Weeds

1
Plant pests Weeds

• Weeds

• Weeds any plant growing in an unwanted location

• Weeds reduce yields of horticultural crops
  through

• Competition for nutrients, water, light

• Allelopathy chemical competition

• Serving as hosts for insect pests

2
Plant pests Weeds

• Weeds

• Weeds successful in reproduction

• Produce hundreds or thousands of seeds that are
  widely dispersed (e.g. Dandelion)

• Reproduce vegetatively (e.g. Quackgrass uses
  rhizomes that produce new plants at each node)

• Long dormancy of seeds (e.g. Chickweed viable for
  more than 30 years)

3
Plant pests Weeds

• Weeds

• Weed seeds can contaminate commercial seed lots
  states have different regulations

• Weeds controlled by

• Physical methods

• Pulling, hoeing, tilling, mowing, mulching

• Often labor-intensive and impractical for large
  production areas

• Chemical methods

• Herbicides chemical used to kill plants

• Herbicides either nonselective or selective
  traditional selective herbicides usually kill
  monocots or dicots but not both, but newer
  herbicides more specialized

4
Plant pests Weeds

• Weeds

• Controlling weeds with herbicides

• Herbicides applied at various times in growing
  season

• Before planting (pre-plant herbicides)

• Before germination (pre-emergence herbicides)

• After growth starts (post-emergence herbicides
  must be selective)

• Herbicides heavily used in U.S.

• More than 5 billion spent on herbicides
  annually more than 650 million pound produced

• Most herbicides used for corn production,
  particularly in Midwestern states (e.g. Illinois,
  Indiana, Iowa)

5
Plant pests Weeds

• Weeds

• Controlling weeds with herbicides

• Most commonly used herbicides

• Atrazine (selective herbicide)

• Dual (metoachlor pre-emergence, selective
  herbicide)

• Lasso (pre-emergence, selective herbicide)

• Round-up (glyphosphate non-selective herbicide
  that interferes with metabolism of plants it
  contacts)

• Largest problem with herbicides is that weeds
  evolve resistance most use more or different
  herbicides over time

6
Plant pests Weeds

• Weeds

• Some weeds controlled biologically

• Biological control agents must

• Selectively feed on weeds

• Reduce weed population to a level that does not
  interfere with crop

• Be adapted to local environmental conditions

• Not be consumed by natural predators

• not disperse from area

7
Plant pests Weeds

• Weeds

• Example of successful biological control

• Opuntia cactus and cactus moth in Australia

• Prickly pear cactus (Opuntia) introduced in
  Australia to produce natural fences in rangelands

• Cactus began taking over valuable grazing areas

• Introduced cactus moth controlled Opuntia cactus

• Unfortunately, few other successes of controlling
  weeds with biological agents