Iconic Cambridge courts are being disfigured, as crows attack the lawns of at least seven colleges. Jesus and Pembroke have had to re-turf entire lawns, with signs up around colleges about the problem, while at least five other colleges, including St Catharine’s, Emmanuel, Murray Edwards, Selwyn and Clare, have seen lawns destroyed, as a constant battle is waged between gardeners and crows.

However, the birds are not the root of the problem. This year central Cambridge has seen an influx of large populations of chafer grubs, the soil-dwelling larvae of chafer beetles. These grubs feed on the roots of the grass, making the lawn vulnerable to damage.

Crows like to feed on them, and the weakened grass roots mean that when they do, they dig up the turf. The large numbers of chafer grubs mean that the crows are constantly feeding on them, giving the turf little time to recover, and creating the mess that we see in Cambridge today.

Many people are asking why some colleges are affected and not others. Paul Gallant, head gardener at Selwyn, explained to Varsity that it all depends on the type of soil. “The grubs like light, sandy soil like the soil at Selwyn. Wolfson and Robinson don’t have the problem because they’re on clay”.

Steve Elstub, Clare's head gardener, expanded on this, saying: “There are light soils around the river Cam because of the silts. This is an open, free-draining soil that helps grubs to grow at a larger rate. This means that colleges closer to the river have been more affected by the grub. Colleges away from the river have heavier land, meaning it is more difficult for the adult chafer beetle to lay in.”

In terms of spreading between college's, Gallant explained that the chafer beetle cannot fly very far or very high, so it is unlikely to travel from college to college. However, he said that Selwyn’s infestation had come over from the Sidgwick site.

Adding insult to injury is the fact that the main control measure for this problem has been outlawed by the EU. In April 2013, the EU enforced a Europe-wide ban of three bee-harming pesticides called neonicotinoids, including imidacloprid, which is the active ingredient in the insecticides previously used to get rid of the chafer grub.

In January 2016, this ban was reviewed, but in April it was confirmed that imidacloprid is highly toxic to bumblebees and it was definitively taken off the market. Elstub said, “this is part of a widespread protocol from the EU to reduce chemical use in general because of their possible damaging effects on wildlife and the potential for them getting into the water system”.

Instead, colleges are opting for the use of nematodes, which are microscopic, whitish-to-transparent worms that eat and destroy the chafer grubs. However, nematodes can only live in wet soil that is not too cold, which means that the soil needs to be irrigated. Large areas of turf are hard and expensive to irrigate, meaning that this natural, biological control measure can only be used to great effect on small areas of roughly 15 to 20 metres squared.

Jesus College have had to re-turf their entire first court lawn as the area affected is too large for the use of nematodes. A post on the College’s Facebook page said: “The situation has become quite severe over the last two to three weeks and large areas of lawn have suffered as a result, First Court in particular. The use of nematodes has proved unreliable.” They added: “Our gardeners will apply topsoil and seed along with fertilizer (which the birds dislike) once the destructiveness has ceased.”

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Clare, only affected in a few, small areas of roughly 4 meters squared in its Memorial Court, are also trying other methods, including rolling the land to make it harder, so that the grubs go deeper into the soil, out of range from crows and birds. They have also tried covering the areas with netting as a deterrent to the crows, allowing the turf time to recover from the grub.

Gallant of Selwyn says that there is a new product, not harmful to crows, which he intends to employ a company to apply. The priority is that the grub removal method does not harm the birds: “As long as I don’t have a lawn full of dead crows,” Gallant said. He ruled out the possibility of re-turfing the lawn as it would require too many hours of labour and would therefore be too expensive.

If noticed early, the threat can be reduced before it really sets in. Elstub said: “The egg-laying takes place in June and July and at that time, if you spot the adult chafer beetles on the lawn, you can mow the lawns accordingly with a rotary mower and collect up the adults. However, there is only about a two-week window for this and sometimes it is difficult to spot them.”

The winter weather could temporarily help solve the problem as the cold will force the grubs deeper into the soil, out of reach of the crows. However, Gallant hopes to remove the grub before they bury down, in order to prevent further problems in spring and, most importantly, for the May Ball. The chafer grub problem goes back ten years or so and colleges have experienced the problem before. It is also a problem nationally.

“We’ll never be rid of the problem,” said Elstub. “It’s a case of managing it and dealing with it in the most appropriate way possible.”