Entomopathogenic nematode

Entomopathogenic nematodes are a type of parasitic roundworm that, in conjunction with certain bacteria, are lethal to insects. These microscopic organisms are often used as a form of natural insecticide to manage pest outbreaks. This function is reflected in the term entomopathogenic, which comes from the Greek word entomon, meaning “insect,” and pathogenic, or “causing disease.” Entomopathogenic nematodes are naturally occurring organisms that live in soil and track their “prey” through movement and the carbon dioxide given off through breathing. Once inside the host, the nematodes release a specific type of bacteria that proves fatal to the insect. The organisms then feed on the insect and reproduce.

Background

Nematodes are a biological classification of multicellular organisms that are also referred to as roundworms. Nematodes typically live in soil or water and can act as parasites in animals and plants. Of the thousands of species of nematodes, most are less than a millimeter long, or about one twenty-fifth of an inch. The largest known nematode can grow to a length of about twenty-six feet (eight meters) and lives inside of sperm whales.

The majority of nematode species are not harmful to humans, although several species can cause serious diseases or medical conditions. A four-inch (ten-centimeter) long species named Wuchereria bancrofti has been responsible for causing a debilitating condition known as elephantiasis. The illness has affected millions of people in developing nations in tropical regions. Other species can cause damage to plants and have been blamed for an estimated $78 billion in lost crops per year. Parasitic nematodes can also affect livestock and can be transmitted to humans though eating improperly cooked meat. Many pet owners try to prevent nematode infestation by giving their pets deworming drugs in pill or tablet form. A nematode species called Dirofilaria immitis causes a serious condition known as heartworm in dogs.

Overview

Entomopathogenic nematodes encompass a number of different species found in two families of nematodes: Heterorhabditidae and Steinernematidae. Nine species are classified as Heterorhabditidae and about forty fall under the category Steinernematidae. These organisms are very small, ranging from about 0.5 millimeters long to 1.5 milometers long. In their earliest stage of life, the organisms are known as infective juveniles. They live in soil and have a closed mouth and cannot feed until they find an insect. They can survive for several months in the soil before dying of starvation. The infective juveniles locate a host by tracking the vibrations the insect makes as its moves through the soil. They can also trace a host by following chemical signals or carbon dioxide emissions exhaled by the insect. Some species lie in wait to ambush a host when it draws near; others burrow underground in pursuit of an insect.

The nematodes enter a host through openings in the insect’s body cavity and eventually make their way into the chamber containing its bodily fluids. Some species can create their own opening in the insect by scratching at its outer shell with a special tooth. Once inside, the infective juveniles release a type of bacteria that is unique to entomopathogenic nematodes. The bacteria infect the host, rapidly multiply, and cause septicaemia, a blood infection that kills the insect within twenty-four to forty-eight hours. The bacteria change the color of the dead insect, indicating the type of nematode that infected the host. Insects turn brown or tan if infected by a Steinernematid, and red if they were infected by a Heterorhabditid.

The bacteria convert the tissue of the dead insect into food for the entomopathogenic nematodes. They also produce chemical substances that preserve the corpse so the nematodes can continue to feed. The infective juveniles mature into adults and begin breeding, producing new generations of infective juveniles. If the host is large enough, several generations of organisms can live and reproduce within an insect. Eventually, when food resources begin to run out, infective juveniles are released back into the soil to seek out new hosts. The corpse of a medium-sized insect can produce an estimated one hundred thousand infective juveniles over a period of about ten days.

The role of entomopathogenic nematodes as an insect parasite was discovered in the seventeenth century; however, it was not until the 1930s that the organisms were first used as a form of pest control. These species of nematodes are not harmful to humans and can be introduced to insect populations without risk or need to wear protective gear. They are often used by farmers and agricultural facilities in place of more harmful chemical pesticides. The type of entomopathogenic nematode used as a pest-control method can be determined by factors such as its range, “hunting” strategy, moisture, temperature, and soil composition.

Certain species of nematode are better suited to target a specific type of insect pest. For example, a species called Steinernema kraussei is effective in killing black vine weevils, a pest that causes billions of dollars of damage annually to the potted plant industry. Heterorhabditis bacteriophora has been used to control populations of ants, beetles, and termites. In China, Steinernema carpocapsae was introduced to protect shade trees from attack by carpenter worms that had been threatening millions of trees. As a result, the carpenter worms were virtually eliminated as a threat in several major Chinese cities.

Entomopathogenic nematodes are effective in controlling pest populations up to soil temperatures of 95 degrees Fahrenheit (35 degrees Celsius) and can survive short-lived freezing temperatures but not long-term exposure. They can be applied using a water can, a spray pump, or through a misting system. Farmers and agricultural engineers typically introduce a population of entomopathogenic nematodes to crops and then wait one to two weeks to follow up with another application. In cases of severe infestation, the organisms can be used every seven to ten days as needed to control the problem.

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