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Summary MBY261 Theme 8-16
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MBY 261: THEME 8Fungal symbiosis: Sirex wood wasp
Four organisms:
1. Wasp: Sirex noctilio
2. Fungus: Amylostereum areolatum
3. Host tree: primarily pine trees (Pinus spp.)
4. Parasite of wasp: Deladenus nematode
- The situation in South Africa: the wasp, fungus and nematode are originally from
Europe, and appeared in 1994 in SA. It has spread from Cape Town all along the
coastal provinces to Mpumalanga. Now these European organisms live on Mexican &
Californian pine trees in subtropical Africa. This unique combination make its
behaviour in SA unpredictable, and new research is necessary to understand its
biology.
,Life cycle
- When female lays eggs, the eggs pass
through the mucus gland, picking up some of
the toxic mucus.
- Then it passes by the mycangium, picking up
some fungal spores
- The eggs are deposited into the sapwood of
the tree through the ovipositor together
with the mucus and fungus
- Toxic mucus and rotting fungus kills the
tree while Sirex larvae develop
- Deladenus nematodes are used for biological control of Sirex, it makes the male and
females wasps infertile. Annually 3 billion nematodes are produced in the FABI
biocontrol centre for the SA forestry industry.
Sirex wood wasp-fungus association
1. What is the origin of the wasp and to which continents has it been introduced?
- Sirex noctilio is endemic to Europe, Asia and North Africa, where it is considered a secondary pest of
various pine species and is of little economic or ecological concern. During the twentieth century, S .
noctilio became established in several countries in the Southern Hemisphere including New Zealand
, (1900s), Tasmania (1952), mainland Australia (1961), Uruguay (1980), Argentina (1985), Brazil (1988)
and South Africa (1994).
- In 2004, S. noctilio was first discovered in North America.
2. What benefits does the fungus get from the symbiosis?
- It is protected in the wasp’s mycangia before the insect drills through the protective bark of the tree
and introduces the fungus directly into a suitable, relatively competition-free, host substrate.
- The mycangia are lined with glands and the secretions produced appear to stimulate fungal growth.
- The mucus has been shown to release A. areolatum from the wax packet that surrounds it while
stored in the mycangia, and stimulates its growth.
3. Why does the fungus rarely reproduce sexually?
- The close association of A. areolatum with its woodwasp symbiont is thought to be a reason that it
rarely reproduces sexually in some areas of its native range.
4. How are the wasps specialized to ensure a continued association with the fungus?
- The wasps are specialized to ensure the continued relationship with their symbiont. Adult females
carry the fungus within paired mycangia that are located at the anterior end of the ovipositor and
open into the oviduct. The mycangia are lined with glands and the secretions produced appear to
stimulate fungal growth. On oviposition, the female woodwasp inoculates arthrospores or fragments
of its symbiotic fungus into the host sapwood through the ovipositor and into a separate drill beside
the egg. At times no egg is deposited but the fungus and the phytotoxic mucus (discussed below) are.
From the second instar, the fungus is transferred from one larval instar to the next and the adult
takes it up into the mycangia when she sheds her pupal skin, thus ensuring the continued association
with the fungus.
5. Name the two factors that contribute to tree death?
- The inoculation of A. areolatum into a tree on its own has no deleterious effect, but when injected
into a tree in combination with the phytotoxic mucus the two function as a pathogen. In tandem, the
fungus and the mucus cause severe physiological stress to the tree, which include impaired water
relations and translocation and this is often followed by tree death
6. What effects does the insect mucus have on the fungus once inoculated into the tree?
- The mucus has been shown to release A. areolatum from the wax packet that surrounds it while
stored in the mycangia, and stimulates its growth.
7. What are the benefits that the insect gain from this symbiosis according to Madden & Coutts (1979),
Coutts and Dolezal (1965) and Gilmour (1965 )?
- first, and some second, instar larvae feed exclusively on the fungus, while later larval stages feed on
fungus-colonized wood.
- Egg eclosion is delayed when conditions in the tree impede fungal growth and larvae may starve if
symbiont growth is inhibited by the presence of other fungi. Larval development relates to fungal
growth and when conditions are optimal for the fungus, larger adults are produced.
- The fungus also modifies environmental conditions. The wood-decaying symbiont dries the wood
substrate providing a more suitable micro-environment for egg and larvae development and wood
degradation by the fungus facilitates tunneling of the larvae.
The impact of a tiny beetle and its deadly fungus on South Africa’s
forests
THE PROBLEM WITH HOST LISTS
-
-
-
-
, CONTROL/MANAGEMENT
- Unstoppable, but good management can reduce the impact
- Cut down & remove heavily infested reproductive hosts
o Dedicated dumping sites (same day)
o Burn, fumigate or solarize wood
o Chipping to small size (< 2 cm), compost chip piles
- Plant resistant tree species
- DIVERSIFY URBAN PLANTINGS, AVOID MONOCULTURE
- Chemical control of high value individual trees
o stem injections
o pesticides
o fungicides
- Chemical lures, repellants and traps
IMPACT OF PSHB
- Not all trees will die
- Urban areas worst affected
- Property & farm values decrease
- Pest risk assessments difficult:
o Varying effects on different tree species
o Tree sensus data not available
- Agriculture: pecans, avocado, others?
- Commercial forestry: wattles, eucalyptus
- Biggest risk: keystone native tree species
o not so visible
o ecosystems change
The Polyphagous shot hole borer (PSHB) invasion in South Africa NB REDUCE
AND SUMMARIZE
Background
The PSHB is a 2mm long ambrosia beetle native to Southeast Asia. The PSHB has a symbiotic
relationship with three species of fungi. The most important of the three fungi is the tree pathogen,
Fusarium euwallaceae. This fungus provides a food source for the beetle and its larvae, but in susceptible
trees, it kills the vascular tissue, causing branch dieback and tree death.
In its native environment in Southeast Asia, it seems as if the beetle and fungus do not cause serious
damage because tree species have evolved with the beetle‐fungus complex and have resistance towards
them, and because there are most likely a suite of natural enemies of the beetle.
Identification of the beetle and fungus
The 'Euwallacea fornicatusspecies complex’ include four closely related, but distinct species. These four
species of Shot Hole Borer are very similar in shape, and can only be distinguished by specialists under a
microscope or with DNA sequences. The correct names of four Shot Hole Borer species in the E. fornicatus
complexare:
1. Tea Shot Hole Borer A [TSHB‐a = Euwallacea perbrevis] {Distribution: Asia, Australia, and
introduced in the USA (Florida and Hawaii)}
2. Tea Shot Hole Borer B [TSHB‐b = Euwallacea fornicatior] {Distribution: Asia}
3. Polyphagous Shot Hole Borer [PSHB = Euwallacea fornicatus] {Distribution: Asia and introduced
in the USA (California), Israel, and South Africa}
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