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Plant pests: Weeds

Plant pests: Weeds. Weeds = any plant growing in an unwanted location. Weeds reduce yields of horticultural crops through:. Weeds. Competition for nutrients, water, light. Allelopathy = chemical competition. Serving as hosts for insect pests. Plant pests: Weeds.

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Plant pests: Weeds

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  1. Plant pests: Weeds • Weeds = any plant growing in an unwanted location • Weeds reduce yields of horticultural crops through: • Weeds • Competition for nutrients, water, light • Allelopathy = chemical competition • Serving as hosts for insect pests

  2. Plant pests: Weeds • Weeds successful in reproduction • Produce hundreds or thousands of seeds that are widely dispersed (e.g. Dandelion) • Weeds • 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 • Weed seeds can contaminate commercial seed lots; states have different regulations • Weeds • 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 • Controlling weeds with herbicides • Herbicides applied at various times in growing season • Weeds • 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 • Controlling weeds with herbicides • Most commonly used herbicides • Weeds • 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 • Some weeds controlled biologically • Biological control agents must: • Weeds • 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 • Example of successful biological control • Opuntia cactus and cactus moth in Australia • Weeds • 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

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