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

However, in some years, very high pest activity results in more severe cob damage, with larvae often tunnelling into the sides of cobs. In such cases grain samples may contain fungus-affected grains and mycotoxins, causing a downgrade in the quality of harvested grain.

What can you do to prevent these losses?

Over recent years Helicoverpa nucleopolyhedrovirus (NPV) has demonstrated its versatility for corn earworm management. One of the major developments has been the effective application of NPV through overhead irrigation, sometimes referred to as ‘chemigation’.

ViVUS Max is currently the only insecticide registered in Australia for application in overhead irrigation water in a wide range of crops.

By adding NPV to irrigation water, growers can artificially inoculate their crop with NPV and achieve a high level of control of helicoverpa larvae.

The main guidelines for NPV use still apply. These include:

• Good coverage is essential as the product needs to be ingested
• Use in the temperature range 25-35°C when larvae are actively feeding
• NPV is more effective against smaller larvae
• Preferably target larvae less than 7 mm in length, but under ideal conditions larvae up to 13 mm in length will be controlled
• Larvae can take up to 8 days to die
• Spray water pH should be neutral (pH 7.0)

There are additional key points to the successful use of NPV via overhead irrigation.

• Application in overhead irrigation water provides the maximum coverage achievable
• Introduce NPV to the irrigation water at the appropriate rate using chemigation equipment
• If the NPV is diluted in water prior to injection into the irrigation water, ensure that the dilution water is clean and not silty with a pH of 7 or less
• Ensure constant agitation in the premix tank
• Ensure any diluted NPV is used within 10 hours of mixing
• Apply in no more than 10 mm of irrigation water

• For one-pass mobile irrigators such as centre pivots, laterals and travellers (guns), continuously introduce the required amount of ViVUS Max into the irrigation water over the course of irrigation.

What rates to use? How often?

For ViVUS Max in maize, the registered rate for normal foliar application is 150 mL/ha. When applied in overhead irrigation water, reduced rate repeat applications have been used successfully. An effective prophylactic strategy would be to make the first application at full tassel emergence (50 to 150 mL/ha depending on larval numbers and size) and then low rate (50 to 75 mL/ha) applications every 5 days or so until late blister/early milk stage.  A total of 4 applications would use about 250 mL/ha (perhaps more under high pressure). This product cost is around $30/ha and will keep things very clean.  

What are the economics of losses to larvae?

A back of the matchbox calculation can be used to give some insight to the damage caused by larvae. An average plant population is 70,000 plants/ha with one cob per plant and one larva per cob. Assume one larva consumes 15 kernels in its lifetime (Note: this value has no validated scientific basis). With an average kernel weight of 4,000 kernels/kg, one larva consumes about 3.8 g. If maize is valued at $300/t, this loss amounts to 262 kg/ha or $79/ha. Based on these rough figures, and assuming a high level of control, there is likely to be an economic benefit from using NPV. Larvae damaging early silks can also reduce pollination, which can result in even greater yield reductions.

Other benefits

As NPV is safe to natural enemies, parasites and predators remain in the crop and keeps working in your favour. This is particularly relevant for the egg parasite Trichogramma which is sometimes very abundant in maize crops. Untreated maize crops can also generate large numbers of helicoverpa moths, so control of larvae in maize can reduce subsequent pressure in nearby crops.

Article by David Murray and Anthony Hawes.  Image of lateral move irrigator by Graham Harris.

Pre-flowering heads of grain sorghum are very attractive to egg-laying moths of the corn earworm. On any individual head, most eggs are laid prior to the start of flowering, as indicated by the presence of yellow anthers.

By the end of flowering, when brown anthers are present at the base of the head, eggs will have hatched and most larvae will be less than 7 mm in length.

A timely spray application of the naturally occurring biopesticide NPV (nucleopolyhedrovirus) remains the best control option for grain sorghum crops under attack from corn earworm.

NPV performs exceptionally well on grain sorghum, with well timed sprays usually achieving greater than 90 per cent control while leaving beneficial parasites and predators to mop up survivors.

If the spread of flowering in a crop is large, it may be better to spray earlier rather than wait until 50% of the crop is at the brown anther stage. This is because caterpillars on the earliest flowering heads may be larger than the ideal size to target with NPV, and they will cause some damage if not adequately controlled with NPV.

Research has shown that early application of NPV creates a disease outbreak and secondary NPV infection will control most caterpillars on late flowering heads.

Other issues to ensure good results with NPV

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
Additives such as Amino Feed, etc. are not recommended when NPV is applied to grain sorghum.

Paying attention to the detail will ensure the best results from NPV.

Article by Dr. Dave Murray

New Beat Sheet contributors

The beat sheet blog team has been expanded and includes two new contributors, Kate Charleston and Zara Ludgate.

Kate is the development extension officer with the entomology team in Toowoomba. Her role is to provide information about IPM in field crops as well as training to growers and industry in managing insect pests according to IPM principles.

Kate joined entomology in June 2008 and has previously worked as a research scientist, agronomist, plant health inspector and extension officer. She started her career with the Department of Primary Industries in Tasmania and joined the Queensland Government in 1999 as an extension officer in a sugar project at South Johnstone in Far North Queensland. Kate has worked with sugarcane, cotton and pulse crops and has considerable training and extension experience.

Zara has just started her career in entomology research. She comes from a rural background and completed a degree in plant and soil science at St Lucia, Brisbane in 2007. For her honours year she investigated the effect of a plant defence compound on the fitness of diamondback moth.

While in Brisbane she provided technical support for research into bio-pesticide production for helicoverpa and green vegetable bug management. She is now based in Toowoomba with the crop protection systems – entomology unit. Her current research interests include insecticide resistance in whitefly and integrated pest management in grain crops.

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

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.