Silverleaf whitefly update

There are reports of large infestations of silverleaf whitefly (SLW) from the Narrabri/Moree region. Exponential growth in whitefly numbers coupled with honeydew on leaves indicates that the whitefly are probably SLW and not East Australian native Bemisia or Greenhouse whitefly (GHW).

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

Silverleaf whitefly in cotton – an update

Silverleaf whitefly (SLW) is a serious pest of cotton. It reduces yield and quality of cotton due to feeding damage and excretion of honey dew. It is a difficult pest to manage due to its ability to rapidly increase in numbers and the development of resistance to many insecticides.

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