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The multitude of golden black-eyed Susan is setting the Autumn Garden aglow.
The multitude of golden black-eyed Susan is setting the Autumn Garden aglow.
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Botanic Garden's chafer grub lawn damage dilemmas

Badger damage close to the Lake, November 2017Badger damage close to the Lake, November 2017
Cambridge University Botanic Garden visitors may be aware of the widespread damage that is being caused to lawns throughout the Cambridge Colleges, as well as to private gardens. The Botanic Garden has not escaped the peril of what is fast becoming the gardeners’ enemy in this region – the chafer grub and the damage it is causing.

These grubs are the soil-dwelling larvae of chafer beetles and they feed on the roots of grass, making lawns vulnerable to damage from crows, magpies and badgers. These animals feed on the grubs, and dig up the turf to get to their fodder. Up until now, Garden staff have worked hard each morning before the Garden opens, to repair overnight damage caused to the lawn by the wildlife. However, now the colder weather has arrived, the Garden’s Head of Horticulture, Sally Petitt, says the Garden will have to stop repairing the chafer grub lawn damage over the coming winter months and visitors will notice less than perfect lawns.

Sally says:“Up to now, we have either rolled back and re-patched the damaged area with turf or sown grass seed on an almost daily basis. However, the decrease in temperatures and the late autumn and winter weather conditions mean it is no longer possible to do this – grass seed won’t now germinate as it’s too cold, and the extent of the problem is too large for us to manage on a daily basis.”

Sally continues: “Visitors to the Garden will notice damage, and we want them to know we’re not ignoring it, but we are just unable to repair it until spring.”

The areas of lawn most affected in the Garden are near the Brookside Entrance, the Main Lawn and the areas around the Systematic Beds and near the Main Walk. The Garden’s horticultural team have tried a number of different methods to control the spread of the grub, including regular mowing to eradicate the eggs. However, so far nothing has worked. In summer 2016, the team attempted to control the outbreak using Terranem nematodes (Heterorhabditis bacteriophora) - a form of biological control which is harmless to humans, but which feed on the contents of the grubs.

We have continued working closely with our biological control advisor, and also with the supplier of the nematodes, and on their advice have this year applied two applications of Nematodes to the Main Lawn and also to Brookside Lawn. Despite these efforts the Main Lawn continues to harbour a population of chafer grubs, and indications are that chafer populations will diminish following successive annual applications of biological control.

Sally continues: "We know that populations of chafers exist beyond the Main Lawn and Brookside Lawn, but have limited treatments to these lawns which we maintain as high profile lawns."

Chafer grubs are becoming particularly prominent in the East of England and the wildlife has remained undeterred. Badgers - which travel many miles to feed - and birds have continued to dig up large areas of lawn to feast on the grubs. It is easy to distinguish which is badger and which is bird damage, as the badgers tend to roll the turf back, while the birds tend to pull it up.

Work to repair the lawns will begin again in earnest in the spring when seeding the patchy areas will restart. However, as with many gardening dilemmas, getting to the root of the problem will take time and patience.

Sally says: “Chafer grubs are becoming ever more prevalent, but particularly in the East of England. While the reason for this is unclear, soil condition and climate change are believed to be contributory factors, and this is an issue which is affecting many other sites. We anticipate the grubs will continue to be a problem. Our priority is that we don’t harm the wildlife that is feasting on the grubs. The best option is to try to eradicate the grubs, but this is difficult given treatments available. We will continue to apply a nematode treatment in successive years, which will slowly reduce the grub population. In the meantime we hope our visitors will understand the dilemmas caused and adapt to our less-than-perfect lawns!”

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