BIOLOGICAL ANTAGONISTS AND BIOLOGICAL CONTROL OF NEMATODES
Biological control is the action of parasites, predators and pathogens in maintaining another
organism's population density at a lower average than would occur in their absence.
Mechanisms of biocontrol of nematodes include predation, pathogenticity and toxicity.
PREDATORS
Several insects and spiders may eat nematodes but this has not been well studied. Many smaller
invertebrates have also been observed to feed on nematodes but are very difficult to culture and
few have been used in biological control studies. Some of these include:
- Mites
- Collembola (springtails)
- Tardigrades
- Annelids
earthworms
enchytraeids
- Nematodes
Mononchus
Dorylaimus
Seinura
Mesodiplogaster
- Turbellarians (flatworms)
- Protozoa
- Fungi
Over 100 species of fungi have been found to be antagonistic to nematodes. Many have
been categorized as "predatory fungi" because they trap nematodes in some fashion and then
penetrate their bodies with mycelia. A germ tube is produced from the hyphae in contact with
the nematode that penetrates the cuticle and ramifies inside the tissues. These fungi are usually
grouped based on the type of trapping mechanism that they use. See figure 4.4 for examples.
Some produce a sticky adhesive that catch the nematode as it crawls by and touches the
fungus. The adhesive material may be:
Produced over the entire surface of the hyphae (Zygomycetes – Stylopage and Cystopage)
or it may be restricted only to the adhesive organs (Deuteromycotina and Basidiomycotina)
- Adhesive knobs = on hyphae (fig 4.4c) or on stalks (fig 4.4d), may or may not detach
(Hyphomycetes, Basidiomycetes)
- Adhesive branch (fig 4.4f) - Monacrosporium cionopagum
- Adhesive networks loops (fig 4.4 e) and loops on loops (fig 4.4b) - Arthobotrys oligospora
Other fungi form rings that physically trap nematodes when they crawl through. Rings
may be "nonconstricting" (fig 4.4 a) which have a small hole that catches the nematode when it
tries to force it's way through the ring (small nematodes can pass straight through). Other rings
are "constricting" which have three cells surrounding a large concentric hole, the inside surface
of which is sensitive to touch. When nematode crawls through and touches the inside surface,
the three cells rapidly increase in volume, reducing the size of the hole and trapping the
nematode (= Dactylaria). Nematodes apparently secrete some substance ("nemin") that induces
trap formation. There is also some evidence that fungi produce substances that attract
nematodes.
PATHOGENS
Protozoa
Viruses
Bacteria
Myxobacteria – only affect rhabditid nematodes, not plant pathogens, cuticle differences
Pasteuria (bacillus) penetrans = one of most pathogenic and used in many biological
control studies. Produces spores that attach to cuticle of juvenile and (30%) germinate about 8
days after nematode penetrates root system. Germ tube penetrates cuticle and forms spherical,
dichotomously branched "thalli" or microcolonies that fragment to daughter colonies. These
continue to proliferate in pseudocoelum and eventually fill body cavity of developing female.
Females decompose and liberate spores that may remain dormant for long periods until contacted
by another nematode.
Fungi
Fungi that do not produce hyphal networks through the soil but produce spores (usually
zoospores) with adhesive which stick onto passing nematodes are usually referred to as parasitic
or pathogenic fungi.
- Endoparasites with ingested conidia (Harposporium) plant pathogenic nematodes unaffected.
- Endoparasites with encysting zoospores (motile spores) - Catenaria anguilulae
Usually the spore forms a germ tube that penetrates cuticle to form hyphae that "dissolve"
nematode and eventually forms zoosporangia inside the nematode. Zoospores leave the
nematode body through an exit tube and swim (flagellated) or crawl (amoeboid) through
soil.
- Endoparasites with adhesive spores (nonmotile spores) - Hirsutella rhossiliensis
Small spores, limited energy reserves, don’t produce extensive hyphae in soil, probably
dependent on nematodes as a food source = obligate parasites. Produces nonmotile spores
which adhere to, penetrate and infect passing nematodes in the soil. Hyphae grow
throughout the still living nematode until, in a few days the nematode is dead and filled
with hyphae. Fungus emerges from cadaver and sporulates.
- Obligate pathogens of females - Catenaria auxiliaris, Nematophthora gynophila
Attacks female cyst nematodes, destroys cuticle body wall internal tissues and eggs.
"Cyst" is not formed. Body loses turgor and is filled with spores in 4 days.
-Egg parasites: Paecilomyces lilacinus, Dactylella oviparasitica, Verticillium
chlamydosporium
Some fungi are egg parasites. Paecilomyces lilacinus attacks egg masses and sometimes
enters female through vulva or anus and can kill female. Nematodes which lay their eggs
in groups (sedentary parasites) are more vulnerable then migratory forms. Once in contact
with eggs the fungus spreads quickly and parasitizes all eggs in stages of embryonic
development, = less effective once there is a juvenile within the egg. Fungus has chitinase
enzymes, chitin is in egg shells but not in cuticle.
Nematoxins
Clostridium pasteurianum and C. butyricum = anaerobic -> produces toxin in saturated soils
that attacks cuticle -> paralyze then decompose.
Desulfovibrio desulfuricans – produces H2S in rice fields
Bacillus thuringiensis – produces toxin that reduces fecundity and egg hatch
Streptomyces avermitilis – produce avermectins, potent broad-spectrum nematicidal properties
Nematoctonus sp. – germinating spores secrete nematoxins, = rapid immobilization and death.
Paecilomyces lilacinus - metabolites or enzymes diffuse into egg and affects development.
Penicillium anatolicum – compounds alter permeability of egg shell, eggs hatch prematurely