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

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