Q What are chafer grubs?
A Chafer grubs are the larvae of large, flying beetles. There are several species, collectively known as chafers. The grubs live in the soil, feeding on plant roots.
Caption: Chafer grubs do most damage in lawns
Q What plants do they attack?
A Chafer grubs can eat the roots of a very wide range of plants, but they are a particular problem in lawns.
Q What damage do they do?
A In established borders the damage is rarely very significant, but they can kill young vegetables, strawberries and bedding plants. Occasionally they may gnaw into tubers, including potatoes, or bulbs. Damage to lawns first appears as yellowing leaves and sparse growth. Large brown patches can appear, especially in dry weather, where chafer grubs have cut or eaten a significant proportion of the roots. In severe cases, the surface of the turf can be rolled up like a carpet. Sometimes more damage is done by birds, such as crows, jays, magpies and rooks, and animals, including badgers and foxes, which dig up the lawn to feast on the grubs.
Q How do I recognise them?
A Chafer grubs live just below the surface. They have a creamy white body curved in a permanent C-shape, with a brown head and six legs. The two species that commonly affect lawns grow to about 20mm long, but others can reach 40mm.
Q Could I mistake them for anything else?
A Vine weevil larvae are a similar shape and colour, but stay much smaller and do not develop legs. Similar brown patches on lawns can be caused by leatherjackets. These are the larvae of crane flies and are dirty brown or grey, about 3cm long, without legs.
Q How do I recognise the adults?
A Four species commonly occur in gardens. The ones most likely to infest lawns are the garden chafer (Phyllopertha horticola) and the Welsh chafer (Hoplia philanthus), most common on sandy soil. These are 10mm to 15mm long, with a brown back. The head and thorax are metallic blue-green on the garden chafer and black on the Welsh. The rose chafer (Cetonia aurata) reaches 20mm long and is a beautiful metallic green all over. The largest is the cockchafer, or may bug (Melolontha melolontha), up to 30mm long with a rosy-brown back and large, fan-shaped antennae. It is a slow, clumsy flier, often attracted to lights at night.
Q What damage do the adults do?
A Chafer beetles feed on the leaves, buds and flowers of a wide range of trees and shrubs, but the damage is usually trivial. Rose chafers can sometimes disfigure flowers by nibbling the buds, but control is rarely practical or necessary.
Q When do attacks occur?
A Chafers are usually a problem in newly cultivated ground where there was previously grass, or in neglected, weedy gardens. The female beetle burrows into the ground and lays eggs near plants in summer and the larvae hatch out a few weeks later. They feed underground for up to five years, according to species, then pupate over winter and hatch as adults in late spring. Attacks are very erratic; they can be a problem some years and not others, or occur in one garden and not in the neighbouring ones. Most damage occurs at the end of the summer, though it may not become obvious until spring.
Q How do I control them?
A In flower borders and vegetable plots numbers rarely build up enough to cause serious damage, especially if grass weeds are regularly removed. Light infestations in lawns can be tolerated if the grass is growing well, but if serious damage occurs you can use the biological control Heterorhabditis megidis. This penetrates the grubs and gives them a fatal bacterial infection. It should be applied in summer when the soil is moist and the soil temperature is between 12°C and 20°C, ideally in August or early September when the larvae are nearest to the surface. The nematodes are sold as Nemasys Chafer Grub Killer, available from most suppliers of biological controls, and are despatched from late July.
Q How do I prevent attacks in future?
A Attacks are very unpredictable, but generally maintaining your lawn in good health will help it survive minor infestations. Keeping on top of grassy weeds will help keep the grubs out of borders and veg plots.