IPM top tips for spring and summer

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 moths 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 in this YouTube video.

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 still well down on previous seasons in many valleys, 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. Many common pests such as mirids, Helicovpera spp. and plant feeding shield bugs are likely to benefit from a greener landscape.

Another group of pests to be on the lookout for with increased rainfall are aphids. Both cotton and green peach aphids have been infrequent for many years but, with rapid lifecycles and increased host abundance in the broader landscape, opportunity exists for rapid population build-up that could spill over 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.

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 better seasons ahead with the prospect of replenishing storages.

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 Central QLD 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 have been badly affected by CBTV disease.

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. Likewise, there are likely to be implications for resistance in Helicoverpa if we see high levels of spraying for FAW in maize and sorghum. When considering control of FAW in these crops keep in mind that Helicoverpa are also likely to be present in these fields and will be impacted as well.
• 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.