Lecture 3: Biological Control of Weeds

Question 1

3 biological control strategies:

Answer 1

• classical control
• conservation control
• inundative control

Question 2

classical control:

Answer 2

the introduction of exotic insects or pathogens from one geographical area to another with the object of controlling naturalised, invasive weeds

Question 3

conservation control:

Answer 3

this refers to the use of indigenous predators & parasitoids, usually against native weeds

Question 4

inundate control:

Answer 4

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

Question 5

classical control of weeds acts on two types of weeds:

Answer 5

• 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

Question 6

Characteristics of classical control: The pathogen or pest is;

Answer 6

• 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

Question 7

problems of aquatic weeds:

Answer 7

• 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

Question 8

Water Fern (Salvinia molesta) is native to:

Answer 8

South America

Question 9

Water Fern (Salvinia molesta) reproduces how?

Answer 9

efficiently by the spreading of buds that easily break off when disturbed

Question 10

Water Fern (Salvinia molesta) affects other plants by

Answer 10

forming floating mats that shade and over crowd native plants. These mates also form in rivers & irrigation ditches which constrains agriculture

Question 11

spread of Salvinia molesta in Australia:

Answer 11

• 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

Question 12

Biological control agent used against Salvinia molesta: Where was it discovered?

Answer 12

Cyrtobagous salviniae (water weevil) was found by CSIRO Entomology scientists in Brazil - the home of the weed

Question 13

Biological control agent used against Slavonia molesta: how does it work

Answer 13

• 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

Question 14

biological control action on Lake Moondara Australia (Agains Salvinia molesta

Answer 14

• 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

Question 15

Where have successful biological controls been used against Salvinia molesta in the world:

Answer 15

• 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

Question 16

biological control of terrestrial weeds: Prickly pear cactus

Answer 16

• Opuntia species,

- controlled by larvae of the moth Cactoblastis cactorum

Question 17

biological control of terrestrial weeds: Japanese knotweed

Answer 17

• Fallopia japonica

- in the UK controlled by a plant louse Aphalara itadori

Question 18

Prickly pear cactus (Opuntia species) General facts:

Answer 18

• native to S. America
• is a succulent up to 7m in height
• stems are fleshy jointed into pads with spines
• large brightly coloured flowers

Question 19

Problems the Prickly Pear cactus causes:

Answer 19

• 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

Question 20

control of prickly pear cactus:

Answer 20

• 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

Question 21

____ is reached between the weed and biological control agent

Answer 21

EQUILIBRIUM

Question 22

When biological controls go wrong:

Answer 22

• 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.

Question 23

biocontrol of Japanese knotweed (Fallopia japonica) in the UK: native to:

Answer 23

Eastern Asia

Question 24

biocontrol of Japanese knotweed (Fallopia japonica) in the UK: introduced into gardens in Europe + spread where

Answer 24

• 200 years ago

- subsequently spread to North America

Question 25

biocontrol of Japanese knotweed (Fallopia japonica) in the UK: description

Answer 25

strong-growing, clump-forming perennial, with tall, dense annual stems.
-stem growth is renewed each year from the deeply-penetrating rhizomes (creeping underground stems)

Question 26

biocontrol of Japanese knotweed (Fallopia japonica) in the UK: growth over the year

Answer 26

• 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

Question 27

biocontrol of Japanese knotweed (Fallopia japonica) in the UK: eradication & damage costs

Answer 27

• 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

Question 28

is japanese knotweed a problem in japan? & WHY

Answer 28

No, it is attacked by 168 insects and 40 fungi

Question 29

how did they source a biocontrol for knotweed?

Answer 29

• 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

Question 30

inundative biological control strategies involve :

Answer 30

the mass release of natural enemies (usually fungal diseases) to kill the weed in a manner analogous to the use of chemical herbicides

Question 31

inundative biological controls in the case of fungal diseases:

Answer 31

(myco-herbicide) spores are sprayed onto a crop containing weeds with the objective of killing the weeds

Question 32

inundative biological controls: Natural enemies must be::

Answer 32

• highly virulent

- highly specific (to avoid damage to other weeds or crops, livestock & humans)

Question 33

plant pathogen example:
-Crop: small grain crops
-Weed: Roundeaf mallow (Malva pusilla)
fungus? Myco-Herbicide?

Answer 33

• Myco-herbicide: BioMal

- Fungus: Colletotricum gloeosporioides

Question 34

Northern Joint Vetch (Aeschynomene virginica) is a

Answer 34

leguminous weed of rice and to a lesser extent sorghum in E. Arkansas USA

Question 35

Northern Joint Vetch (Aeschynomene virginica) competes with crop how?

Answer 35

• erect annual about 2m tall and it competes very successfully with the rice by shading it
• also severely contaminates rice crop with vetch seeds

Question 36

Northern Joint Vetch (Aeschynomene virginica) Old control + disadvantages with it

Answer 36

• 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

Question 37

biocontrol for Northern Joint Vetch: naturally:

Answer 37

• 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

Question 38

biocontrol for Northern Joint Vetch: commercially

Answer 38

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.

Question 39

how is College marketed?

Answer 39

• as a dry formulation (15% viable spores, 85% inert ingredients)
• each pack contains 757 billion spores (treats 2.5 hectares)

Question 40

development of mycoherbicide

Answer 40

• 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

Question 41

advantages of biocontrol :

Answer 41

• 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

Question 42

disadvantages of biocontrol

Answer 42

• 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

Question 43

biological control is not appropriate when

Answer 43

• 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).