Army worms are the highly destructive larval stage of several moth species, known for consuming vast amounts of turf and agricultural crops quickly. These caterpillars gain their name from their characteristic behavior of moving together in large numbers, often migrating en masse from one feeding site to another. Understanding the natural enemies that prey upon these agricultural pests is a central component of effective integrated pest management. Identifying the organisms that naturally control army worm populations provides the basis for biological control strategies that reduce reliance on chemical treatments.
Defining the Army Worm and its Phases of Vulnerability
The life cycle of the army worm involves four distinct stages: egg, larva, pupa, and adult moth. The adult moth is nocturnal and focuses on mating and laying eggs, often in clusters of hundreds on plant foliage. The larval, or caterpillar, stage is the most damaging phase, lasting approximately two to three weeks, during which the worm consumes the majority of its lifetime food intake.
This feeding stage is when the army worm is most exposed to predation and parasitism. Once the larva has matured, it typically burrows into the soil to enter the pupal stage, transforming into a moth. While protected by the soil, the immobile pupa remains vulnerable to certain subterranean enemies that can still locate and attack it.
Key Insect Parasites and Predators
Invertebrate enemies represent the most specialized and effective natural controls against army worms. These organisms are categorized as either true predators or parasitoids, each employing a different mechanism to eliminate the pest. True predators, such as ground beetles (Carabidae), immediately consume the army worm larva upon capture. Damsel bugs and ant species also act as generalist predators, hunting and feeding on the caterpillars they encounter on the plant surface.
Parasitoids are insects that lay their eggs either on or inside the army worm, with the developing larvae consuming the host from the inside out. Tiny parasitic wasps, including the Trichogramma genus, target the egg masses laid by the adult moth, preventing the caterpillars from hatching. Other specialized wasps, such as those from the Cotesia genus, attack the younger larval instars, with their progeny emerging from the caterpillar after its death.
Parasitic flies, notably Tachinid flies, are also significant parasitoids of the army worm larva. The female fly deposits its white eggs directly on the caterpillar’s body, usually near the head, or lays them on foliage for the host to ingest. The resulting fly maggot burrows into the army worm and feeds internally, eventually killing the host before emerging to pupate. The combined effect of these specialized parasitoids is a significant reduction in army worm populations, often leading to the collapse of an infestation.
Vertebrate Predators: Birds, Mammals, and Reptiles
Larger animals also consume army worms, acting as generalist predators that opportunistically feed in outbreak areas. Avian species are effective, often congregating in fields experiencing heavy infestations. Birds such as starlings, crows, and meadowlarks forage along the ground and on plants, picking off exposed larvae.
Small mammals, including shrews and mice, contribute to the control of army worms by hunting them on the surface and digging up pupae in the soil. Skunks also consume large numbers of the caterpillars and pupae as they forage through turf and crop fields. These vertebrate predators are drawn to areas where the army worms are most abundant, but their consumption depends on the local availability of other food sources.
Amphibians and reptiles, such as toads, frogs, and various lizard species, also feed on army worms. These animals wait for the caterpillars to move across the soil or low-lying foliage. Although their presence offers a localized benefit, the impact of these generalist vertebrates is often less sustained than that of the specialized insect parasitoids.
Microbial Agents and Disease
Microbial agents also play a role in regulating army worm populations. Naturally occurring pathogens can cause debilitating diseases that lead to the death of the caterpillar. The bacterium Bacillus thuringiensis (Bt) is one of the most widely recognized agents, with specific strains like Bt aizawai being effective against army worms.
When the army worm ingests the bacterial spores, a toxic protein is released in its gut, paralyzing the digestive system and causing the caterpillar to stop feeding and die. Viruses, such as the Nucleopolyhedrovirus (NPV), are also host-specific and lethal to army worms. The virus multiplies inside the caterpillar, which often appears flaccid and discolored before liquefying, releasing the viral particles back into the environment to infect other worms. Entomopathogenic fungi complete this microbial arsenal by infecting the army worm externally, with fungal spores germinating on the cuticle and penetrating the host’s body.