What are chafer grubs?

Chafer grubs are the soil-dwelling larvae of chafer beetles. Depending on the species of chafer they either feed on decaying plant material or plant roots. Some, which are not garden problems are found in borders and compost heaps. However, several species that feed on the roots of grasses can cause problems in lawns.

Symptoms

Some species of chafer grub eat the roots of grasses and other plants. Evidence of their activities can be seen in a number of ways:

• Damage to lawns is most obvious between autumn and spring when the grubs are reaching maturity
• Patches of the lawn may become yellowish
• Birds, particularly of the crow family (jays, magpies, rooks and crows), badgers and foxes tear up turf in order to access the grubs to feed on them
• Damaging infestations can be very localised and sporadic
• Chafer grubs can be found in the soil under the loose turf. They have stout white bodies curved in a C shape, light brown heads, with three pairs of legs at the head end. They are bigger than the adult beetles and, if straightened out, can be up to 18mm (almost ¾in) long
• Chafer grubs, dung beetles and stag beetles all have similar looking grubs. Only those grubs found in turf are likely to be chafers damaging lawn roots 
• Other less troublesome species of chafer grubs can also occur in turf and garden borders, such as the cockchafer (Melolontha melolontha), summer chafer (Amphimallon solstitialis) and brown chafer (Serica brunnea). These can have larvae up to 30mm (over an inch)
• Similar root damage in lawns can also be caused by leatherjackets but churning up of the turf by other animals is less likely where leatherjackets are the problem

Control

Non-pesticide control

• Repair damaged turf by re-sowing with grass seed or laying turf in mid to late spring when the chafer grubs have moved deeper into the soil to pupate
• Poorly maintained lawns are thought to be more susceptible to damage, so attention paid to feeding, watering and moss prevention may help avoid damaging infestations
• Biological control: You can buy pathogenic nematodes, usually Heterorhabditis bacteriophora, which attack the larvae by infecting them with a fatal bacterial disease. These microscopic animals can be watered into the lawn when the ground is moist and soil temperature range between 12-20ºC (55-68ºF). the nematodes are available by mail order from some biological control suppliers and from some garden centres. The turf around the edge of affected areas should be targeted to deal with larvae spreading from infestation “hot spots”. However, by the time areas of infestation become apparent, the soil may be too cold for nematodes to be effective. As a preventive measure nematodes can be applied in July to September. Nematodes should be applied as soon as possible after purchase, following the suppliers’ instructions. It may be necessary to water the lawn before and after application to ensure the soil is sufficiently moist for nematode activity and survival

Pesticide control

There are currently no pesticide controls for chafer grubs in lawns which can be applied by home gardeners.

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Biology

The chafer species most often found damaging lawns are the garden chafer, Phyllopertha horticola, and the welsh chafer, Hoplia philanthus; the latter is often found in sandy soils and is not confined to Wales. Adults of P. horticola are about 9mm (3/8in) long and have a metallic green head and thorax with light brown wing cases. Hoplia philanthus is a similar size with a black head and thorax and reddish brown wing cases.

Adult chafer beetles feed on the foliage of many plants but are generally not a problem. They will, however, sometimes damage the flowers of roses and other plants. In heavily infested gardens, the adult beetles fly up from turf in large numbers at dusk from late May to June. Eggs are laid in the turf and these hatch a few weeks later. The grubs feed on roots but do not cause significant damage until early autumn, by which time the larvae are becoming fully grown. They overwinter as larvae and pupate in the soil in the spring.

Two other species, which are generally less damaging, are the summer chafer (Amphimallon solstitialis) and the brown chafer (Serica brunnea). The latter is generally found in or near wooded areas on sandy soils and is about 9mm long and reddish brown in colour. The summer chafer is light brown, about 16mm long, and distinctly hairy.

One species of chafer grub is often found in compost heaps, the larvae of the rose chafer (Cetonia aurata).  This is a species that feeds on decomposing organic matter and as such is a useful garden insect. The adult beetles are about 2 cm (1 inch) long and metallic green.

Chafer grubs can  appear similar to the larva of the stag beetle (Lucanus cervus). However, larvae of the stag beetle are always associated with dead wood, usually tree roots and can reach 11 cm long (nearly 4 inches). Stag beetles are uncommon and cause no damage in gardens and should be welcomed.