Lecture 3: Biological Control of Weeds Flashcards
3 biological control strategies:
- classical control
- conservation control
- inundative control
classical control:
the introduction of exotic insects or pathogens from one geographical area to another with the object of controlling naturalised, invasive weeds
conservation control:
this refers to the use of indigenous predators & parasitoids, usually against native weeds
inundate control:
this uses pathogenic fungal spores in the same way as chemical herbicides i.e. spores are sprayed onto a crop containing weeds with the objective of killing the weeds
classical control of weeds acts on two types of weeds:
- Aquatic weeds
- Water fern (Slavinia molesta) controlled by the water beetle Crytobagous salviniae)
- Terrestrial weeds
- -Prickly pear cactus (Opuntia species) controlled by the moth Cactoblastis cactorum
- Japanese knotweed (Fallopian japonica) in the UK
Characteristics of classical control: The pathogen or pest is;
- only introduced once
- must be very host specific
- must have a negative impact on plant individuals and the population dynamics of the target weed
- must be prolific
- must be good colonisers
- must thrive & become widespread in all habitats & climates that the weed occupies
problems of aquatic weeds:
- clogging of grids and sluiced in hydro-electric plants
- inteference with navigation & fisheries
- depletion of oxygen from the water leading to death of fish
- act as habitats for vectors of human diseases
- increases water loss through evapo-transpiration
Water Fern (Salvinia molesta) is native to:
South America
Water Fern (Salvinia molesta) reproduces how?
efficiently by the spreading of buds that easily break off when disturbed
Water Fern (Salvinia molesta) affects other plants by
forming floating mats that shade and over crowd native plants. These mates also form in rivers & irrigation ditches which constrains agriculture
spread of Salvinia molesta in Australia:
- 1952 :Introduced into Australia
- 1976 : Widespread in rivers& lakes (as there was a lack in parasites/pathogens & because it posses a v rapid growth rate
- 1978: Largest infestation in Lake Moondara in N. Queensland
- 1979: Use of herbicides stopped
- Search for a biological control agent began
Biological control agent used against Salvinia molesta: Where was it discovered?
Cyrtobagous salviniae (water weevil) was found by CSIRO Entomology scientists in Brazil - the home of the weed
Biological control agent used against Slavonia molesta: how does it work
- adult female lays her egg sin a cavity that she creates by chewing into the leaf bud
- larvae that hatch feed on the base of the leaf bud and eventually tunnel into the rhizomes
- Weevil larvae become adults in 17-28 days during the warmer parts of the summer
biological control action on Lake Moondara Australia (Agains Salvinia molesta
- 15000 Water weevils individuals were released in June 1980
- by June 1981 the weed was under control
- Salvinia has been controlled in tales 13 other tropical countries has a direct result of the research
Where have successful biological controls been used against Salvinia molesta in the world:
- Australia (Lake Moondara)
- Sri Lanka
- Botwana
- Senegal
- Mauritania
- Benin
- South Africa
- USA
- Papua New Guinea
-Is a problem in many tropical & sub tropical areas in thew world
biological control of terrestrial weeds: Prickly pear cactus
- Opuntia species,
- controlled by larvae of the moth Cactoblastis cactorum
biological control of terrestrial weeds: Japanese knotweed
- Fallopia japonica
- in the UK controlled by a plant louse Aphalara itadori
Prickly pear cactus (Opuntia species) General facts:
- native to S. America
- is a succulent up to 7m in height
- stems are fleshy jointed into pads with spines
- large brightly coloured flowers
Problems the Prickly Pear cactus causes:
- propogates by seed or pads in contact with the ground
- Nasty spines
- if eaten by livestock spines cause damage to the animals mouth
- forms dense thickets and excludes livestock & other species
control of prickly pear cactus:
- 1925: prickly pear had spread from being used as a paddock divider on a few farms in South Australia, to covering over 4 million hectares of farming land unusable for livestock
- 1928 - Cactoblastis cactorum moth larvae was introduced (from Argentina) to prickly pear populations in New South Wales and Queensland.
- Within ten years the Cactoblastis larvae had virtually wiped prickly pear out.
- One of the first major successes in biological control
____ is reached between the weed and biological control agent
EQUILIBRIUM
When biological controls go wrong:
- during 1950’s C. cactorum was introduced to the Caribbean island of Nevis to control prickly pear
- however, the adult moth crossed 3km of weak to the nearby island of St Kitts
- More recently C. cactorum has spread more widely in the Caribbean, and in 1989 it reached the mainland USA in Florida.
- By 1999, alarm was growing for endemic prickly pear species in the USA and Mexico.
- Opuntia is a key member of major ecosystems and an important agent in the fight against desertification and in soil regeneration
- It has a wide range of uses in food products, medicines, cosmetics, agriculture and in energy production.
- In Mexico 360,000 ha of Opuntia are under cultivation and 3 million hectares of wild Opuntia are also utilized.
- The sector is estimated to be worth some US$80 million per annum, with exports valued at $30 million.
biocontrol of Japanese knotweed (Fallopia japonica) in the UK: native to:
Eastern Asia
biocontrol of Japanese knotweed (Fallopia japonica) in the UK: introduced into gardens in Europe + spread where
- 200 years ago
- subsequently spread to North America
biocontrol of Japanese knotweed (Fallopia japonica) in the UK: description
strong-growing, clump-forming perennial, with tall, dense annual stems.
-stem growth is renewed each year from the deeply-penetrating rhizomes (creeping underground stems)
biocontrol of Japanese knotweed (Fallopia japonica) in the UK: growth over the year
- winter: plant dies back beneath ground but by early summer the bamboo-like stems shoot to over 2.1m (7ft) suppressing all other growth
- grows through concrete & destroys properties forcing building contractors to abandon infested building sites
- in the UK half-million homes are currently uninsurable
biocontrol of Japanese knotweed (Fallopia japonica) in the UK: eradication & damage costs
- eradication requires determination as it is very hard to remove by hand or with chemicals
- damage & removal costs £288 million per annum in the UK
is japanese knotweed a problem in japan? & WHY
No, it is attacked by 168 insects and 40 fungi
how did they source a biocontrol for knotweed?
- 6 years ago Dr Richard Shaw & colleagues at CABI Switzerland brought back 40 potential control agents for testing
- literature search of each species diet narrowed it down to 9
- these 9 were tested on 90 British plant species & close relatives of Japanese knotweed
- 2 mm long plant louse (psyllid) Aphalara itadori was found to be host specific
- 2010 UK government gave permission for a limited field trial at isolated site
- after 3 months knotweed was infested with louse
- Released at 8 sites across Eng & Wales has been approved
- its too early to assess impact
inundative biological control strategies involve :
the mass release of natural enemies (usually fungal diseases) to kill the weed in a manner analogous to the use of chemical herbicides
inundative biological controls in the case of fungal diseases:
(myco-herbicide) spores are sprayed onto a crop containing weeds with the objective of killing the weeds
inundative biological controls: Natural enemies must be::
- highly virulent
- highly specific (to avoid damage to other weeds or crops, livestock & humans)
plant pathogen example:
-Crop: small grain crops
-Weed: Roundeaf mallow (Malva pusilla)
fungus? Myco-Herbicide?
- Myco-herbicide: BioMal
- Fungus: Colletotricum gloeosporioides
Northern Joint Vetch (Aeschynomene virginica) is a
leguminous weed of rice and to a lesser extent sorghum in E. Arkansas USA
Northern Joint Vetch (Aeschynomene virginica) competes with crop how?
- erect annual about 2m tall and it competes very successfully with the rice by shading it
- also severely contaminates rice crop with vetch seeds
Northern Joint Vetch (Aeschynomene virginica) Old control + disadvantages with it
- for a long time only control was the use of proposal and pheenoxy herbicides
- however:
- -they injure the rice
- they fail to kill the vetch unless applied at exactly the right time
biocontrol for Northern Joint Vetch: naturally:
- Anthracnose (Colletotricum gloeosporioides) an endemic disease of vetch, present in low amounts in the environment
- it is dispersed by rain splash & therefore doesnt spread very far very fast
biocontrol for Northern Joint Vetch: commercially
by applying heavy dose of spores in the form of the mycoherbicide Collego
- field trials aerial sprays of fungal spores killed 99% of the vetch plants
- farmers mix the formulated dry spored with a wetting agent & mix with 250L of water
- The suspension is sprayed from the air when the crop is well watered and relative humidity is likely to be high for the next 12 h.- Within a week the vetch plants begin to show lesions and within 5 weeks are dead.
how is College marketed?
- as a dry formulation (15% viable spores, 85% inert ingredients)
- each pack contains 757 billion spores (treats 2.5 hectares)
development of mycoherbicide
- Field search for specimens of the target weed showing symptoms of infection.
- Isolate and grow the fungus in the laboratory so that it produces spores.
- The effectiveness/specificity of the pathogen is then tested by spraying spores onto detached leaves and seedlings.
- If the fungus passes the preliminary screen a much wider range of species will be tested for specificity.
- It must grow and produce spores in cultures in industrial conditions.
- The developer must apply for a patent, which means that the fungus must not have been exploited before.
- Other pesticides applied to the crop must not harm the fungus.
- A suitable formulation in which to apply the fungus must be designed to ensure that the fungus has the maximum chance of infecting adult plants.
- Very expensive to produce a mycoherbicide
advantages of biocontrol :
- Target specificity
- Continuous action
- Long-term cost effectiveness
- Often gradual in effect therefore environmentally friendly
- Self dispersing (even into difficult terrain)
- Can be used as part of an IPM strategy
- Reduces the vigour and competitiveness of a weed
disadvantages of biocontrol
- Initially high research cost -Protracted time until impact likely
- Uncertainty over ultimate scale of impact
- Uncertain ‘downstream effects in ecosystems (hard to predict or quantify)
- Only targets a single weed species
- Suitable agents may not exist Irreversible
biological control is not appropriate when
- A species is a weed in one area but valued in other situations
– blue weed (Echlum vulgate) is a serious pasture weed in the
USA but is also a desirable plant for honey production. - Weeds are close relatives of economic crops.
- Lots of weeds are present. As biological control agents are highly specific to one weed species little would be gained from controlling one weed amongst a large variety of weed species.
- Eradication of the weed is desired (e.g. poisonous weeds).
