Chafer grubs, the larvae of the chafer beetle, are a fairly common pest of turf in Britain. They damage grass plants by feeding on the roots, which can result in serious thinning of the sward. However, more harm can be caused by predators, such as rooks, crows, badgers and foxes, excavating large areas of turf in search of the grubs. In this short article I shall look at the biology of these insects and the measures we have to control them writes Graham Paul.
First, some facts about beetles in general. The simplified diagram (first image in the photo gallery) may help to clarify some of the terminology used.
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Beetles are classified in Coleoptera, the largest order of insects with over 370,000 species known globally. They vary considerably in shape but most species have two pairs of wings; the forewings (elytra– singular: elytron) are tough, hardened and serve to protect the inner wings and body when not in flight. Elytra are sometimes known as wing cases. The name Coleoptera translates from modern Latin and as ‘sheath-wings’.
Chafer beetles belong to a very large family of insects called the Scarabaeidae, often referred to as the scarab beetles, which number some 20,000 species worldwide. One of these scarab beetles, the dung beetle Scarabaeus sacer was held sacred by the Ancient Egyptians, as a symbol of Khepri (a ‘subordinate’ of the sun god Ra) whose job was to roll the morning sun across the sky!. The connection arises from the behaviour of the dung beetles, seen by ancient scholars, rolling balls of dung across the ground. Khepri is often depicted in Egyptian drawings and carving as the figure of a man with a scarab beetle head.
Scarab beetles have distinctively ‘clubbed’ antennae, the club consisting of a number of flaps (lamellae) arranged in the shape of a fan. This feature, which enhances the sense of smell, is especially noticeable in the Cockchafer who seem to be the ‘punk rockers’ of the scarab beetle community.
Chafer grubs are widely distributed across most of the UK. They are particularly common on light sandy and chalky soils in non-irrigated turf such as golf roughs, fairways and ornamental turf. For the most part they are not usually present in sufficient numbers to cause serious damage and warrant control measures. However, should an area suffer a large infestation, turf grass loss can result and this can be particularly unattractive. It is therefore important to understand this pest and know how to deal with it should the situation arise. Chafer grubs have a habit of infesting the same area year after year so effective control measures are essential for those routinely affected areas.
There are five species that can be found damaging turf in the UK: - Garden Chafer (Phyllopertha horticola), Cockchafer (or ‘May Bug’ Melolontha melolontha), Welsh Chafer (Hoplia philanthus), Summer Chafer (Amphimallon solstitialis) , and Brown Chafer (Serica brunnea). The two main species found in turf (in appreciably large numbers) are the Garden Chafer and the Cockchafer.
The Garden Chafer is probably the most important in amenity turf; its grubs can often be found in large numbers in the rootzone.
The adult beetles are about 10mm in size and have a metallic green head and pronotum with bronze wing cases (eletra). They usually appear in May and June and are sometimes seen on the surface of turf during the daytime – especially when the infestation is heavy.
The Adult Cockchafer, at 25-30mm in length, is much bigger than the Garden Chafer. They have a dark head with a shiny black pronotum covered by short, closely set hairs. The body is chestnut brown in colour and exclusive to this species of chafer beetle are several forward-pointing white arrowheads on the abdomen just below the elytra.
They have a dull black abdomen and a long, flat pygidium (stern). The antennae are tipped with large, fan-like clubs consisting of seven blades in the male, whilst the female has only six. Males have longer antennae than females. Adult beetles are mainly seen at night during May and early June and sound like bumblebees when flying.
The larvae of chafer beetles have a curved, creamy white body with a nut brown head and a pair of legs on each of the front three segments of the thorax, which is typical of many beetle species. Cockchafer larvae are much bigger than those of the Garden Chafer (30-35mm c/w 15mm).
The life cycle of the chafer grub varies depending on the species and local climatic conditions but can be generalised as follows: -
Adult beetles emerge from their pupal cases and begin to fly at dusk from late May to June. They mate on nearby trees and shrubs until dawn, at which point the adults return to the soil. Several mating flights may be made but eventually the females lay 15-20 eggs in a 2-5 day period.
The eggs are laid about 15cm deep in the soil and hatch after approximately two weeks. If moisture levels are good the larvae move up toward the surface and begin to feed on plant roots. However, in drier conditions they remain lower in the soil. Larvae continue to feed until late September when they move deeper into the soil to over-winter. Pupation takes place in the following spring (around mid-May) normally below the surface. The Garden Chafer completes its life cycle in a single year but other species, such as the Cockchafer, feed below ground for 2-3 years before maturing into adult beetles and larvae of these may be found in infested soil at any time of the year, although there will be higher numbers in spring and autumn.
Chafer grubs feed on organic matter, including the roots and thatch of all turf grasses and initial damage has the appearance of drought stress. Heavily infested turf first appears off-colour, grey-green and wilts rapidly in hot sun. Continued feeding will cause turf to die in large irregular patches. The tunnelling of the larvae makes the turf feel spongy underfoot and it may be easily rolled back like a carpet, due to the severed roots. In some instances grub populations themselves may not cause observable turf injury, however, severe damage to turf can be caused by predators feeding on the grubs. Large birds (especially crows), badgers, and foxes are the main culprits and show no regard for the grass as they excavate the surface to get at the tasty grubs!
Control Measures
Cultural Control by rolling grub-infested turf to restrict their movement and feeding activities has been tried but gives little benefit to improving the health of the grass and does not resolve the problem.
Some degree of biological control can be achieved with entomopathogenic nematodes (EPN’s – nematodes that are pathogenic to insects), applied between June to August when the soil is warm enough to sustain them. Various products are available such as ‘Nemasys G’. They are usually diluted with water and applied with a sprayer – with the nozzle filters removed!
EPN’s are transparent, microscopic, nematode worms that are parasites of certain insect larvae and have a symbiotic relationship with a bacteria carried in their intestines. As soon as the juvenile nematode enters the host it regurgitates the bacteria, which is very toxic to the chafer grub, killing it within 24 hours. The bacteria help break down host tissues to feed the nematode whilst it completes the rest of its life cycle within the grub and, after reproducing, the carcass bursts open releasing hundreds of new juvenile nematodes back into the soil where they search for new hosts to ambush. These EPN’s are obligate parasites, existing in the soil for short periods as free-living juveniles but are only able to reproduce inside the host grub.
Chemical Control can be achieved with the residual insecticide imidacloprid, marketed under the brand name ‘Merit Turf’. It is only available for UK turf in granular form, which is applied at the rate of 30 Kg/ha and must be immediately washed into the soil with sufficient irrigation to move the active ingredient through the thatch, wetting the top inch of soil. The low application rate for these granules (3g/m²) calls for specialist application equipment, as bulking the product with carrier material to facilitate the low dose can give uneven distribution.
Imidacloprid breaks down in strong sunlight, thus application should take place towards the end of the day if possible. Imidacloprid is a systemic insecticide that translocates rapidly through plant tissues following application and is effective by contact or ingestion. It can be applied at any time of year, however, maximum control is achieved with an application prior to egg laying during spring or early summer, when the adult beetles are active. This period can normally be anticipated when the beetles are seen in full flight, as this is when they mate and then lay the eggs in the soil. This recommendation corresponds with the most susceptible stages of the pest’s life cycle (from egg hatch to the 1-2nd larval Instar stage). Pheromone traps can be used to indicate when the adults have emerged.