Outlined in today’s blog are the results for whitefly parasitism assessments, whitefly species composition and whitefly resistance testing from the 2009-2010 cotton season.
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
While soybeans are more likely to be attacked by helicoverpa from flowering onwards, severe infestations can occur as early as the seedling stage. In recent years, severe early infestations have been reported on the Downs and in the coastal Burnett with the affected crops experiencing significant yield losses.
Control of corn earworm, Helicoverpa armigera, in maize has generally not been practised because of the high cost associated with repeated insecticide application required during silking. In most years it is a case of forsaking the top of every cob to larval damage
Good IPM starts in winter
Start low – It’s a numbers game
If you have pest populations in weeds and dirty fallows in spring, you run the risk of early infestation of crops, and potentially larger crop losses.
Research has shown that the risk of population outbreaks in SLW is primarily driven by two factors; the size of the initial population in spring and the summer temperatures. Higher starting numbers give the whitefly the edge over their natural enemies and reduce the number of generation it takes to reach outbreak levels.
What risks do the pests pose?
Cotton aphid
There are high aphid populations on volunteer cotton in some cotton growing areas. In last season’s disease survey, bunchy top was commonly observed on volunteer cotton plants surviving over from the previous season. The earlier CBT is transmitted to cotton, the greater the potential yield loss. Combined with industry concerns about aphid resistance to neonictinoids, the removal of overwintering hosts where aphids are present should be seen as a priority.
See the earlier blog for another discussion of the importance of crop hygiene in minimising the risk of bunchy top (http://thebeatsheet.com.au/cotton/farm-hygiene-i…est-management)
Mealybug
In addition to Burdekin and Central Highlands Solenopsis Mealybug have now been confirmed in cotton growing areas of Dawson, Balonne, and Darling Downs.
Weedy field perimeters and poor channel hygiene was also associated with mealybug infestations.
Will mealybug be a problem next season?
In June and July, entomologists from DEEDI conducted repeat surveys of five fields, in the Emerald region. These fields had a history of severe mealybug infestation last season. Mealybug were still present in both surveys despite crop destruction after harvest. Mealybug were found on volunteer and ratoon cotton. Fields with a high incidence of volunteers consistently had higher mealybug numbers. Other problem weeds noted as stand outs – in terms of being abundant across fields and having a relatively high frequency of mealybug incidence include pigweed, bladder ketmia and bellvine. Mealybug were found on the plants and also as far as 5cm below the surface – perhaps overwintering here.
Establishing a host free period for mealybug, to minimise carryover, is going to be a challenge this season. Research by DEEDI entomologists has demonstrated that mealybugs can survive up to 60 days without food, and during that period can continue to produce offspring.
There are a lot of things we don’t know about mealybug or the potential for another outbreak next season. However, the clear association between in-crop weediness and volunteers and mealybug hotspots makes it clear that farm hygiene should contribute to minimizing the likelihood of a high starting population.
Managing ratoons and volunteers
Managing volunteers/ratoon cotton is always challenging. In field, cultivation and herbicides can be effective in controlling volunteer cotton, however the following points should be considered:
Control of volunteers growing outside fields (along channels, roads and fences) is even more challenging as herbicide control is not always possible.
Manual chipping is sometimes the only effective option, particularly where volunteers are well established. While this is an intensive strategy, it is a good investment when taking into account the potential costs and losses caused by pests such as SLW, aphids and now mealybugs.
Article by Susan Maas, Melina Miles and Kate Charleston