1. Steinernema feltiae

Classification: Steinernema feltiae belongs to the phylum Nematoda (nematodes), class Secernentea, order Rhabditida and family Steinernematidae. They are entomopathogenic nematodes (EPNs), which means that they parasitize and kill insects.
Life cycle and mechanism of action:
Infective juvenile stage (IJ): This is the only free-living and infective stage. IJs actively or passively seek out host insects.
Penetration into the host: IJ penetrate the host body through natural openings (mouth opening, anus, stomata) or in some cases through thinner parts of the cuticle.
Release of symbiotic bacteria: Upon entering the host haemocoel (body cavity), Sf release symbiotic bacteria of the genus Xenorhabdus (specifically Xenorhabdus bovienii).
Septicemia and death of the host: The bacteria multiply rapidly in the host hemolymph, producing toxins and enzymes that degrade host tissues and cause septicemia. The host usually dies within 24-72 hours. The bacteria also produce antibiotics that inhibit the growth of saprophytic microorganisms, thus protecting the cadaver for nematodes.
Nematode reproduction: nematodes feed on bacteria and decomposed host tissues, mature and reproduce (one or more generations).
Emergence of new IJs: when nutrients in the cadaver are depleted, new IJs develop and leave the cadaver in search of new hosts.
Primary hosts: Sf are known to be effective against the larvae of dicotyledons (e.g. Sciaridae, Thysanoptera), some Lepidoptera larvae and other soil pests. Adult insects are generally less susceptible.
 

2. Application of Steinernema feltiae against ants

Ants (family Formicidae) are social insects with complex hierarchies and defense mechanisms, which pose significant challenges for biological control using EPNs.
Potential targets in the anthill
Larvae and pupae: They are the most vulnerable stages, have softer cuticle and are less mobile. Infection of larvae could lead to a reduction in the number of future workers.
Worker larvae: Adult worker larvae are active, have a harder cuticle, and exhibit strong social behaviors (e.g., mutual grooming - allogrooming) that may eliminate IJs.
Queen: She is the key to the survival of the colony. Her infection would be fatal to the anthill, but she is usually deep in the nest and well protected.
Challenges in ant control applications
Nest Penetration: Ant hills can be extensive, with deep underground tunnels. Transporting sufficient IJ to the vicinity of sensitive stages is challenging.
Ant defense mechanisms
Physical: Bites, stings (in some species).
Chemical: Formic acid, various pheromones and repellent secretions. Some of these substances may be toxic or repellent to nematodes.
Social: Allogrooming (mutual scavenging) can physically remove IJ before they can penetrate. Ants also remove dead or diseased individuals from the nest (necrophoric behavior), which could limit the spread of the nematode in the colony.
Specificity Sf: S. feltiae are not primarily adapted to parasitizing ants. Their main hosts are other insect groups. Laboratory studies may show some mortality of ants at high concentrations of Sf, but this may not translate into effective control in the field.
Environmental conditions: Sf require high humidity and optimum temperatures (typically 15-25 °C for Sf) for activity, survival and infectivity. The indoor environment of an anthill may be regulated by ants and may not always be ideal for nematodes, especially in drier or cooler periods. The outer parts of the anthill are exposed to UV radiation, which is harmful to IJ.
 

3. Application methods and factors affecting effectiveness

Formulations:Sf are supplied in a variety of carriers (e.g., clay, gel, water dispersible granules).
Suspension preparation: the product is mixed with water according to the instructions.
Application
Watering/Spraying: Application of the aqueous suspension directly to the anthill and its inlets. It is important to ensure sufficient soil moisture before and after application to allow IJ to migrate.
Injection: Direct injection of the suspension into the deeper parts of the anthill using injector applicators.
Success Factors
Moisture: A key factor. Dryness rapidly inactivates and kills IJs.
Temperature: optimal temperature range for Sf. Outside this range, their activity and effectiveness decreases.
UV: Direct exposure to sunlight is lethal to IJ. Application should take place in the evening, morning or on cloudy days.
Soil type: Sandy soils allow better mobility of IJ than heavy clay soils.
IJ Concentration: Higher rates may be required to overcome ant defence mechanisms and reach target individuals.
Ant species: Different ant species have different colony sizes, nest structures and defence strategies.
 

4. Scientific evidence and research

Most research on EPNs focuses on pests where they demonstrate high efficacy (e.g., stink bugs, weevil larvae).
Studies specifically targeting Steinernema feltiae and ants are less numerous and results are often equivocal or show limited efficacy compared to other methods or against other hosts.
Some studies show laboratory pathogenicity of Sf against certain ant species, especially larvae, when forced exposure is used. However, transferring this pathogenicity to the complex environment of the anthill and achieving colony collapse is considerably more difficult.
There are other EPN species (e.g., some Heterorhabditis species or other Steinernema spp.) that may exhibit higher virulence against certain ant species, but even here, practical application to eradication of entire colonies is problematic.
Research may also focus on indirect effects: e.g., if Sf disrupts a colony, ants may abandon it and relocate, which may appear to ‘destroy’ the original ant colony, although the colony may survive.
 

5. Advantages of using EPNs (in general)

Biological agent: Safe for humans, pets, birds and plants when applied correctly.
Environmentally friendly: Does not leave toxic residues in soil or water.
Specificity (relative): They have a limited host range, minimising impact on non-target organisms (e.g. earthworms). Sf are considered relatively safe for bees.
Impossibility of resistance (so far): Insects have difficulty developing resistance to the complex attack of the nematode and its symbiotic bacterium.
 

6. Limits and disadvantages of using Sf against ants

Low effectiveness against ants: Ants are not the primary hosts of Sf. Their defence mechanisms are often too strong.
Difficulty of application to reach deep parts of the nest.
High sensitivity to environmental conditions (humidity, temperature, UV).
Cost: They are more expensive than conventional chemical insecticides, repeated applications or high doses are often needed.
Short shelf life: IJs are living organisms and have a limited life span even when stored optimally.
Slower onset of effect: Compared to chemical insecticides, IJs may take longer to show a visible effect (if at all).
 

7. Alternative and complementary methods

Chemical insecticides: baits (slow action, workers carry the poison into the nest to the queen and larvae), contact insecticides (rapid but often superficial action).
Physical methods: boiling water (limited reach, non-ecological), nest excavation (may lead to fragmentation and new colonies).
Other biological methods: some fungal species (e.g. Beauveria bassiana, Metarhizium anisopliae) are being investigated for ant control but face similar challenges to EPNs.
Integrated Pest Management (IPM): Combination of different methods.
 

Summary
The use of Steinernema feltiae for the direct and complete destruction of established anthills has limited and very often unsatisfactory efficacy. Ants as a target organism pose a significant challenge to Sf due to their social structure, defence mechanisms and protected nest.
Sf may have some effect in:
very young or small colonies.
Application at high concentrations under optimal humidity and temperature conditions.
Targeting larval stages if IJ manages to penetrate to them.
Causing stress and disturbance to the colony, which may lead to colony relocation.
It is not recommended to rely on Steinernema feltiae as the primary or only method of eradicating large and established ant hills. Their effectiveness in such scenarios is low and unpredictable.
For serious ant problems, targeted insecticide baits that workers carry into the nest are often more effective.
Steinernema feltiae remain an excellent biological control agent for other, more sensitive pests such as stink bugs in greenhouses and pots, where they demonstrate high efficacy.
When considering the use of Sf against ants, it is crucial to have realistic expectations and understand the biological interactions and limitations of this method.
 

My personal opinion
When you use Sf against ants, you're usually brewing up a bigger problem than you had. Stress in a colony will trigger a common response strategy:
Loss Signaling: Ants in a colony communicate primarily by chemistry (pheromones) and touch. A reduced number of larvae, or a change in the behavior of the workers tending the larvae, can signal to the queen and other colony members that there is a problem and a loss of offspring.
Compensatory reproduction: the queen is the primary reproductive individual in the colony. If the colony loses future workers (larvae), it is in her "interest" (from an evolutionary perspective) to replace that loss as quickly as possible to ensure the survival and prosperity of the colony.
Redirection of resources: in response to the loss of larvae, the workers can feed the queen more intensively and provide her with better care. Higher quality food supply and better conditions allow the queen to increase egg production.
Regulation of colony size: Ant colonies have mechanisms to regulate their size and structure. The loss of larvae is a signal that they need to "add gas" in reproduction.
Repeatedly applying Sf in the garden will force the ants to look for safer places to survive, they will end up under your stepping stones for example, and as the colony is ‘stressed’ the new colony is likely to be much larger for the reasons given above. So in this case, you will have a problem with both the location of the anthill and its size. 
The economic aspect is an absolute debacle. A small packet of nematodes costs about 7€, the application ideally is on 10m2. Using boric acid with a price of 7€ you can produce 20-100l of bait without any problems. I generally use a total of 20-50ml of bait to kill 1 anthill. In the case of boric acid baits you kill the queen and you have solved the problem. In the case of using nematodes, you can make a small problem into a new and much bigger one. Don't be influenced by influencer marketing and save yourself the trouble and the money...