and Chocolate Lilies. Image by Freya Headlam

In the home garden, many insects benefit when vegetable gardeners grow herbs as well, and let some plants flower and go to seed. We can mow less often: let lawns flower and seed, so providing food and materials and shelter for insects (and birds and lizards). Or even dig up the lawn (or the nature strip), and plant small native plants instead. In any garden, we can add a variety of indigenous plants around or under existing plants, especially the indigenous grasses and small strappy plants which the study of Melbourne green spaces found were best at attracting insects. These and other small local plants are often charming in a low-key way and usually rather tough.

Obtaining locally indigenous plants

To purchase plants native to your local area you will need to seek out a local indigenous nursery where tube stock costs only $2 to $4 .  Or you could find out what plants are locally indigenous and source them from a general nursery.  Online searches which include the words "indigenous plants" and the name of your area will usually be helpful.

Notes:

  1. My thanks to Armon Aristidou for his help suggesting some sources for this article.
  2. The species pictured here are indigenous to a region in Victoria and are not intended to be promoted for use Australia-wide.

References

  1. Cardoso P. et al.  2020.  Scientists’ warning to humanity on insect extinctions.  Biological conservation 242:108426
  2. Goulson D. 2021. Silent earth:  averting the insect apocalypse. Jonathan Cape.
  3. Wyver C. Reeves L.  2020 (Feb 5). Plants are flowering a month earlier. The Conversation.
  4. Martin A.E et al. 2018. Flying insect abundance declines with increasing road traffic. Insect conservation and diversity 11, 608 – 613.
  5. Le Page M. 2021. Improving even tiny green spaces boosts urban wildlife. New Scientist, 18:10.
  6. Threlfall C. et al. 2017. Increasing biodiversity in urban green spaces through simple vegetation interventions. Journal of applied ecology, vol 54:1874 – 1883.
  7. ABC TV Landline. 2020: April 5.
  8. Fitzpatrick G. 2021. Seeing the urban forest for the trees: considerations and recommendations for the Bayside Urban Forest Strategy Unpublished paper.
  9. Fitzpatrick G. The Gardener’s guide to the birds of south-east Melbourne. (recent self-publication)

by Freya Headlam