Nematodes have become very important in biological pest control. These nematodes are tiny, white or colorless, and can only be seen with a strong magnifying glass. More than 20,000 species have now been discovered and new ones are constantly being added. The parasitic groups among the nematodes can cause problems for humans, animals and plants. However, among the insect infecting groups, several species have been identified that are excellent biological pest control agents. As early as the 1930s, the first successes were achieved in America with nematodes to get rid of the larvae of the Japanese beetle. However, it was not until the early 1990s that nematodes were used against the dreaded slugs in Europe. Today the roundworms,
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Areas of application of the nematodes
The principle of biological pest control is to target specific nematode species where they, as natural enemies, decimate the number of pests. A complete eradication of living beings that are considered harmful is not the purpose of this principle, but rather control while maintaining a minimum level of biodiversity. As a classic example, cats have been bred for more than 10,000 years to keep mice out of the food pantry. The use of roundworms is one of the most modern methods of combating pests in an environmentally friendly way. Five genera have so far proved particularly successful: Steinernema feltiae (SF nematodes), Phasmarhabditis hermaphrodita (PH nemantodes), Steinernema carpocapsae (SC nematodes),
codling moth
- with SF nematodes
- large-scale treatment
- with neighbors
- Apply from 8° Celsius
- moderate control success
fungus gnats
- with SF nematodes
- combats the 5 mm long larvae
- Application all year round from 8° Celsius
- Monitoring with yellow boards
- good control success
vine weevil
- HM nematodes in April and June
- SK nematodes February to April
- SC nematodes May to September
- also available as a bait trap for beetles
- good chance of success
garden beetle
- with HM nematodes
- in the period July to October
- additional pheromone traps in June/July
- good success rates
somersault beetle
- with HM nematodes
- Larvae eat many roots
- apply in June and July
- works only on grubs in the 2nd year
snails
- with PH nematodes
- apply from April to September
- in the greenhouse all year round
- good chance of success
hair midge
- with SF nematodes
- up to 300 larvae per female
- Application March to May
- from 8° Celsius air temperature
- medium effectiveness
meadow crane
- with SC nematodes
- in September and October
- possibly in combination with Bacillus thuringiensis
- required when ground temperature is below 10° Celsius
Dung beetle
- with HM nematodes
- causes damage to the lawn
- Application in April and May
- from 11° Celsius ground temperature
- sufficient effectiveness
Asseln
- with SC nematodes
- lay out in beech leaves as bait
- Application all year round
- good chance of success
mole cricket
- with SC nematodes
- Application from April to July
- the adult animals are fought
- good chance of success
Feldmaikäfer
- with HM + SF Nematoden
- dreaded crop garden pest
- root feeding of the larvae
- apply July to September
- good success if 3 years in a row
ants
- with SF nematodes
- good alternative to ant poison
- Application March to October
- Effect: the ants resettle
- good success
Specialist shops supply the nematodes in a dormant state, in the form of a powder to which a clay mineral has been added.
Various options for deployment
The application of the nematodes with a sufficient amount of water has proven to be very effective. As a rule, around 500,000 nematodes per m² of application area are required for the treatment to be successful. This apparently quite high number of nematodes is necessary because they do not search for the pests in a targeted manner, such as the lacewing larvae, but rather come across them by chance. Experience has shown that twice the amount is required for mushroom cultivation, whereas 50,000 nematodes per m² can be sufficient when fighting snails.
During the nematode treatment, this replaces the usual watering. This is not a problem outdoors; in the greenhouse, on the other hand, care must be taken not to completely saturate the substrate with water, because nematodes are non-swimmers and then cannot move. The watering can is ideal for distributing the water-nematode mixture in the hobby garden. Inexpensive dosing devices are available from specialist retailers for areas larger than 200 m². Some manufacturers also offer nematodes in the form of a liquid suspension that can be incorporated into the plant substrate. This type of application is particularly suitable for potted plants. If the pot depth is 5 cm, 10 million nematodes per m³ should be released.
Limits of use of nematodes
Biological pest control using nematodes can only lead to satisfactory success if the following requirements are met:
- the type of pest is well defined;
- the timing of the fight is right;
- the appropriate nematode species is used;
- the nematodes are kept for a maximum of 2 days;
- the soil is sufficiently moist;
- there is the right temperature;
- do not apply under blazing sun;
- the dosage is strictly adhered to;
- the earth is not completely submerged;
- better water later;
- keep the soil moist for 2 weeks.
The delivered nematodes can be kept for a maximum of 2 days at a temperature between 4° and 8° Celsius and should be released from this point at the latest. Without host animals, roundworms can survive in the soil for a few months; however, their population is then reduced so drastically that it is necessary to apply fresh nematodes again.
The further development of the action of nematodes
Research into the possible uses of nematodes in biological pest control does not stand still. On the contrary, we have now succeeded in further developing the procedure so that not only the larvae of the pests are destroyed, but the adult beetles themselves. The mode of action is presented below using the example of the vine weevil:
The vine weevil is a beetle that is mainly up to mischief in the kitchen garden, but also doesn’t stop at rhododendrons or roses. From April to October, the adult beetles attack the young shoots, leaves and buds of the plants. The female beetles lay their eggs directly on the roots, from which the 10 mm long larvae then develop, which feed on the roots. Depending on the weather, several generations of this pest develop each season.
The only 1 mm small nematodes are among the natural enemies of these larvae. Once applied with the water, they kill the vine weevil larvae within 24 to 48 hours. However, it is rarely possible to get rid of all the larvae with the help of the roundworms. Nevertheless, a whole series of adult specimens of this pest developed, and chemical agents had to be used to combat them.
But now the scientists have succeeded in developing a beetle trap in which the vine weevils become infected with the SC nematodes, which contributes to a significant reduction in the number of pests. The trap is constructed in such a way that there are several grooves on a wooden board containing a gel consisting of around 2.5 million nematodes. The beetle trap is laid out on the damp ground with these grooves facing downwards, whereby 1 trap is sufficient for 10 m² and a duration of 6 weeks. Even if the attracted beetles only come into contact with this gel for a short time, they are infected with the nematodes and die. After 6 the grooves are refilled with fresh gel and reinserted. In this way, the hobby gardener has a combined method of biological pest control at hand:
SK nematodes against the larvae
- February to April and September to October
- from a soil temperature of 5° Celsius
HM nematodes against the larvae
- April to June and Aug. to Oct.
- from a soil temperature of 12° Celsius
SC nematodes as a beetle trap
- from May to September
This further development of research is promising and raises the hope that it can also be extended to other pests that survive the larval stage and make life difficult for the hobby gardener as adult specimens.
combination with other means
After you have applied the nematodes, you can safely fertilize the soil with minerals or organics. The beneficial insects are not harmed by this, apart from lime. You should avoid using it for the first two months. If the soil has been covered with bark mulch, the nematodes can easily penetrate a layer 5 cm to 7 cm thick. If the hobby gardener has spread the mulch in a thick layer, he should rake it aside before spreading the nematodes. Then the nematodes can migrate deep enough and do their job. The mulch layer can then be spread out again.
The simultaneous use of insecticides containing thiacloprid as an active ingredient has been discussed controversially. The experts from the manufacturer group Bayer report that no damage to the nematodes by this agent has been reported so far, but still advise against a combination. The proponents argue that the nematodes do not take up any food outside their host, including thiacloprid. Since the development of the beetle trap with a gel made from nematodes was recently successful, the environmentally conscious hobby gardener prefers to use this agent and does not use any chemicals.
Conclusion
Scientists assume that nematodes make up more than 80% of all animal organisms. These include numerous species that may harm humans and animals. However, just as many nematodes can also be considered useful, especially in the biological control of pests. The tiny little nematodes, 1 mm long, take on codling moths, spider mites, fungus gnats, snails and even the vine weevil. Provided that the pest has been accurately identified, the specialist trade will have exactly the right type of nematode ready to take action against it in an environmentally friendly way.
