Chafer grubs: identification, treatment & prevention
Chafer grubs are larvae of different beetle species. While some can do a lot of damage, others can be useful helpers in the garden.
The grubs are the larvae of the scarab beetle family Scarabaeoidea. The most common beetles of this family are the May beetle (also cockchafer or doodlebug), June beetles (summer chafer), the garden chafer, the flower chafer and rhinoceros beetles. While the adult beetles feed on leaf matter (causing only minimal damage), the grubs find their food underground and can, for example, inflict significant damage to the plant roots. But not all grubs are pests.
Chafer grubs: identifying the useful species
The larvae of the flower chafer and rhinoceros beetles are beneficial organisms. The rhinoceros beetle (Oryctes nasicornis) is the largest representative of the scarab beetle family and is a protected species. It can grow between 2.5 and 4 cm in size. The males of the rhinoceros beetles carry a horn bent backwards on their head, which strongly resembles the horn of their heavyweight mammal namesakes. The female animals possess a small protrusion on the same place. The larvae have the typical shape of all scarab beetle larvae. They are very large, coarse and white in colour. Three relatively long leg pairs and the brown head capsule are on the front part of the body. The entire body of the larva is C-shaped.
The flower chafer (Cetoniinae) also belongs to the protected beetle species. Its carapace has a shiny green to copper-golden colour. Its underside is quite hairy. It is often found on rose petals and therefore bears the common name flower chafer. But it is not really harmful for these plants.
The larvae of the flower chafers and the rhinoceros beetles are very useful. They feed mainly on dead plant matter and wood remains and thus provide for the formation of humus. The females of both beetle species also like to lay their eggs in compost heaps.
In order to recognise the “good” chafer grubs, here a few clear characteristics:
- The flower chafer grub is white with a greyish shade. Its front part is a bit slimmer than its rear part. If it is laid on a smooth surface, it stretches, turns and crawls away on its back. He stretches three pairs of small legs at chest height.
- Three pairs of legs and a thickened rear part are also typical for the larvae of the rhinoceros beetle. However, it towers above the larva of the flower chafer by a double and can grow up to 10 cm long. The development of the grubs of the flower chafer is two to three years. The rhinoceros beetle development is even longer and lasts between two and five years. The different times are influenced by the climate of the animals’ habitat.
Both beetle species feed on fallen or dead plant material or rotting wood and bark. They therefore do not damage living plants through root feeding, like the grubs of the other beetle species. Only once they reach adulthood, they feed on leaves. Nonetheless, they are not considered pests.
Chafer grubs: identifying the harmful species
The larvae of May beetles (Melolontha), June beetles (Amphimallon solstitiale) and garden chafers (Phyllopertha horticola) are pests. They feed mainly on the roots of living plants.
The grubs of the three beetles mentioned above are quite similar in appearance to one another. All have a C-shaped curved body, a brownish head capsule and three pairs of legs in the chest area. However, they differ in size. The larva of the garden chafer is the smallest with about 2 cm. The June beetle can measure up to 3 cm and the May beetle up to 6 cm. But beware: younger stages of the cockchafer larvae can easily be confused with those of the June beetle. If you want to be absolutely sure, you have to get help from a professional to identify them. The front and rear parts of all three species have approximately the same diameter. Based on the way the grubs move, they can also be distinguished from the useful grubs of the flower chafer and rhinoceros beetles. While they stretch out and crawl away on their backs, the May bug larvae maintain a curved posture, whereas the June beetle and garden chafer stretch out but try to make progress in the prone position.
Here is a short description of the adult beetles:
The grubs of the May beetle (also referred to as cockchafer) remain in the ground for three to five years, depending on the climate of their habitat. The beetle has brown wings, a black neck shield and a black and white pattern reminiscent of triangles on the sides. Its antennae look like tiny fans.
The larvae of the June beetle hibernate twice and pupate in the spring of the third year. The beetles also have fan-like antennae. The colour of their carapace ranges from brown to dark yellow. The neck shield stands out because of how dark it is and is divided by a light midline.
The entire life cycle of the garden chafer from the egg to the adult beetle is between one and three years, depending on the climate. They grow to about 1 cm long. Their elytra are light brown in colour. The rest of the body is black-green and shiny metallic.
The grubs of the May, June and garden chafers feed on the largest roots on lawns and in potted plants.
Combatting chafer grubs: how to get rid of them
The larvae of May, June and garden beetles can be found in many gardens. They might be present even if you might not notice them. If there are only a few of them, the root feeding is not threatening for the plants and they can regenerate well. If, however, there is a large number of grubs, you should consider controlling the pests to protect your garden.
Grubs in flower pots and raised beds
If a plant in a flower pot is miserable and you want to find out whether grubs are the cause of this, dip the pot completely into a bucket of water. Wait an hour or two. The little animals don’t like to be drenched in water at all and soon appear on the surface. Then you can collect the pests.
If you want to be on the safe side, water the plants with water mixed with nematodes of the genus Heterorhabditis bacteriophora. These are available in all well-assorted specialist shops and are by far the most effective method of eliminating grubs. The almost invisible nematodes attach themselves to the pest, parasitize on it and then kill it. The same applies here: the environmental conditions determine the success or failure of the nematode application.
Further useful tips and facts on the use of nematodes can be found here.
You should never place flower pots in the garden without the drainage trays. In addition to having your plants potted correctly, it helps to grow geraniums. The grubs greatly dislike geraniums. They also steer well clear of larkspur (Delphinium).
In a raised bed, it would also be advisable to pour the soil through a sieve before sowing or replanting. Then you can be absolutely sure that there are no grubs in the soil.
Last but not least, do not forget that a few grubs are a natural occurrence even in the most well-kept gardens. In a limited number they do not cause any significant damage.
Grubs in the lawn
If you find yellow patches on your lawn, grubs could be the reason. They eat away the roots of the grasses, causing them to atrophy. To find evidence, remove a small turf of the lawn. The larvae of the aforementioned one or more species of beetles live just below the surface. If crows, blackbirds or other birds also scour the bald spot for the protein-rich prey, the damage to the lawn becomes all the greater.
Nematodes, also referred to as roundworms, belong to the genus Heterorhabditis bacteriophora can be used to eliminate grubs in lawns or pots. However, nematodes require specific conditions to be successful. Make sure that you use nematodes at a temperature of at least 12 °C so that they are effective.
Traps direct the insects to other places in the garden. To make a trap for grubs you need large pots or buckets. These are filled up to 10 cm with compost or horse manure. These containers are dug into the garden to a depth of about half a metre and the left-over edge is filled with soil. The best time to place the grub traps into the ground is in the spring months and near the affected area. After a year, they can be dug out, refilled and put in the soil again.
Tip: Let plenty of dandelion grow all around.
If you create a paradise for the grubs in one limited area in the garden, you can then embed vertical plates (for example rhizome barriers) deep into the soil to contain the area further. To prevent an infestation of the lawn, pour garlic brew over the area. This will deter the grubs.
Tip: Many gardeners think that frequent mowing of the lawn can minimise the amount of cockchafer grubs. However, too frequent lawn mowing or scarifying can, in fact, make it easier for grubs to enter the soil. Therefore, ensure a dense and thick lawn cover and let the lawn grow a little longer. What the lawn may lack in aesthetics, it can balance out in its practicality. A taller lawn is a great advantage for pest control and can reduce infestation by grubs up to 70%.
For more tips on how to control grubs in the lawn, click here.