NPV (a viral biopesticide) is registered for use against helicoverpa on a range of field crops in Queensland, usually with higher rates in cotton and maize to compensate for difficult coverage due to the presence of sheltered feeding sites and/or a more complex plant structure.
Note that infected larvae reduce their feeding but can take over a week to actually die.
Coverage
NPV must be ingested to become active. Select the correct combination of volume, nozzles, and pressure to suit the spraying conditions, and target the areas where larvae are most likely to be active.
For high-volume, water-based sprays, a minimum of 30 L water/ha is recommended for aerial application, and 100 L water/sprayed ha for ground rig application.
Use pressures of 300 kPa (3 Bar) or higher to provide the right application volume. Refer to manufacturer´s guidelines to ensure that the pressure required to give the appropriate volume per hectare will still deliver spray droplets in the desired spectrum. Aim for a minimum droplet density index of 60 droplets/cm2. A light wind (4-15 km/h) at application will help distribute the spray on the target.
Finer droplets are often associated with a more rapid virus uptake, due to the increased probability of larvae encountering the product. However, under very low relative humidity it may be necessary to increase the droplet size and application volumes to ensure thorough coverage and rapid uptake.
Aerial ultra low volume (ULV) applications of NPV (with the addition of spray oils) at 3 L/ha total spray volume have been effective against helicoverpa in sorghum and cotton during very hot, dry conditions. The reduced cost of water cartage is balance by the higher expense of oil carriers. A key advantage is that aerial ULV sprays enable prompt treatment of large areas. Always read the label regarding appropriate use.
Low rates of NPV are effective under certain conditions, such as targeting very small and small larvae, promoting an epizootic (natural) outbreak of the virus, or in mixture with other insecticides to maximise control. Low rates will not control above-threshold populations. Some growers routinely apply NPV in early season applications of other products (e.g. fungicide sprays), but the cost:benefit of this approach has not been confirmed.
Timing
Helicoverpa larvae can be rapidly infected with NPV (within an hour of spray application). Focus on applying NPV when conditions (e.g. temperature and humidity) are most suitable.
- Apply when temperatures are between 25-35°C.
- Below 18°C, helicoverpa larvae are relatively inactive and unlikely to acquire an effective dose.
- High humidity can improve droplet penetration and coverage.
- Light showers may help disperse the NPV particles more thoroughly across plants. Delay application if heavy rain is expected within the hour.
NPV is significantly more effective on larvae <7mm, so target larvae before they get too large or take shelter in areas of the plant that are difficult to reach with sprays.
In sorghum, apply NPV three days after 50% of heads in the field have completed flowering (brown anthers to the base of the head). This (i) targets larvae up to 7mm and (ii) conserves parasitoid Microplitis wasps. As moths lay eggs in developing sorghum heads just prior to flowering, waiting allows for larval sizes up to just over 7 mm in length (early third instars). Microplitis prefers larvae between 3-7 mm. Provided there are at least three days between Microplitis parasitism and NPV infection, a Microplitis larva will be able to complete its life cycle inside its caterpillar host before the caterpillar dies of NPV infection.
In sorghum crops where there is a large spread of flowering, it is better to spray before 50% of the heads are at the brown anther stage, as secondary infection by NPV can kill a large proportion of the caterpillars that hatch after the NPV application.
Additives
Using molasses-based additives containing the reducing sugars glucose and fructose (such as Optimol® or AminoFeed®) can significantly improve NPV performance, possibly acting as a feeding attractant or stimulant, improving distribution of the spray formulation over the plant surface, or helping activate the virus in the insect gut. Additives are not required in sorghum as NPV alone usually achieves sufficient control.
Ensure water used in spray mixes has a pH of 7 or less. Alkaline water will seriously reduce NPV performance.
For more information, search the Beatsheet for individual articles relating to NPV use.