Role of Entomopathogenic nematodes in Insect pest management
Nematodes commonly referred to as roundworms, eelworms, or threadworms are bilaterally symmetrical, colorless, unsegmented, translucent, usually elongate and more or less cylindrical throughout their body length. The body is covered by a non-cellular elastic cuticle, which differs chemically from the chitinous cuticle of arthropods and lack appendages. Nematodes may be free-living, predaceous, or parasitic.
Nematodes occur as free-living organisms and as facultative or obligate parasitises of plant and animals. Those parasitic on insects are known “entomogenous nematodes”. Parasitism by entomogenous nematodes can have various deleterious effects on their hosts including sterility, reduced fecundity, reduced longevity, delayed development, or other behavioral, physiological or morphological aberrations and in some cases, rapid mortality.
EPNs are not actually microbial organisms but are always included as part of the microbial control agents. In fact, the non-feeding infective juveniles (IJs) possess attributes of both insect parasitoids or predators and microbial pathogens. Like parasitoids/ predators they have chemoreceptors and are motile; like pathogens they are highly virulent, killing their host quickly.
Important Entomopathogenic Groups
Parasitic associations with insects are described from 23 nematode families. Seven of these families contain species that have the potential for biological control of insects. The most commonly studied genera found useful in the biological control of insect pests- Steinernema and Heterorhabditis belonging to families Steinernematidae and Heterorhabditidae, respectively Obligate pathogens in nature. The number of species, within both the families, is increasing rather rapidly. Recently, 67 and 17 species are recognized in Steinernematidae and Heterohabditidae respectively.
Nematode Bacteria Association
Nematodes in the families Heterorhabditidae and Steinernematidae, both belonging to Order Rhabditida (Class Secernentea), have the ability to quickly kill (1-4 days) their hosts thanks to their mutualistic association with bacteria in the genus Photohabdus and Xenorhabdus, respectively. These bacteria are facultative anaerobes in the family Enterobacteriaceae. EPNs recovered from soils throughout the world and are very common soil organisms. Each nematode species is specifically associated with one symbiont species, although a symbiont species associated with more than one nematode species.
Bacteria receives following benefits from nematodes
- It is unable to survive in soil, the bacterial symbiont requires the infective juvinile nematode for protection by being housed in its intestine.
- Lacking invasive ability, it is dependent upon the infective juvenile (IJs) to transport it into the host’s hemocoel.
- It receives protection from the nematode that inhibits the hosts anti bacterial defences.
Nematode receives following benefits from bacterium
- bacterium kills the host quickly and creates a suitable environment for the nematode to develop by producing antibiotics that suppress competing microorganisms and protect insect cadaver.
- The bacterium transforms the host tissues into a food source for the nematode
- It serves as a food source for the nematode.
Biological Characters of EPNs
- The development of entomopathogenic nematodes has three stages: egg, larva, and adult
- The larvae have four instars, the 1st instar, 2nd instar, 3rd instar and 4th instar.
- The 3rd instar larva can live outside host body and it is solely instar larva having infective ability, namely the infective juvenile (IJs) or dauer stage.
- The infective juvenile has a sheath that doesn’t get off in the 2nd instar, which has functions on resisting harmful environments and increasing virulence of nematodes.
Pathogenicity and Lifecycle or Mode of Action
Both the groups are obligate pathogens in nature. The only stage that survives outside of a host is the third stage non-feeding infective juvenile. The IJs carry cells of their bacterial symbionts in their intestinal tract.
- After locating a suitable host the infective juvenile enters the insect host through the mouth, anus, spiracles, or by direct penetration through the cuticle. there must be a continuous film of water between nematode and insect target for infection to occur.
- The nematodes may take different infective-pathways for different hosts. If the mode of entry is by mouth or anus, the nematode penetrates the gut wall to reach the hemocoel. If by spiracles, it penetrates the tracheal wall. When the infective juvenile reaches the hemocoel of a host, it releases the bacteria, which multiply rapidly in the hemolymph.
- When the infective juvenile reaches the hemocoel of a host, it releases the bacteria, which multiply rapidly in the hemolymph. Usually, the insect dies within 24-72 hours.
- Even though the bacterium is primarily responsible for the mortality of most insect hosts, the nematode also produces a toxin that is lethal to the insect
- The bacteria propagate and produce substances that rapidly kill the host and protect the cadaver from colonization by other microorganisms. The nematodes start developing, feed on the bacteria and host tissues metabolized by the bacteria, and go through 1-3 generations.
- The infective juvenile becomes a feeding third-stage juvenile, feeds on the bacteria and their metabolic by-products and molts to the fourth stage and then to males and females of the first generation.
- After mating, the females lay eggs that hatch as first-stage juvenile that molt successively to second, third, and fourth stage juveniles and then to males and females of the second generation.
- The adults mate and the eggs produced by these second generation females hatch as first stage juveniles that molt to the second stage.
- The late second stage juveniles cease feeding, incorporate a pellet of bacteria in the bacterial chamber, and molt to the third, stage retaining the cuticle of the second stage as a sheath, and leave the cadaver in search of new hosts.
- Depleting food resources in the host cadaver leads to the development of a new host.
- The major difference between Steinernema and Heterorhabditis is that adults of latter are hermaphrodites in the first generation. But amphimictic in the following generations, whereas Steinernema adults always amphimictic.
Dispersal Mechanism of Infective juveniles
- The juveniles of steinernematids and heterohabditids disperse vertically and horizontally both actively and passively.
- Passively dispersed by rain, wind, soil, humans, or insects.
- Active dispersal measured in centimeters whereas passive dispersal by insects measured in kilometers.
Survival of Infective juveniles
The infective juveniles do not feed but can live for weeks on stored reserves as active juveniles and for months by entering a near anhydrobiotic state.
The length of time that juveniles survive in the soil in the absence of a host depends upon such factors as temperature, humidity, natural enemies, and soil type
Better in a sandy soil or sandy-loam soil at low moisture and with temperature from about 15-25°C than in clay soils and lower or higher temperature.
Mass production and formulation
Entomopathogenic nematodes produced by in-vivo or in-vitro methods
- Most common insect host used for in vivo laboratory and commercial EPN production is the last instar of the greater wax moth, Galleria mellonella due to its high susceptibility to most nematodes, ease in rearing, wide availability and ability to produce high yields.
- In invtro production technology is used when large scale production is needed at reasonable equality and cost. The in vitro solid culture on a three dimensional rearing medium consisting of crumpled polyether foam. Liquid culture offers increased cost efficiency relative to other production methods. But it also demands greater capital investment and a higher level of technical expertise
Advantages of EPMs
- Extraordinarily lethal to many important soil insect pests
- Safe for plants and animals
- Nematode applications do not require masks or other safety equipment; and re-entry time, residues, groundwater contamination
- Nematodes working with their symbiotic bacteria, kill insects in 24-48 hr
- No adverse effects shown against non targets in field studies
- They do not require specialized application equipment
- Application through irrigation systems has improved grower acceptance
- Insecticidal nematodes are virtually without competition from other biological agents for control of soil inhabiting and plant boring insects.
Disadvantages of EPNs
- High cost of production
- The survival of IJs on the leaf surface incresed due to development of improved formulations containing antidesicants and surftants.