Chafer Grub Control

Currently there is no chemical of last resort to help control chafer grubs which have exceeded the desired threshold for a particular turf surface. The threshold is a quantity above which unacceptable damage would occur and this is a subjective opinion in many cases, although there will be a figure (depending upon the species concerned) above which significant damage will be caused. It will, however, vary according to the needs of the user, manager, budget, resources available and quality of the surface and this figure would act as the trigger to use chemicals as a means of control.

Just because a number of chafer grubs are present within a turf doesn’t mean any chemical action is needed, although a typical approach in the past would have been to apply a pesticide (well, when there was at least one approved for such use) without giving some reflective thought as to whether this was really needed.

Prior to any chemical application and control (which should be a last resort), good practice will, or should, see that cultural, physical and then biological control (where applicable) take place to help reduce the chance of the threshold being reached in the first place. This now needs to be the focus of turf care, although it should really have been the approach taken in the past anyway, but like many things which are easy, it was quicker and less thought needed in just applying a pesticide, so it was understandable why this was a typical approach to take.

Building a strong, healthy and resilient sward is required to help defeat many situations of chafer grub infestation; this isn’t achieved just by a bit of extra aeration, or similar, but by considering all turf maintenance activities and how they impact on the health and stress caused to a turf surface.

The simplest and most beneficial gain is made by raising the height of cut to as high as realistically practical for any given turf surface; there has been a drive over recent years to cut as close as possible but with significant negative effects, just look at many surfaces where turf easily kicks out during play – there is a direct link between grass height of cut and root growth and development, yet this is often overlooked in the quest to produce a surface that ‘looks good’. Drilling down into what is need to maintain this ‘good look’ starts to show up many practices which cannot be classed as sustainable; the whole approach to the application of good science is questionable in many cases.

Anyhow, onto the chafer grubs themselves:

Chafers

There are several, at least six, different species of chafer grub. The Cockchafer is probably the most destructive to turf, as well as the most common, although the Garden chafer is frequently stated as being the commonest, (I’ve always seen the Cockchafer wherever I’ve inspected turf damage from chafers, so I wonder if this is a bit a ‘gardening myth’ which has stuck, or not?) but it will also cause extensive damage.  Know which one it is though, as this can affect any control programme being undertaken.

The chafer grubs will eat grass roots and cause the turf to die back and turn brown where there is a high infestation of grubs. Other chafer grubs which might cause occasional and localised damage to turf include Brown Chafer, Summer Chafer and Welsh Chafer. The Rose Chafer grub is unlikely to be a cause of any damage to turf surfaces.

Cockchafer

A white grub, about 30 to 40mm in length, with three pairs of front legs. Its head and six legs are light brown in colour, whilst its body is whitish (see below).

Cockchafer grub

Cockchafer grub

The adult beetle is about 30 to 35mm in length, having a brown body with white stripes. It is also known as the May bug, due to the prominence of the beetles flying about in the month of May, but they are also present in June. The Cockchafer will also fly about at night and is attracted to light. It causes variable amounts of damage to a turf surface, primarily being a minor pest, although localised damage can be quite significant.

Cockchafer beetle

Cockchafer beetle

Cockchafers have a life-cycle of 3 to 4 years. Damage is similar to that produced by leatherjackets. To check the grub which is causing damage to your turf it is important to take some cores, as this will determine the method of control. Severe damage can result in turf being able to be rolled up, because the roots have been totally severed from the shoots. Its scientific name is Melolontha melolontha.

Garden Chafer

A small white grub, 15 to 20mm in length, with three pairs of front legs. The adult beetle is typically up to about 10 to 15mm in length, having a greenish head and abdomen, but with light brown wing cases. This chafer has a one year life cycle and the adult beetle is typically active during May and June, especially during the daytime, but will typically settle down at dusk, unlike the Cockchafer, which will still be ‘on the wing’. Its scientific name is Phyllopertha horticola, and also has another common name of ‘Bracken clock’.

Garden chafer beetle

Garden chafer beetle (source: Charles J Sharp [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Chafer grub control

These might be controlled in a number of ways:

  1. Cultural and physical control, especially by aeration and scarification, as this can help to improve air circulation and reduce any thatch blanket that can protect grubs during cold winter weather. Following any egg laying and grub development, the grubs will gradually work their way deeper into the soil – especially by the autumn where they pass into the pupae stage. Try and keep this surface and immediate layer relatively open to the weather.
  2. Biological control – Use a Biopesticide: These are living micro-organisms, in particular nematodes, that are specific individual ‘products’; they attack a specific targeted enemy and have a limited period of activity to avoid them becoming rampant and out of control in the environment. An example is Nemasys, which attacks the chafer larvae, being used when soil temperatures are above 12C.
  3. Chemical control, but there may not be any approved chemicals for use (not in February 2019 anyhow).

Other essential cultural and physical controls, include:

Intensive aeration to open up, and keep open, the surface (with solid tine and hollow tines) over the autumn and early winter. This will, potentially, expose buried grubs to inclement weather conditions and this might reduce their development or even kill some of them off.

Regular slit tining can aid root pruning and root development, helping to create a comprehensive root biomass that can potentially counter the feeding requirements of the grubs ensuring adequate root mass remains for continued grass development. Unless ether is a very large quantity of grubs the turf sward will be able to survive many an infestation of chafer grubs, all without having to apply a pesticide.

The Verti-knife (Charterhouse)

Verti knife (Charterhouse)

Verti knife (source: Charterhouse)

and Rotoknife (Imants)

Roto-knife (Imants)

Roto-knife (source: Imants)

are two pieces of aeration equipment which could be considered as having the potential to crush, squash or slice grubs within a soil profile, with the knives cutting through the turf. If used as part of a regular aeration programme these could potentially be of a significant benefit in the physical control of chafer grubs.

Rolling has been suggested as a possible control method, however, considering the ground pressures applied by rollers it is unlikely that this can be an effective control method without causing significant detrimental effects to the soil profile and sward quality; it would therefore not be recommended.

Correct mowing practices (i.e. not shaving or too close mowing) to enable a strong and deep root system to develop: this will provide good resilience to the sward in that whilst the grubs will consume a certain quantity of root mass, there will be a suitable and adequate quantity remaining (due to good cultural practices) to maintain a suitable surface and aid rapid recover following their departure as beetles.

A systematic approach to managing turf grass surfaces is needed to be undertaken on a pro-active basis: essentially good working practices, besides the above, this will also include correct nutrition, irrigation, scarification etc. with particular emphasis on soil and weather conditions and knowing and understanding when it’s best to apply maintenance activities and materials for your particular surface at your locality with your micro-climate.

Having a final fall-back of a suitable pesticide would be ideal if thresholds were exceeded, however, currently this is not something which is available as Plan C.

Chris Gray, 11th February 2019