Biology

Steinernematids and Heterohabditids are obligate natural parasites.

The entomopathogenic nematodes are small (0.4–1.1 mm in length) not segmented roundworms. Adults and larvae are similar. Ontogenesis consists of 3 phases: an egg, a larva, an adult. The larva has 4 ages. All the phases of the life cycle of an entomopathogenic nematode take course inside the body of a host-insect (phytophagan), except 3^rd larva stage. The whole cycle of development is 7–30 days.

The first age larva develops being inside the egg. The second age larva leaves it, molts and can infest an insect-hosts. The infestation of insects occurs through natural holes (mouth, spiracles, anus) or thin areas of the cuticle of a host (usually Heterohabditids), as a result of this a nematode penetrates into the hemocoel of an insect. Then infective larvae or juveniles make the release of the bacteria of the genus Xenorhabdus and Photorhabdus (dependently on species) into the upper part of the intestine of the host. Multiplying intensively, mutualistic bacteria produce substances that kill the host (the immune system of insects cannot resist to bacterium activity) and defense its dead body from colonization of other microorganisms. The death of the insect results from septicaemia during 24–72 hours. The larva feeds on the putrefied tissues of the insect when the transition to 4^th age proceeds. After that the adult individual of the nematode forms. A female can lay to 1500 eggs during its lifespan. The majority of them perishes, females and males emerge from survived ones. The males die after mating. When the availability of food reserve is sufficient, the female lays eggs inside the body of the host. If food reserve is not sufficient the larvae of 1^st and 2^nd ages begin to develop inside a female body. Having reached 3^rd age the infective larvae leave inhabitation and begin to search a new host. However, it can be two or more generations inside the body of the same host dependently on the availability of food source. Only 3^rd age larvae can exist out of a host body because they don't need nutrition. They are covered with cuticle of 2^nd stage. 3^rd stage is always infective. The infective larva is resistant to many synthetic chemicals.

There are following staging points in the life activity of the infective larvae: the search of hosts, penetration into the host body, propagation in the body of the host, the emergence of a new generation of the infective larvae from the host body eaten from inside, the search of a new host.

The main difference between Steinernematids and Heterohabditids is that all but one species in the former group are amphimitic, whereas species in the latter group are hermaphroditic in the first generation but amphimitic in the following generations.

The association between the nematode and bacteria is absolutely mutualistic because of the next factors: firstly, the nematode depends on the bacteria because they quickly kill the insect; secondly, they form suitable medium for its development, producing antibiotics, which suppress competitive organisms; thirdly, they transform the tissues of the host-insect into accessible food resource; fourthly, the bacteria themselves are food resource. At the same time, the bacteria need the nematodes for defense from external influences, for penetration into the hemocoel of the host and the inhibition of the anti-bacterium proteins of a prey.