The relationship between humans and cockchafer larvae has always been fraught with conflict. Until the larvae transform into flying harbingers of spring, they become greedy grubs and incur the displeasure of gardeners and farmers. This guide highlights ecological control measures, gives tips for effective prevention and takes a look at the career of a May beetle.
How do you fight cockchafer larvae ecologically in the garden?
May beetle larvae, also known as grubs, are considered pests because they eat plant roots and thus damage the garden. Ecological control agents such as nematodes can kill them without poison, while natural garden care prevents infestation.
- The cockchafer larva is considered harmful because its diet consists of plant roots.
- Mass occurrences with devastating damage are a thing of the past.
- Nematodes fight local infestations with cockchafer grubs without poison. Organic garden care is the best prevention.
May beetle larva – pest or beneficial?
The cockchafer larva owes its bad reputation to its menu. Plant roots of all kinds are an essential part of the diet of cockchafer grubs. A grub spends between three and five years of its development deep in the ground and feeding on roots. The cockchafer larva owes its status as a pest to this unfortunate diet. Dramatic harvest losses or serious forest damage are only to be complained about when there are mass quantities with a population of 100 to 200 larvae per square meter.
The days of devastating damage caused by cockchafer larvae are of course long gone. In many places, only grandparents have witnessed a plague of May beetles. Until the 1960s, there was even a day off school during a so-called May beetle year so that children could collect beetles and larvae. Today, children and young adults may still know the beetles from “Max and Moritz” by Wilhelm Busch or as Sumseman from “Peterchens Mondfahrt”.
Selective infestation cannot be ruled out
Locally, cockchafer larvae cause headaches for hobby gardeners when nests with a dense swarm of grubs form in the bed or under the lawn. An infestation can be recognized by the stunted growth of perennials, bushes and trees. Yellow spots appear in the lawn, which gradually enlarge and dry out. Against this background, it is little consolation for affected gardeners that lady cockchafers primarily prefer garden soils as nurseries that are managed in an ecologically exemplary manner.
Fighting cockchafer larvae with beneficial insects – This is how it works
Did you find a single cockchafer larva? Then this is an opportunity to thoroughly search the bed or lawn for additional specimens. If you find a large number of grubs or entire nests under the lawn, control measures are an option. To ensure that the infestation does not become a plague, it is not necessary to resort to chemical poison. Instead, you rely on the powerful protection of nematodes of the genus Heterorhabditis. How to fight May beetle larvae with beneficial insects:
- Best time for control is 6-8 weeks after the flight period
- Buy fresh nematodes (e.g. 12 million for 24 m²) as well as watering bars (€13.00 on Amazon) for spreading with a watering can
- Dissolve nematodes in the clay granules in water according to the supplier's instructions
- water the affected area with nematode water
- keep treated soil constantly slightly moist for 6 to 8 weeks
- Do not lime the bed or lawn before or after (fertilization does not bother nematodes)
The nematodes, which are approximately 1 mm small, actively search for May beetle larvae and penetrate the bodies. There the beneficial insects release a bacterium that is toxic to the grubs. The larvae are destroyed within 2 to 3 days. The nematodes have their sights set on cockchafer and June beetle grubs alike. Pupated larvae and adult beetles are not part of the nematode's prey pattern.
Only with emergency approval – soil fungus kills cockchafer larvae
Fungal infections kill cockchafers painfully
When looking for alternatives to DDT and other poisons against cockchafer larvae, the scientists found soil fungi. Beauveria brongniartii has been useful in the fight against mass numbers of grubs and cockchafers for more than 100 years. The fungus infects its host with spores, which neither larvae nor adult beetles survive.
The control agent is applied in the form of grains that have the fungus growing around them. The spores form at a depth of 5 to 10 centimeters in the soil and infect existing cockchafer larvae. In Germany, the use of the drug still requires emergency approval. In Austria, the preparation is available upon presentation of a plant protection certificate. Switzerland has approved the product for use in small and home gardens.
Excursus
Rose beetle larva or cockchafer larva?
With their shiny metallic green wings, rose beetles are a feast for the eyes. The neat appearance makes identifying the noble beetles child's play, even for the layman. The distinction between rose beetle larvae and cockchafer larvae is more problematic. Both species belong to the scarab beetle family and look very similar at first glance. This similarity ends tragically for the protected rose beetle grubs. The location provides an important clue as to which larva you discovered in the garden. The rose beetle larvae prefer to live in compost because their diet consists exclusively of dead, decomposing plant parts. The cockchafer larva colonizes the soil under beds and lawns to nibble on the roots of bushes, trees, perennials, grasses and vegetables.
Prevention is the best control
Hedgehogs love larvae and grubs
Targeted prevention can successfully avoid direct confrontation with cockchafer larvae. For this purpose, a wide range of effective methods are available to ensure that female cockchafers do not lay their eggs in the bed or lawn in the first place. This is how it works:
Insect protection nets
After the last frost, the main flight time of cockchafers begins. Now the finished beetles leave their pupal shell, in whose protection they spent the winter. Males and females don't hesitate to start families. Within a short time, the winged girls are mated and are looking for a place to lay their eggs. By covering the garden with insect protection nets, you can put a stop to the lady beetles. In the wake of global warming, preventative measures should be noted in the calendar for the beginning/mid-April.
Attract natural enemies
The declared enemies of cockchafer grubs include moles, hedgehogs, badgers, starlings and crow. Numerous other biological opponents like to eat the juicy larvae. In order for predators to find their way into the garden and hunt the cockchafer larvae, suitable conditions are advantageous. The following tips & tricks may inspire you for a natural garden in which harmful cockchafer larvae and other grubs have a bad hand:
- consistent avoidance of pesticides
- Garden design with wild corners, piles of leaves, rotten tree trunks as places of retreat
- Create mixed and Benje hedges as hiding places for beneficial insects
Keeping chickens and ducks is one of the most effective control methods in the natural garden. Pecking chickens and busy ducks discover every little cockchafer egg and every grub in no time. A positive side effect is that running ducks thwart the plans of voracious slugs to attack flowers, perennials and vegetable plants.
Scarify the lawn
Regular scarifying prevents grub infestation
The lawn is a popular place for female cockchafers to lay their eggs. However, a well-kept green area with thick turf is not welcomed by the beetle ladies. Regular scarifying is one of the smart preventive measures against grubs, especially since the best time is when the eggs are being laid. The scarifier not only combs out thatch and makes room for strong noble grasses. Cockchafer eggs that have already been laid are also destroyed.
Lawn protection fabric
If you are laying a new lawn, it is worth investing in a protective fabric between the coarse subgrade and fine subgrade. This precaution is always advisable if you or your neighbors have ever had to deal with an increased number of May beetles. This preventative measure does come at a price. In return, you not only repel cockchafer larvae. Other uninvited guests, such as moles, also stay away from your lawn carpet.
Background
Mass arrival every 30 to 45 years
Scientists have found that a mass emergence of cockchafer larvae occurs every 30 to 45 years. When the larvae, with a population of 200 grubs per square meter, cavort in the forest floor and nibble on tree roots, affected trees die across the board. However, cockchafer plagues are usually reported locally, as was the case in a district in southern Hesse in 2010. Those responsible there used a chemical insecticide to combat the pests. This triggered nationwide protests from nature conservation associations. The NABU (Nature Conservation Association of Germany) explicitly pointed out in a widely acclaimed position paper in 2006 that natural regulatory mechanisms can get rid of the problem just as quickly as toxic chemicals.
There are no more cockchafers Maybe their departure scares me and maybe I conclude that the cockchafers are just a little ahead of us (Reinhard May)
Cockchafer larva in portrait – profile to get to know
May beetle larvae grow up to 5cm in size
The life of a cockchafer larva begins as a tiny, 3 mm long, yellowish egg. When the larva hatches from the egg after 4 to 6 weeks, the parents have long since passed away. Male cockchafers die immediately after mating. Females carefully choose different locations to lay 15 to 100 eggs about 10 cm deep in the ground. With this work, female cockchafers have fulfilled their purpose in life and also die. The following profile summarizes important characteristics of grubs:
| May beetle larva | |
|---|---|
| Insect family | Scarabaeidae |
| Genus | Cockchafer (Melolontha) |
| Size | 3-5 cm |
| Body shape | roll-shaped, curved |
| Extremities | 3 pairs of legs in front |
| Coloring | light to cream with brown head |
| Development | 3 to 5 years |
| Hatch and excursion | from the end of April/beginning of May |
| habitat | Soil up to a depth of 100 cm |
| Food | Plant roots |
Immediately after hatching from the egg, cockchafer larvae begin to eat. In the first few days the grubs mainly eat mulch and extremely delicate roots. In proportion to their growth, the larvae feed on thicker roots, which they locate using root exudates. A powerful mouthpart allows the insatiable larvae to attack almost all plant roots in the soil. When there is drought, grubs seek out deeper layers of soil with a higher moisture content. Damp weather attracts them to higher altitudes, but not into daylight.
Tip
The grubs of the rhinoceros beetle are of a completely different caliber than cockchafer larvae. With a body length of up to 12 centimeters, rhinoceros beetle larvae are among the giants among beetle offspring. In most cases, organic gardeners with their own compost heaps have the privilege of admiring a rhinoceros beetle larva or adult beetle.
Frequently asked questions
Are cockchafer larvae poisonous?
Hobby gardeners and forest owners blame cockchafer larvae on the fact that plant roots are at the top of the menu. Depending on the population in the garden or forest, the insatiable grubs cause damage to perennials and woody plants of all kinds. While herbaceous plants and deciduous trees can recover after being damaged by adult cockchafers above ground, cockchafer grubs deprive plants of their livelihoods with no prospect of regeneration. The harmful effect is based on the insatiable appetite. May beetle larvae do not release toxic substances into the plants or the soil.
There is a cockchafer larva in our garden. How long does it take to develop from the grub to the finished May beetle?
A cockchafer larva spends three to five years in the ground. On average, the development from a newly hatched grub to a winged cockchafer takes four years. During this time, the diet consists exclusively of roots of flowers, perennials and trees. Under favorable conditions, a May beetle larva overwinters twice, pupates in August of the third year, hibernates in the pupal cradle and flies out of the ground as a finished beetle in April or May of the fourth year.
Are there manual control methods against local infestation with May beetle larvae in flower and perennial beds?
If you suspect that cockchafer larvae are in the bed, dig up the affected plants and their root balls. Collect by hand any grubs you discover in the root network. You cannot be sure that you have caught all of the May beetle larvae. For this reason, we recommend reinserting the plants into the bed in a spacious pot with fresh substrate. In this way, hidden specimens cannot spread further in the garden.
We are struggling with a high number of May beetle larvae in the garden. Shouldn't we just mill the contaminated soil?
Only if all control methods fail should you consider using a tiller against cockchafer larvae. The tiller plows through the earth to a depth of 10 centimeters and destroys all grubs. Further control agents are no longer required. However, all other soil creatures also have to pay with their lives for the radical tillage. Furthermore, the soil structure is completely destroyed. The subsequent regeneration takes a long time.
Tip
The fight against a gigantic population with billions of May beetles in Avignon, France, was not successful. In 1320, the pests were ordered by a court order to “withdraw within three days to a field marked by boards, where there would be sufficient food for them. Violators would be treated as outlaws and exterminated, it said. Although the announcement shows the first signs of an ecological attitude, the measure came to nothing.
