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The biology and ecology of the entomopathogenic nematodes, Heterorhabditis spp. (Heterorhabditidae) and Steinernema spp. (Steinernematidae)

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Molyneux, A S (1983) The biology and ecology of the entomopathogenic nematodes, Heterorhabditis spp. (Heterorhabditidae) and Steinernema spp. (Steinernematidae). Unspecified thesis, University of Tasmania.

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Abstract

The influence of a number of key environmental factors on the
behaviour and survival of various species of the entomopathogenic nematodes
of the genera HetePOPhabditis and SteineT'nema was studied in
relation to the soil habitat and the presence or absence of the insect
host. In addition, the head region of adult nematodes was examined
using a scanning electron microscope in order to differentiate characters
of taxonomic importance because there was difficulty in separating
many of the nematode strains by other means.
The association of six labial papillae with six lips was found to
be characteristic of the genus HetePOPhabditis and the paired structures
of the cephalic papillae to be a potentially useful taxonomic character
at the species level. The unusual lobe-like structures of the head
region of an undescribed steinernematid Ql were found to constitute a
new genus within the family Steinernematidae.
In a comparison between two HetePoPhabditis spp. (HetePOPhabditis
sp. Dl and H. heliothidis strain T327) and two SteineT'neTTll spp.
(S. feltiae Agriotos strain and S. glasepi strain KG) it was found that
in the absence of insect hosts, the steinernematids were able to survive
for longer periods of time in sand than were the heterorhabditids.
HetePOPhabditis sp. Dl, H. heliothidis strain T327 and s. feltiae Agriotos
strain showed an inverse relationship between survival time and
temperature whereas S. glaser'i strain KG survived for many months at
each temperature tested. Infective juveniles of S. glasePi strain KG
became quiescent in sand when insect hosts were not available; those of
the two HetePOPhabditis spp. and S. feltiae Agriotos strain did not.
Hete1"07"habditis sp. Dl and S. feltiae Agriotos strain infective
juveniles survived for longer periods of time in aerated water compared
with infective juveniles kept in moist sand at the same temperature.
Within the pH range 4.5-6.5, the survival of Hete1"01"habditis sp. Dl
infective juveniles did not appear to be affected by pH.
In a further comparison of the two genera, the steinernematids were
active at lower temperatures and were able to parasitize insects over a
greater temperature range. The temperature range of infectivity for
insects differed between nematodes of the same genus and between species
isolated from the same geographical area. However, in general, the
temperature range pertaining to the nematodes' original locality tended
to determine the temperature limits for nematode behaviour.
The effect of soil moisture content on the infectivity of Hetepo-
1"habditis sp. Dl and S. g"laser'i strain KG was investigated using postfeeding,
third instar Luaitia aup1"ina larvae in various soil types.
Infection occurred over a wide range of soil moisture potentials and in
a loamy sand, infection occurred at moisture potentials equivalent to or
below the permanent wilting point of plants ( pF 4. 2). In contrast,
infection of L. aupPina larvae ceased at a lower moisture potential in a
sandy clay loam and fine sand.
The infectivity of 13 species/strains of nematodes for nine species
of pest insect was compared at different temperatures. All species/
strains of nematodes were able to kill insects of each species. The
degree of infectivity of each of the different nematodes varied considerably
for different hosts and no one species/strain was the most infective
for all insect species. S. feltiae, the only nematode species
tested by most other workers, was never the most infective for any of
the insect species tested and was least infective in two instances.
For the majority of soil-dwelling insects tested, the infectivity
of most of the heterorhabditids was greater than that of the steinernematids.
HetePoPhabditis and Steinernema infective juveniles were
observed entering insects via mouth, anus and/or spiracles. However,
the possession of an anterior terminal tooth by Hete1"o1"habditis infective
juveniles enabled them to penetrate the external cuticle of
insects. This additional route of entry may explain the greater infectivity
of HetePOPhabditis compared to SteinerneT!ll for some insect hosts.
Parasitization of an insect occurred in a temperature range that
was greater than that permitting nematode development and reproduction
but was less than the temperature range of growth for XenoPhabdus spp.
The range of temperature allowing development and reproduction inside
cadavers was different for each nematode species/strain tested. Different
strains of the same species were also found to have different temperature
limits for development and reproduction. There were also substantial
differences in the number of infective juvenile nematodes
produced per cadaver. Within this temperature range, cadavers resulting
from exposure to high dosages of infective juvenile nematodes invariably
became fetid and the nematodes failed to develop and reproduce.
Developmental rate was different for each nematode species/strain
tested with most heterorhabditids taking longer to complete their life
cycle than did the steinernematids at various temperatures. The
relationship between developmental rate and temperature was linear. For
the first time, in this area of nematology, theoretical threshold temperatures
for development were calculated which gave a close approximation
to the observed lower temperature limits for development.
Three particular methodologies were found to contribute to a clearer
understanding and evaluation of nematode/insect bioassays:
1) The use of moist sand in specimen jars provided a uniform and
easily replicated method of testing the susceptibility of soildwelling
insects to nematode infection;
2) The relationship between dosage and mortality in the nematode/
insect bioassays was described statistically by pro bit analysis;
and
3) Post-feeding, third instar L. oupPina larvae were ideal for testing
the influence of various environmental factors on nematode behaviour
and survival in soil because:
a) large numbers of larvae of uniform age could be easily obtained
from laboratory cultures, and
b) at this stage of their development, L. oupPina larvae naturally
burrow into soil.

Item Type: Thesis (Unspecified)
Keywords: Insects, Soil nematodes, Helminths
Copyright Holders: The Author
Copyright Information:

Copyright 1983 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Bibliography: leaves 158-180. Thesis (M.Ag.Sci.) - University of Tasmania, 1983

Date Deposited: 03 Feb 2015 03:04
Last Modified: 11 Mar 2016 05:56
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