FARMERS concerned about large populations of silverleaf whitefly (SLW) moving from cotton into emerging grain crops have been urged not to panic.
Queensland Department of Agriculture senior entomologist Dr Melina Miles said extremely high levels of SLW, which infested many cotton crops across NSW during summer, have been observed moving into grain crops around Moree and the Liverpool Plains.
She said the situation was also likely to be similar in the Riverina region.
But she said SLW were a warm-season pest and activity should drop off as temperatures cooled.
The SLW move in large numbers once cotton crops are defoliated, spreading to surrounding vegetation which can include emerging grain crops, Dr Miles said.
While the number of SLW adults landing in seedling canola, wheat, faba beans and cereals can look dramatic, previous experience has shown the impact of these infestations on the growth of the grain crops is negligible.
In 2014 we saw a similar scenario for the first time, which enabled us to investigate and establish the risk level for SLW in winter crops, which we found to be negligible.
SLW adults cause very little direct damage to crops as a result of their feeding. Their pest status in cotton is associated with the production of large quantities of honeydew that contaminates the open cotton, and in horticulture, they have potential to transmit viruses that affect crop growth and productivity.
Neither of these impacts are issues for grain crops.
Dr Miles said the impact of nymphs on plants is potentially significant, but the likelihood of nymphs surviving long in grain crops is considered low.
This season we have seen SLW laying eggs in canola, faba beans and wheat, but as temperatures cool, SLW activity will drop off, she said.
SLW are a warm-season pest and do not persist over winter in crops where temperatures drop below 20°C. Adults live for up to two weeks and we expect that they will die or move from these largely unsuitable hosts in this timeframe.
Seeing SLW in wheat, for example, shows very clearly that they are not deliberately seeking out these crops, but landing in them as they disperse from cotton or broadleaf weeds.
Grasses are known to be poor hosts for SLW.
In addition to many of the grain crops being unsuitable for SLW, SLW nymphs are very susceptible to neonicitinoid seed treatments, so it is unlikely that any nymphs from eggs laid in grain crops will persist in treated crops.
Adult SLM are small 0.8 to 1.2mm long, sap sucking, flying insects. The adults hold their white wings vertically titled, and their bodies are yellow. You will find them congregated on the underside of leaves.
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