NPV (nucleopolyhedrovirus)

NPV can be used in a range of field crops.

• In sorghum, NPV is highly effective (should reduce population levels by more than 90%, if you follow the application guidelines), and the preferred product to manage helicoverpa.
• In other crops, use a molasses-based additive. Control varies depending on a range of factors, including temperature and larval feeding behaviour.

NPV can persist for years in a protected environment such as the soil, but is killed by exposure to ultraviolet light in sunlight, and high temperatures.

Current commercially-available formulations of NPV are registered for use against helicoverpa species. These products are not toxic to beneficial insects in the crop, but may impact larval parasitoids indirectly (if there is insufficient time to complete development in infected larvae).

You do not need to protect beneficial populations in chickpeas, as predator and parasitoid populations are naturally low due to the presence of malic acid on the leaves.

After ingesting NPV, infected larvae will eat normally for a couple of days before reducing feeding substantially. Diseased larvae typically climb to the top of the plant to die. Shortly after death, the body becomes flaccid and its skin ruptures, releasing millions of infectious virus particles. When many larvae are living close together, they can infect each other and create an outbreak.

Helicoverpa are unlikely to develop resistance to NPV. Both the naturally occurring viruses and the commercial products are a mix of many strains, each with a slightly different infection strategy.

NPV particles are called polyhedral inclusion bodies (PIBs), and must be eaten by the larvae for infection to occur.

Once the PIB is ingested, the virus infects the gut cells, spreading to the blood within 24 hours, and then to almost all the tissue in the body. Even a single PIB is usually enough to kill a caterpillar.

NPV can kill young larvae within 4 days of ingestion, and older larvae within 7 days, depending on dose and temperature. Cooler temperatures can prolong the larva's survival to more than 10 days.

Temperature has a major impact:

• Daytime temperatures of 25–35°C are ideal for NPV activity.
• NPV activity is slower under cooler conditions (do not apply below 18°C).
• Larvae may stop feeding below 15°C, and take a while to recover. Delay applying NPV until temperatures have warmed to above 20°C following a cold night.
• Larvae death may take longer in cooler weather, but this will not influence overall damage levels.

Infection relies on the larva ingesting the virus. Larvae in sheltered positions are more difficult to reach.

• Levels of 40–90% (average 60%) larval infection are possible on young cotton; however, in flowering cotton, larvae can shelter in squares and flowers.
• NPV will not control larvae sheltering in maize cobs, but will affect larvae on the silks.