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The changing status of invertebrate pests and the future of pest
management in the Australian grains industry
Ary A. Hoffmann
A,D
, Andrew R. Weeks
B
, Michael A. Nash
A
, G. Peter Mangano
C
and Paul A. Umina
A
A
Centre for Environmental Stress and Adaptation Research (CESAR), Bio21 Molecular Science Institute,
Department of Zoology, The University of Melbourne, Parkville, Vic. 3010, Australia.
B
CESAR, Bio21 Molecular Science Institute, Department of Genetics, The University of Melbourne,
Parkville, Vic. 3010, Australia.
C
Western Australian Department of Agriculture and Food, South Perth, WA 6151, Australia.
D
Corresponding author. Email: ary@unimelb.edu.au
Abstract. The Australian grains industry is dealing with a shifting complex of invertebrate pests due to evolving
management practices and climate change as indicated by an assessment of pest reports over the last 20–30 years.
A comparison of pest outbreak reports from the early 1980s to 2006–07 from south-eastern Australia highlights a decrease in
the importance of pea weevils and armyworms, while the lucerne flea, Balaustium mites, blue oat mites and Bryobia mites
have increased in prominence. In Western Australia, where detailed outbreak records are available from the mid 1990s, the
relative incidence of armyworms, aphids and vegetable weevils has recently decreased, while the incidence of pasture
cockchafers, Balaustium mites, blue oat mites, redlegged earth mites, the lucerne flea and snails has increased. These changes
are the result of several possible drivers. Patterns of pesticide use, farm management responses and changing cropping
patterns are likely to have contributed to these shifts. Drier conditions, exacerbated by climate change, have potentially
reduced the build-up of migratory species from inland Australia and increased the adoption rate of minimum and no-tillage
systems in order to retain soil moisture. The latter has been accompanied by increased pesticide use, accelerating selection
pressures for resistance. Other control options will become available once there is an understanding of interactions between
pests and beneficial species within a landscape context and a wider choice of ‘softer’chemicals. Future climate change will
directly and indirectly influence pest distributions and outbreaks as well as the potential effectiveness of endemic natural
enemies. Genetically modified crops provide new options for control but also present challenges as new pest species are likely
to emerge.
Introduction
Agricultural production in southern Australia has intensified as
farming systems have moved from low intensity grazing of native
grasslands to wide-scale cropping and high intensity grazing
based on introduced pastures containing perennial ryegrass
(Lolium perenne) and subterranean clovers (Trifolium
subterraneum) (BRSA 2001). The present shift towards annual
cropping systems –often based on shorter rotations of pasture and
increased frequency of wheat, barley and canola –is driven by an
economic rationale that has coincided with an increased use of
fertilisers [nitrogen (N) and phosphorus], and an increased
reliance on the chemical control of pests, similar to the
worldwide trend (Tilman et al. 2002).
Invertebrate pest control in Australia has relied on chemical
applications and host plant resistance for over 100 years. Modern
crop varieties are often susceptible to pests due to a loss of plant
host resistance mechanisms (Norton et al. 1999). This is typified
in canola, a derivative of rapeseed that has reduced levels
of erucic acid (<2%) and glucosinolates (<30 mmol/g of
aliphatic glucosinolates) –chemicals that typically increase
plant resistance (Busch et al. 1994). Since the adoption and
subsequent increases in the area planted to susceptible canola
varieties, the amount of pesticide usage has increased (e.g. Moens
and Glen 2002). Increased chemical applications often lead to
changes in pest complexes (Pimentel et al. 1991; Huffaker et al.
1999). Species with relatively high levels of tolerance or the
ability to evolve resistance to chemicals can increase in
frequency. Increased chemical use can also lead to more
complex changes at higher trophic levels that influence the
structure of pest communities (Stark et al. 2007).
Changes in climate, and farm management responses to these
changes, are also starting to influence pest complexes.
Traditionally, Australian growers are adept at making the most
of limiting factors, which increasingly includes low moisture
availability (Perry et al. 1977). By changing management
practices to control availability of water in agricultural
systems, production can be maintained. In wetter southern
areas where production is expanding, new technologies are
being used such as controlled traffic, raised beds to alleviate
waterlogging and conservation tillage that retains crop residues
(stubble) on the soil surface. In northern areas, including the flood
plains of northern New South Wales and southern Queensland
CSIRO PUBLISHING Overview
www.publish.csiro.au/journals/ajea Australian Journal of Experimental Agriculture, 2008, 48, 1481–1493
CSIRO 2008 10.1071/EA08185 0816-1089/08/121481
that lie on the temperate/tropical overlap, unreliable winter
rainfall means sowing of crops is dependent on subsoil
moisture rather than rainfall during the growing season, and
growers maintain moisture and soil structure by avoiding over-
tillage and compaction. These changes can lead to new pest
pressures.
Although there is much speculation about the effects of
management practices, climate change and altered pesticide
applications on shifts in pest complexes, these are poorly
documented. There is no validated monitoring of changes in
pest pressures over time, or of the relative importance of different
pests in crops. There is also no systematic evaluation of how
landscape level changes (e.g. shelterbelts, revegetation) and
genetically modified (GM) crops might impact on pest control
in the future.
This paper attempts to gain some understanding of the nature
of changing pest pressures over the last few decades in southern
Australia along with future prospects for pest control. Changes in
pest importance are identified and interpreted in terms of likely
drivers including altered management, pesticide pressures and
climatic variability. The possible effects of climate change on the
distribution and abundance of pest and beneficial organisms are
considered. The issue of pest control in GM crops is briefly
discussed now that the moratorium on GM plantings for food
crops has been lifted across several Australian States and the
impact of possible landscape level changes on pest control are also
considered.
Tracking changes in pest pressures
There are no routine quantitative assessments of the importance of
different pests in the Australian grains industry. Nevertheless,
there are indirect ways in which changes might be assessed. Here
we consider changes evident from pest report bulletins in two
geographically distant regions: Victoria/southern New South
Wales, and Western Australia.
The Victorian/southern New South Wales bulletins were
started in the early 1980s by the Agricultural Division (now
known as the Department of Primary Industries) of the Victorian
Government. These bulletins listed records of invertebrate pest
outbreaks (including pest outbreaks that were encountered and
controlled as well as ones that were difficult to control) in Victoria
and some parts of New South Wales, and were typically
produced on a monthly basis. We obtained bulletins covering
three periods: 1980–84, 1985–89, and 1990–94. In addition, we
considered information from the more recent Pestfacts South-
Eastern bulletins that have been disseminated through the Centre
for Environmental Stress and Adaptation Research at The
University of Melbourne. These reports were based on
information from a network of agronomists and extension
workers in Victoria and New South Wales. Data from the
bulletins were available for the 2006 and 2007 growing
seasons. Pestfacts South-Eastern is issued on an ‘as needed’
basis and there are typically 13 bulletins over the winter
growing season (from April to November).
The Western Australian information was collated from the
PestFax bulletins that have been produced by the Department of
Agriculture and Food of the Western Australian Government.
These bulletins are produced on a weekly basis over the winter
growing season; typically 26 bulletins from April to November.
PestFax bulletins have been issued concurrently since 1996 and
are generated from an extensive network of researchers,
agronomists and extension workers.
Outbreak records are an imperfect means for assessing the
relative importance of different pests. They rely on assessments of
pests by extension workers even though the pest species might
not always be correctly identified. Some minor or localised
outbreaks might go undetected or unreported. Thus, these
records do not fully represent the entire matrix of invertebrate
pest-crop outbreaks. These records also do not indicate the
relative economic effects of particular pests, only the presence
of an outbreak. Specific pests of less widely grown crops
(e.g. some pulses) may not be reported as frequently as other
species attacking common crops such as wheat and canola.
Nevertheless, the bulletins are likely to reflect common pest
issues and provide an indication of changes in pest status
across years.
All records of invertebrate pest outbreaks reported in the
bulletins were included here. There was no attempt at
controlling the quality of the information; however, the
majority of reports were from trained and experienced
agronomists, State department staff and other industry
representatives. For the Victorian/southern New South Wales
outbreak data, there were 258, 132, 244, and 295 records for the
1980–84, 1985–89, 1990–94 and 2006–07 periods, respectively.
For Western Australia, outbreak numbers were higher and,
therefore, considered on a yearly basis rather than within
periods. Records increased over time, ranging from 61 in 1996
to 199 in 2007. As we were interested in changes in the relative
incidence of invertebrate pests over time, the summation of
reports for each period (Victoria/southern New South Wales)
or year (Western Australia) for an individual pest was divided by
the number of records for pests in that period or year. For instance,
if there was a total of 100 records in a period, consisting of 38
records of one species, this species would be designated as having
a relative incidence of 0.38 for that period.
The Victorian/southern New South Wales outbreak data
indicate substantial changes in the relative incidence of pests
over the last 25 years (Fig. 1). Outbreaks of armyworms (Family:
Noctuidae), which were common in the 1980s are rare in recent
times. Cutworms (Agrotis spp.), loopers (Family: Geometridae),
wireworms (Family: Elateridae), pea weevils (Bruchus pisorum),
underground grass grubs (Oncopera fasciculate) and pasture
webworms (Hednota spp.) also appear less frequently in recent
records than they did in the early 1980s. In contrast, lucerne fleas
(Sminthurus viridis) and the redlegged earth mite (Halotydeus
destructor) seem to have increased in abundance. New pests that
have appeared in outbreak records in the period 2006–07 for the
first time include Bryobia mites (Bryobia spp.), blue oat mites
(Penthaleus spp.), Balaustium mites (Balaustium medicagoense)
and slaters (Order: Isopoda). Native budworm (Helicoverpa
puntigera), pasture cockchafers (Family: Scarabaeidae) and
aphids (Family: Aphididae) have remained important pests
throughout the 25-year period.
In Western Australia, where more complete records were
available, the relative incidence of pests was regressed against
the year to identify groups that tended to increase or decrease
over time. Aphids, native budworm, diamondback moth
1482 Australian Journal of Experimental Agriculture A. A. Hoffmann et al.
(a) Victoria/southern New South Wales
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Aphids (various)
Armyworms (various)
Balaustium mite
Blue oat mite
Bryobia mite
Cabbage centre grub
Cabbage white butterfly
Cockchafers (various)
Cutworms (various)
Diamondback moth
False wireworm
Black field cricket
Grasshoppers (various)
Loopers (various)
Lucerne flea
Native budworm
Pasture webworm
Pea weevil
Redlegged earth mite
Rutherglen bug
Sitona weevil
Slugs (various)
Snails (various)
True wireworm
Underground grass grub
Weevils (misc)
Early 1980s
Late 1980s
Early 1990s
2006–07
(b) Western Australia
0
0.05
0.10
0.15
0.20
0.25
0.30
African black beetle
Aphids (various)
Armyworms (various)
Balaustium mite
Blue oat mite
Bronzed field beetle
Bryobia mite
Cockchafers (various)
Cutworms (various)
Desiantha weevil
Diamondback moth
European earwig
False wireworm
Grasshoppers (various)
Loopers (various)
Lucerne flea
Native budworm
Onion fly
Pasture day moth
Pasture webworm
Pea weevil
Redlegged earth mite
Rutherglen bug
Slugs (various)
Small lucerne weevil
Snails (various)
Thrips (various)
Vegetable beetle
Vegetable weevil
Weed web moth
Weevils (misc)
Incidence of pest outbreaks (proportion of total)
Pest taxa
Fig. 1. Relative incidence of outbreaks of pests in (a) Victoria/southern New South Wales from the early 1980s and (b) Western Australia from 1996. All outbreaks in a period were summed
and the proportion associated with a particular pest determined.
Changing status of invertebrate pests of grains Australian Journal of Experimental Agriculture 1483
(Plutella xylostella), lucerne flea and mites are important pest
species (Fig. 1). Only groups where linear regressions indicated
effects of year accounting for a substantial proportion of the
variation (R
2
>0.25), are presented. These analyses indicated that
the relative incidence of aphid pests has decreased over the last
10 years, along with the relative incidence of armyworms and
vegetable weevils (Listroderes difficilis) (Fig. 2). In contrast, the
relative incidence of lucerne flea, Balaustium mites, blue oat
mites, redlegged earth mites, snails and pasture cockchafers has
increased (Fig. 3).
Some of these changes may reflect shifts in the type and
total acreage of crops being grown and the areas where they are
sown. In other cases, an increase in the status of a new pest
might reflect a new invasion. Many of Australia’s broadacre pests
are introduced species, and the pest status of an invader will
increase as it colonises new areas. The recent occurrence of
Bryobia mites as a pest in Victoria/southern New South Wales
may reflect a new incursion. However, dramatic changes in
the pest status of species can occur even when they have
been resident for some time. For instance B. medicagoense
appears to have been present since the 1930s, around which
time it was probably introduced from South Africa (Halliday
2001). This species was initially regarded as a beneficial mite
in the 1990s (James et al. 1995). However, in the last 10 years,
this mite has clearly been identified as responsible for crop
damage in both western and south-eastern Australia.
Improved species identification and increased awareness
may also have led to some changes in the importance of pest
species. For instance, the damage caused by blue oat mites in
southern Australia has previously been underrepresented owing
to their frequent misidentification as another pest species, the
redlegged earth mite. It is only recently that the correct
identification of these species has been shown to be important
for control (Robinson and Hoffmann 2001; Umina and Hoffmann
2004). This has increased awareness and may be responsible for
the increase in reports of blue oat mites (Fig. 1). Similarly, the
decrease in vegetable weevil (Listroderes difficilis) incidence in
Western Australia may be partially attributed to increased
awareness of other pest weevil species such as desiantha
(Steriphus diversipes) and small lucerne weevil (Artichonotus
taeniatulus) (Mangano and Severtson 2008). Mandolotus
(Mandalotus spp.) weevil and grey banded leaf weevil
(Ethemaia sellata) have also recently increased in prominence
in eastern Australia (http://cesarconsultants.com.au/services/
pestfacts-3.html, verified 5 October 2008).
Pesticide usage as a driver
The use of pesticides has been, and continues to be, the most
common management option for the control of Australian
invertebrate pests (Beaulieu and Weeks 2007). Over
$1.8 billion was spent nationally on pesticides in 2004, with
over 5000 t of the most widely used insecticide group, the
organophosphates, being applied annually (Radcliffe 2002).
Synthetic pyrethroids were introduced as an alternative to the
environmentally unsuitable organochlorines, cyclodienes and
some organophosphorus chemicals (Morton and Collins 1989).
Pyrethroids possess an inherently high activity and can be applied
at extremely low doses for the control of a huge range of
agricultural pests. They are generally considered to have lower
impacts on wildlife in the surrounding environment, degrade
quickly, are immobile in the soil (Elliott 1989), and are a cheaper
alternative to most other chemical classes. An application of
a-cypermethrin, a pyrethroid commonly used to control several
Australian grain pests, has a minimum cost of only $0.60 per ha
(Moore and Moore 2007). These attributes have resulted in the
widespread adoption of synthetic pyrethroids, which now
(a) Aphids
R2 = 0.3364
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
1994 1996 1998 2000 2002 2004 2006 2008
(b) Armyworms
R2 = 0.3204
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
1994 1996 1998 2000 2002 2004 2006 2008
(c) Vegetable weevils
R2 = 0.2517
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1994 1996 1998 2000 2002 2004 2006 2008
Incidence of pest outbreaks (proportion of total)
Year
Fig. 2. Changes in the incidence of outbreaks of pests that have decreased
in relative importance in Western Australia between 1996 and 2007.
1484 Australian Journal of Experimental Agriculture A. A. Hoffmann et al.
account for more than 25% of the world insecticide market
(Hemingway et al. 2004).
Many of the changes observed in pest species over the last
20–30 years could have been driven by increases in chemicals
applied to Australian agroecosystems, particularly the use of
synthetic pyrethroids. There are numerous pests, including the
redlegged earth mite, blue oat mites, Balaustium mites and the
lucerne flea, which have increased in relative incidence in
Western Australia and Victoria/southern New South Wales.
Blue oat mites, lucerne flea and Balaustium mites have
recently been shown to have a higher natural tolerance to a
range of chemical classes when compared with other groups
(Umina and Hoffmann 1999; Robinson and Hoffmann 2000;
Arthur et al. 2008; Roberts et al. 2008). In particular, these species
have a relatively high tolerance level to several synthetic
pyrethroids that are routinely used in the grains industry.
Balaus tium mites
R2 = 0.5009
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Blue oat mites
R2 = 0.4961
0
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
Cockchafers
R2 = 0.5664
0
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040 Lucerne fleas
R2 = 0.2500
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Redlegged earth mites
R2 = 0.4265
0
0.05
0.10
0.15
0.20
0.25
Snails
R2 = 0.385
0
0.005
0.010
0.015
0.020
0.025
0.030
0.035
Incidence of pest reports (proportion of total)
1994
1996
1998
2000
2002
2004
2006
2008
1994
1996
1998
2000
2002
2004
2006
2008
1994
1996
1998
2000
2002
2004
2006
2008
1994
1996
1998
2000
2002
2004
2006
2008
1994
1996
1998
2000
2002
2004
2006
2008
1994
1996
1998
2000
2002
2004
2006
2008
Year
Fig. 3. Changes in the incidence of outbreaks of pests that have increased in relative importance in Western
Australia between 1996 and 2007.
Changing status of invertebrate pests of grains Australian Journal of Experimental Agriculture 1485
The indiscriminate nature of broad-spectrum pesticides often
results in the death of natural enemies and other beneficial
invertebrates, which can potentially create or enhance pest
problems (Readshaw 1975). Moreover, it is widely accepted
that a common outcome of the heavy reliance on chemicals for
control is the emergence of secondary pests (Jutsum et al. 1998;
Hopper 2003). Given their high natural tolerance, it is likely that
blue oat mites, Balaustium mites and lucerne fleas are persisting
after chemical control of other pests. For examples, previous
research has shown that earth mite species compete with each
other (Weeks and Hoffmann 2000; Umina and Hoffmann 2005)
and other pests such as the lucerne flea (Michael et al. 1997),
suggesting the suppression or eradication of one pest species will
lead to an increase in the relative incidence of another. This may
account for some of the changes in earth mite pests found in
Western Australia and Victoria/southern New South Wales
(Fig. 1). Redlegged earth mites are often relatively more
susceptible to pesticides (Robinson and Hoffmann 2000;
Arthur et al. 2008; Roberts et al. 2008). However, the
development of tolerance to organophosphorus pesticides
(Hoffmann et al. 1997) and more recently resistance to
synthetic pyrethroids (Umina 2007) could be contributing to
the resurgence of this pest.
Climate change as a driver
Grain production appears to be influenced by climate change. For
wheat, maize and barley, there has been a negative response of
global yields due to decreased rainfall and increased temperatures
(Lobell and Field 2007). Over the next few decades, climate is
further expected to decrease grain yields. For example, in south-
eastern Australia, wheat yields are expected to decrease by around
25% unless this can be offset by crop breeding and changes in
farming practices (Anwar et al. 2007). There are several scenarios
about the impacts and opportunities associated with climate
change for Australian farmers in the future. For example,
temperatures during winter months will be warmer and there
will be less rainfall and increased levels of carbon dioxide (CO
2
)
(CSIRO 2007).
Climate change is already likely to have contributed to
changes in the relative importance of invertebrate pests in
recent times. The decrease in incidence of armyworm
outbreaks in recent years appears to be associated with the
advent of drier conditions. Typically, large infestations of this
pest occur after heavy autumn rains. For instance, in 1988 and
1989, there were large outbreaks in south-east Queensland and
parts of New South Wales following heavy rains (McDonald et al.
1995). Outbreaks depend on the establishment of large
populations on winter grass hosts and migration to crops
(McDonald et al. 1995). However, high winter rainfall
conditions have been rare in the last two decades (see
http://www.climatechangeinaustralia.gov.au/recentchanges.php,
verified 5 October 2008), and this has probably prevented the
build-up of populations of this pest. Another pest that is likely to
be influenced by the changing availability of breeding resources
away from cropping areas is H. punctigera. This species
builds up to high numbers on non-crop plants in central
Australia over winter and then migrates to cropping regions
in spring (Gregg et al. 2001). Outbreaks are linked to rainfall
events that allow for build-up of host plants and the direction of
weather patterns facilitating the migration of moths into
agricultural areas (Fitt et al. 1995; Maelzer et al. 1996).
When the distribution of pests is limited by climatic
variables, future distribution limits may expand or contract
directly in response to climate change.
For two species of blue oat mite, Penthaleus major and
P. falcatus, moisture and heat have been identified as the two
main factors limiting their inland distribution in south-eastern
Australia; however, it is not clear what limits their northern
distribution (Robinson and Hoffmann 2001). The distribution
of redlegged earth mites has also been linked to temperature and
moisture (Wallace and Mahon 1971a) although it is also not clear
if these variables can explain shifts in the distribution of this
species over the last 20–30 years (Weeks and Hoffmann 1999;
Robinson and Hoffmann 2001). The inland distribution of blue
oat mites and redlegged earth mites will probably contract as
conditions become drier. However, changes in the distribution of
other mite species are harder to predict. In particular, the
distribution of a third blue oat mite pest, Penthaleus tectus,
which occupies disjunct areas in the Victorian Mallee and
inland New South Wales (Robinson and Hoffmann 2001) as
well as in Western Australia (A. Arthur, pers. comm.), is not easily
linked to climatic variables (Robinson and Hoffmann 2001). A
greater understanding of species distribution shifts requires
mechanistic models that consider the biological attributes of
species and how they interact with the environment (Kearney
and Porter 2004). Such models and biological information are not
yet widely available for grains pests although predictive models
of population growth and migration patterns have been developed
and tested for Helicoverpa spp. (Fitt et al. 1995; Duffield and
Steer 2006).
Slugs in cropping systems are another group likely to respond
to climate change. The key requirement for activity in slugs is
moisture (Young and Port 1991; Chio et al. 2004). The present dry
conditions may be reducing outbreaks of slugs overall, as
suggested by patterns from past records (Fig. 1). Additionally,
the species composition of slug communities may be altered. The
main pest across southern Victoria is currently Deroceras
reticulatum (Horne 2004); however, there is an increase in
Milax gagates in some slug communities particularly
following drought conditions (Nash 2008). M. gagates,a
burrowing species, appears to be behaviourally better adapted
to drier environments (South 1992). This shift would have direct
consequences on crop protection, as M. gagates is harder to
control using conventional molluscicides and can cause
more plant damage per individual (Crawford-Sidebotham
1970; Godan 1983).
Climate change can influence pests and beneficial organisms
through interactions with plant responses to CO
2
. In general,
elevated levels of CO
2
reduce the nutritional value of plants,
decreasing the performance of invertebrates feeding on them
(Zvereva and Kozlov 2006). This might result in an increased
level of plant damage because the pests have to consume more
plant tissue to acquire similar levels of nutrition. However, there
might also be negative effects on beneficial predators and
parasitoids if their prey is smaller and of poorer quality.
1486 Australian Journal of Experimental Agriculture A. A. Hoffmann et al.
Conversely, the number of prey consumed by predators might
then be increased and lead to improved pest control through
beneficial invertebrates. Thus far, few evaluations relevant to the
Australian grains industry have been undertaken. Coll and
Hughes (2008) considered the interaction between an
omnivorous pentatomid bug (Oechalia schellenbergii) and
the cotton bollworm (Helicoverpa armigera) when feeding on
peas. In this case the pea plants had reduced N content when
grown under higher levels of CO
2
and this in turn influenced
the size of the bollworm larvae. Moreover, the predators
were more effective under higher CO
2
levels because they
appeared to be better at subduing the smaller cotton bollworm
larvae. This suggests that higher CO
2
could make generalist
predators more effective. However, Gao et al. (2008)
suggested that the aphid Aphis gossypii may become a more
serious pest under elevated CO
2
concentrations because of
increased survivorship of the aphid and the longer
development time of one of its main predators, Propylaea
japonica.
The effects of climate change on beneficial invertebrates, and
hence the levels of control they are likely to exert, will be complex
and depend on factors other than those mediated through changes
in plant host quality. Other factors that need to be considered are
the relative thermal resistance of beneficial species and their hosts
including acclimation effects, the impact of temperature on
population growth, host searching by beneficial species and
competitive interactions, the ability of prey to counter
predators and parasitoids, and interactions between thermal
conditions and symbionts inside predators and prey (Stacey
2003; Hance et al. 2007). At present, we know very little
about interactions between grains pests, natural enemies and
environmental factors. Until more information becomes
available on the effects of temperature, CO
2
and moisture on
tritrophic interactions within cropping environments, it is
difficult to make predictions about whether control imposed by
natural enemies will improve or decline under climate change
(Hoover and Newman 2004).
Changing farming practices as a driver
Changes in farm management practices over the last 20–30 years
are likely to have had direct and/or indirect effects on invertebrate
pests. Changes in sowing times have often occurred in
response to climate, and these may impact on spring pests
(Glen 2000). Minimum or no-tillage systems are becoming the
norm in the Australian grains industry partly in response to
altered rainfall patterns (Ugalde et al. 2007). These systems
are likely to increase invertebrate biodiversity (Stinner and
House 1990; Robertson 1993), but may also lead to changes in
pest complexes. For instance, no-tillage systems with stubble
retention often increase seedling loss due a more favourable
microenvironment for slug (Glen et al. 2003) and snail
(Baker 2002; Lenard et al. 2003) populations. Management
changes also influence many beneficial species that help
control pests. For example, carabid beetles (Family:
Carabidae), effective predators of slugs, wireworms and
moth larvae, are favoured in reduced tillage systems
(Andersen 1999, 2003).
Irrigation practices can influence the distribution of pest
species. One pest whose importance appears to be increasing
due to irrigation is the lucerne flea, S. viridis. This species depends
on warm and dry summers to successfully complete diapause, and
adequate moisture in winter to allow populations to develop on
lucerne and other host crops (Wallace and Mahon 1971b). These
constraints have restricted the northerly and easterly distribution
of lucerne flea in southern Australia within the 250- and 225-mm
isohyets, respectively. However, lucerne flea has recently spread
outside this zone, particularly in areas where lucerne and pastures
as well as other crops are irrigated (Bishop et al. 2001;
J. K. Roberts, pers. comm.). Irrigation probably provides
suitable moisture for population survival and increase in areas
where summer weather patterns disrupt the development of egg
diapause (Bishop et al. 2001).
Changes in paddock rotation strategies and cropping intensity
have almost certainly influenced pest species, although such
effects are difficult to detect and quantify. The amount of
canola grown in Australia has steadily increased over the last
20–30 years (ABARE 2007). However, canola is a relatively poor
crop plant at establishment and is particularly prone to attack
by various pests including earth mites, lucerne flea and weevils
(Gu et al. 2007; Micic et al. 2008). The increases found in these
and other pests (see Fig. 3) could, in part, reflect the increase in
canola sown in Australia. In contrast, pasture is now less often
used in rotations and this factor as well as rainfall changes may
have contributed to a reduction in armyworm pest reports as
discussed above.
Changes in weed management in Australia have probably
had direct and large impacts on the abundance of invertebrate
pests and particularly aphids in grains. Widespread resistance
to multiple chemicals has developed in various pest aphid
species (Edwards et al. 2008) and increasing temperatures
over the last 20–30 years have resulted in quicker aphid
generation times and larger population sizes (Satar et al. 2005;
Nourbakhsh et al. 2006; Salman 2006; Iversen and Harding
2007). Despite this, the relative number of recorded pest
outbreaks due to aphids has remained relatively constant
across south-eastern Australia and has decreased in Western
Australia. Aphids survive the summer in low numbers on
perennial grasses, summer weeds, or volunteer crops that may
appear after periodic rain events (Edwards et al. 2008). They then
move into winter crops, and the timing of aphid flights is
correlated with the occurrence and extent of rainfall events
during the late summer and early autumn (Thackray et al.
2005). The development of new herbicide chemistries, the use
of crop rotations and the introduction of integrated weed
management programs in Australia have resulted in better
control of summer weeds (Turner 2004). This has likely
impacted on the survival of aphids and other invertebrate pests
that use weeds as refugees over summer.
Other improvements in management practices have
probably influenced the relative importance of some pest
species. The fall in the relative importance of aphids in
Western Australia may, at least partly, reflect improvements in
aphid management strategies. In particular, tools developed
by the Western Australian Department of Agriculture and
Food allow risk assessments of aphid-transmitted viruses to
Changing status of invertebrate pests of grains Australian Journal of Experimental Agriculture 1487
be made in the lead up to the cropping season, improving
control decisions for aphids (http://www.agric.wa.gov.
au/content/PW/PH/DIS/PLANT_DISEASE_FORECAST.htm,
verified 5 October 2008).
GM crop plants and their potential impacts
The introduction of GM plants not only has the potential to
transform agriculture, but also the influence of invertebrate pests
found in different systems. Since the development of commercial
GM crop plants in the early 1990s, the total area annually
planted to GM crops worldwide has increased to over
100 million ha (Dill et al. 2008). In 2007, this included over
2 million ha of herbicide tolerant GM crops, 20 million ha of
insecticide transgene GM crops and 22 million ha of ‘stacked’
(combined herbicide and insecticide) crops (James 2007).
Plantings of GM crops are growing annually by an average of
12% worldwide, with the adoption of stacked GM crops
increasing at the staggering rate of 66% in 2007 (James 2007).
The moratorium on GM food crops in Australia has recently been
lifted in several States and the first GM herbicide tolerant food
crops [e.g. Roundup Ready canola (Monsanto)] were planted
in 2008.
GM crops will represent one of the most significant changes in
the Australian grains industry over the next decade and lead to
substantial changes in farm management practices, particularly
around how invertebrate pests are managed. There are numerous
crop varieties worldwide that now incorporate Bacillus
thuriengensis (Bt) transgenes that target lepidopteran pests
(Tabashnik et al. 2008). Other modifications for insect
resistance, such as proteinase inhibitors and lectins which are
typically not species-specific, are currently being developed.
The number of insecticide sprays is generally reduced in Bt
GM crops compared with conventional crops (Knox et al.
2006; Sisterson et al. 2007). In Australia, the introduction of
Bt cotton (Ingard and Bollgard II) led to large reductions in
insecticide applications throughout a growing season and
reduced the environmental impact of growing cotton (Knox
et al. 2006). Incorporating insecticidal transgenes into grain
crops in Australia should also reduce the number of sprays and
reliance on broad-spectrum insecticides, and potentially
reduce negative effects of chemical applications on non-target
organisms.
One of the major concerns of insecticidal transgenic crop
plants is the development of resistance in target pest
populations. The selection pressure that these crops exert on
pest populations is considerably higher than for conventional
insecticide sprays (Bates et al. 2005), and laboratory selection
experiments predict that resistance to single Bt toxins can occur
relatively quickly (Tabashnik et al. 2005). Yet, in over 10 years of
field selection, there is only one lepidopteran pest that has
developed field resistance (Helicoverpa zea)toaBt toxin
(Cry1Ac) in cotton and control failures for this pest are still
extremely rare (Tabashnik et al. 2008). Insect resistance
management strategies, based on the ‘refuge strategy’, are
likely to be responsible for the delay in field resistance
(Tabashnik et al. 2005), and should form an integral part of
management strategies for insecticide GM crops in the Australian
grains industry.
Questions have been raised over the effects insecticidal (and
herbicidal) transgenes in crops have on non-target organisms
(Wolfenbarger and Phifer 2000). Research has shown that while
non-target organisms can be exposed to transgene-derived
proteins such as Bt toxins, there are generally no adverse
effects associated with these proteins (O’Callaghan et al.
2005). This, however, may not always be the case, and any
new insecticidal transgene incorporated into crop plants
must be thoroughly tested on a range of organisms and
ecosystems. For instance, field peas resistant to pea weevil
have been developed and tested under glasshouse conditions.
These peas contain an a-amylase inhibitor, a seed protein toxic
to weevil larvae, which is present in the seeds of some
legumes such as beans. Transgenic peas that express the gene
producing this inhibitor are protected in that there is high
mortality and delayed development of the pea weevil
(De Sousa-Majer et al. 2007). However, transgenic expression
of the a-amylase inhibitor isolated from beans in field peas
can lead to inflammation in mice and other animals that feed
on the peas (Prescott et al. 2005). This inflammation occurs
because the inhibitor that is naturally present in beans is
modified in peas.
After the initial trials in 2008 in Victoria and New South
Wales, the area in Australia planted to Roundup Ready canola is
predicted to increase greatly over the ensuing years. Research
from overseas indicates that the adoption of herbicide
resistant transgenic crops will result in large reductions in the
application of herbicides (Phipps and Park 2002). This will
change the spectrum of plants available to pest herbivores and
alter invertebrate pest pressures, particularly at establishment
where pest abundance is often increased in the presence of
weeds (Gu et al. 2007). The introduction of stacked GM crops
(herbicide and insecticide) may provide combined control
of invertebrate pests and weeds, but stacked crops are
unlikely to be available in the foreseeable future for pests in
Australian grains. It is more likely that other strategies may
need to be used in the interim in conjunction with Roundup
Ready canola.
Other developments towards future integrated
pest management
In addition to the incorporation of insecticidal transgenes into
crop plants, future developments in integrated pest management
(IPM) within the grains industry are likely to involve the adoption
of selective chemicals, the recognition and management of
shelterbelts and other landscape-level changes as tools for
assisting pest control, and the development of new ways for
fostering invertebrate biodiversity on farms.
Recently, there has been a push to develop improved
methods of controlling pest species, including the use of
selective ‘soft’chemicals over conventional broad-spectrum
pesticides. Broad-spectrum pesticides have negative impacts
on a variety of soil biota (Radcliffe 2002; Hopper 2003;
Panda and Sahu 2004) and in many situations are often
applied in an ad hoc fashion. Chemicals have either direct
(e.g. toxic) or indirect (e.g. depletion of natural enemies and
beneficial invertebrates) effects, and these issues are never
completely addressed during the registration of pesticides
1488 Australian Journal of Experimental Agriculture A. A. Hoffmann et al.
(Alassiuty and Khalil 1995; Martikainen et al. 1998; Van Zwieten
2004).
In horticulture and viticulture in particular, selective
chemicals and biological control are used as part of a
formulated IPM strategy (Smith and Papacek 1991; James and
Whitney 1993; Radcliffe 2002; Thomson and Hoffmann 2007).
However, the switch from broad-spectrum pesticides to selective
pesticides is limited by the current scarcity of economically-
viable options available within the Australian grains industry.
This is primarily due to the lower profit margins in the grains
industry, which delays the release of selective chemicals for
broadacre settings.
Nevertheless, selective chemicals have played a role in the
implementation of IPM strategies by Australian cotton growers.
While the cotton IPM strategies are primarily based around
transgenic Bt cotton varieties, selective chemicals are
applied to conserve and enhance beneficial insect activity
(Wilson et al. 2004). This has resulted in a reduction of up to
80% in pesticide use by the cotton industry (Knox et al. 2006).
Furthermore, in the last decade some softer chemical options
have become available in the grains industry. These include a
select few foliar sprays, such as Bt, which target several
lepidopteran pests, and pirimicarb, which is a specialised
aphicide (Overton 1996). New insecticide seed dressings have
also become widely available in recent times, which offer a more
targeted and environmentally preferred option than broad-
spectrum chemicals. Although generally more expensive, seed
dressings provide chemical protection at the plant–insect
interface and have little or no direct effect on many natural
enemies (Albajes et al. 2003).
The utilisation of landscape features to promote pest control
has rarely been considered in broadacre agriculture within
Australia. Apart from recent work showing positive effects of
shelterbelts on earth mite control (Tsitsilas et al. 2006) and
remnant grass verges on slug control (Nash et al. 2007; Nash
2008), there is almost no recognition of the value of landscape
features for pest control. This stands in stark contrast to the
situation overseas and particularly in Europe, where numerous
studies have demonstrated the value of vegetation in harbouring
beneficial organisms. Adjacent vegetation influences the
diversity of beneficial insects in agricultural ecosystems
including natural enemies like beetles, spiders, parasitoids and
predatory mites and flies (Symondson 2002; Schmidt et al. 2004;
Thorbek and Bilde 2004; Thies et al. 2005; Olson and Wackers
2007). These increases in natural enemies can provide benefits
in terms of pest control, although the relationship between
natural enemies and pests is not always clear (Gurr et al. 2000;
Olson and Wackers 2007) and needs to be considered on a case-
by-case basis to develop useful recommendations. Nevertheless,
pest control through natural enemies present in agricultural
ecosystems is usually most effective when there is a complex
of beneficial species (Rosenheim 1998; Symondson 2002).
Careful manipulation of vegetation adjacent to farms (Collins
et al. 2003) might help promote beneficial species by providing
nectar sources, shelter and alternate hosts to allow these species to
persist. In Victoria, there is already a push to revegetate some
farming areas in a mosaic for developing areas where biodiversity
might be maintained under climate change and for carbon capture
(Victorian Government 2008). Ecological services on farms
including pest control could benefit if such efforts recognise
the potential of properly maintained vegetation to harbour
beneficial species.
Control of pests through endemic and introduced natural
enemies may become increasingly important in the future, as
the input costs of fertilisers, fuel, and pesticides start to rise due to
finite resources. This makes it increasingly important to
maintain an agricultural environment where beneficial
invertebrates provide vital ecosystem services, such as
pollination, pest control, organic decomposition and the
maintenance of soil structure. Stubble retention, decreased use
of broad-spectrum pesticides, landscape changes and a reduction
in tillage can all help provide an environment where invertebrate
biodiversity remains high and services provided through
invertebrates are maximised. Practical guidelines are now
available to assist growers in identifying and promoting the
maintenance of diverse populations of invertebrates (Horne
and Page 2008). Yet there has been very little research on the
impact and maintenance of beneficial invertebrates and
answers to key questions are unavailable at this time. For
example, what predators and parasitoids are the most
effective in controlling particular pests? How can useful
invertebrate biodiversity on farms be measured easily? Can
these measures be linked to production and economic returns?
What practices and landscape changes promote beneficial
biodiversity?
Conclusions
The distribution of pest outbreaks in the grains industry has
shifted markedly in the past few decades. Patterns of pesticide
use, climate change, farm management responses, changing
cropping patterns and environmental concerns are all likely to
have contributed to these shifts. In the future, even more
significant changes in the distribution and status of some pests
are likely as conditions become drier, migration patterns of
invertebrate pests change, and complexes of pests and
beneficial species that provide endemic control respond
differently to climate change. The introduction of GM crops
will also potentially change the distribution of pests in grains.
While grain prices are increasing along with food prices,
production costs are also rising rapidly and there are external
pressures to maintain more biodiverse agricultural ecosystems.
These factors represent challenges but also opportunities to
promote more sustainable ways of controlling pests through
the careful use of chemicals and the promotion of invertebrate
communities that provide effective pest control and other
services. Pest species and key beneficial species need to be
monitored effectively so that changes in pest status can be
identified and eventually used in a predictive framework.
Acknowledgements
We are grateful to Peter Ridland for providing access to pest bulletins fromthe
Victorian Department of Primary Industries, Garry McDonald for discussions
and two anonymous reviewers for comments. Thanks to Paul Mitrovksi,
Emily Thomson and Dusty Severtson for technical assistance. Our research on
pest and beneficial species in broadacre agriculture is supported by the Grains
Research and Development Corporation including the National Invertebrate
Changing status of invertebrate pests of grains Australian Journal of Experimental Agriculture 1489
Pest Initiative, while Ary A. Hoffmann is supported by a Federation
Fellowship from the Australian Research Council.
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