Have you ever wondered why the crows have been so interested in tearing up your lawn recently? It turns out they are in search of the tasty grub snack known as the European chafer beetle (Rhizotrogus majalis).
Working in a botanical garden, we often receive questions on how to manage this invasive turf pest. It is simple really. Like any healthy ecosystem, a healthy lawn is the key!
What is the European chafer beetle?
The European chafer beetle originates from Western and Central Europe. By the 1940s it had been identified in the bustling nursery-growing area of Newark, New York. Forty years later it had spread to neighbouring states and southern Ontario via nursery stock, topsoil transportation, and by hitchhiking in soil and debris on vehicles. Fast-forward to today and you can now see the European chafer beetle successfully living throughout the Lower Mainland.
Lifecycle and Damage
In late winter to early spring crows, skunks and raccoons are all in search of the delicious and fatty chafer larvae overwintering underneath what was your grass! Unfortunately, by the time you see these signs of damage at the surface level, the real damage to the grass root systems has already happened.
The lifecycle of the European chafer beetle lasts one year: nine months of which they spend as grubs in your lawn. Generally, from August to May they feed on fibrous roots in preparation to pupate into an adult beetle. Grass roots are their caviar, although they have been known to munch on the occasional perennial or juvenile tree. In areas with mild, wet winters (like Vancouver) they are often only 2.5 to 5 cm (1 to 2 inches) below the surface. Most of this damage remains unnoticed until the spring when warmer temperatures cause the remaining grass to show signs of stress. By this point, patches of turf can often be easily picked up with your hand and/or the beak of a crow.
After a short pupation cycle, the brown adult beetles emerge at sunset in June to take flight to a nearby tall tree and mate. The sound is said to be like that of swarming bees. The females will then fall/fly back to the ground around the tree to deposit eggs. Here is the interesting part: to lay their eggs the females generally prefer moist, not waterlogged, close-mown turf and will often not congregate in fallow ground or in long grass. The eggs will then hatch in 2 weeks and remain in a delicate first larval stage for an additional 3 weeks (through July and August) before growing into large grubs.
How many grubs is too many grubs?
In fall, winter or early spring, peel up a 30 x 30 cm (1-foot) square of grass. In a well maintained lawn, over 10 grubs can indicate a problem. In poorly maintained lawns, only 4 or 5 grubs are sufficient to cause extensive damage.
A word of warning – not all grubs are European chafer beetle! Many beneficial beetles can have similar looking larvae. To confirm you do have the correct grub, visit this website for more information.
What you can do?
Over the last couple of decades our interests in keeping and maintaining lawns has changed considerably. Many people no longer desire the perfect green sea of grass that was so popular in the 1950s. With more water restrictions and environmentally conscious ways of looking at open spaces, our tastes are changing and our strategies must likewise adapt.
With this in mind, good lawn maintenance is key to mitigating the damage done by the European chafer beetle grubs. Stronger, better grass root systems will enable your lawn to weather the fall, winter and spring feeding tendencies of this pest.
Choose the right grass:
Monoculture turf-grass crops are never a great idea unless the turf is a golf course green or lawn bowling rink. Start thinking about diversifying the types of grass and plants within your lawn to help encourage a better, more environmentally friendly ecosystem. If possible, stay away from the water- and fertilizer-dependent Kentucky bluegrass (Poa pratensis) and focus your attention on red or creeping fescues (Festuca rubra spp.) which are both drought and shade tolerant and can survive infertile soils. Their rooting systems are also deeper making it a more robust choice for chafer beetle proofing. Since fescues can take longer to germinate, it is often recommended to mix in some perennial ryegrass (Lolium perenne) to speed up the growing process. When deciding whether to purchase seed or sod, look for “shade” mixes with over 50% fescue and some perennial ryegrass.
Diversify your lawn with specific “weeds”:
A weed is any plant deemed to be in the wrong place. In regard to turf, this can often include many non-grass plants that can actually benefit the overall ecology of your lawn by promoting insect diversity, drought tolerance and nutrient cycling. Consider adding grass alternatives such as micro-clover (short-stemmed and small-leaved Trifolium repens selections), western yarrow (Achillea millefolium var. occidentalis), English daisies (Bellis perennis) or sweet violets (Viola odorata). On the other hand, it’s best to remove larger broad-leaved plants, such as dandelions (Taraxacum officinale), plantain (Plantago spp.) and dock (Rumex spp.), which all have a smothering effect on turf-grasses.
As part of the Horticulture Training Program at the UBC Botanical Garden, we are currently experimenting with overseeding the open lawn with the following mix. Pop by this summer to see how it is faring!
- 45% red fescue (Festuca rubra subsp. rubra)
- 15% chewing fescue (Festuca rubra subsp. commutata)
- 35% perennial rye (Lolium perenne)
- 4% micro-clover (Trifolium repens var. pirouette)
- <1% western yarrow (Achillea millefolium var. occidentalis)
Annual lawn maintenance:
Like any plant, grass requires water, air, food and light to grow healthy, deep roots. Maintaining a balance of these requirements is very important, considering the amount of traffic and abuse turf can take. If you are not comfortable renting and using the required yourself , there are many contractors in the Vancouver area that provide this service. Here are some annual maintenance items you should keep in mind to keep your grass healthy:
- Whether you are enjoying a picnic, walking your dog or driving a car on your lawn, all of these activities lead to compaction issues. Compaction reduces the soil’s ability to allow water to infiltrate, to retain water, to drain adequately, to promote gas exchange between the roots and soil and to take up nutrients. It is often the number one reason why a lawn fails to thrive. To maintain good air and water relations in the soil you should aerate your lawn. My mentor once taught me to stab a screwdriver into the soil to test the compaction. If you find there is a lot of resistance, then your ground is probably compacted.Make sure to select an aerator that physically pulls up a “plug” of soil and distributes it onto the grass. No need to rake this away! It will decompose and become your new lawn.
- Top dress:
In March or April, consider adding 2.5 cm (1 inch) of topsoil over your existing turf. Purchase a topsoil product with at least 70% sand to ensure ongoing drainage. After evenly raking-out the material, spread grass seed on top by hand or with a broadcast spreader. Lightly rake-in, being careful not to drag all the seed into one area. To discourage birds from stealing your golden seed, walk over the top dressed area with a large roller to compress your new layer. This also ensures direct contact between the seed and the soil and improves germination success.
Like many plants, grass prefers a near-neutral soil pH to grow best. In Vancouver our soil is slightly acidic with a pH around 5.5. Although fescues can thrive in lower pH soils, it can lead to nutrient deficiencies and reduced vigour in perennial ryegrass. To reduce the overall acidity, spread dolomite lime in the spring and fall.
With proper top dressing and mowing techniques, I suggest steering away from using fertilizers entirely. The added soil and decomposing grass clippings should assist in replacing the required nutrients. It’s true that fertilizers will make your grass grow quicker and a darker green, but ask yourself if you really need it!
For turf areas previously impacted by chafer beetles, in addition to the above practices you can apply a light coating of high nitrogen fertilizer at ¾ the suggested manufacturer’s rate in May. This will help build the turf more quickly. To reduce the possibility of fertilizer burn on your grass, only apply on a dry day after the dew has evaporated. There is no need to apply fertilizers in the fall since the grass is going dormant for the winter and the added nitrogen only encourages prodigious moss growth in this area.
Every couple of years you may need to dethatch or “verticut” your lawn prior to aerating and top-dressing. Thatch is the spongy decomposing layer of plant debris between the grass roots and living foliage. This layer also hosts beneficial microorganisms that are important to healthy plants and soil Too much thatch can block water uptake and gas exchange. This is especially important to keep in mind if you have a lawn with a high proportion of fescue-type turf-grasses. Aim for a thatch layer under 1 cm thick, otherwise it is time to dethatch!
Think about de-thatching like brushing your dog after years of neglect, imagine all the hair that will be piled at your feet! In addition, de-thatching cuts the grass rhizomes, which encourages new growth.
Cool season grasses have naturally evolved to go dormant in high heat and low moisture conditions. Summer dormancy in turf-grass means that the grass blades die back to a perennial crown at soil level. By using fescue species in your lawn, and especially accepting a certain amount of brown in the lawn we can significantly reduce the irrigation dependencies of these spaces. In hot weather locally, a lawn requires 2.5 to 3.5 cm (1 to 1.5 inches) of water per week to stop it from going dormant. Note that Kentucky bluegrass and perennial rye requires almost double that amount to stay green! If your area does not have a track record of chafer beetle infestations, I would encourage you to not water your lawns.
To keep a healthy lawn, never cut more than 1/3 of the grass blade at one time. This also provides a small enough clipping that can remain on top your lawn to decompose naturally. This will help you gauge if you need to mow every two weeks or more frequently. In general, turfgrass species look best when kept under 8 cm (3 inches) long. However, when the adult chafer beetles are landing to lay eggs in June/July, it’s best to allow your grass to grow longer to discourage them from landing in your lawn!
The application of pesticides for residential lawns is banned in many municipalities in British Columbia. Furthermore, I feel that the reason we have turf areas are to enjoy playing and relaxing on the grass. Nobody wants to sit on grass that has been treated with pesticides. In addition, by applying pesticides you may also wipe out any beneficial organisms in your lawn and gardens.
Nematodes (e.g., Heterorhabditis bacteriophora) can help reduce infestations if applied at the right time, but the cost is high and the benefits not assured, compared to adopting good annual maintenance strategies.
When applied during late July, these microscopic roundworms parasitize the egg and first larval stages of the chafer beetle. This will assist in reducing the overall beetle population, but will usually still result in a crow buffet next winter and spring unless proper turf upkeep is adopted. You can place an order for dormant nematodes at your local retail garden store in spring. To apply nematodes first water your turf well, then apply the nematodes as per the instructions. Keep your lawn moist for several days and avoid fertilizing at this time.
For more information on European chafer beetle, please see the following references.
- Read Douglas Justice’s follow-up article on the use of neonics when dealing with European chafer beetles.
- Tashiro, H. (1987) Turfgrass Insects of the United States and Canada: Comstock
- City of Vancouver Website
Submitted by Kerrie van Gaalen, UBC Botanical Garden staff, April 13, 2016