Biology Reference
In-Depth Information
price value of the crop at the farm gate (Pimentel 1997). Consequently, because
nonindigenous weeds cause more extensive crop losses than native weeds, they
cause a greater increase in the cost of food.
Weeds, both native and exotic, are also a problem in pastures, where 45% of the
weed species are nonindigenous (Pimentel 1993). US pastures provide approxi-
mately $10 billion in forage crops annually (USDA 2006), and the losses due to
inedible weeds are estimated to be $2 billion per year. Forage loss due to nonindig-
enous weeds, therefore, amounts to about $1 billion per year.
Some introduced weeds, such as leafy spurge (
Euphorbia esula
), are toxic to cat-
tle and other ungulates (Trammel and Butler 1995). In addition, several nonindige-
nous thistles have reduced native forage plant species in pastures, rangeland, and
forests, thus reducing cattle grazing (Cotton Thistle 2007). According to Babbitt
(1998), ranchers spend about $5 billion each year to control invasive nonindigenous
weeds in pastures and rangeland; nevertheless, these weeds continue to spread.
Control of weeds in lawns, gardens, and golf courses makes up a significant
proportion of the total management costs for lawns, gardens, and golf courses of
about $36 billion per year (USCB 2007). In fact, Templeton et al. (1998) estimated
that each year, about $1.3 billion of the $36 billion is spent on residential weed,
insect, and disease pest control. Because a large proportion of the residential weeds,
such as dandelions (
Taxaxcum officinale
), are exotics, the estimate is that $500
million is spent to control exotic weeds in residential areas and an additional
$1 billion is spent to control nonindigenous weeds on golf courses. Weed trees also
have an economic impact. For instance, $3 to $6 million per year is spent in efforts
to control the melaleuca tree (
Melaleuca alternifolia
) in Florida (Pimentel et al.
2000). Valuable cropland may be devalued in the USA because too contaminated
by Silverleaf Nightshade (
Solanum elaeagnifolium)
(Mekki 2007).
1.2.1.2 PlantPathogens
There are an estimated 50,000 parasitic and nonparasitic diseases of plants in the
United States, most of these are fungi (USDA 1960). In addition, more than 1,300
species of viruses are plant pests in the US (USDA 1960). Many of these plant
microbes are nonnative and were introduced inadvertently with the seeds and other
parts of host plants that were introduced. Including the introduced plant pathogens
and other soil microbes, it is estimated that conservatively more than 20,000 species
of microbes have invaded the United States.
US crop losses to all plant pathogens total about $33 billion per year (Pimentel
1997; USCB 2007); $21 billion each year of these losses are attributable to nonin-
digenous plant pathogens. In addition, growers spend $720 million each year on
fungicides; about $500 million of that is used to combat nonindigenous plant patho-
gens specifically. The total damage and control costs of nonindigenous plant patho-
gens therefore amount to about $22 billion per year. In addition, on the basis of the
fact that 65% of the plant pathogens are exotic, the estimated control costs of plant
pathogens in lawns, gardens, and golf courses are at least $2 billion per year.
