Nematodes are microscopic, unsegmented roundworms used in the biological control of insect pests. Entomopathogenic nematodes (EPNs) have evolved a deadly partnership with bacteria to kill a wide variety of insects, often targeting those that live in the soil. The use of these organisms offers an environmentally conscious alternative to chemical pesticides for managing pest populations. Whether EPNs can effectively kill ants is complex, depending heavily on the specific ant species and the environmental context.
The Biological Mechanism of Nematode Action
The ability of entomopathogenic nematodes to kill insects relies on a symbiotic relationship with specific bacteria. Nematodes in the genera Steinernema and Heterorhabditis carry bacteria (Xenorhabdus and Photorhabdus, respectively) within their gut.
These infective-stage nematodes actively seek out a host insect in the soil by sensing cues like carbon dioxide and temperature. Once a host is located, the worm penetrates the insect’s body cavity, typically through natural openings.
Inside the insect’s hemolymph, the nematode releases the symbiotic bacteria from its gut, which multiply rapidly and cause lethal septicemia within 24 to 48 hours. The bacteria convert the insect’s tissues into a food source, allowing the nematodes to feed and reproduce before new infective juveniles emerge from the carcass.
Targeting Ants: Effectiveness and Specific Species
The success of using entomopathogenic nematodes against ants varies based on the ant species and its defensive behaviors. Certain species, such as Steinernema feltiae and Heterorhabditis bacteriophora, are commercially marketed for use against fire ants and carpenter ants. They often target the vulnerable larvae and pupae within the colony, as these are soft-bodied and less mobile than the hard-exoskeleton adults.
However, field trials against imported fire ants (Solenopsis invicta) suggest that nematodes are not a reliable solution for eliminating entire colonies. Ant colonies possess defenses that counteract the nematode’s attack, including rapidly detecting and removing infected or dead individuals to prevent pathogen spread. Furthermore, the mobility of ants allows them to quickly abandon a treated area, relocating the colony and queen to a new site. This relocation often results in the splitting of the colony rather than its total destruction.
Nematodes are generally more effective at causing localized dispersal or treating ants that nest in confined spaces, like carpenter ants in decaying wood. The difficulty of ensuring the nematodes reach the queen and the entire nest structure deep underground remains the primary challenge for complete ant colony control. Their ability to eliminate large, established colonies in the field is limited by the ant’s behavioral defenses and the environment.
Selecting and Applying Nematodes for Pest Control
Successful application of nematodes requires attention to environmental conditions to ensure the viability and mobility of the worms. Nematodes are sensitive to heat and ultraviolet (UV) light, so they should be applied during the cooler times of the day, such as early morning or evening. Applying them on a cloudy day further helps minimize exposure to damaging UV rays. The soil temperature is also a factor, with optimal activity occurring when temperatures are above 55°F (13°C) but below 90°F (32°C).
The nematodes require a moist environment to move effectively through the soil, using water films between soil particles to travel and locate hosts. Before application, the area to be treated should be moistened, and the soil must be kept consistently damp for at least two weeks afterward to maintain nematode activity. The correct nematode species must be selected based on the target pest; for instance, Steinernema feltiae is often used against surface-dwelling pests, while Heterorhabditis species are better for deeper soil targets.
When applying, the nematodes are mixed with water and should be continually agitated, as the worms are heavier than water and will sink to the bottom of the sprayer or watering can. For ant control, the mixture should be applied directly into the mound or nest entrance through a drenching application, to increase the chance of contact with the colony’s interior. This direct approach maximizes the concentration of nematodes near the ant population, giving them the best opportunity to find and infect the vulnerable larvae.