Lecture 4: Chemical Control of Weeds

Question 1

The global pesticide market was values at $___ in 2012 with the total value expected to reach ___ in 2017

Answer 1

$46.1 billion

$65.3 billion

Question 2

Synthetic pesticides were worth a total of $__ in 2012 and ___ in 2017

Answer 2

$44 billion in 2012

$61.5 billion in 2017

Question 3

Biopesticides were worth a total of $__ in 2012, and will surpass ___ in 2017

Answer 3

$2.1 billion in 2012

$3.7 billion in 2017

Question 4

pesticide use is greatest in

Answer 4

developed countries

Question 5

herbicides are chemicals used to control

Answer 5

weeds

Question 6

herbicides make up __% of pesticides sales worldwide

Answer 6

45%

Question 7

the greatest use of herbicides is in

Answer 7

developed countries

Question 8

two type of herbicides:

Answer 8

contacts or systematic

Question 9

2 type of treatment by herbicides

Answer 9

Pre-emergence or post-emergence of crops etc

Question 10

classification of herbicides:

Answer 10

• type of herbicide
• type of treatment
• chemical nature
• mechanisms of action

Question 11

contact herbicides:

Answer 11

• only kill the portion of green tissue contacted (they are not taken up by the plant)
• uniform spray converge is required to kill the plant

Question 12

contact herbicides: how do they kill the plant?

Answer 12

due to their phytotoxicity (usually by rupturing cell walls)

Question 13

contact herbicides are used against:

Answer 13

annual weeds only - do not kill perennial weeds

Question 14

contact herbicides e.g.:

Answer 14

• Paraquat for non selective control

- Methyl arsenic acid for control of grasses

Question 15

Systematic herbicides:

Answer 15

-enter the plant and are translocated in the phloem and/or the xylem to other parts of the plant

Question 16

systematic herbicides are applied as

Answer 16

either a foliar application or as a soil drench

Question 17

systematic herbicides require how long to achieve good control

Answer 17

days to weeks

Question 18

systematic are more effective against annual/ perennial weeds?

Answer 18

PERENNIAL

Question 19

examples of systematic herbicides:

Answer 19

• Glyphosate for non selective weed control

- Atrazine for certain annual grasses & broad-leaved weeds

Question 20

for systematic herbicides to be an effective weed management tool it must:

Answer 20

1. contact the plant surface
2. remain on the plant surface long enough to be absorbed
3. reach a cellular site and disrupt a life supporting process (e.g.amino acid synthesis) or structure (e.g. membranes)
4. Degrade into non-toxic metabolites

Question 21

contact herbicide positives:

Answer 21

-kill the plant quickly & they are not very persistent in the soil

Question 22

contact herbicide negatives:

Answer 22

• have to be sprayed frequently often at quite high doses to kill weeds
• only annual weeds are controlled

Question 23

systematic herbicide positives:

Answer 23

• do not have to cover the whole plant therefore they can be applied at lower doses
• can control perennial weeds

Question 24

systematic herbicide negatives:

Answer 24

• must be applied carefully so that mist doesnt affect the crop plants
• tend to be more persistent in the soil

Question 25

chemical structure of herbicides:

Answer 25

organic & inorganic compounds, some of which occur naturally & some of which are synthesised in labs by chemical companies

Question 26

type of treatment: total weed control-

Answer 26

herbicides are applied with the intention of killing all plants present e.g. on railway tracks or in industrial areas

Question 27

type of treatment: selective weed control:

Answer 27

- application of a herbicide is meant to kill only the weed species - crop plants should not be harmed agricultural situations

Question 28

mode/mechanism of action of systematic herbicides:

Answer 28

- inhibitors of normal growth (auxin mimics) - photosynthetic inhibitors - amino acid synthesis inhibitors - inhibitors of cell division - inhibitors of pigment synthesis - inhibitors of lipid biosynthesis

Question 29

inhibitors of normal growth (auxin mimics) are used primarily for controlling:

Answer 29

broadleaf perennial weeds in cereal crops e.g. picloram (cordon) fluroxypyr (starane)

Question 30

inhibitors of normal growth (auxin mimics) are absorbed where

Answer 30

through both roots & foliage and are translocated in the xylem & phloem

Question 31

inhibitors of normal growth (auxin mimics) have what effects on the plant

Answer 31

- growth hormones regulate cell elongation, protein synthesis, and cell division - injury symptoms on susceptible plants include growth & reproduction abnormalities

Question 32

photosynthetic inhibitors:

Answer 32

disrupt photosystem || or photosystem 1 e.g. simazine or paraquat - plant but be undergoing active photosynthesis for the herbicide to be effective (tf more effective on sunny days) - injury symptoms may be sdelyeaed under cloudy conditions and become evident after the first sunny day after application - causes build up of e- in the leaves

Question 33

amino acid synthesis inhibitors: act by..

Answer 33

inhibiting an enzyme (acetolactate synthase -ASL) involved in a.a synthesis

Question 34

a.a synthesis inhibitors act over..

Answer 34

a number of days - weeks

Question 35

typical symptoms of a.a synthesis inhibitors:

Answer 35

chlorosis, purple colouration around the veins, secondary root proliferation

Question 36

a.a synthesis inhibitor example:

Answer 36

probably most famous herbicide - Glyphosate

Question 37

production of a herbicide:

Answer 37

- chemicals (with specific or random structure) are produced in an chemistry lab - applied to a small number to TEST weed species as a spray or systematic application -PRIMARY SCREEN - any chemical which kills the plant goes to a 2nd CREEN (adult & seedling, stage, different concentrations etc.) - tested for toxicity on crop species animals & Humans - tested for persistence in the soil - long term field trials & more thorough tests on possible toxic side effects

Question 38

production of a herbicide takes __ years and costs how much

Answer 38

10-15 years from discovery to market & costs $10-15M

Question 39

advantages of herbicides:

Answer 39

- they control multiple weed species - some can move into roots and control perennial weed species - some can move into roots & control perennial weed species - some possess residual soil activity and can control weeds over a long period of time - many cause little or no injury to the crop plant (As many are selective) - They can be applied over a large area rapidly - they control weeds quickly

Question 40

disadvantage of herbicides:

Answer 40

-may be toxic to humans & Other animals -may negatively impact the soil ecosystem -may leach into the water supply may persist in the soil for too long and damage a susceptible crop improper application may result in injury to the crop -repeated use of a herbicide may lead to herbicide resistant weeds

Question 41

integrated weed management strategies involves:

Answer 41

- carefully managed use of different control tactics - (e.g. biological, cultural, chemical or GM ) - there are limitations to integrating these methods - need to understand the biology of the weed, the biological control agent and the chemical herbicide to determine the best time to spray

Question 42

e.g. of integrated weed management:

Answer 42

Control of the Water Hyacinth (Eichornia crassipes)

Question 43

Water Hyacinth (eichornia crassipes) facts

Answer 43

- LAKE VICTORIA - native to S. America & was introduced into florida in 1880's - very high growth rate - population doubles in 12 days - prevents sunlight & oxygen getting into the water - crowds & shades out submersed plants & reduces biological diversity - prevents fishing & blocks waterways - until the introduction

Question 44

Water Hyacinth (Eichornia crassipes) is now controlled by a combination of

Answer 44

- classical biocontrol agents - mycoherbicide (innundative control) - occasionally a chemical herbicide - mechanical clearance

Question 45

classical biocontrol control agents used to control Water Hyacinth (Eichornia crassipes)

Answer 45

- Water hyacinth weevils (Neochetina spp) - -both adults & larvae feed on various parts of the plants - Water hyacinth moth larvae (Sameodes spp)

Question 46

Mycoherbicide used to control Water Hyacinth (Eichornia crassipes)

Answer 46

-composed of spores of the fungus Cercospora rodmanii • The pathogen exists at low levels in the population but because it is spread by rain splash it never reaches epidemic proportions. • Therefore water hyacinths are inoculated with spores ro create an epidemic • These control measures are used in Florida, Australia, Fiji, Honduras, India, Malaysia, S. Africa, Thailand (and other countries).