Taking the fight to aphids

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                                                        This article appeared in the December 2009 edition of the Kondinin Group’s monthly magazine Farming Ahead. The Kondinin Group
                                                        holds the copyright on the article. Reproduction of this text in whole or part by any other publication or for any other purpose is not
                                                        permitted without permission of the Farming Ahead editor. For more information contact Kondinin Group on 1800 677 761.
photo: David McClenaghan, CSiro Entomology

                                                                                                                                  Taking the
                                                                                                                              fight to aphids
                                                  for CSIro ENTomology

                                                                                                               Aphids are pests of crops and pastures all over Australia. in this overview,
                                             Natural enemies: Predators such as ladybirds
                                             can provide significant control of aphids                         Farming Ahead with CSiro looks at the main aphid species on Australian
                                             and improved strategies for monitoring and                        grain and pasture plants and the best options for managing them.
                                             conserving natural enemies could enhance
                                             their effectiveness.

                                                                                                               Aphids can severely damage Australian                                 support large aphid numbers, the aphids
                                                                                                               grain crops such as cereals, oilseeds and                             produce winged morphs that disperse with
                                               At a glance                                                     legumes. Feeding aphids damage plants                                 the wind. In agricultural monocultures,
                                                                                                               in several ways, through direct feeding,                              most of these individuals will find a
                                              Although feeding aphids remove                                   transmitting plant viruses, injecting toxins                          suitable host thus letting aphid numbers
                                              nutrients from plant cells, crops                                and secondary fungal growth (sooty mould)                             build unhindered. Asexual reproduction
                                              usually only sustain significant                                 on honeydew.                                                          and the telescoping of generations (females
                                              damage when aphids are present                                                                                                         giving birth to live young in which embryos
                                              in large numbers. However,                                       Although feeding aphids                                               of the next generation are already
                                                                                                                                                                                     developing) mean aphid populations can
                                              low aphid numbers can cause                                      remove nutrients from plant                                           rapidly expand.
                                              substantial damage if they
                                              transmit plant viral diseases.
                                                                                                               cells, crops usually only                                                In areas such as southern Victoria and
                                                                                                               sustain significant damage                                            Tasmania where plants grow year round,
                                              In mediterranean-style climates,                                                                                                       aphids are present all year. In Mediterranean
                                              such as the Western Australian                                   when aphids are present in                                            climates, such as those of WA, South
                                              wheatbelt, aphids survive the                                    large numbers.                                                        Australia and southern New South Wales,
                                              summer in low numbers on                                                                                                               aphid numbers plunge during summer
                                              perennial grasses, summer weeds                                     However, low aphid numbers can cause                               with populations re-establishing following
                                              or volunteer crops.                                              substantial damage if they transmit plant                             autumn rain. Many aphid species can also
                                                                                                               viral diseases. Some species, such as                                 survive and reproduce at near-freezing
                                              In order to lower farm inputs                                    Russian wheat aphid (Diuraphis noxia),                                temperatures, which lets them thrive in
                                              and reduce the risk of insecticide                               inject saliva containing toxins into plants.                          winter cropping areas.
                                              resistance, aphid management                                     Last but not least, the honeydew secreted by
                                              practices need to aim at reducing                                aphids can lead to sooty mould, which                                 Cereals under attack
                                              insecticide use.                                                 inhibits photosynthesis and can cause grain                              The most common aphids in southern
                                                                                                               handling problems.                                                    Australian cereal crops are oat aphids
                                              Aphids are best managed by                                                                                                             (Rhopalosiphum        padi),    corn    aphids
                                              developing and using varieties                                   Engineered to feed                                                    (R. maidis), rose-grain aphids (Metapolophium
                                              resistant to aphids or viruses or                                  Aphids, with their piercing and sucking                             dirhodum, except in WA), and the grain
                                              both, conserving natural enemies                                 mouthparts, are designed to feed from the                             aphids, Sitobion miscanthi and S. frageriae.
                                              and, when necessary, using                                       cells transporting plant nutrients. Under                             Direct feeding damage can occur on stems,
                                              insecticides selectively.                                        ideal conditions, their numbers build                                 leaves and heads, usually in the tillering and
                                                                                                               rapidly and, when the plant can no longer                             later stages of crop growth. During dry

                                                                                                                                                                        Farming Ahead December 2009 No. 215   49

    periods, R. rufiabdominalis can build up on           insects. These build up in crops along with      Soybean
    roots of winter cereals where it is difficult to      the aphids and can maintain aphid                   The soybean aphid (Aphis glycines), first
    control.                                              populations below threshold levels.              discovered in Australia during 2000, can be
       In cereals, most damage is caused by                  The green peach aphid (GPA), Myzus            a problem in coastal NSW and Queensland.
    aphids transmitting viruses; in particular            persicae, can sometimes cause significant        High populations reduce plant vigour and
    barley yellow dwarf virus (BYDV). However,            early season crop loss. It is also the most      yield, and lead to sooty mould growth.
    there is often little yield loss from BYDV            important vector of beet western yellows         Again, predators play an important role in
    because infection occurs late when plants             virus, which could become a serious problem      aphid suppression.
    are less susceptible.                                 in southern Australia in the near future.
       No wheat varieties have resistance to
                                                          lupins and pulses                                   Aphids have been key pests of legume
    BYDV, however there is effective resistance
                                                            The blue-green aphid (Acyrthosiphon            pastures, an important component of ley-
    available in barley and some tolerance
                                                          kondoi), the cowpea aphid (Aphis craccivora)     farming systems of southern Australia,
    available in oats. Varieties with some
                                                          and GPA commonly feed on cultivated              since the 1980s when a number of species
    resistance are potentially beneficial for
                                                          lupins in Australia, usually during budding      were introduced. Depending on soil type
    growers in high-rainfall zones, which are
                                                          and flowering. Feeding can reduce yields,        and climate, these pastures are usually
    generally high aphid risk areas.
                                                          especially in low-rainfall areas, when plants    dominated by annual medics (Medicago
       Seed dressings containing imidacloprid                                                              spp.) or clovers (Trifolium spp.). Cowpea
    can reduce the incidence of aphids and                are stressed and less able to compensate for
                                                                                                           aphid and blue-green aphid will colonise
    BYDV. However, there should be a follow-              aphid damage. Yield loss depends on lupin
                                                                                                           both medics and clovers. Another species,
    up foliar spray of a synthetic pyrethroid             variety. Yellow lupin varieties, such as
                                                                                                           Therioaphis trifolii, has two different forms:
    seven weeks after crop emergence. If no               Wodjil, are usually very susceptible to aphids
                                                                                                           spotted alfalfa aphid (T. trifolii f. maculata),
    seed dressings are used and BYDV is a risk,           whereas most narrow-leafed lupin varieties,      which attacks lucerne and some medics;
    then two synthetic pyrethroid applications            such as Kalya and Tanjil, are not. However,      and spotted clover aphid (T. trifolii), which
    are recommended, at three and seven weeks             there have been reports of GPA colonising        can severely damage clovers in some areas.
    after emergence.                                      resistant narrow-leafed lupins.                  Aphid-transmitted viruses can also be a
       In northern Australian growing areas                                                                problem, particularly BYMV in clover and
    (central NSW to central Queensland), low              While chickpeas are not                          alfalfa mosaic virus in medics.
    numbers of the oat aphid and the corn aphid           generally at risk from aphids,                      Most varieties of the barrel medic
    are always present in cereal crops. Then,                                                              (M. truncatula) have resistance to both blue-
    about every 5–7 years, enormous numbers               both faba bean and lentils                       green aphid and spotted alfalfa aphid, with
    develop during early spring, particularly in          are particularly susceptible to                  limited protection against pea aphid and
    barley, and these may reduce yield through                                                             spotted clover aphid. There are also aphid-
    feeding damage. However, most populations             cowpea aphid.                                    resistant lines for the strand medic,
    will be reduced to sub-economic levels by                                                              M. littoralis. So far, resistant clovers have
    natural enemies. If spraying is economic,                There are thresholds for treatment against    eluded breeders however moderate tolerance
    growers     generally     use     dimethoate,         aphid damage on lupins and these have            to blue-green aphid is available in some
    chlorpyrifos or pirimicarb.                           been extrapolated to other pulses.               varieties of subterranean clover. No resistance
                                                          Dimethoate, methidathion, omethoate, and         exists against spotted clover aphid.
    Canola carnage                                        pirimicarb are among the recommended
                                                          insecticides but registration varies between        While organophosphate (dimethoate) and
       Both the cabbage aphid (Brevicoryne                                                                 carbamate (pirimicarb) insecticides are
    brassicae) and the turnip aphid (Lipaphis             states.
                                                                                                           registered for use against aphids in legume
    erysimi) can cause considerable feeding                  In pulses, aphids transmit viruses such as    pastures, growers in high-risk areas should
    damage to canola shoot tips. However, as              cucumber mosaic virus (CMV) and bean             first consider aphid-resistant varieties.
    aphid colonies develop on flowering spikes,           yellow mosaic virus (BYMV). CMV can also         Otherwise, seed treatments are a good
    canola compensates by producing more                  cause significant yield losses in lupins, but    option. Before resorting to sprays, growers
    spikes. Control needs to be considered if             the risk depends on when the aphids arrive.      should check for natural enemies which
    plants are drought stressed, as aphids target         Sowing lupin seed infected with CMV is the       could provide natural aphid control. More
    stressed plants, which are less able to               main source of infection, so using clean         natural enemy-friendly, ‘softer’ insecticides
    compensate for feeding damage. In marginal            seed is the best control method.                 should be favoured. Because of their anti-
    areas where drought stress is more likely,                                                             feeding effects, pyrethroid insecticides are
    aphid damage can result in high yield losses.         Sorghum                                          likely to be the most effective at preventing
    Cultivars such as Hyden and Beacon appear               The corn aphid is an occasional pest of        aphids from introducing viruses.
    to be the least susceptible to aphid                  sorghum panicles where their honeydew
    colonisation.                                         causes sticky grain, with consequent
                                                                                                           Host plants: In agricultural monocultures, it is easy
       Pirimicarb, the only chemical registered           harvesting and handling difficulties. Natural
                                                                                                           for dispersing winged forms of aphids to find a
    for use on aphids in canola, has the                  enemies are very important in reducing           suitable host, thus letting aphid numbers increase
    advantage of being kinder to beneficial               panicle infestations.                            unhindered.
    photo: Carl Davies, CSiro plant industry

50 Farming Ahead December 2009 No. 215

                                                                                Alternative hosts take advantage of the rain       continue to be effective control tools for
                                                                                and aphid numbers can build on these.              some time. Imidacloprid insecticides are
                                                                                Because these hosts are often reservoirs for       very effective against aphids and no evidence
                                                                                aphid-transmitted viruses, the same rainfall       for resistance to them was found in a recent
                                                                                events are also linked to virus outbreaks.         survey.
                                                                                   Simulation models that forecast aphid and       future management practices
                                                                                virus (BYDV, CMV and BYMV) outbreaks in
                                                                                the WA wheatbelt are based on this                    In order to lower farm inputs and reduce
                                                                                association between rainfall and aphid/virus       the risk of insecticide resistance, aim to
                                                                                risk. Virus incidence and potential yield loss     reduce insecticide use to manage aphids.
                                                                                can be estimated from these models.                Aphid-resistant varieties are economical but
                                                                                                                                   not always available. Nor do they always

                                                      photo: CSiro Entomology
                                                                                Insecticide resistance                             prevent virus spread. Aphids can also evolve
                                                                                  Prophylactic spraying to control aphids          biotypes that overcome resistance thus
                                                                                increases the risk of insecticide resistance       reducing the useful life of resistant varieties.
                                                                                developing, in both target and non-target             Research on how aphids feed and how
                                                                                species.                                           plants defend themselves could lead to
                                                                                                                                   more durable resistant varieties. It appears
Insecticide resistant: Green peach aphid, which
is common in the grainbelts of Australia, has                                   gpA, which is common in the                        that plant resistance is often mediated by an
                                                                                                                                   immune factor that acts like an ‘antibody’,
developed resistance to more classes of insecticide
than any other insect species.
                                                                                grainbelts of Australia, has                       recognising a factor associated with aphid
                                                                                developed resistance to more                       feeding and triggering the plant’s defences.
                                                                                                                                   Other research is looking at engineering
Biological control                                                              classes of insecticide than                        plants so the genes that allow GPA to feed
   Parasitoids and predators can help control                                   any other insect species.                          on resistant varieties of narrow-leaf lupins
aphids in cropping systems. One indicator                                                                                          will be silenced. However, these ‘high-tech’
that parasitoids are around is the presence                                        While it is a more serious pest of              solutions will take at least 10 years to reach
of mummified aphids with their hardened                                         vegetables, it can damage canola and narrow-       the market.
and darkened cuticle. Aphidius spp.,                                            leafed lupins. GPA can also feed on resistant         Meanwhile, base aphid management on
Lysiphlebus spp. and Diaraetiella rapae are                                     lupin varieties. In addition to sprays targeted    developing and using varieties resistant to
the most effective parasitoids in Australian                                    at it, GPA is often exposed to sprays while        aphids or viruses or both, conserving natural
grains, while Aphelinus spp. are generally                                      feeding on broadleaf weeds in other crops.         enemies and, when necessary, using
restricted to summer crops such as sorghum.                                     Insecticide resistance in Australian GPA has       insecticides selectively. Avoid prophylactic
Some exotic parasitoids such as D. rapae                                        been present since the early 1990s.                treatment because of the problems they
probably arrived in Australia with introduced                                                                                      cause. Seed treatments can also greatly
                                                                                   A study of GPA from most of Australia’s
aphids while others were introduced                                                                                                reduce non-target effects.
                                                                                grain growing areas revealed that almost
deliberately as biological control agents.
Many parasitoids were introduced in the                                         every aphid collected had organophosphate             Research is needed into more strategic
1980s when the pea aphid, blue-green aphid                                      resistance caused by a mechanism that also         methods of applying insecticides. One
and spotted alfalfa aphid were accidentally                                     provides some cross-resistance to pyrethroid       promising approach is using border
introduced. Predators, such as ladybirds                                        insecticides. This mechanism is unusual            treatments to kill immigrant aphids but the
and hoverfly and lacewing larvae are also                                       because it can be ‘switched on’ in response        greatest benefit could come from improved
effective against aphids.                                                       to pesticide exposure. As a result, aphid          strategies for monitoring and conserving
                                                                                populations can quickly adapt to survive           natural enemies. It is difficult for growers to
   Check crops for natural enemies before                                       insecticides, particularly organophosphates        decide whether biological agents will
using insecticides. Beneficials are generally                                   such as dimethoate. This mechanism is so           suppress aphids to the point that insecticides
more sensitive to insecticides than aphids                                      widespread that organophosphates are               are not necessary. A combination of these
so more are killed and this could result in an                                  unlikely to be a useful control tool in the        approaches should provide Australian grain
increase in aphids. If insecticides are                                         future.                                            growers with effective options to manage
needed, use ‘softer’ options. These include                                                                                        aphids into the future.
seed treatments, which only affect the sap                                         Knockdown resistance (kdr) to pyrethroids
suckers and pirimicarb, which is the ‘softest’                                  is also common in GPA in Australian grain
                                                                                crops. In northern Victoria and southern           AckNowledgemeNt this overview of
of the registered contact insecticides.                                                                                            aphids in Australian pastures and grain crops
                                                                                Queensland, kdr was particularly frequent,
   In areas of Australia with consistent                                        perhaps as a result of recent exposure to          is a summary of a paper by owain edwards,
rainfall, aphids and their predators are                                        pyrethroids used in vegetables in these            Bernie Franzmann, deborah thackray and
present all year. In Mediterranean climates,                                                                                       Svetlana micic * on Insecticide resistance and
                                                                                areas. Because of this, it appears that any
with wet winters and long, dry summers,                                                                                            implications for future aphid management
                                                                                use of pyrethroids would quickly lead to
aphid and natural enemy populations                                                                                                in Australian grains and pastures: a review
                                                                                resistance. Unfortunately, prophylactic            which appeared in the Australian Journal of
crash during summer. When the weather                                           pyrethroid sprays targeting cereal aphids in
cools, aphids reproduce faster than their                                                                                          experimental Agriculture 48 (12): 1523–1530.
                                                                                cereal crops will probably induce resistance
natural enemies and so increase their                                           in GPA feeding on broad-leafed weeds in            owain edwards is from cSIRo entomology,
numbers. With the arrival of warmer                                             these crops. This could lead to control            Bernie Franzmann from the department of
spring temperatures, natural enemies can                                        problems in adjacent canola or lupin crops.        Primary Industries and Fisheries Queensland,
catch up and become effective. However,                                                                                            deborah thackray from the University of
this is often too late to prevent significant                                      Prophylactic spraying can also select for       western Australia and Svetlana micic from the
crop damage.                                                                    resistance in non-target organisms. High           department of Agriculture and Food western
                                                                                level resistance to synthetic pyrethroids in       Australia.
Predictive modelling                                                            the redlegged earth mite (Halotydeus
  In the Mediterranean-style climate of the                                     destructor) in WA almost certainly resulted
WA wheatbelt, aphids survive the summer                                         from the use of synthetic pyrethroids for           CSIro research
in low numbers on perennial grasses,                                            prophylactic aphid control, since pyrethroids
                                                                                                                                    coNtAct owain edwards
summer weeds or volunteer crops. The                                            have not been registered for use against it.        cSIRo entomology
timing of aphid flights into crops during the                                      The mechanism conferring resistance to               (08) 9333 6401
winter growing season correlates with late                                      carbamates such as pirimicarb has not been   
summer and early autumn rainfall.                                               found in Australian GPA so they should

                                                                                                                         Farming Ahead December 2009 No. 215   51

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