Steinernema carpocapsae

Steinernema carpocapsae is an entomopathogenic nematode, a member of the family Steinernematidae. It is a parasitic roundworm that has independently formed an insect killing symbiosis with bacteria, similar to its kin, the Heterorhabditidae. This parasite uses its bacterial symbionts as a weapon by unleashing them in the direct host, insects, that they infiltrate. S. carpocapsae is particularly effective against lepidopterous larvae, including various webworms, cutworms, armyworms, girdlers, some weevils, and wood-borers. This species is a classic sit-and-wait or "ambush" forager, standing on its tail in an upright position near the soil surface and attaching to passing hosts. Consequently, S. carpocapsae is especially effective when applied against highly mobile surface-adapted insects (though some below-ground insects are also controlled by this nematode). S. carpocapsae is also highly responsive to carbon dioxide once a host has been contacted, thus the spiracles are a key portal of host entry. It is most effective at temperatures ranging from 22 to 28 °C.^[1]

Mechanism

The immunomodulatory and pathogenic properties of parasitic nematodes are largely attributed to the excretory/secretory (ES) products they release during infection. The infective larvae also start spewing out a complex cocktail of proteins. When researchers isolated and examined this mixture more closely, they found that it was made up of 472 different proteins – many of them are proteases. For the average insect, a lethal dose of Xenorhabdus nematophilus consists of about 3500 bacterial cells. But, each S. carpocapsae only carries 20—200 cells of S. carpocapsae – well below the lethal dose. The fact that a single worm is enough to kill an insect host with so few bacterial cells means that S. carpocapsae is not just relying on the bacteria to do all the work.^[2]

Morphology

The nematodes are formulated and applied as infective juveniles, the only free-living and therefore environmentally tolerant stage. Infective juveniles range from 0.4 to 1.5 mm in length and can be observed with a hand lens or microscope after separation from formulation materials. Disturbed nematodes move actively, however sedentary ambusher species (e.g. Steinernema carpocapsae, S. scapterisci) in water soon revert to a characteristic "J"-shaped resting position. Low temperature or oxygen levels will inhibit movement of even active cruiser species (e.g., S. glaseri, Heterorhabditis bacteriophora). In short, lack of movement is not always a sign of mortality; nematodes may have to be stimulated (e.g., probes, acetic acid, gentle heat) to move before assessing viability. Good quality nematodes tend to possess high lipid levels that provide a dense appearance, whereas nearly transparent nematodes are often active but possess low powers of infection.

Insects killed by most steinernematid nematodes become brown or tan, whereas insects killed by heterorhabditids become red and the tissues assume a gummy consistency. A dim luminescence given off by insects freshly killed by heterorhabditids is a foolproof diagnostic for this genus (the symbiotic bacteria provide the luminescence). Black cadavers with associated putrefaction indicate that the host was not killed by entomopathogenic species. Nematodes found within such cadavers tend to be free-living soil saprophages.^[3]

Life cycle

The infective juvenile stage (IJ) is the only free-living stage of entomopathogenic nematodes. The juvenile stage penetrates the host insect via the spiracles, mouth, anus, or in some species through intersegmental membranes of the cuticle, and then enters into the hemocoel.^[4] Both Heterorhabditis and Steinernema are mutualistically associated with bacteria of the genera Photorhabdus and Xenorhabdus, respectively.^[5] The juvenile stages release cells of their symbiotic bacteria from their intestines into the hemocoel. The bacteria multiply in the insect hemolymph and the infected host usually dies within 24 to 48 hours. After the death of the host, nematodes continue to feed on the host tissue, mature and reproduce. The progeny nematodes develop through four juvenile stages to the adult. Depending on the available resources one or more generations may occur within the host cadaver and a large number of infective juveniles are eventually released into the environment to infect other hosts and continue their life cycle.^[6]

Habitat

Steinernematid nematodes are exclusively soil organisms. They are ubiquitous, having been isolated from every inhabited continent from a wide range of ecologically diverse soil habitats including cultivated fields, forests, grasslands, deserts, and even ocean beaches. When surveyed, entomopathogenic nematodes are recovered from 2% to 45% of sites sample.^[7]

Distribution

Asia, Africa, North, Central, South America and Caribbean, Oceania, and Europe.^[8]

External links

• Rhabditida: Steinernematidae & Heterorhabditidae
• Entomopathogenic Nematodes

References