1. Be prepared for increased pest activity, particularly aphids.
Pest activity has been relatively subdued in recent seasons due to reduced cotton acreage and a very dry surrounding landscape. Although the cotton area for 2020-21 is well down on previous seasons, increased rainfall will see major changes in the surrounding landscape that will in turn influence pest populations that will affect crops now and in future seasons.
With increasing rainfall, aphids are a pest to be on the lookout for. Whilst aphids have been infrequent pests for several seasons, their rapid lifecycle and an increasing abundance of hosts in the broader landscape creates the opportunity for rapid population build-up that may spill into cotton crops.
Fortunately, aphids have many natural enemies that commonly occur in cotton crops. Ladybirds, hover flies, lacewings and parasitic wasps can all exert effective and timely biological control. If aphids are encountered during crop scouting, take the time to examine the status of the colony as this can provide useful clues about the future trajectory of a population. For example, the presence of isolated winged aphids accompanied by a small number of juveniles indicates that the population is newly established. In comparison a leaf with a mixed population of all sizes has been established for much longer. Keep an eye out for patches of honeydew on lower canopy leaves as this can indicate the presence of aphids (or mealybugs) in the overlying canopy.
Take the time to look for predators that may be amongst an aphid colony. Hoverfly or ladybird larvae are not always immediately obvious. Similarly, ladybird or lacewing eggs maybe nearby. The presence of predator eggs and larvae is a positive sign that biological control is taking place and that, provided aphids are below threshold (see the CPMG) or that honeydew is not presenting a risk for lint contamination, it is likely that biological control will subsequently suppress aphid populations.
If other pests such as mirids require control, be mindful of the impacts that different insecticides can have on natural enemies and the aphid population. Some insecticides are more disruptive to natural enemies than others whilst some products are registered for control of aphids as well as mirids. Refer to the CPMG for impacts on beneficial species. This year’s guide included a handy pull out impact table that you can keep in your glovebox or place on the office or shed wall.
2. Maintain good farm hygiene
There is an old saying that goes: “The best time to plant a tree was yesterday. Failing that the next best time is today”. Similarly, if you have not caught up with feral ratoon and volunteer cotton removal in and around your farm, today is the best day to start getting things ready for next season (that will hopefully see replenished storages and increased cotton production).
Key areas to inspect include tail drains, supply channels, roadways and fallow fields in and around your farm. A survey of perennial feral cotton plants taken in 2013 found that 63 per cent of plants sampled in CQ along farm roadways, supply channels, drainage and fence lines were infected with Cotton Bunchy Top Virus (CBTV).
A similar pattern was found in the St George irrigation area and Darling Downs with 29 per cent and 53 per cent of feral growing cotton plants found to be infected with CBTV. These diseased plants act as reservoirs for both the virus and the cotton aphid vector within the farm landscape. Winged aphids infected with virus can disperse from these plants onto new season crops taking CBTV with them. During previous La Niña phases a number of cotton crops, particularly on the Darling Downs, have been badly affected by CBTV disease.
3. Keep an eye out for fall armyworm
With fall armyworm (FAW) detections throughout Queensland and well into NSW, many agronomists are taking a closer look at the caterpillars they come across in different crops.
Importantly, please note that FAW have not been detected in in any cotton crops (either Bollgard® 3 or unsprayed non-Bt cotton refuges) grown over the last 7 months in Northern Australia. Host preference field studies conducted by Dr Brian Thistleton and his team at the Department of Primary Industries and Resources in the NT also found no evidence for FAW moth laying eggs in in field grown cotton plots, however their study did show that FAW could feed and develop on conventional cotton when confined within containers in the lab without alternate host choices.
The incursion of FAW in Australia is still very new and therefore it is too early to definitively rule particular scenarios in and out. However, at this stage field observations and studies suggest that cotton is not a preferred host for egg laying FAW moths. Given that >90 per cent of the crop grown is Bollgard® 3, people are unlikely to encounter this pest on cotton during the coming season. For cotton crop managers the clear message is be alert not alarmed.
If you find or suspect that larvae of this pest are present in your cotton fields (on either Bollgard or non-Bt cotton refuges) please notify your local CottonInfo REO or the industry’s IPM Technical Lead (Paul Grundy) so that steps can be taken to verify an identification. This is important as many researchers are currently trying to understand the pest status and host range for FAW. Fall armyworm can be very difficult to distinguish from Helicoverpa spp. when they are small (<15mm) in size, becoming easier to identify as they become larger. Some of the key characteristics are depicted on the larval image below and this YouTube video.
4. Be mindful of insecticide resistance.
Resistance levels for many pests have subsided during recent seasons due to the drought-related reduction in cropping and changes in insecticide stewardship. Reduced resistance levels are a positive for the industry but as cropped area increases there are some trends to be aware of:
- Testing by Dr Lisa Bird (NSW DPI) for Helicoverpa resistance has found that resistance to indoxacarb (Steward®) has declined from 9.8 per cent in 2018-19 to 5.7 per cent this year with the most marked reductions occurring in Central and North Qld (Fig 1). This decline is the likely result of drought related declines for pulse production and subsequent reduced spraying. If high usage of indoxacarb re-occurs when the drought breaks it is likely that resistance will again increase, highlighting the need for mindful usage.
- Testing from last season by Dr Jamie Hopkinson (DAF) confirmed low levels of resistance to Spirotetramat in silverleaf whitefly (SLW) from Emerald and Dalby. The detected level of resistance will not affect field efficacy but is a reminder that care needs to be taken with this product, which is why usage is now restricted to a single application per season for SLW. The double application strategy for Solenopsis mealybug control when warranted is an allowable exception, but an IPM approach focused on good farm hygiene and conserving natural enemies will provide a better result for the long term management of pest mealybug.