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They’re back, the grubs!
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You may have already noticed that there is damage to your lawn. Do you struggle with a damaged lawn with scattered large brown and dead patches? Do you notice soft, loose, and spongy sections of lawn even though you water the lawn adequately? In addition, are birds trying to rip out large patches of lawn, while raccoons and skunks like to dig up large lawn areas of lawn and roll them back every night?
If you answered yes to the above questions, various pests have established their nursery on your lawn. In such situations, there is often talk of white grubs or leatherjackets. What are grubs, what do they do with your grass, and how can you control grubs?
Let's start with what exactly grubs are.
What Are White Grubs?
The words “white grubs” refers to the collective name for the larvae of different species of beetles. They are the larvae of a particular beetle family, the “Scarabaeidae”, also known as scarab beetles. These include the May/June beetles (Phyllophaga spp.), the Masked Chafers (Cyclocephala spp.), the Asiatic Garden beetle (Maladera castanea), etc. In North America, it is quite often the European Chafer beetles (Rhyzotrogus majalis) and Japanese beetles (Popillia japonica) that cause problems for our beautiful lawns. Challenges with June beetles (Phyllophaga anxia) are also very common.
Those rather unwelcome comrades go in search of food. They eat grass roots, just below the growing point, resulting in the turf becoming detached from the mineral substrate. The result is the drying out of the turf when rainfall is reduced. Secondary damage is due to the ripping-up of the lawn by birds, raccoons and skunks searching for these grubs. Grubs can become one of the causes of sleepless nights for some greenkeepers, sports field managers, garden enthusiasts, and homeowners.
Yet, not all grubs are pests! The dung beetle larva also looks like a grub, but these can be found in compost heaps, helping to recycle nutrients. To ensure you're dealing with the right grubs, it's important to recognize them in order to ascertain whether it is necessary to combat them.
How Do You Recognize White Grubs?
Although there are different types of beetles, the larvae of the beetles in the scarabidae family are often similar in appearance. In principle, these morphological features are particularly useful:
White to white-yellow abdomen that is somewhat translucent and gray toward the end of the body due to underlying intestinal contents.
C-shape body position when disturbed
Hard orange to brown colored head
Three pairs of orange to brown colored small legs
The length of the larger larvae varies from species to species: 1.5 to 3.5 cm. It is important not to confuse grubs with leatherjackets, the larvae of crane flies. They are thinner and not curved, soft and brown-grey, have no legs, and you can hardly recognize the head or tail.
Now that you know what the appearance of the white grubs is, you can easily recognize them. Does this description match the appearance of the critters you have encountered on your lawn? If so, then we can safely conclude that you are dealing with white grubs. But how do these critters live?
What Is the Life Cycle of White Grubs?
The life cycle of white grubs consists of adult, egg, and three larval stages. The duration of the life cycle of grubs in the larval stage depends on the species, but can also be influenced by climate and location. As a consequence, damage depends on the larval stage and species present. In North America, the development of the June beetle can take three years whereas, for the most damaging species in the lawn, the European Chafer beetle and the Japanese beetle, the development takes a typical one-year life cycle.
For a generalized life cycle of an annual white grub on a lawn, the beetles fly from mid-June to early July, emerging from the soil to mate. They then lay up to 60 eggs at approx. 5-10 cm in depth during the early summer season. During July through August, these eggs hatch, typically 2 weeks after being laid, so the newly born larvae (first instar stage) immediately start feeding on the roots of the grass of your lawn. These grubs go through a second and then a third stage. As they grow, grubs consume more roots and feed more aggressively. Damaged turf often appears at this moment. Most grubs reach the third instar by the middle of September, but they may continue feeding well into October. They migrate deeper into the soil following the first freezing air temperatures and remain just below the frost line during the winter. They move upward as the soil temperature begins to warm again in the spring—March, returning to the root zone where major damage is done as they continue to feed for 4 – 8 weeks before pupating. They go slightly deeper into the soil and turn into pupae. After 7 to 10 days—from mid-June to early July, adults emerge from pupae to renew the cycle.
The larval stages of grubs with annual life cycles present a challenge due to feeding damage being less apparent during spring and usually most visible above ground in late summer when significant damage has been caused, and control is inadequate.
Some stages in the grubs’ life cycle are more vulnerable, thus more susceptible to target for control. Understanding the life cycle of grubs can tell you whether you have a problem and how or when these pests can best be controlled.
How Do You Track Down White Grubs?
Keeping an eye out for signs and symptoms of pest infestations can provide critical information for pest control. You need to observe the flights of the beetles. The time of the flights gives a good indication of which grubs are present on the site. The beetles are easier to spot than the grubs. The European Chafer beetle is light reddish-brown, with a slightly darker head and thorax and wings covered with longitudinal grooves and minute punctures. The Japanese beetle is about half an inch long and has a shiny metallic-green body, with dark legs and a green head and thorax. June beetles are light to reddish-brown or black.
Yet, white grubs can be distinguished by observing the pattern of hairs and spines, called raster, on the rear ends—pre-anal—with a hand lens. For example, a European Chafer beetle has a Y-shaped anal slit while a Japanese beetle has a V-shaped anal slit. In case of doubt, there is always the possibility to have the grubs identified by a specialist before the start of pest control.
In heavily infested places from mid-June, the adult beetles fly up from the grass in large numbers at dusk and lay eggs. The damage to the lawn is only visible when the larvae are on the lawn. The first signs are small yellow spots. This is a sign that larvae are predominant from July, and the damage is most visible. The presence of birds is also a sign of white grubs. They forage in the wilted grass patches to find the tasty grubs, plucking them out of the soil and causing secondary damage to the lawn. To confirm that white grubs are the reason for yellowing or dying patches of lawn, use a spade to cut three sides of a 1-square-foot (0.3 m2) piece of sod, 2 inches (5 cm) deep, on the edge of dying grass. If grass pulls up easily and has few roots, and there is an average of three to seven per square foot, then treatment may be warranted.
Now that you know the life cycle of white grubs, and you know if you have them in your lawn, note that if you do nothing about the white grubs, they may persist for years and years.
Biological Control of Grubs
Did you know that you can use beneficial nematodes to control white grubs? For many people who like to adjust it naturally, this is a logical choice, as nematodes are environmentally friendly, with no consequences for the environment and nature. Nematodes penetrate the grubs and, once inside, release certain toxic bacteria that kill them within 24-48 hours. What is special is that nematodes feed on the dead grubs and multiply, and then young nematodes—infective juveniles, carrying their bacterial partners, look for a new grub to infect to eat.
The nematodes are supplied in formulations such as sponge, gel, or vermiculite that are ready to be mixed with water. You can easily spray the lawn with a watering can or sprayer where they will be absorbed into the soil. However, It is important to follow the product directions as to when to apply nematodes.
Most effective control results can be achieved when early instar grub larvae are targeted. It is recommended to use beneficial nematodes when the larvae are young and vulnerable; usually between the beginning of July and the middle of September, depending on the product used. Several species are available including, Heterorhabditis bacteriophora, Steinernema carpocapsae and Steinernema feltiae. Since there are different types of beetles, the ideal time to control them depends on which type of beetles or grubs you want to fight.
It is best to buy nematodes directly from a trusted source for quality control and prompt shipment. Nematode viability should be checked before application by examining a small amount of the formulated material in water using a hand lens or a microscope. This can prevent you from applying dead nematodes.
For effective control, it is important to use the nematodes at exactly the right time or the right stage of the grub life cycle. Soil temperature should be above 10ºC if you want the nematodes to do their job well. It is always recommended to apply the nematodes early in the morning or in the evening, as the nematodes cannot tolerate sunlight well. The soil must be moist for optimal control; therefore the soil should be pre-moistened before application, and the treated area should be kept moist for about 2 weeks after treatment to create ideal conditions for nematodes to move more easily.
An infected white grub can be recognized by the color. Normally grubs are white, yet, after infection by nematodes of Heterorhabditis bacteriophora, the grub color change to reddish brown while grubs killed by Steinernema species are yellow.
Learn more about the use of beneficial nematodes in lawns here. Find out more about the use of beneficial nematodes for pest control here.
Check out our infographic on “How to use beneficial nematodes to combat insect pests in your garden”.