Biology Reference
In-Depth Information
10.1 Introduction
Invasive weeds cause significant economic losses and ecological disruptions in both
commercial and native forest ecosystems. They also cause serious problems in natu-
ral areas, some of which are wooded, contain scrublands, wetlands, grasslands, and
biologically diverse ecosystems. Invasive weeds by their very nature are highly suc-
cessful colonizers and compete for resources such as light, nutrients, and water,
which results in the suppression of young or naturally regenerating trees (Green 2003).
The replacement of a relatively diverse native ecosystem by monotypic stands of
alien species is a serious disruption of the ecosystem (Randall 1996). In Hawaii, 946
out of a total of 2,690 plant species are alien, with about 800 native species endan-
gered and over 200 endemic species believed to be extinct because of alien species
(Pimentel et al. 2005; Vitousek 1988). In Florida, about 25% of some 3,500 plant
species are nonindigenous and about 60 species are said to be highly invasive
(Simberloff 1997). According to Campbell (1998), an estimated 138 alien tree and
shrub species including salt cedar (
Tamarix pendantra
Pall.,
Tamaricaceae
), euca-
lyptus (
Eucalyptus
spp.,
Myrtaceae
), Brazilian pepper (
Schinus terebinthifolius
Raddi,
Anacardiaceae
), and Australian melaleuca [
Melaleuca quinquenervia
(Cav.)
S.T. Blake,
Myrtaceae
], have invaded native US forest and shrub ecosystems
(Pimentel et al. 2005).
Melaleuca quinquenervia
was spreading at a rate of 11,000 ha/
year throughout the forest and grassland ecosystems of the Florida Everglades, caus-
ing damage to the natural vegetation and wildlife (Pimentel et al. 2005). This rate of
spread appears to be abating with the introduction of two insect biocontrol agents,
the melaleuca weevil,
Oxyops vitiosa
(Center et al. 2000), and the melaleuca psyllid,
Boreioglycapsis melaleucae
(Pratt et al. 2004).
Successful biological control programs ultimately reduce, and sometimes elimi-
nate, the need for conventional methods of control for invasive weed species that have
escaped from their natural pests and pathogens. The benefit-to-cost ratio of successful
biological control can be very high, especially for countries repeating earlier success-
ful introductions from another country (Waterhouse 1998). In this chapter, we describe
several successful and early-stage biological control programs using microbial agents
to illustrate the application strategies (i.e., inoculative [classical biocontrol] vs. inunda-
tive [bioherbicide] approaches) and the weed-pathogen systems involved. For over-
views of biological control of weeds by using plant pathogens, the readers are referred
to Charudattan (2001a), Gardner (1992), and Yandoc-Ables et al. (2006a, b).
10.2 ClassicalBiologicalControl
Classical biological control of weeds involves the introduction, establishment, and self
sustenance of pathogens from the native range of the target weed into an area where
the weed has naturalized and become a problem (Briese 2000). The aim is to achieve
a long-term equilibrium between the population of natural enemies and the weed, and
