Abstract
Heterorhabditis bacteriophora and Steinernema carpocapsae are microscopic entomoparasitic nematodes (EPNs) that are attractive, organic alternatives for controlling a wide range of crop insect pests. EPNs evolved with parasitic adaptations that enable them to “feast” upon insect hosts. The infective juvenile, a non-feeding, developmentally arrested nematode stage, is destined to seek out insect hosts and initiates parasitism. After an insect host is located, EPNs enter the insect body through natural openings or by cuticle penetration. Upon access to the insect hemolymph, bacterial symbionts (Photorhabdus luminescens for H. bacteriophora and Xenorhabdus nematophila for S. carpocapsae) are regurgitated from the nematode gut and rapidly proliferate. During population growth, bacterial symbionts secrete numerous toxins and degradative enzymes that exterminate and bioconvert the host insect. During development and reproduction, EPNs obtain their nutrition by feeding upon both the bioconverted host and proliferated symbiont. Throughout the EPN life cycle, similar characteristics are seen. In general, EPNs are analogous to each other by the fact that their life cycle consists of five stages of development. Furthermore, reproduction is much more complex and varies between genera and species. In other words, infective juveniles of S. carpocapsae are destined to become males and females, whereas H. bacteriophora develop into hermaphrodites that produce subsequent generations of males and females. Other differences include insect host range, population growth rates, specificity of bacterial phase variants, etc. This review attempts to compare EPNs, their bacterial counterparts and symbiotic relationships for further enhancement of mass producing EPNs in liquid media.
Highlights
Entomoparasitic nematodes (EPNs) Heterorhabditis bacteriophora and Steinernema carpocapsae are utilized as biocontrol agents against various insect pests of agricultural significance (Figure 1)
This review will 1) identify the biological differences in the life cycles of H. bacteriophora and S. carpocapsae; 2) describe the nutritional relationship between EPNs and their bacterial symbionts; and 3) briefly describe potential biological processes occurring during host interaction that may benefit mass production processes
Unlike H. bacteriophora, juvenile stages resulting from endotokia matricida in S. carpocapsae do not develop into infective juvenile (IJ) until they exit the maternal nematode
Summary
Entomoparasitic nematodes (EPNs) Heterorhabditis bacteriophora and Steinernema carpocapsae are utilized as biocontrol agents against various insect pests of agricultural significance (Figure 1). Symbiotic bacteria Photorhabdus spp. and Xenorhabdus spp. provide Heterorhabditis and Steinernema nematodes, respectively, with diverse services within the host insect: 1) produce virulence factors that neutralize and kill the insect host [2,3,4]; 2) bioconvert the host into nutritional components; 3) serve as the main food source for their nematode partners [5,6]; and 4) produce antimicrobials (Figure 2) which prevent putrification of the insect host by competing microbes [7,8,9,10] These bacterial based services produce an optimal environment for nematode growth and reproduction [11]. This review will 1) identify the biological differences in the life cycles of H. bacteriophora and S. carpocapsae; 2) describe the nutritional relationship between EPNs and their bacterial symbionts; and 3) briefly describe potential biological processes occurring during host interaction that may benefit mass production processes
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