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

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

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

It is thought that increased host plant availability from a wet winter/spring, warm conditions and a decline in natural enemies due to the use of broad spectrum insecticides has contributed to the high SLW numbers in this more marginal area of occurrence.

DPI&F entomologists will be visiting Narrabri on Thursday, 11 February 2009 and Moree on Friday 13 February 2009 where they will meet with growers and consultants and speak about SLW and the management options that are available.

Cotton fields around St George are at or reaching high densities of SLW. Reports indicate Admiral® has been applied on many fields to suppress SLW populations. Parasitism levels of 50% and 70% were recorded from two fields in the St George area. This should help to keep SLW in check later in the season even if they start to re-infest crops post Admiral® spray.

SLW numbers in Biloela and Theodore are reportedly dropping off. This may be in part due to parasitism levels. Recent testing for insecticide resistance in populations of SLW from Biloela and Theodore show no alarming results for Admiral®. These results were expected due to the minimal use of Admiral® in central Queensland this season.

GHW are in moderate densities in the Norwin region on the Darling Downs. While GHW will produce honeydew it does not normally cause the same problems as SLW which has a wider host range, higher reproductive rate, develops resistance to insecticides rapidly, and is adapted to high temperatures. Where populations are a mix of SLW and GHW, consider treating as if all are SLW.

The report on managing silverleaf whitefly by Richard Sequeira and Tracey Farrell can be accessed through the cotton CRC using the link below: http://www.cottoncrc.org.au/files/5743fee2-f978-4a79-a9d1-9b1800e899cf/Whitefly_Management[1].pdf.
This document outlines sampling, thresholds and management options for SLW. Remember only one spray of Admiral® is allowed per season.

Article by Zara Ludgate

Resistance testing for the 2007-08 season indicated no alarming results for Admiral® (pyriproxyfen) or Pegasus® (diafenthiuron) in cotton areas. The IRMS guidelines for Admiral® require that only 1 spray may be applied per season. Apart from the cost involved, more then one Admiral® spray has been shown to increase the development of resistance.

As part of the management strategy for whitefly it is important to know what species or biotypes are present as these will significantly impact on the management decision that is required. Refer to past beatsheet articles to read more on identifying the different species and biotypes of whitefly.

We are seeing mixed populations of whitefly across Queensland. On the Darling Downs, greenhouse whitefly (GHW) has made up >90% of the population in the Norwin area. A sample from Theodore showed the whitefly population was made up of 70% SLW and 30% GHW.

In Emerald, a limited number of Pegasus® sprays have been applied for SLW. Pegasus® is best used for early season suppression of SLW at low insect densities or as a late season knock down to prevent honey dew contamination of open bolls.

Very few (if any) Admiral® sprays have been applied so far this season. Admiral® may be applied after 1450 day degrees if SLW numbers reach high densities. Remember that only one Admiral® spray may be applied per season to limit the potential for resistance to develop.

An excellent publication has been produced through the Cotton CRC by Richard Sequeira and Tracey Farrell That outlines thresholds and sampling methods for SLW in cotton in more detail (www.cottoncrc.org.au/files/5743fee2-f978-4a79-a9d1-9b1800e899cf/Whitefly_Management.pdf). This should be referred to when making management decisions for this pest.

In Emerald, there were high levels of natural control of whitefly from the parasitic wasps, Eretmocerus hayati and Encarsia formosa. Parasitism levels of 40% and 75% were recorded in two fields in the Emerald Irrigation Area. In a field at Biloela, parasites were so abundant that the tiny wasps were clearly visible walking around on leaves.

It is likely that the high parasitism levels recorded in Emerald and Biloela are due, in part, to very limited mirid sprays earlier in the season. This has avoided flaring SLW and allowed beneficials to multiply and offer a free service to growers and consultants in controlling whitefly.

Whitefly numbers are reportedly building up at St George/Dirranbandi. DPI&F entomologists will be visiting the area next week to collect samples for resistance monitoring and check parasitism levels.

While whitefly are definitely starting to make their presence felt in cotton fields across Queensland, their presence does not necessarily warrant action. Monitor fields often as whitefly can build up exponentially, identify what species/biotypes are present, use the available thresholds and avoid flaring whitefly by minimising the use of disruptive insecticides and maintaining beneficials in the system.

Article by Zara Ludgate

This posting has been compiled from information and discussion with Richard Sequeira (Principal Entomologist, Emerald) and Paul Grundy (Senior Entomologist, Ayr) who have considerable experience in managing SLW. The aim is to provide information that may help in making decisions about the need for and timing of control, and the appropriate control option.

It appears that there is a larger than predicted population of SLW in cotton this season, especially as the season is average in terms of temperature. Why is this so particularly on the Downs where it is predicted that outbreaks would only occur in hotter than average seasons?
The answer to this is that outbreaks of SLW are driven not only by temperature, but by two factors, the size of the initial population in spring and the summer temperatures. In the 2007-08 season we have experienced temperatures that are close to the long term average, but we would have started the season with a large carryover from the outbreak in 2006-07. The rainfall and weed growth in winter and spring would have provided hosts for the SLW to carryover from last season to this one.

Over the last week or so, SLW populations seem to be increasing with the percentage infestation rising rapidly. In Central Queensland (CQ) it is usual to see a rapid increase in the percentage infestation at the 5th node once the crops cut out. It looks as though the population is increasing in size, but what is actually happening is movement of the population up towards the top of the plant as it stops putting on nodes. In CQ the quality of the leaves lower on the plants also starts to decline around this time and becomes unsuitable for SLW, forcing them higher in the canopy. Paul suggested that there are visual clues to whether the population is increasing in number. Observing the amount of honeydew on the lower leaves can be instructive. Over a week, an increase from a light speckling to larger droplets is indicative of a population increase.

As a SLW population approaches threshold natural enemies will not contain it. It is interesting to note that there has been very little parasitism recorded from samples taken in St George, and on the Downs this season. Parasitism levels in CQ are also low. Richard’s interpretation of this is that wet weather and high humidity has a negative impact on the parasitoids.

As SLW populations approach, or exceed, the treatment threshold, questions arise around the issues of whether to control, when and what product to use. The SLW thresholds are available in the Cotton Pest Management Guide, and on the web at
http://www.dpi.nsw.gov.au/agriculture/field/fibres/cotton/cotton-pest-management-guide Section 2, pages 11-13

To use the threshold recommendations it is necessary to calculate the day degree (DD) accumulations for the season, this will enable you to match the crop stage to the appropriate threshold and management options. The current DD data for a range of cotton-growing regions is presented in the table below.

Which whiteflies are out there?
Results of whitefly samples identified from Downs cotton crops this week indicated variable incidence of SLW. In all cases except St George, identifications were based on immature (scale) stages used to differentiate Bemisia tabaci (SLW and Native Bemisia) from Greenhouse Whitefly (GHWF). These results infer that all Bemisia were SLW. Very few parasitoids were found in collections.

Anecdotal evidence supports the above findings; that in some cases the whitefly infestations have been predominately GHWF. Where pyrethroids have been applied to control stink bugs (green vegetable bug and cotton stainers), whitefly numbers have been suppressed. While GHWF is susceptible to pyrethroids, SLW is likely to have some level of resistance to pyrethroids and will survive treatments. The risk is that use of pyrethroids and other broad spectrum insecticides used to control bugs may well flare SLW by disrupting natural enemies.

The Downs is considered a marginal area for problems with SLW because in an average season there is not enough heat and generally too few generations to create a problem. There is a perception that the 2007/08 season has been cooler than normal. Day Degree accumulations indicate that 2007/08 has been marginally cooler than average (see Table 1). Surprisingly, it has not been as cool as many have expected.

Management of SLW
Cotton crops are approaching the stage where decisions must be made on whether to control whitefly present in crops. Firstly it is important to know which whitefly is present. Is it SLW?

Given positive SLW identification, are they at levels that will present a problem later on in the crop cycle? To answer this question, careful monitoring of adult numbers is required.

Monitoring SLW infestations
Percentage infestation is based on a fixed sampling protocol, where a leaf is considered infested if 2 or more adult whiteflies are present. Full details of the sampling protocols and threshold values are available in the Cotton Pest Management Guide or the Cotton CRC website http://www.dpi.nsw.gov.au/agriculture/field/fibres/cotton/cotton-pest-management-guide

Control options
The objective of SLW control is to manage the population early to minimise exposure of open cotton to honeydew contamination.

Management options as defined by crop stage, accumulated Day Degrees and corresponding whitefly thresholds – include CONTROL, KNOCKDOWN and SUPPRESSION.
The CONTROL option involves application of an Insect Growth Regulator (IGR) and is appropriate for moderate to high densities after 1300 DD.
The KNOCKDOWN option involves application of conventional chemistry and may be used on low to moderate densities after 1600 DD to minimise contamination of lint from lower bolls.
Low to moderate densities after peak flowering (1300 DD) may also be treated with conventional chemistry for population SUPPRESSION so as to avoid the need for a later application of IGR.

Use of Insect Growth Regulators (IGRs)

• Where % infested leaves is consistently equal to or above the corresponding threshold value (> 1 adult/leaf after 1300 DD), use IGR from 1500 DD but no later than 1600 DD (first open boll).
• IGR – Pyriproxifen (Admiral®) – is highly effective against SLW, gives excellent control across a broad density range and is the corner stone of effective SLW management in cotton as it is also very selective, allowing survival of predators and parasites.
  
• Ensure only a single application of Admiral® occurs within a season.
• Delaying the IGR application beyond 1600 DD and > 2 adults/leaf could result in yield loss, lower efficacy of the IGR, substantial lint contamination or all of the above.

Use of Conventional Chemistry

• Adult SLW densities of 0.5-1 adult/5th node leaf beyond 1600 DD (first open boll to leaf drop) are sufficient to contaminate lint from earliest open bolls.
• After first open boll (>1600 day degrees) and densities <>
• Single applications of Pegasus® or endosulfan (if window permits) used early (1300 – 1450 DD) on SLW densities <>

What contribution might natural enemies make to SLW control?
Natural enemies make an important contribution to the suppression of SLW when populations are low. Natural enemies include ladybeetles that will feed on scale, the parasitoid wasps, and probably a number of predatory bugs. However, their potential impact is not significant enough to be taken into consideration once the SLW population gets to the point that in-crop control (IGR application) decisions are being made.