ineffective fumigation, posing both an immediate problem for commodity losses and
contributing to phosphine pesticide resistance. In the worst case, resistance could
lead to the loss of phosphine fumigation as an effective pest control [12].
Other organisms that destroy plants that must be considered are the nematodes
and molluscs. For the most part, nematodes, which are small, unsegmented worms,
dwell in the soil and feed on plant roots. Some, however, invade bulbs, stems, or
leaves. In so doing, they produce damage to the plant themselves and also open the
way for further damage from fungi. Methyl isothiocyanate and formaldehyde are
commonly used nematicides.
Slugs and snails are especially damaging in a wet spring. They damage many leaf
and root vegetables and produce considerable economic loss. The most effective
chemical against them has been a polymeric form of acetaldehyde called meta. Upon
contact with meta, desiccation occurs from excessive loss of water from slime
secretion.
7 Pesticide Control of Nuisance Plants and Other
Organisms
7.1 Terrestrial Plants
Attention also should be directed to a large category of miscellaneous terrestrial
plants that compete with the plants that humans have chosen to cultivate. Chemical
weed killers or herbicides for the control of these plants are widely used in agriculture, for lawns, and along roadways and railways.
When applying chemicals in plant control, care should be taken to determine
whether the nuisance plant serves any less evident function like a food source for
beneficial organisms such as pollinators.
Herbicides are called pre-emergents when they are sprayed before the weed plant
appears. The germinating seed absorbs the herbicide until a lethal concentration
occurs within the young plant. Foliage application or postemergent application is
made directly on the plant.
Herbicides may be selective or nonselective, i.e., they may have a biochemical
specificity that permits selective killing of weeds and spraying of cultivated plants,
or they may be broad-spectrum nonselective herbicides for total kill programs.
The herbicides can be further classified on the basis of hormonal or nonhormonal
action. The phenoxyacetic acids (2,4-dichlorophenoxyacetic acid and related compounds) are the oldest of the hormonal herbicides. They produce their effect by
competing with the action of natural hormone, resulting in lethally abnormal growth.
The excessive root thickening produced in treated plants is a typical effect of
2,4-dichlorophenoxyacetic acid (2,4-D) poisoning. The phenoxyacetic acids have
been the herbicides of choice for broad leafed weed control in grass and cereal crops.
10 Environmental Control of Pests and Vectors
401
contributing to phosphine pesticide resistance. In the worst case, resistance could
lead to the loss of phosphine fumigation as an effective pest control [12].
Other organisms that destroy plants that must be considered are the nematodes
and molluscs. For the most part, nematodes, which are small, unsegmented worms,
dwell in the soil and feed on plant roots. Some, however, invade bulbs, stems, or
leaves. In so doing, they produce damage to the plant themselves and also open the
way for further damage from fungi. Methyl isothiocyanate and formaldehyde are
commonly used nematicides.
Slugs and snails are especially damaging in a wet spring. They damage many leaf
and root vegetables and produce considerable economic loss. The most effective
chemical against them has been a polymeric form of acetaldehyde called meta. Upon
contact with meta, desiccation occurs from excessive loss of water from slime
secretion.
7 Pesticide Control of Nuisance Plants and Other
Organisms
7.1 Terrestrial Plants
Attention also should be directed to a large category of miscellaneous terrestrial
plants that compete with the plants that humans have chosen to cultivate. Chemical
weed killers or herbicides for the control of these plants are widely used in agriculture, for lawns, and along roadways and railways.
When applying chemicals in plant control, care should be taken to determine
whether the nuisance plant serves any less evident function like a food source for
beneficial organisms such as pollinators.
Herbicides are called pre-emergents when they are sprayed before the weed plant
appears. The germinating seed absorbs the herbicide until a lethal concentration
occurs within the young plant. Foliage application or postemergent application is
made directly on the plant.
Herbicides may be selective or nonselective, i.e., they may have a biochemical
specificity that permits selective killing of weeds and spraying of cultivated plants,
or they may be broad-spectrum nonselective herbicides for total kill programs.
The herbicides can be further classified on the basis of hormonal or nonhormonal
action. The phenoxyacetic acids (2,4-dichlorophenoxyacetic acid and related compounds) are the oldest of the hormonal herbicides. They produce their effect by
competing with the action of natural hormone, resulting in lethally abnormal growth.
The excessive root thickening produced in treated plants is a typical effect of
2,4-dichlorophenoxyacetic acid (2,4-D) poisoning. The phenoxyacetic acids have
been the herbicides of choice for broad leafed weed control in grass and cereal crops.
10 Environmental Control of Pests and Vectors
401