Start low – It’s a numbers game

If you have pest populations in weeds and dirty fallows in spring, you run the risk of early infestation of crops, and potentially larger crop losses.

 Research has shown that the risk of population outbreaks in SLW is primarily driven by two factors; the size of the initial population in spring and the summer temperatures.  Higher starting numbers give the whitefly the edge over their natural enemies and reduce the number of generation it takes to reach outbreak levels. 

 If you start with more pests in the spring, even with careful management and favourable climate (for population suppression) you are going to reach threshold quicker.  For SLW this could be the difference between no control (Zone 1), suppression control (zone 2) or an IGR (Zone 3). 

What risks do the pests pose?

Cotton aphid

There are high aphid populations on volunteer cotton in some cotton growing areas.  In last season’s disease survey, bunchy top was commonly observed on volunteer cotton plants surviving over from the previous season.  The earlier CBT is transmitted to cotton, the greater the potential yield loss. Combined with industry concerns about aphid resistance to neonictinoids, the removal of overwintering hosts where aphids are present should be seen as a priority. 

See the earlier blog for another discussion of the importance of crop hygiene in minimising the risk of bunchy top (http://thebeatsheet.com.au/cotton/farm-hygiene-i…est-management)

Mealybug

In addition to Burdekin and Central Highlands Solenopsis Mealybug have now been confirmed in cotton growing areas of Dawson, Balonne, and Darling Downs. 

In February 2010, a survey was conducted in the Emerald region to determine whether there were any specific conditions associated with the severity of the mealybug outbreak in this region. Results of the survey showed that where mealybug infestations were severe, weeds and ratoon cotton had been present in fallows.  The higher the incidence of weeds and ratoon cotton in-crop the greater the severity of mealybug infestation.

Weedy field perimeters and poor channel hygiene was also associated with mealybug infestations.

 Will mealybug be a problem next season?

In June and July, entomologists from DEEDI conducted repeat surveys of five fields, in the Emerald region. These fields had a history of severe mealybug infestation last season. Mealybug were still present in both surveys despite crop destruction after harvest.  Mealybug were found on volunteer and ratoon cotton. Fields with a high incidence of volunteers consistently had higher mealybug numbers.  Other problem weeds noted as stand outs – in terms of being abundant across fields and having a relatively high frequency of mealybug incidence include pigweed, bladder ketmia and bellvine. Mealybug were found on the plants and also as far as 5cm below the surface – perhaps overwintering here.

Establishing a host free period for mealybug, to minimise carryover, is going to be a challenge this season.  Research by DEEDI entomologists has demonstrated that mealybugs can survive up to 60 days without food, and during that period can continue to produce offspring. 

There are a lot of things we don’t know about mealybug or the potential for another outbreak next season. However, the clear association between in-crop weediness and volunteers and mealybug hotspots makes it clear that farm hygiene should contribute to minimizing the likelihood of a high starting population. 

Managing ratoons and volunteers

Managing volunteers/ratoon cotton is always challenging.  In field, cultivation and herbicides can be effective in controlling volunteer cotton, however the following points should be considered:

• The effectiveness of registered herbicides is generally limited to volunteers no more than 4-6 leaf.  The size of the volunteers needs to be assessed before a herbicide selection is made.
• Larger plants will be more difficult to control in a single pass.
• Using the recommended water volumes for application is imperative for effective control.
• Other weeds within the field should be taken into consideration when making a herbicide selection.
• It is important to read all labels to confirm the correct application timings and rates.  Label directions must be followed.

Control of volunteers growing outside fields (along channels, roads and fences) is even more challenging as herbicide control is not always possible. 

Manual chipping is sometimes the only effective option, particularly where volunteers are well established.  While this is an intensive strategy, it is a good investment when taking into account the potential costs and losses caused by pests such as SLW, aphids and now mealybugs.

Article by Susan Maas, Melina Miles and Kate Charleston

SPECIES COMPOSITION

There has been no Q biotype detected by DEEDI entomology staff since surveillance commenced in April 2009. While Q biotype may be present in Australia at very low densities, it certainly does not pose a current management risk to growers.

 Silverleaf whitefly (SLW) was recorded at threshold levels from the Gwydir and Lower Namoi valleys for the second consecutive season. Threshold levels of whitefly were also recorded in cotton fields at Narromine in the Macquarie valley. This suggests that SLW is getting a ‘foot hold’ in these more southern areas and it can be expected that SLW will now be an annual cotton pest in these regions.

Greenhouse whitefly (GHW) was also recorded in high numbers at some locations on the Darling Downs this season. GHW is not a pest of cotton and does not require control but its resemblance to SLW highlights the importance of correct identification.

 Please see previous blogs for more information on species identification http://thebeatsheet.com.au/

PARASTISM ASSESSMENTS

Parasitism of SLW ranged between two percent and ninety percent across the different cotton growing regions. There is a strong trend for increasing parasitism levels as the season progresses in the absence of broad spectum insecticides. This strongly supports IPM practises for avoiding early season broad spectrum insecticides.

Parasitism levels have noticeably built up in the Gwydir area compared to last season when only low levels of parasitism were recorded. This is not unusual as there is often a lag between a pest arriving in a new locality and parasitoid numbers building up. This season fifty percent parasitism was recorded in one field in the Gwydir valley. This indicates that whitefly parasitoids are present at levels that can have considerable impact on the whitefly population.

Healthy whitefly nymph; nymph parasitised with Eretmocerus sp. (note asymmetrical position of yellow organs); nymph parasitised with Encarsia sp (note dark body of developing wasp).

For anyone who would like to learn more about how to check for parasitism or if there are any groups that would like to organise a day to look at parasitism, please contact Zara Ludgate at Toowoomba DEEDI Entomology.

RESISTANCE MONITORING RESULTS FOR COTTON IN 2009-2010

For Admiral® (pyriproxyfen), the resistance frequencies from cotton fields were low with no changes in resistance frequency from previous years. This means that we have no concerns about SLW resistance to Admiral in cotton at this stage however resistance is present in non-cotton crops. Cotton growers should not become complacent about managing whitefly resistance.

Studies from non-cotton fields indicate resistance to Admiral has developed in QLD and NSW. It is possible that this is a result of multiple applications of Admiral per season which led to selection of resistant whitefly. The cotton IRMS strategy of maximum of 1 spray per season of Admiral® must be followed.

Results for Pegasus® (Diafenthiron) show that the resistance frequencies from cotton were low with no changes in resistance frequencies from past seasons. There are currently no concerns about resistance to Pegasus®.

Bioassay set up to test the resistance status of whitefly to Pegasus®

Feedback

DEEDI entomology staff would like to hear from you about areas of whitefly management that require more research or extension. We encourage users to leave a comment on this page. This will help us to better direct future research and extension activities.

Article by Zara Ludgate

Be on the lookout for caterpillar pests and use the guide below to help with identification. Correct identification is important in management and timing of insecticide application.

Common Armyworm Leucania convecta
Common armyworm is generally considered a spring pest but heavy infestations have been recorded in oats in the Jondaryan area. At this time of year common armyworm can defoliate seedling crops and pasture when in high numbers. In spring, as winter cereals are maturing, infestations of armyworm arise from large migrations of moths from inland Australia. Infestations now do not mean that these fields or farms will necessarily get armyworm in spring.

Common armyworm is a hairless, striped caterpillar with three bands immediately behind the caterpillar head. They are nocturnal, hiding under plants during the day and are often observed curled up in a ‘C’ shape. Colour is not a useful identifier for armyworm as it is highly variable depending on diet and population levels, but the three white bands immediately behind the head are always present.

Pupa of the wasp parasitoid Cotesia sp. are present in high numbers and will be an important natural enemy for this pest.

Common armyworm is nocturnal so any insecticide should be applied at dusk/night.  Small (early instar) armyworms can be controlled with Bt which will also preserve natural enemies but it is not effective on large caterpillars. Large caterpillars can be treated with a range of chemicals including chlorpyrifos. More chemical options can be found on http://services.apvma.gov.au/PubcrisWebClient/welcome.do

Be careful if treating infested pastures as there are long withholding periods for cattle grazed on this pasture with reference to export slaughter intervals and export grazing intervals. These intervals are not always marked on insecticide labels.

The export slaughter interval for chlorpyrifos is 42 days and the export grazing interval is 56 days. For  information about slaugher and grazing intervals relating to other chemicals, please contact us via the blog. 

Sorghum Head Caterpillar Cryptoblabes adoceta
Sorghum head caterpillar has been reported in the Chinchilla area in sorghum. Sorghum head caterpillar is a grain feeder with most severe damage occurring at the soft dough stage.

The most obvious identifier of sorghum head caterpillar is webbing in the grain. Caterpillars are similar in appearance to common armyworm but smaller, reaching a maximum size of about 13mm long. Sorghum head caterpillar does not have the three white bands immediately behind the head, and its body is tapered at both head and tail end.

Webbing can make it difficult to get good contact with the larvae when treating infestations with insecticide.

Please refer to the following DEEDI link for more information http://www.dpi.qld.gov.au/26_13758.htm

Castor Oil Looper Achaea janata
Castor oil looper infestations have been reported across the Downs http://thebeatsheet.com.au/general/castor-oil-looper-outbreak/ . Castor oil looper is a pest of broadleaf crops like soybean although most outbreaks in the past few weeks have been in pastures where caterpillars are feeding on broadleaf weeds like pig weed and marshmallow weed.

Castor oil looper is identifiable by a dark band running across its back. It is highly variable in colour and may be striated with various colours including red, pink and black.

Insecticidal control is best targeted at small grubs however outbreaks of castor oil looper are relatively uncommon in crops. They could potentially create a dilemma in pigeon pea refuges as Bt products (e.g. Dipel) can not be used in a Bollgard® cotton refuge.

Article by Zara Ludgate

The castor oil looper is a minor and sporadic pest but larger outbreaks are possible in some seasons. Large flights of castor oil looper moths have been reported over Southern Queensland during March.

Identification of castor oil looper

The mature castor oil looper is purple-brown, has both light and dark forms and has two conspicuous black spots on the back behind the front legs. Loopers are up to 60 mm long and in addition to 3 pairs of true legs, also have four pairs of prolegs towards the rear of the body. Their body tapers noticeably towards the head. Loopers, as their name indicates, move with a distinctive looping action.

The adult moths are a mottled dark brown with darker lines. The hindwings have a white band across the wing and three evenly spaced white blotches along the outer margin of the wing. The wings are usually folded roof-like at rest. Moths are 25 mm long and 55 mm across the out-stretched wings.

 Lifecycle

Little is currently known of the lifecycle of castor oil loopers. The egg stage is estimated to last about 5 days, with the caterpillar (looper) stage taking about four weeks and the pupal stage lasting about 10 days. Unlike Helicoverpa which pupate in the soil, looper larvae usually pupate on the plant under leaves in a thin silken cocoon.  

Damage

Loopers are mainly leaf feeders but can occasionally feed on seed. Larger caterpillars can consume whole leaves and large infestations can cause severe defoliation.

Management

Loopers are easily controlled with biopesticides such as Bt (e.g. Dipel) when they are small.  Regular inspection, at all crop stages, is necessary to detect infestations early so as to enable control measures to be applied before serious damage is done.

Loopers are attacked by numerous predators and parasites.  Many of these also attack Helicoverpa (e.g. predatory bugs, tachinid flies, braconid wasps and ichneumonid wasps).  Loopers are frequently parasitised by small wasps (Apantales sp.) with scores of parasite larva developing per looper host.  The use of Bt for looper control will help preserve beneficial insects and also reduce the risk of subsequent whitefly and mite attack. 

Outbreaks of looper viruses are frequently observed in crops with high looper populations.  However, larvae are usually not killed by virus until they are a medium-large stage (instars 4-5).  Looper virus is not the same as Helicoverpa nuclear polyhedrosis virus (NPV). Application of NPV (e.g. VivusMax)  will not control loopers.

Other caterpillar pests

We have received a few reports of caterpillar infestations in pastures. Without specimens or good quality photos the entomology team is not able to identify these pests.  If you have any concerns about pests in crops or pastures please forward photos or samples of the pests to Zara Ludgate at DEEDI, PO Box 102, Toowoomba 4350 Qld or send images to Zara.Ludgate@deedi.qld.gov.au

Article by Kate Charleston

Parasitism levels appear to be a little down from last year, but still good levels have been recovered. Parasitism so far has ranged from 50-90% in Emerald, 35% in St George, 20% on the Downs and 10% in lower Namoi.

Whitefly parasitoids are small wasps. There are two parasitoids that are commonly encountered in cotton, Eretmocerus and Encarsia. Eretmocerus hayati was released for biological control of whitefly in cotton and horticulture and is the most effective and abundant parasitoid in the cotton system.

The parasitoid wasps attack early instar whitefly nymphs. The female wasp is capable of laying hundreds of eggs which she deposits individually underneath the nymphs. When the parasitoid larvae hatches, it tunnels into the whitefly and eats it from the inside–out.

Identification of Parasitised Whitefly

Unparasitised whitefly nymph

• Early juvenile stages will be clear with two red eyes and two bright yellow organs (mycetomes) at the tail end (fig. 1).

• As the nymph reaches late 4th instar it will change from clear to a bright yellow with white spots appearing as the wings form (fig. 2).

• The exuviae (or spent pupal case) will be a transparent white (fig. 1).

Figure 1. (left) Early 4th instar whitefly nymph and two exuviae (spent pupal cases).
Figure 2. (right) Late 4th instar whitefly nymph.

Parastised whitefly nymph

• Nymphs are dull or dirty yellow/brown – Eretmocerus parastioid (fig. 3)
or
• Nymphs are dark brown or black – Encarsia parastioid (fig. 4)

• Sometimes the dark semi-circle of the developing parastoid larvae is visible (fig.5).

• Myecetomes appear non-symetrical or irregular.

• Exuviae is ‘dirty’ in appearance.

• Exuviae has a round exit hole where the wasp parasitoid has chewed its way free of the dead whitefly.

Figure 3. (left) Whitefly parasitised with Eretmocerus sp.
Figure 4. (centre) Whitefly parastised with Encarsia sp.
Figure 5. (right) Larvae of developing parasitoid visible inside whitefly nymph.

Correct use of a hand lens

A common complaint for why consultants do not check for parasitism or species composition in whitefly is that the whitefly are ‘too small’. Correct use of a hand lens will make it possible to identify species composition and parasitism.

The correct way to use a hand lens is to hold the hand lens right up to your eye, as close as your sunglasses would be (fig. 6). Then, bring the object into focus by moving the object, not the hand lens.

Often, users hold the hand lens away from their face which gives far less magnification making identification of parasitised whitefly impossible.

Figure 6.  Correct use of a hand lens requires users to hold the hand lens very close to their eyes to get the most magnification from the hand lens.

Article and images by Zara Ludgate

The exotic species of mealy bug is commonly known as the Solenopsis mealybug (Phenacoccus solenopsis).

The Solenopsis mealybug is a native of North America – first collected and described in New Mexico in 1897. In 1990, it was reported as a pest of cotton in Texas. From there it moved into Central and South America, and is now known to occur in Ghana, Nigeria, Israel, Pakistan, India, Indonesia, Thailand and China.

The Solenopsis mealybug is a polyphagous pest which means that it feeds and reproduces on a wide range of plants. In Pakistan it has been recorded on 154 plant species including field crops, vegetables, ornamentals, weeds, and trees.

Other key reasons for the fast spread and difficulty to control this pest include:

• The bugs possess a waxy coating that protects them from insecticides and natural mortality factors
• They have a high reproductive rate
• They have the ability to hide in soil cracks and crevices.
• They are spread through natural carriers such as raw cotton seeds, wind, water, rain, birds, humans, farm equipment and animals.

Identification and lifecycle of the Solenopsis mealybug
The female mealybug is wingless with a 3-4 mm long oval shaped body which is covered with white hydrophobic (water repellent) mealy wax. There are dark bare spots on the thorax and abdomen, which appear as dark longitudinal lines.

The adult male is about 1 mm long, with a grey body and a single pair of transparent wings. Two filaments of white wax project from the end of its abdomen. The adult male has no feeding mouthparts and causes no damage.

Mature females lay eggs in waxy pouches called ovisacs. Each ovisac contains between 150- 600 eggs, the majority of which are female. The eggs hatch after three to nine days into nymphs called ‘crawlers’, which are very mobile.

No parthenogenesis (asexual reproduction) is reported in the literature and therefore it is assumed that the species reproduces through sexual reproduction.

The female crawler undergoes four larval instars before turning into an adult (there is no pupal stage). The total life span of a female mealybug is 30–48 days, which includes 21 days as adult.

Male crawlers undergo three larval instars over 13–17 days before spinning a cottony cocoon in which it passes a pupal stage for 6-8 days. A male adult lives for only 3-5 days. Mealybugs can have 12–15 generations in a year.

The species (as eggs in ovisacs or in other life stages) can survive cold conditions, both on the host plant and in the soil. In warm climates, mealybugs reproduce all year round.

Mode of damage
Mealybugs have sucking mouth parts at all stages of their life cycle, which they use to extract large amounts of plant sap. During the feeding process a significant amount of sap oozes out as honeydew which forms a sticky deposit on the leaves and stem. Honeydew promotes the growth of sooty mould fungi which inhibit photosynthesis.

Symptoms of plants infested during the vegetative phase include:
• Distorted and bushy shoots
• Crinkled and/or twisted and bunchy leaves
• Stunted plants that dry completely in severe cases.

Symptoms of late season infestations during the reproductive crop stage include:
• Fewer, smaller and deformed bolls
• Reduced plant vigour
• Early crop senescence.

Mealybugs can also stain cotton lint and reduce quality hence this pest has the ability to cause damage to both the quality and quantity of cotton.

Methods of spread
Mealybugs are generally disseminated as crawlers. The crawlers can move from an infected to a healthy plant as well as infected field to adjacent healthy fields. While this type of movement is localised, bugs have the means to travel long distances and infest new areas. The waxy coating on the mealybug crawlers facilitates passive transport of the insect by sticking onto equipment, other insects (e.g. bees), birds, animals or people. Small crawlers are also readily transported by wind and rain or in water in irrigation channels. Long-distance movement through the transport of infested plants is also possible.

Ants are also a significant factor in the spread of mealybugs. In return for providing the honey dew that the ants feed on, they spread the mealybugs, protect them from natural enemies and keep their colonies clean.

Clusters of tiny crawlers – ready for dispersal  (image by Zara Ludgate)

Management of mealybugs
There are no insecticides registered for the control of mealybugs in cotton. However there are a number of management options that can reduce infestations and the overall impact of this pest.

• Mealybugs multiply on different hosts and may initially breed on weeds before migrating to cotton crops
• Weeds in and around fields should be removed.
• Do not throw uprooted weeds into water channels.
• The removal of affected plants at the early stage of infestation may reduce mealybug numbers in the rest of the crop.
• Avoid physical contact with infested plants as mealybugs easily adhere to clothing and implements.
• Practice good farm hygiene and clean all equipment that has been in affected fields.
• Consider the insecticides that are used in control of other insect pests to conserve natural enemies of mealybugs.

Trials in Pakistan and India show some promising results in terms of suitable insecticides. Insecticide trial work to control mealybug will commence in Queensland in the near future.

The main priority at this point is to try and limit further spread of the mealybug from and within the Emerald and Burdekin areas. With harvesting of crops to commence soon in the Emerald area, the Queensland Government is working closely with the cotton industry to develop wash-down and decontamination protocols for harvesting machinery. More information about these protocols will be provided in a blog next week.

Biological control
A survey in Pakistan recorded as many as nine species of predators feeding on Solenopsis mealybugs. These predators were present throughout the cotton season but their effectiveness was not sufficient to keep mealybugs under control.

In Australia, ladybird beetles and their larvae, and green lacewing larvae were abundant in mealybug-infested fields. One of the most conspicuous predators is the mealybug ladybird (Cryptolaemus montrouzieri), whose larvae look like an oversize mealybug. To date no Solenopsis mealybug parasitoids have been recorded in Australia

Meanwhile in India, a small wasp (Aenasius bambawalei) is reported to parasitise about 60 percent of the mealybug population under field conditions.

Article by Kate Charleston and David Murray.  Images by Zara Ludgate and Greg Kauter.

Whitefly samples identified from the MacIntyre and Lower Namoi areas have been 100% B biotype (SLW). Parasitism levels recorded from this area are low at this stage (between 0-10% parasitism).

Figure 1.  B biotype, Bemisia tabaci   (Photo: R. Lloyd)

Whitefly numbers are building quickly with infestation levels rising rapidly from well below threshold to threshold levels. Rainfall in the next week may assist in reducing population build up by dislodging eggs and nymphs from leaves and slowing development during cooler weather.

Whitefly infestations have put additional pressure on supply of registered insecticides. No new supplies of whitefly insecticides will be available until the week of 8 February 2010. Admiral® will be available on the 8 February and Pegasus® will be available later in February.

So what is the management strategy given the chemical shortage?

For early planted cotton, there will be no Pegasus® available in time to benefit these crops. Therefore, whether infestation levels are in the ‘suppression’ zone in the threshold matrix or the ‘IGR’ zone (see fig. 2) becomes irrelevant because the only option will be an Admiral® application.

Do not use Admiral® too early. One well-timed application of Admiral® will take the crop through to harvest. Going too early risks having to come in with another spray. Only one application of Admiral® is allowed per season under the IRMS.

For growers who already have open cotton and are above threshold, the options are:

1)      Apply Admiral® as soon as it becomes available

2)      Apply a knockdown now and follow up with Admiral® as soon as it becomes available

Provided whitefly infestation levels are not too high, delaying treatment up to 10% open cotton should not result in damage from whitefly. Given the weather forecast for rain and cooler weather in the next few days, consider waiting for Admiral® to become available. This will put you into the 3C zone, ‘IGR + knockdown’, eg Admiral® + Talstar® + PBO.

Figure. 2. Threshold Matrix for whitefly management in cotton

In a situation where hot weather is predicted and where cotton fields are looking visibly ‘sticky’, then a knockdown spray followed by Admiral® is suggested. However, a knockdown application will only give around 3 days of relief and will destroy any natural enemies that are working for you.

For later planted cotton, Pegasus® may be available for use against moderate infestations. However, as early cotton is defoliated, late cotton may be subject to mass migration of whitefly from early cotton fields and so Admiral® may again be the best option for control.

Whitefly levels in other regions

Emerald - whitefly were generally at moderate levels and were managed with a Pegasus® application that also targeted mite and aphid infestations. Parasitism levels were high ranging between 45-90% for this area.

St George – whitefly are building up with parasitism levels generally around the 35% level at this stage.

Darling Downs – there are reports of B biotype building up in the west as well as high populations of Greenhouse whitefly in the Brookstead area. Greenhouse whitefly do not cause sticky cotton so always check the identity of the whitefly as part of your management strategy.

Further References

The whitefly threshold matrix and all the information for sampling can be found on the cotton CRC website under ‘Managing silverleaf whitefly’:

http://www.cottoncrc.org.au/content/Industry/Publications/PestsandBeneficials/Whitefly.aspx

Distinguishing ‘brown’ stink bugs from each other.

 Adults brown stink bugs (Dictyotus caenosus) are shield shaped, matt brown, and smaller than green vegetable bug (GVB), about 7 – 8 mm long (Plate 1). They may be confused with glossy shield bug (Cermatulus nasalis) which is a slightly larger predatory bug, and with rice spotting bug (Eysarcoris distinctus) which is smaller (5 – 6 mm) and has two pale elongated marks on the top (Plate 1). 

  Brown stink bugs lay pale cream eggs in twin row rafts (Plate 2).

Eggs are similar in shape to GVB eggs, but egg rafts of GVB are seldom in two rows. Newly hatched nymphs are orange with dark markings and a black head (Plate 3).  These are indistinguishable from other shield bug nymphs.  As they grow they change colour to have a pale brown abdomen and transverse dark and pale markings at the centre of the abdomen (Plate 3, fourth instar nymphs pictured).

 The latest published thresholds for helicoverpa in vegetative soybeans (Rogers and Brier, 2010) show that while soybeans can tolerate damage inflicted by moderate helicoverpa populations up to 7 larvae/m2 without yield loss, severe yield loss is inflicted by populations >7 larvae/m2 at a rate 4-5 times greater than during the pod-fill stage. The conundrum therefore is that while vegetative soybeans are far more tolerant of low to moderate helicoverpa populations (< 7/m2) than podding soybeans, they are markedly less tolerant of populations > 7/m2 than are pod-filling soybeans (see Figure 1).

The reason for this severe yield loss is that unlike most leaf-feeders such as loopers, helicoverpa also attack the plant’s auxiliary buds and vegetative terminals, completely destroying these structures.

Damage to auxiliary buds potentially reduces yield as these structures are the precursors to the plant’s flowers and (subsequently) pods. Damage to vegetative terminals is potentially bad for yield. The reason for this is that while plants may compensate by setting additional side branches, pods formed on these are often closer to the ground and are more difficult to harvest.

Where helicoverpa populations are not excessive, damage is spread over a number of plants and the crop is able to recover and compensate without yield loss.

However, once populations exceed critical level in vegetative crops (about 7/m2), damage per plant reaches a critical level beyond which the subsequent plant growth is severely affected.

 To reduce the risk of severe early helicoverpa damage, the sampling guidelines have been revised to “sample crops twice weekly from the seeding stage onwards”. This more-intense sampling regime maximises the chance of helicoverpa larvae being detected while they are still small (ideally <7 mm) and thus able to be controlled with a Helicoverpa virus biopesticide, eg VivusMax or Gemstar.

Remember that the use of biopesticides in pre-flowering soybeans is a key in the “Go Soft Early” strategy to minimise the risk of silverleaf whitefly (SLW) attack in soybeans. The “Go Soft Early” strategy also promotes the build up of beneficial insects attacking other pests such as helicoverpa and loopers.

Vegetative soybean crops can tolerate populations up to 7 larvae/m2 and it is not necessary to kill every helicoverpa larva in a crop. Assuming only 70% control, even populations as high as 20 larvae/m2 can be reduced to below the critical 7 larvae/m2 level in vegetative crops.

Very small plants have fewer nodes and hence fewer auxiliary buds and a given helicoverpa population will damage a greater proportion of auxiliary buds per plant. Severely damaged plants will also be more susceptible to subsequent helicoverpa attack.

Note that small larvae often feed in leaf terminals, so inspect these and look for the tell tale damage symptoms, small holes in the leaflets and frass. For the latest guidelines about applying helicoverpa virus, refer to the previous blog of 8th January 2010. In seedling and early vegetative crops, pesticide costs can be halved by banding the spray over the crop, and blocking off nozzles over the bare inter-row.

References:
Rogers D.J. and Brier H.B. (2010). Pest-damage relationships for Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on vegetative soybean. Crop Protection 29(1): 39-46.
Rogers D.J. and Brier H.B. (2010). Pest-damage relationships for Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on soybean (Glycine max) and dry bean (Phaseolus vulgaris) during podfill. Crop Protection 29 (1): 47-57.

Article by Hugh Brier. Images by Hugh Brier and Joe Wessels

Control of corn earworm, Helicoverpa armigera, in maize has generally not been practised because of the high cost associated with repeated insecticide application required during silking. In most years it is a case of forsaking the top of every cob to larval damage

However, in some years, very high pest activity results in more severe cob damage, with larvae often tunnelling into the sides of cobs. In such cases grain samples may contain fungus-affected grains and mycotoxins, causing a downgrade in the quality of harvested grain.

What can you do to prevent these losses?

Over recent years Helicoverpa nucleopolyhedrovirus (NPV) has demonstrated its versatility for corn earworm management. One of the major developments has been the effective application of NPV through overhead irrigation, sometimes referred to as ‘chemigation’.

ViVUS Max is currently the only insecticide registered in Australia for application in overhead irrigation water in a wide range of crops.

By adding NPV to irrigation water, growers can artificially inoculate their crop with NPV and achieve a high level of control of helicoverpa larvae.

The main guidelines for NPV use still apply. These include:

• Good coverage is essential as the product needs to be ingested
• Use in the temperature range 25-35°C when larvae are actively feeding
• NPV is more effective against smaller larvae
• Preferably target larvae less than 7 mm in length, but under ideal conditions larvae up to 13 mm in length will be controlled
• Larvae can take up to 8 days to die
• Spray water pH should be neutral (pH 7.0)

There are additional key points to the successful use of NPV via overhead irrigation.

• Application in overhead irrigation water provides the maximum coverage achievable
• Introduce NPV to the irrigation water at the appropriate rate using chemigation equipment
• If the NPV is diluted in water prior to injection into the irrigation water, ensure that the dilution water is clean and not silty with a pH of 7 or less
• Ensure constant agitation in the premix tank
• Ensure any diluted NPV is used within 10 hours of mixing
• Apply in no more than 10 mm of irrigation water

• For one-pass mobile irrigators such as centre pivots, laterals and travellers (guns), continuously introduce the required amount of ViVUS Max into the irrigation water over the course of irrigation.

What rates to use? How often?

For ViVUS Max in maize, the registered rate for normal foliar application is 150 mL/ha. When applied in overhead irrigation water, reduced rate repeat applications have been used successfully. An effective prophylactic strategy would be to make the first application at full tassel emergence (50 to 150 mL/ha depending on larval numbers and size) and then low rate (50 to 75 mL/ha) applications every 5 days or so until late blister/early milk stage.  A total of 4 applications would use about 250 mL/ha (perhaps more under high pressure). This product cost is around $30/ha and will keep things very clean.  

What are the economics of losses to larvae?

A back of the matchbox calculation can be used to give some insight to the damage caused by larvae. An average plant population is 70,000 plants/ha with one cob per plant and one larva per cob. Assume one larva consumes 15 kernels in its lifetime (Note: this value has no validated scientific basis). With an average kernel weight of 4,000 kernels/kg, one larva consumes about 3.8 g. If maize is valued at $300/t, this loss amounts to 262 kg/ha or $79/ha. Based on these rough figures, and assuming a high level of control, there is likely to be an economic benefit from using NPV. Larvae damaging early silks can also reduce pollination, which can result in even greater yield reductions.

Other benefits

As NPV is safe to natural enemies, parasites and predators remain in the crop and keeps working in your favour. This is particularly relevant for the egg parasite Trichogramma which is sometimes very abundant in maize crops. Untreated maize crops can also generate large numbers of helicoverpa moths, so control of larvae in maize can reduce subsequent pressure in nearby crops.

Article by David Murray and Anthony Hawes.  Image of lateral move irrigator by Graham Harris.