insect-pathogenic Nematodes - Beneficials for insect pest control
Fig. 1: Front end of an infective juvenile of Heterorhabditis bacteriophora.
The hook is used to srape holes into the insect's skin to facilitate penetration. (Scanning electrone micrograph)
Nematodes: a fascinating class of animals
Nematodes are a similarly successful class within the animals as the insects. They occur in almost every habitat
ranging from the deep sea to the highest mountains. No other group of animals are more abundant in number than
the nematodes. Most species are harmless free living soil inhabitants. Some species, however, cause severe
damage to plants, and there are a number of dangerous animal and human parasites. Surprisingly, though the habitats
occupied are so different, the basic construction of the nematodes is very uniform between species. It is simple
and efficient, an ever lasting success, comparable to the bicycle or the VW-beetle. One main concept is parsimony:
The animals do not need arms or legs, they only need longitudinal muscles. The body is robust and reliable, hence
the regeneration or repairing is poorly developed. The number of cells is limited and the development from the egg
to the full-grown Nematode is same for every individual and it can be exactly predicted which cell-lineage will
finally build which organ of the Nematode. It is partly due to this excellent repeatability, that most research
in developmental biology and genetics of animals is nowadays performed on a Nematode, the bacterial feeder,
Caenorhabditis elegans. It was the first multicellular Organism, which genetic code has been fully deciphered.
By chance, it is a close relative to the netomopathogenic nematodes, which have evolved from free-living
bacrterial feeders.
Fig. 2: infective juvenile of the Nematode Steinernema feltiae. The particular shows the
fromt end of the intestine where the symbiotic bacteria are sequestered in a typical pouche. Having entered
the insect's haemocoel, the nematodes releases the bacteria which subsequently propagate and kill the insect.
The Nematode feed on the bacteria, which proliferate rapidly inside the dead insect. (Foto: Ehlers)
insect-pathogenic nematodes: Acquiring new resources by symbiosis
Insect-pathogenic nematodes of both genera Heterorhabditis and Steinernema have independently entered
a symbiotic relationship with bacteria. The bacteria belong to the family of Enterobactericeae, and are
hence closely related to Escherichia coli, the probably best studied bacterium which lives in our intestine.
The Nematode symbionts, however, are not harmful for humans and they can not grow at temperatures of more
than 35°C. It is major advantage of this symbiosis, that the insects entered by the Nematode dyes relatively
quick. There are a number of closely related nematodes without symbiotic bactreria, that do also enter insectsbut that subsequently have to wait until the insect dyes for other reasons. Et other species only enter dead
insect carcasses. By carrying the symbiotic bacteria, these two genera have conquered a new ecological niche
and a new food source. The same holds true for the bacteria. Most of the species can not kill the insects
unless they are carried into the insect's haemocoel.
Fig. 3: Infective juveniles of the species Heterorhabditis bacteriophora. One juvenile is
approximately 0.7 mm longe and 0.023 mm wide (Foto: Peters).
That's how it works: Life cycle of entomopathogenic nematodes
The soil-lving infective juveniles (or dauer-juveniles, from the German expression dauer = enduring) (Fig. 3)
carry the symbiotic bacteria in the intestine (Fig. 2). The nematodes find their hosts by either cruising through,
like the cruise-forager Heterorhabidtis spp., or they sit and wait until an insect passes by, like the
ambush-forager Sterinernema carpocapsae. Many species show a mixture or intermediate behaviour between
these two extremes. After an insect has been found, the Nematode enter into the haemocoel through the insect's
skin, or they take the route through the mouth, anus or the respiratory system of the insect. The nematodes of
the genus Heterorhabditis are equipped with a tooth serving as a penetration tool (Fig. 1).
Fig. 4: Life-cycle of an insect-pathogenic nematodes in a white grub. Following the
propagation in the grub, the nematodes leave the carcass and scane the surrounding for further hosts to infest.
Having reached the haemocoel, the nematodes release the bacteria, which propagate rapidly and kill the host within
3 days. The bacteria and the digested insect tissue is the nutrient source for the nematodes which proliferate
enormously. Two to 3 weeks later, up to 300,000 new nematodes may emerge from one infested white grub. On the other
hand, it only takes 2- 3 nematodes to kill the insect.
Fig. 5: One of the bioreactors at e~nema. Billions of nematodes are produced in these
completely sterilsed vessels.
An insect of 80.000 kg: industrial production of nematodes
There is an overwhelming number of very efficient antagonsist of pest insects, which, however, can not be
produced in large quantities at low cost. Insect-pathogenic nematodes, however, grow and proliferate very
well in conventional bioreactors. The trick is to mimic the situation in an infested insect. The nematodesand the symbiotic bacterium are grown in a sterile environment, free from other Organisms. To date, the biggest
reactor ever used for the production of insect parasitic nematodes contained 80.000 l.
Reliable service: Quality control
Nematodes are living Organisms. By nature the quality may differ between single production batches. A consequent
and continual quality control is therefore indispensable. We check each the nematodes of each production batch
for their ability to kill insects. Only those batches passing our strong internal thresholds are out on the market.
In line with the honest relationship to theor distributors, e~nema has taken the initiative to develop and
validate quality assessment procedures together with key-distributors in Germany. One output of this project was
a simpe standard assessment procedure.
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