Whitefly management options

In the last week, reports from the Downs are suggesting that silverleaf whitefly (SLW) numbers have increased rapidly, and now there are a number of fields that have a population at or exceeding the treatment threshold.

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.

Weighing up the options
In making a decision about SLW control, it is important to consider not only the level of infestation but also the age of the crop (DD accumulation) , how long it has to go until open cotton or leaf drop, and what is going on in neighbouring crops.

The situation on the Downs currently is that there are crops at SLW threshold that do not yet have open cotton. In this situation, Paul suggests that a second check be made on the population a week after it is first recorded to be at threshold, just to make sure the population really is at that level. If it is, it is necessary to weigh up the options for control.

Richard’s sense was that without open cotton, there is no urgency to control the population. Potentially you can wait a week or two until closer to the first open boll before applying Admiral®. Certainly, the application of Admiral® should be closer to 1600 DD than 1500 DD (first open boll is at 1650 DD). The aim of this strategy to decrease the chance that there could be a resurgence of the population post treatment, still with time to contaminate open cotton. However, on the Downs, the expectation is that the temperatures will be cooling through March and into April, and the risk of a second large population is unlikely.

Another approach is to treat populations at threshold now with Admiral®, particularly knowing that it has 3 week residual, and that SLW will not start to breed in the crop until April. With this timeframe, particularly in cooling conditions, it is unlikely that SLW will build up to threshold before the end of the season.

Pegasus® will knock a population of SLW, and control aphids in the crop. However, given the length of season many crops still have to go, the use of Pegasus ® now may require a re-treatment with Admiral ® later. Whilst Pegasus ® is the less expensive option, it is more likely to require re-treatment if used now than the application of an Admiral ®. Pegasus ® is an option for late crops where SLW populations do not reach threshold until there is open cotton and a quick knockdown is needed late in the season. If using Pegasus® remember that it is a contact product and good coverage is essential for good control.

Whitefly Update

Reports of increasing numbers of whitefly are filtering in from across the Downs. In some cases, numbers are sufficient to suggest that control of Silverleaf Whitefly (SLW) may be warranted. Correct identification and regular monitoring of populations is essential to determine if intervention is necessary.

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.

Whitefly in crops this season

There is an increasing number of reports of whitefly in cotton crops on the Downs. Typically, whitefly start appearing in crops in mid to late summer, once populations have built up on weed hosts or other host crops (e.g. sunflower) adjacent to cotton crops.

So far this season we have been able to make collections of whitefly adults and immatures (scale) from two sites in the Jimbour – Macalister area. In both cases 75% of the population was Greenhouse Whitefly, and the remainder Silverleaf Whitefly (SLW) (or Bemisia B-type). The native Bemisia was rare in the samples.

We are keen to make further collections to get a picture of the whitefly populations across the region.

Typically, the Downs is at low risk of a SLW outbreak, except in seasons like 2006/07 when conditions are considerably hotter than average. In a relatively cool season like the current one, the expectation is that an outbreak is unlikely. This expectation is supported by the results so far (although limited) that indicate SLW populations to be low.

St George and Emerald are areas where conditions are suitable for a SLW outbreak in average years.

Whitefly as pests – a brief background
Whilst populations of Greenhouse Whitefly can build up in cotton, they are susceptible to a wide range of pesticides, as is the native Bemisia species. As a result Greenhouse and native Bemisia are often controlled incidentally when the crop is sprayed for other pest species.

Both Greenhouse and Silverleaf Whitefly produce honeydew as they feed. However, the honeydew of SLW persists on the lint through to processing, causing sticky cotton. See the photo (right) of open cotton severely contaminated with honeydew, and the black mould that grows on the honeydew. Sticky cotton is difficult to process, and as a result is highly undesirable in the market. Management of SLW is focused on preventing populations outbreaks and limiting the amount of honeydew, rather than limiting numbers to prevent direct damage to the crop from feeding.

The SLW’s pest status is as a result of it being well adapted to warm conditions, having a high reproductive rate, a wide host range, resistance to a wide range of insecticides, and the ability to rapidly develop resistance with exposure.

SLW has resistance to organophosphates, carbamates, synthetic pyrethroids, imidacloprid, endosulfan, bifenthrin, and amitraz. The level of resistance varies from region to region.

In cotton, infestations of SLW require careful management to prevent killing the parasitoids (wasps) that can contribute to the control of low numbers. Spraying to control sucking pests such as stinkbugs, mirids and aphids requires careful consideration in the presence of SLW. Where possible, avoid flaring SLW by choosing options other than broad spectrum insecticides such as synthetic pyrethroids.

Identifying whitefly species
There are three species of whitefly that can infest cotton in Australia, the Greenhouse Whitefly (Trialeurodes vaporariorum), the native Bemisia tabaci, and the Silverleaf Whitefly (Bemisia tabaci B-biotype).

Knowing which species is present in your crops is important, because only one of the three species, the Silverleaf Whitefly (SLW) has the potential to cause significant losses in cotton.

The adults of the Greenhouse and Bemisia species can be readily distinguised by eye. The Greenhouse Whitefly is about twice the size of the Bemisia species, and has overlapping wings, held flat over the body. The SLW and native Bemisia hold their wings tent like over the body and there is a visible split between the wings. See pictures left (Greenhouse) and right (SLW).

Juvenile stages of the Greenhouse Whitefly (the scale and pupae) can be distinguished easily from those of the Bemisia species with a good (x20) hand lens or microscope. Greenhouse Whitefly scales are white and a fringe of long hairs protruding from them. The Bemisia species pupae are yellowish in colour and are hairless (see pictures left and right above).

Eggs are laid haphazardly on the undersides of leaves, and are brown shortly before the scales hatch from them. When there are large numbers of eggs the undersurface of the leaf can look like sandpaper. See photo (right) of high density egg lay on the underside of a leaf.

The SLW and native Bemisia are indistinguishable from each other, and can only be separated by enzyme or DNA-based tests in the laboratory. This is the technique used to identify two collections made on the Downs this week.

More information on Silverleaf Whitefly and its management can be found on the website of the Cotton Catchment Communities CRC.

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

Monitoring insecticide resistance in SLW populations
The management of SLW is highly dependant on a limited number of insecticides, particularly the insect growth regulator pyriproxifen (Admiral®). Together with the history that SLW has for rapidly developing resistance when exposed repeatedly to insecticides, insecticide resistance is a real threat. For this reason it is important to monitor populations of SLW from different regions to determine the levels of resistance they may have to key insecticide groups, and use this information to provide advice on appropriate resistance management strategies for SLW.

The DPI&F Entomology group in Toowoomba is doing this resistance monitoring for the cotton industry, and will be screening collections of whitefly from the field during the season.

We need your help to identify populations of SLW that can be included in the screening.
Contact Richard Lloyd at DPI&F in Toowoomba by phone on 46881315, or email: richard.lloyd@dpi.qld.gov.au.