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Engraved grubs are not only found on plants in the home garden, but also in the compost. Many hobby gardeners ask themselves whether and how they can fight the supposed pests and which home remedies are best suited for this. However, what many people do not know is that grubs in compost soil are very useful and are not allowed to be combated by law!
What are grubs?
Engerlings are the beetle larvae of scarab beetles. This is the superfamily Scarabaeoidea, which includes around 28,000 subspecies worldwide. Accordingly, grubs can be the larvae of a wide variety of beetles. Some of these show strong differences, both in their appearance and in their way of life. Because among the scarab beetles there are specimens that feed on dung or plants, but also some that are predatory. Because of this, the beetle larvae can be both beneficial and harmful to agriculture. The larvae of the following beetle species are very common in home gardens:
- cockchafer
- June beetle
- garden beetle
- rose chafer
- rhino beetle
Engerling - beneficial or pest?
Depending on which grubs they are, they can be useful or harmful to plants and compost in the home garden. The larvae of May, June and garden beetles, for example, prefer to feed on humus, grass, herb and tree roots, which is why they can pose a danger to the garden in large quantities. Rose beetle and rhinoceros beetle larvae, on the other hand, feed on rotting wood and plant material and therefore do not pose any danger, but are actually very useful - especially for the compost soil!
- Pests: May, June and garden chafers
- Beneficial insects: rose and rhinoceros beetles
How to distinguish grubs?
If you come across beetle larvae in your garden, you can use the site where they were found to determine which grub species it is. May, June and garden chafers feed on living plant matter, so they tend to be around plants. Rose and rhinoceros beetles, on the other hand, feed on rotting wood and plant matter, which is why they are often found in the compost heap. Additionally, the larvae can be differentiated based on the following differences:
May and June beetle larvae
- about the same thickness in front and behind
- about 2 - 3.5 cm long
- whitish or slightly yellowish in color
- strong long and thick legs with a clear kink
- biting tools on the head
Cockchafer grubs. Source: Botaurus, Melolontha-melolontha-20-VII-2007, crop from Plantopedia, CC0 1.0
Rose and rhinoceros beetle larvae
- thicker at the back than at the front
- little stubborn legs
- curved body
- about 1.5 - 3.5 cm long
Distinguish grubs based on locomotion
The beetle larvae can not only be distinguished visually, because an important distinguishing feature is the way they move. To do this, the grub is simply lifted and placed on a flat surface, such as a tile. The larva will now try to move, which in turn gives information about their assignment:
- Rose beetle larvae: turn on their backs, crawl forward like caterpillars
- May, June and garden chafer larvae: move sideways in a curved manner
grubs in the compost
May, June and garden beetle larvae can definitely cause damage to lawns and meadows, as well as useful and ornamental plants in the home garden. As already mentioned, these grubs feed on living roots. However, if you come across the beetle larvae in the compost, you will most likely get to know the larvae of the rose beetle or rhinoceros beetle. However, the larvae are not harmful there - quite the opposite! The rose beetle larvae are considered beneficial and contribute to the following in the compost soil:
- loosen the compost material
- accelerate decomposition
- produce humic substances during digestion
- Humic substances are degradable components of permanent humus
Notice: Permanent humus is very valuable for the long-term fertilization of plants, as they release the nutrients gradually and slowly.
Fight rose beetle larvae in compost?
grub of the rose chafer. Source: Fritz Geller-Grimm, Scara fg03, cut from Plantopedia, CC BY-SA 3.0
Anyone who finds rose beetle larvae in the compost can look forward to a visit from the beneficial insects. Fighting is not necessary - and not even allowed! According to the Federal Species Protection Ordinance, rose beetles are among the specially protected species and may not be injured, killed or caught. Damaging their breeding and resting place is also not allowed and can be punished with high fines! Accordingly, the larvae must be allowed to exist in the compost heap
- which is ultimately very beneficial anyway!
- fighting is not allowed!
- not even with home remedies!
Establish grubs in the compost
Instead of fighting the rose beetle larvae, hobby gardeners can take advantage of them and settle them in the compost heap. This not only promotes the decomposition of the compost, but also protects species. However, there are a few points to consider here, as the larvae need around two to three years before they have fully developed into beetles. During this time, the compost heap must be cared for accordingly so that the larvae feel completely at ease:
- Year 1: Gather and rearrange material
- 2nd year: Deposit material
- The compost heap needs to reach about 20-30 cm into the ground
- Larvae and beetles are very active in digging
- Add sawdust and wood shavings
- Keep compost heap moist
- not implement before early/mid-May
- Sieve the compost through a coarse sieve
- Collect larvae and transfer to fresh compost soil
- Collect beetles and set them down on the compost edge
Notice: The wood shavings must be free of paint residues, boric acid and copper salts.