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.
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.
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.
Contact Richard Lloyd at DPI&F in Toowoomba by phone on 46881315, or email: [email protected].