Success means different things to different people. Unfortunately, the success or failure of weed bio-
logical control projects is often evaluated by nonparticipants lacking knowledge of the original goals
set by project architects. Criteria for success should match objectives and goals clearly articulated so
that success can be properly archived for future synthesis. The Australian tree Melaleuca quinque-
nervia (Cav.) S.T. Blake, an aggressive invader of the Florida Everglades, may be the largest plant
ever targeted for biological control. We realized early on that biological control agents would not
remove the many tons of woody biomass that comprised these infestations and so would be unlikely
to reduce the infested acreage. Control of this plant by other means, however, was complicated by the
billions of canopy-held seeds that are released following injury to the tree. A plan was developed in
coordination with land management agencies wherein the goal of biological control was to curtail me-
laleuca expansion and suppress regeneration while using other means to remove mature trees. Three
insect species have been released and others are under consideration. These agents, supplemented by
the impacts of an adventive rust fungus and a scale insect, have met established goals and this project
shows signs of an emerging success based on the established goals.
‘Success has many fathers while failure dies an or-
phan’. This oft-quoted aphorism illustrates the political
necessity of highlighting successes when they occur so
that one’s endeavors continue to be supported in the
future. Unfortunately, weed biological control proj-
ects are rarely undertaken based on the likelihood of a
successful outcome (Peschken and McClay, 1995). In-
stead, biological control is often the method of last re-
sort after other methods against recalcitrant weeds have
failed. This is not conducive to improving the overall
statistical success rate but is often the most responsible
or economic option. Biological control of many seri-
ous weed problems would likely never be attempted if
target choice was based primarily on maximizing the
probability of success as advocated by Peschken and
Many investigators have focused on the perfor-
mance of individual agents to gauge success, primarily
with an aim toward predicting which taxonomic groups
make the best biological control agents. Such post-hoc
analyses suggest a low success rate for weed biologi-
cal control, with only a small proportion of success-
fully established agents producing effective control
(Crawley, 1989a,b). Critics have used these statistics to
advise against the use of biological control as a weed
management tool (Louda and Stiling, 2004). McFa-
dyen (1998, 2000), however, strongly disagreed with
this advice and emphasized the need for project-based
US Department of Agriculture, Agricultural Research Service, Invasive
FL 33312, USA.
US Department of Agriculture, Agricultural Research Service, Inva-
FL 32608, USA.
US Department of Agriculture, Agricultural Research Service, Austral-
Industrial Research Organization, Entomology, Long Pocket Labora-
tories, 120 Meiers Road, Indooroopilly, QLD 4068, Australia.
Corresponding author: T.D. Center
firstname.lastname@example.org, email@example.com, min.rayamajhi@ars.
usda.gov, firstname.lastname@example.org, email@example.com>.
© CAB International 2008
firstname.lastname@example.org, email@example.com, min.rayamajhi@ars.
usda.gov, firstname.lastname@example.org, email@example.com>.
© CAB International 2008
XII International Symposium on Biological Control of Weeds
Most authors use the term ‘success’ to refer only
to ‘complete success’, wherein no other measures are
needed to reduce the weed populations to acceptable lev-
els. However, this neglects the importance of partially
successful projects that have value when less effort
is subsequently required to control the weed, because
the density or extent of weed populations is reduced,
or the weed is less able to reinvade cleared areas or
is slower to disperse (Hoffmann, 1995). Success and
failure are at the extreme ends of a continuum of many
possible outcomes and even moderate amounts of
stress can reduce the competitive ability of a weed and
render it less invasive (Center et al., 2005; Coetzee et
Successful biological control agents often act by
preventing continued expansion of a weed population,
rather than by reducing population densities (Hoffmann,
these proceedings). Hoffmann also noted that it may be
necessary to model weed outbreaks that never happen
to perceive biological control effects. Documentation
of such effects is difficult, at best, which explains why
so many projects are incompletely evaluated and even
successful projects may be undervalued or forgotten.
Thus, statistical success rates should be viewed with
circumspection, inasmuch as only obvious successes
are reported. Furthermore, weed declines may occur in-
crementally over many years or even decades and may
not be easily observed, especially when observational
baselines shift over time, project funding terminates, or
personnel changes interrupt collection of critical data.
Success of projects should be assessed in terms of
the project’s original goals and objectives. Hence, a
project can and should be deemed successful whether
or not the density of the weed is reduced so long as the
goals set out by the project architects are met. In this
sense, it is possible to have complete success without
complete control so long as the project goals are clearly
stated, understood and documented. A recent proj-
ect aimed at the control of Melaleuca quinquenervia
(Cav.) S.T. Blake (melaleuca) in South Florida as part
of a broader Everglades restoration effort is used herein
to illustrate this concept.
Melaleuca is a large tree (up to 30 m tall) of Australian
origin that was introduced into southern Florida during
the latter part of the 19th century (Dray et al., 2006). It
has invaded wetland habitats, especially fire-maintained
Everglades ecosystems, where the burning regime now
favours melaleuca over less fire-tolerant native species.
As a result, vast areas of these heterogeneous marshes
have been transformed into swamp forests consisting of
melaleuca monocultures. Melaleuca rapidly dominates
infested areas after its initial colonization (Laroche and
Ferriter, 1992) and at its peak was estimated to occupy
at least 607,000 ha of conservation lands in the south-
ern part of Florida (Bodle et al., 1994).
Control of melaleuca is complicated by the fact that
it grows in areas that are hazardous and strenuous to
work in and where access is difficult. These difficul-
ties are exacerbated by the tree’s reproductive biology.
Melaleuca flowers numerous times each year, often
several times on the same stem axis due to indetermi-
nant growth, forming spike-like clusters composed of
multiple, dichasial groups of three flowers each (Tom-
linson, 1980). Each cluster contains up to 75 individual
flowers. Fruits arising from these flowers are persistent
serotinous capsules that each contains 200–350 minute
seeds (Meskimen, 1962). These generally remain in the
fruits until disruption of the vascular connection causes
the capsules to desiccate and open, often en masse,
after a fire, freeze, drought, or herbicide treatment
(Meskimen, 1962). A few (about 12% per year) open
continuously, as radial growth of the stem separates the
vascular connection, producing a light, perpetual seed
rain of about 3 billion seeds/ha/year
form a rather short-lived soil seed bank with a half-life
of less than 1 year (Van et al., 2005). A single large
tree located within a dense stand retains about 50 mil-
lion seeds in its canopy with stands holding as many
as 25 billion seeds/ha (M. Rayamajhi, unpublished
data). An isolated tree may hold twice as many seeds
as one of similar size in a dense stand. Surprisingly, a
large proportion (85–90%) of these are actually hollow
seed coats (Rayachhetry et al., 1998; Rayamajhi et al.,
2002). Nonetheless, the remaining 10–15% of embry-
onic seeds create an enormous regenerative capacity
capable of producing seedling densities of up to 2256
(Franks et al., 2006) following a massive
herbicide application. These may grow into thickets of
up to 130,000 small trees/ha (Van et al., 2000). As the
stand matures, it thins to about 8000–15,000 trees/ha
comprised mostly of mature trees with an understory
of suppressed saplings (Rayachhetry et al., 2001). The
standing biomass in these forests has been estimated at
129–263 metric tonnes (t)/ha (Van et al., 2000).
Isolated individual trees constitute a seed source for
further encroachment. The seeds, when released, gener-
ally fall within 15 m of the parent tree (Meskimen, 1962).
They often grow into dome-shaped clumps or ‘heads’
with the parent trees in the centre and progressively
younger trees toward the periphery. These eventually
coalesce with others blanketing vast acreages of wet-
lands with dense swamp forests. The isolated ‘outliers’
therefore are regarded as potential new infesta-
tions and, as part of a quarantine strategy, are first
priority for control operations (Woodall, 1981).
The trees within these stands produce multiple ad-
ventitious roots that form an intertwined skirt at the
waterline or on saturated soil (Meskimen, 1962). These
contribute biomass to the forest floor and trap large
amounts of litterfall as well as organic debris causing
soil accretion (White, 1994), thus increasing the local
Biological control of Melaleuca quinquenervia: goal-based assessment of success
elevation (T. Center, personal observation). Altering
the elevation of the Everglades even by a few centime-
ters dramatically shifts plant community composition
(Ogden, 2005), thus these newly created melaleuca is-
lands forever change the physiography and ecology of
the area. There is also evidence that essential oils in
melaleuca litter may be allelopathic (Di Stefano and
Fisher, 1983). These changes render infested habitats
unsuitable for many native species making restoration
difficult if not impossible.
The South Florida Water Management District in con-
junction with the Exotic Pest Plant Council convened
a meeting of the major agencies that were managing
the melaleuca problem. They developed a ‘Melaleuca
Management Plan for Florida’, published during 1990
and revised in 1994 and 1999.
Two points were evident during the development of
this plan. First, biological control could not eliminate
the huge amounts of woody biomass present; herbicidal
and mechanical control would therefore be needed to
reduce the infestations to a maintenance level. Second,
public agencies could not expend public funds to con-
trol melaleuca infestations on private lands that often
abutted cleared tracts of public lands. These unassail-
able stands provided an invasion front and a potential
seed reservoir to support reinvasion of cleared sites. The
role of biological control in this plan was to neutralize
the reproductive potential of these remaining stands by
reducing seed production, seedling recruitment and re-
generation; thereby inhibiting spread, reducing reinva-
sion of cleared areas and facilitating traditional control
measures. However, implementation of biological con-
trol would take time, whereas chemical and mechanical
control could be employed rapidly. So the plan relied
on an early deployment of traditional control measures
that would gradually be supplanted by biological con-
trol as agents became available (Figure 1).
Insects associated with melaleuca were enumerated in
Australia during the late 1980s and early 1990s (Bal-
ciunas, 1990). These inventories revealed an entomo-
fauna of over 400 species (Balciunas, 1990; Balciunas
et al., 1993a,b, 1994, 1995a,b,c; Burrows et al., 1994,
1996). The most promising species were studied fur-
ther and three have now been released.
The first insect evaluated was the weevil Oxyops vi-
being a flush feeder on growing stem tips, was desir-
able because of its ability to disrupt flower production,
which depends on continual growth of the stem axis.
It proved to be host-specific (Balciunas et al., 1994;
G. Buckingham, unpublished report) and was released
during 1997 (Center et al., 2000). Its need to pupate
in dry soil (Purcell and Balciunas, 1994; Center et al.,
2000), however, limited it to habitats that were not per-
manently under water. Field and laboratory assessments
of a mirid, Eucerocoris suspectus Distant, in Austra-
lia (Burrows and Balciunas, 1999) suggested that its
host range was limited to melaleuca and a few close
relatives. Follow-up studies in US quarantine facilities
failed to confirm this so it was dropped from consider-
ation. The host range of the pergid sawfly Lophyrotoma
and Balciunas, 1997; Buckingham, 2001), but after
discovering that larvae synthesize toxic octapeptides
The strategy employed to control melaleuca in south Florida
involved early use of traditional control methods to reduce
biomass while biological controls were being developed and
(Oelrichs et al., 1999), we elected not to release it out
of concern over potential negative effects to insectivo-
The melaleuca psyllid Boreioglycaspis melaleucae
Moore was found to be host-specific (Purcell et al.,
1997; Wineriter et al., 2003), and was released during
2002 (Center et al., 2006, 2007). It feeds mainly on the
new growth but will also utilize older leaves and the
green stems. Furthermore, it completes its life cycle en-
tirely on the plant so it is less restricted by habitat. The
tube-dwelling pyralid Poliopaschia lithochlora (Lower)
was highly rated because of its ability to damage mela-
leuca and its preference for low-lying, humid habitats
(Galway and Purcell, 2005), but its use of an ornamental
species, Melaleuca viminalis (Sol. ex Gaertner) Byrnes,
during testing diminished its prospects (M. Purcell, un-
published data). A fergusoninid gall fly, Fergusonina
turneri Taylor, and its mutualistic nematode Fergusobia
melaleucae Davies and Giblin-Davis, also proved to be
highly specific (Giblin-Davis et al., 2001) and were first
released during 2005 (Blackwood et al., 2006). It has
proven difficult to establish but efforts are continuing.
Most recently a stem-galling cecidomyiid, Lophodiplo-
sis trifida Gagné, has proven to be host-specific (S. Win-
eriter et al., unpublished data) and should gain approval
for release. A bud-feeding weevil Haplonyx multicolor
Lea and a leaf-galling cecidomyiid Lophodiplosis in-
Two adventive organisms have also recently infested
melaleuca trees in Florida. A pestiferous, undescribed
scale insect (Pemberton, personal communication) was
detected in Florida during 1999. It attacks melaleuca
trees as well as some 300 other plant species (Pem-
berton, 2003; R. Pemberton, unpublished data). The
guava rust Puccinia psidii G. Winter (Basidiomycetes:
Uredinales), which infects mainly young foliage, ap-
peared during 1997 (Rayachhetry et al., 1997) and is
The effects of the biological
Numerous studies aimed at determining the impacts
of O. vitiosa and B. melaleucae have been conducted or
are ongoing. However, determinations of the individual
effects of one have been confounded by the presence of
the other, as well as by the presence of the adventive rust
fungus and scale insect. These studies have included
comparisons of melaleuca stands with and without the
agents, caging studies, defoliation experiments, insec-
ticide and fungicide exclusion experiments, and before
and after comparisons of stand dynamics.
Flower and seed production
The effects of herbivory by O. vitiosa on melaleuca
performance were possible early during the release
program when none of the other organisms were pres-
ent. Pratt et al. (2005) compared flowering frequency in
melaleuca stands where the weevil had been released to
stands without them. They found that the likelihood of
flowering increased with tree size but that undamaged
trees were 36 times more likely to reproduce than dam-
aged trees in similar habitats (Figure 2). Overall, about
45% of the weevil-free trees were flowering compared
to about 2% of infested trees.***
In another study, Pratt et al. (2005) enclosed the
canopies of small (2.9 cm diameter at breast height or
Diameter at Breast Height (cm)
Release of the weevil Oxyops vitiosa profoundly affected flowering of mela-
leuca trees. The proportion of the trees that produced flowers was much lower
after being damaged by the weevils regardless of size.
dbh) trees with sleeve cages and introduced weevil lar-
vae into the enclosures, either once or twice, to produce
one or two defoliations of the young foliage. The sec-
ond defoliation was done about 10 weeks after the first.
These treatments were compared to controls with no
defoliation or to trees artificially defoliated by manually
removing all foliage. Flower production on all trees
was monitored monthly for 1 year. The control trees
flowered normally during this period, whereas trees ar-
tificially defoliated failed to produce any flowers. Trees
defoliated once or twice by the weevil larvae produced
a few flowers but numbers were not statistically dif-
ferent from each other or from the artificial defoliation
treatment (Figure 3).
Interestingly, in a comparison of ten herbivore-
impacted trees with ten non-impacted trees at simi-
lar, nearby sites at Estero, Florida, Rayamajhi et al.
(unpublished data) found that herbivory by O. vitiosa
resulted in higher rates of capsule abortion when com-
pared to sites without natural enemies. Mean number of
capsules in herbivore-impacted infructescences was re-
duced by nearly 50% compared to the herbivore-absent
site. This decreased density of capsules was apparent as
gaps in the capsule clusters caused by abortion of the
undeveloped fruits. The herbivore-impacted trees were
very similar to those near Brisbane, Australia where the
average infructescence was 5.7 cm long but contained
only 18 capsules (Rayamajhi et al., 2002). The average
numbers of seeds per capsule were similar in both the
Florida and Australian sites.
Rayamajhi et al. (unpublished data) have also found
that when the trees were subjected to attack by O. vi-
did seed viability and germination ability. Seed viabili-
ty and germination tests (Van et al., 2005) also revealed
reductions in both measures of seeds from herbivore-
attacked trees compared to controls.
Franks et al. (2006) described the effects of the wee-
vil larvae and the psyllids, alone and in combination,
on growth and survival of melaleuca seedlings by cag-
ing the insects on 26 cm-tall seedlings in field plots.
They compared these results to a natural infestation of
the insects on nearby seedlings. O. vitiosa larvae had
no effect on seedling height, leaf number, or survival,
whereas psyllids caused all of these measures to de-
crease by about 55–60% over the 5-month term of the
study. About 95% of seedlings survived when protected
from psyllids as compared to only 40% when exposed
to herbivory (Center et al., 2007).
In another study, Tipping et al. (unpublished data)
found that after becoming infested by both the psyllid
and the weevil, melaleuca trees recruited a much lower
density of seedlings than trees without either herbi-
vore. They also compared densities of saplings in plots
that were periodically treated with insecticide to
plots were located in an area that had burned during
June 1998, resulting in a massive seed rain and thick-
ets of about 1000 seedlings/m
. By the time the study
and had grown to about 70 cm in height. Densities in
Small trees were caged then subjected to herbivory by Oxyops vitiosa either
once (Herbivory 1) or twice (Herbivory 2) or to mechanical defoliation and
compared to undefoliated controls. Defoliated trees, regardless of the manner
of defoliation, produced very few flowers relative to the controls.
the protected plots were virtually unchanged during the
5-year period of the study as compared to those in the
unprotected plots which declined by almost half.
Tipping et al. (unpublished data) conducted two in-
secticide exclusion studies on the growth of melaleuca
saplings in common garden experiments over about
a 3-year period. The first experiment investigated the
effect of the melaleuca weevil, O. vitiosa, and supple-
mental irrigation on the growth of small trees. The second
examined the effects of herbivorous insects (both
the psyllids and the weevils) and plant chemotype
(nerolidol or viridifloral). In both cases, plants treated
with insecticide more than doubled in stature, where-
as those not treated grew very little. In the first study,
plants attacked by O. vitiosa grew at a much slower
rate compared to the protected plants (Figure 4). The
unprotected plants produced more stem tips per unit of
height, creating a shorter, bushier habit, which provided
more resource for the tip-feeding insects. Supplemental
irrigation improved the growth of insecticide-treated
trees but had no effect on trees that were not treated
with insecticide. Chemotype had no apparent effect on
the impact of the insects. Seed capsule production was
much lower among unprotected plants in both studies.
Rayamajhi et al. (2007) studied the dynamics of me-
laleuca stands before and after the widespread impacts
of the biological control agents. They found that the
average density of the trees in mature stands declined
by 72% overall from 15,800 trees/ha during 1996 to
4400 trees/ha during 2003. Interestingly, the standing
biomass based on harvesting studies increased some-
what from an initial average of 263 t/ha to 274 t/ha
during the latter harvest. This was because most of the
mortality was among the smaller suppressed trees in
the understory that represented a small proportion of
the biomass. The density of small trees, those with a
dbh of less than 10 cm, decreased 83% from 12,600 to
2200 trees/ha; density of intermediate-sized trees with
a dbh of 10–20 cm decreased 46% from 2600 to 1400
trees/ha; density of large trees >20 cm increased from
600 to 800 trees/ha.
Another study (Rayamajhi et al., 2007) showed that
densities decreased between 1997 and 2006, in part due
to self-thinning. The decline accelerated after the ef-
fects of biological control became apparent and the rate
of decline was inversely related to the position of the
trees within the stands. Densities of trees at the periph-
ery, which consisted mostly of small individuals, de-
creased by about 6076 individuals/ha/year before 2001
Insecticide, Rainfall + Irrigation
No insecticide, Rainfall Only
No insecticide, Rainfall + Irrigation
Small trees grown in a common garden plot were treated with insecticide to exclude herbiv-
orous insects and given supplemental irrigation and compared to unprotected and unwatered
trees. Protected trees grew vigorously and those receiving supplemental irrigation grew the
most. Unprotected trees grew very little and the supplemental irrigation seemed to have lit-
as compared to 16,725 individuals/ha/year after 2001.
Densities in the inner portions of the stands, which con-
tained higher proportions of larger trees, decreased at
relatively constant rates. This further demonstrated the
greater effect of herbivory on smaller trees. The average
diameter of the trees increased, not because they grew
but because of selective mortality of smaller individu-
als. This was corroborated by a decrease in or leveling
off of total basal area coverage during the post-release
period in contrast to a prior increasing trend.
Despite the finding that the surviving larger domi-
nant trees accounted for most of the biomass, biomass
allocation changed due to extensive defoliation of all
of the trees. The foliage of large trees growing in dense
stands was limited to the upper branches at the treetops
and this accounted for only 5.1% of the total biomass
during 1996, before insect-induced defoliation. This de-
creased from 17 to 8 t/ha to represent only 1.5% of the
total biomass during 2003 (Figure 5). The biomass allo-
cated to seed capsules decreased by 85% from 6.7 t/ha,
or 0.46% of the total biomass to 1.0 t/ha, or 0.29% of
the total biomass.
Litter-traps were placed under mature melaleuca
stands to collect leaf litter in an attempt to measure the
activity of the biological control agents in the canopy of
taller trees. The proportion of fallen leaves that exhib-
ited weevil damage symptoms was analysed. Though
the weevil releases began in 1997, the first weevil-dam-
aged leaves did not appear in the traps until 1999 (rep-
resented by 5% of the trapped leaves) and by 2005, the
proportion of damaged leaves reached approximately
45% (Rayamajhi et al., 2007). This increased percent-
age of damaged leaves reflected the decreasing propor-
tions of leaf biomass (stem to leaf biomass), increasing
tree mortality, and decreasing tree densities.
The ability of melaleuca to sprout from cut stumps
complicates control. This requires follow-up herbicide
treatment to prevent coppicing and stand regeneration.
Several studies have indicated that the flush of foliage
associated with this regrowth is highly attractive to both
psyllids and weevils, as well as to the rust fungus. Pratt
et al. (unpublished data) found that insecticide exclu-
sion of biological control agents led to an increase in
leaf and stem biomass compared to unprotected stumps
(Figure 6). Chronic attack over an 18-month period led
to mortality of almost half the unprotected plants. In
a similar insecticide- and fungicide-exclusion study,
Rayamajhi et al. (unpublished data) found that the rust
fungus, P. psidii, played an important additive role.
Proportion of photosynthetic tissues and the mortality
of stems were higher in treatments involving both in-
sects (O. vitiosa and B. melaleucae) and rust (P. psidii)
together than in treatments using either alone. Death of
the regrowth often led to the death of the stump itself
(M. Rayamajhi et al., unpublished data). These data
indicate that biological control can compliment, and
in some cases replace, the use of herbicides for stump
Clearly, many melaleuca stands have undergone sig-
nificant declines and remaining trees are now in poor
condition. However, vast stands of melaleuca still exist
that overtly appear unchanged. Yet after closer scrutiny,
we have revealed that the dynamics of these stands
have changed in very significant ways. Fewer trees
now produce flowers, those that do flower produce
The proportion of the total tree biomass allocated to foliage declined dramati-
cally due to defoliation primarily by Oxyops vitiosa and the psyllid Boreiogly-
fewer inflorescences and the inflorescences produced
contain fewer individual blossoms. Many of the fruits
abort and those that do manage to set seed produce a
smaller proportion of viable seeds. The constant defo-
liation of the stem tips causes the capsules to desiccate
and release seeds during drier periods when conditions
are unfavourable for germination. Those that do fall,
lodge in a favourable site and manage to germinate are
infested by psyllids that kill a large proportion before
they attain a significant size. If they survive, they grow
slowly due to constant defoliation and produce few
flowers. Meanwhile, existing stands have nearly been
removed from publicly held lands and those on private
lands are less invasive. Hence, the goals of the project,
as stated above, have been met so the project should
be considered a success. It is not yet a ‘complete’ suc-
cess in that biological control is more effective in some
habitats and during some periods than others but addi-
tional agents that are currently under development may
fill these gaps.
The research reported herein was supported by funding
from the South Florida Water Management District,
the Florida Department of Environmental Protection,
the US Army Corps of Engineers, the Miami-Dade
County Department of Environmental Resource Man-
agement, and Lee County as well as by the USDA-
Agricultural Research Service Areawide Projects. We
thank all past and present staff of the USDA–ARS Aus-
tralian Biological Control Laboratory and the Invasive
Plant Research Laboratory. We are also indebted to
the Student Conservation Service and the AmeriCorps
program for the tremendous support provided by the
many conservation interns that have been involved in
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