Whether you have the most perfectly mowed lawn in the neighborhood or a lush, biodiverse garden where insects happily buzz around, there are always bare or yellow spots where the grass won't grow. But how do these places actually arise? And more importantly, how can you tackle them? Furthermore, what role do birds play in this?
Yellow and bare spots in the lawn
What is an eyesore for many people are yellow or bare spots in the lawn. Sometimes the neighbor's cat peeing is blamed for this, other times the neighbor's dog. However, the actual cause is much closer! Under the turf there are voracious insect larvae that are not immediately visible, but do have clear consequences. The vast majority of these larvae are grubs or leatherjackets.
What are grubs and leatherjackets?
Grubs and leatherjackets are collective names for different types of larvae of a large group of beetles (grubs) and crane flies (leathers). Among the beetle larvae in the Netherlands we mainly deal with the cockchafer, the June beetle and the rose beetle. As far as crane flies are concerned, we mainly encounter the meadow crane fly and the cabbage crane fly. The different types of grubs are difficult to distinguish from each other, just like the two types of leatherjackets. Fortunately, distinguishing between the different species is not necessary for controlling the larvae.
How do you recognize grubs and leatherjackets?
Both grubs and leatherjackets are larvae that live underground. It can be difficult to tell them apart, but fortunately there are clear characteristics you can look for to identify them.The leatherjacket can grow up to 5 centimeters in size and is gray-brown in color. A clear characteristic is that these larvae have no clear front or back and also have no legs. During the day the larvae remain hidden, but at night they gnaw the green parts of plants, usually as close to the ground as possible. This feeding method can seriously damage the plant, causing everything above the damaged areas to die, causing bare spots in the grass. In addition to grass, leatherjackets also prefer young plants in vegetable garden containers, such as lettuce.
The grub can grow up to 4 centimeters and is white with a reddish-brown head. Unlike the leatherjacket, the grub has visible legs. Additionally, the grub typically curls up into a "C" shape when you pick it up. The grubs first feed on available organic material, but over time switch to eating plant roots. When they start feeding on this en masse, the grass above the ground dies, resulting in yellow or bare spots in the lawn.
Although you can get an idea of whether you are dealing with a grub or a leatherjacket based on the damage, it is better to actually look for them and observe them. Below we have an image showing the most noticeable differences between the two.
Development of grubs and leatherjackets
The life cycle of grubs begins with adult cockchafers and other beetles flying around from late April through June, looking for suitable places to lay their eggs. These eggs hatch after several days to weeks, usually about four weeks after the adult beetles have been observed. The larvae then continue to develop until they are large enough to pupate and fledge as adult beetles, which then lay eggs. In the Netherlands it usually takes 3 years for the life cycle to complete.The life cycle of leatherjackets is slightly different. Here too, the adult crane flies fly from April to June, but there is also a second wave of adult crane flies from August to October. During this period the female lays about 350 to 1300 eggs, which hatch after a few weeks. The larvae then feed on young plants until they go dormant in winter. In the spring they become active again and continue feeding until they are large enough to pupate into adult crane flies from April onwards. 
Natural control of grubs and leatherjackets with nematodes
To prevent or reduce bare or yellow spots in the lawn, it is important to combat grubs and leatherjackets. An effective method for this is the use of nematodes. These microscopic organisms penetrate the larvae and kill them by means of a bacterium. This cooperation between the nematode and the bacteria is called symbiosis, which means that both parties benefit. In contrast, the relationship between the larvae of the pest insects and the nematodes is parasitic: one party (in this case the pest insect) suffers damage from the other party (the nematode).The optimal time to use nematodes against grubs is when the larvae are smaller than 2 cm. At that time, the nematodes can effectively penetrate the larvae. The best time to treat is usually between early June and late September, about two months after the beetles are spotted. If no beetles have been seen, but you still want to be sure, treatment is recommended in August and September.
For the control of leatherjackets it is also essential to tackle the larvae as early as possible. It is most effective to treat 1 to 2 months after seeing the leatherjackets, preferably in the summer when the larvae are still small. If the crane flies have not been observed but you do not want to take any risks, treatment at the end of October is a safe option.
The most effective nematodes against grubs are Phora , which are active from 12 degrees, but are most effective at temperatures above 18 degrees. Carpo are the most effective nematodes for leatherjackets, preferably at soil temperatures above 12 degrees. At lower temperatures, the use of Felti is recommended, as this strain is more effective below 12 degrees.
And what about the birds? Natural predators against grubs and leatherjackets
In nature the principle of eating and being eaten applies. As previously discussed, nematodes can benefit from an insect infestation, but they are not the only ones that enjoy consuming grubs and leatherjackets. In addition to nematodes, moles and birds also love these delicacies. So if you notice birds constantly pecking at your lawn, it could be a sign that you are dealing with grubs or leatherjackets. These feathered friends are looking for a tasty meal.By using natural predators such as birds against grubs and leatherjackets, you can contribute to the control of these pest insects in a natural way. So pay close attention to the behavior of the birds in your garden, they can be a valuable indication of possible problems with grubs and leatherjackets.
