Getting the most from NPV sprays on grain sorghum

Some issues have recently been raised on the Darling Downs regarding the use of Helicoverpa nucleopolyhedrovirus (NPV) against corn earworm on grain sorghum. These issues involve the delay in time to kill and the level of control not necessarily meeting growers’ expectations.

Seasonal conditions
The last couple of weeks on the Downs have been cooler than normal. The average daily screen temperature for Dalby for the week ending 11 December was 25.5°C, while for the week ending 4 December it was just 22°C. Daily minimums were as low as 14°C. These lower temperatures will influence the time to death of NPV-infected caterpillars.

Temperature affects the development rates of caterpillars, their feeding, and the rate at which they die from NPV once they are infected. The lower the temperature, the slower caterpillars develop and the longer it takes for NPV-infected caterpillars to firstly stop feeding, and then to die.

Studies by Chris Monsour (formerly DPI&F) investigated these relationships. At 30°C, an NPV-infected 6-day old caterpillar (late second instar) will feed normally for 2 days before feeding is greatly reduced. At 20°C, caterpillars feed normally for 5 days before feeding is reduced. Importantly, the total amount of food consumed by these NPV-infected caterpillars is similar, whether at 30°C or 20°C (see Figure). Also remember that most caterpillars feed on anthers until late fourth instar (21 mm in length) and this feeding will not affect yield.

Time to death from NPV

At 30°C, it takes on average 4.5 days after infection for a 6-day old caterpillar to die from NPV. This compares with 6.2 days at 25°C and 7.5 days at 20°C. Under recent field conditions, many larvae are taking more than 8 days to die from NPV after spray application.

Figure. Consumption of artificial diet by healthy and NPV-infected 6-day old caterpillars and time to death from NPV at three temperatures (Data from Chris Monsour).

High grub infestations
If we accept that NPV will kill 90% of caterpillars, a starting infestation of 10 caterpillars per head results in 1 caterpillar per head surviving. This would not normally be an issue, but with the current high value of grain sorghum, this number of survivors may be above the economic threshold. You need to ask, is any other product going to do a better job? The answer is ‘No’.

Beneficial safety
NPV is safe on beneficials (parasites and predators) and these are important in helping mop up any survivors, as well as ensuring aphids are not flared. The end result is generally better than a 90% job.

What do we need to do to make sure of good results with NPV?

Timing
Consider applying NPV sprays earlier i.e. before 50% brown anthers, particularly if the spread of flowering is large. In this way most early flowering heads will be fully protected and secondary infection will control most caterpillars on the late flowering heads. It is best to target caterpillars less than 7 mm in length when using NPV, and this is the size of caterpillars on heads that have just finished flowering.

Water quality
Water used in spray mixes should have a pH of 7. Alkaline water will seriously reduce the performance of NPV, so buffer water with Li700 or equivalent to neutralise pH.

Water volumes
For high-volume, water-based sprays, a minimum of 30 L water/ha is recommended for aerial application, and 100 L water/ha for ground rig application.

Coverage
NPV must be ingested to be effective, so the challenge is to achieve good coverage of the target. This means paying particular attention to water volumes, nozzles, operating pressure, weather conditions, etc. You want to spread NPV over as much of the head as possible to ensure caterpillars have a high chance of picking up a lethal dose as they feed on the head.

Additives

NPV historically performs very well on grain sorghum, usually achieving greater than 90% control when used alone. For this reason, additives such as AminoFeed, etc. are not recommended when NPV is applied to grain sorghum.

However, for all crops other than sorghum, the use of a molasses-based additive containing the reducing sugars glucose and fructose (such as AminoFeed) is recommended when applying NPV.