1 Life Cycle
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The life cycle of the entomopathogenic nematodes Steinernema
and Heterorhabditis consists of two important phases: a freeliving phase in the soil and a parasitic phase inside the insect host.
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The life cycle takes place 10–15 cm below the soil.
l
The life cycle consists of the stages of the egg, juvenile 1, 2,
3, and 4 (J1, J2, J3, and J4), and adult.
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The third stage (J3) infective juvenile (IJ) or dauer juvenile
(DJ) is the only free living non-feeding stage of the nematode
outside of their insect host.
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IJ with its closed mouth and anus is sheathed in a two-layered
cuticle and can persist for months in the soils in the absence of a
suitable host.
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During favorable environmental conditions, IJs can search for
susceptible hosts in the environment. Steinernematidae IJs use
ambusher strategy and Heterorhabditidae IJs use cruiser strategy to locate their insect host.
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Once the insect is detected, the IJ expels its external cuticle to
expose the mouth and anus, and the parasitic phase starts when
IJ enters the insect host through natural openings like mouth,
anus, and spiracles or cuticle.
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Once inside the host, the nematodes invade along the intestinal
wall into the insect hemocoel or blood cavity and release their
symbiotic bacteria.
l
The bacteria produce secondary metabolites that suppress the
immune function of the insect larvae.
l
The metabolites are also lethal to the larvae and the host insect is
killed by septicemia or toxemia within 24–48 h [94] after
infection.
l
The bacteria rapidly proliferate, breaking down the host tissues,
and convert the insect cadaver into an effective growth base for
the nematode. The bacteria and the degrading tissues of the host
cadaver serve as food sources for nematode growth and
reproduction [21].
l
Owing to the association of various symbiotic bacteria with
EPN, steinernematid nematodes turn the insect cadaver tan,
ochre, gray, or dark gray, whereas heterorhabditid nematodes
turn the host cadaver red, purple, orange, yellow, brown, or
sometimes green.
l
In the process, the bacteria create a secured niche for the nematode and themselves by developing antibiotics with bactericidal,
fungicidal, and nematicidal properties that inhibit other microorganism competitors [95].
20
Biology
l
The life cycle of the entomopathogenic nematodes Steinernema
and Heterorhabditis consists of two important phases: a freeliving phase in the soil and a parasitic phase inside the insect host.
l
The life cycle takes place 10–15 cm below the soil.
l
The life cycle consists of the stages of the egg, juvenile 1, 2,
3, and 4 (J1, J2, J3, and J4), and adult.
l
The third stage (J3) infective juvenile (IJ) or dauer juvenile
(DJ) is the only free living non-feeding stage of the nematode
outside of their insect host.
l
IJ with its closed mouth and anus is sheathed in a two-layered
cuticle and can persist for months in the soils in the absence of a
suitable host.
l
During favorable environmental conditions, IJs can search for
susceptible hosts in the environment. Steinernematidae IJs use
ambusher strategy and Heterorhabditidae IJs use cruiser strategy to locate their insect host.
l
Once the insect is detected, the IJ expels its external cuticle to
expose the mouth and anus, and the parasitic phase starts when
IJ enters the insect host through natural openings like mouth,
anus, and spiracles or cuticle.
l
Once inside the host, the nematodes invade along the intestinal
wall into the insect hemocoel or blood cavity and release their
symbiotic bacteria.
l
The bacteria produce secondary metabolites that suppress the
immune function of the insect larvae.
l
The metabolites are also lethal to the larvae and the host insect is
killed by septicemia or toxemia within 24–48 h [94] after
infection.
l
The bacteria rapidly proliferate, breaking down the host tissues,
and convert the insect cadaver into an effective growth base for
the nematode. The bacteria and the degrading tissues of the host
cadaver serve as food sources for nematode growth and
reproduction [21].
l
Owing to the association of various symbiotic bacteria with
EPN, steinernematid nematodes turn the insect cadaver tan,
ochre, gray, or dark gray, whereas heterorhabditid nematodes
turn the host cadaver red, purple, orange, yellow, brown, or
sometimes green.
l
In the process, the bacteria create a secured niche for the nematode and themselves by developing antibiotics with bactericidal,
fungicidal, and nematicidal properties that inhibit other microorganism competitors [95].
20
Biology