Jo Holloway, Research Entomologist, NSW DPI, Wagga Wagga
Reports are coming in that there are large numbers of heliothis in crops and that two species, the native budworm (Helicoverpa punctigera) and the corn earworm or cotton bollworm (H.armigera), are present in roughly equal numbers. This means control of these pests may be hard, as H. armigera is known to have developed resistance to a wide range of insecticides (synthetic pyrethroids and some carbamates). Part of this is due to its life cycle: while H. punctigera fly in from inland Australia, H. armigera is known to overwinter in the region. This means
successive generations are exposed to insecticides from season to season, allowing resistance to develop, whereas with H.punctigera each season is a new, non-exposed generation.
To distinguish between the two species, medium-larger caterpillars of H. armigera have white hairs around the head, a “saddle” of darker pigment on the 4th abdominal segment and dark coloured legs; H. punctigera have dark hairs around the head, no saddle and light coloured legs.
Heliothis caterpillars feed on leaves, flower buds, flowers, developing pods, fruits and seeds. In most crops young caterpillars (<7 mm long) will graze only on the leaves, moving on to feeding on pods, grain, bolls and cobs once they are third instar or older (>8 mm long).
However, in some crops (e.g.. mung beans, cotton) caterpillars infest reproductive structures (e.g.. flowers) as soon as they hatch. Concealment in these structures makes them more difficult to control with insecticides.
Most feeding (90%) is done by third instar caterpillars (8-13 mm) onwards. Large heliothis (>24 mm; 5th-6th instars) are the most damaging stage and consume about 80% of their overall diet. Therefore, to minimise crop damage, it is important to control caterpillars when they are small (<7 mm).
Image: H. punctigera (left) and H. armigera caterpillars. Note the white hairs around the head and dark pigment on the 4th abdominal segment that distinguish H. armigera from H.punctigera
Timing and coverage are critical to achieving control of heliothis caterpillars. Sprays should target:
- Very small (1-3 mm) and small (4-7 mm) caterpillars (that require a lower dose to kill);
- Feeding or caterpillars that are moving in the open, and therefore more easily exposed to spray droplets;
- Good coverage, especially with some of the ingestion-active products, because the caterpillar must actually feed
on the plant material covered with the spray.
Two non-chemical, pathogen formulations are also available for control of heliothis. NPV (nucleopolyhedrovirus) and the bacterium Bacillus thuringiensis (commonly known as Bt). The commercial NPV is highly selective and only infects H. armigera and H. punctigera, and is harmless to humans, wildlife and beneficial insects. Bt is available as a selective spray that only kills moth larvae. Both formulations are faster acting in warmer
temperatures and are more effective on small caterpillars, killing them within 4 days of ingestion, while larger ones may take 5-10 days. As with all chemicals and spray formulations, it is important to read the label to ensure effective use. Economic thresholds (Table 1) will assist in determining at what point it is cost effective to use a spray to control the heliothis caterpillars.
As with other pests, weed management in and around crops, as well as management options that support retention of natural enemies (predators and parasitoids) will also assist in preventing a build-up in the number of heliothis.
|K - Loss|
|P - grain|
|C - cost of|
|ET - larvae|
|Chickpeas - desi||30||900||10||0.3|
Table 1: Yield loss estimates for five pulses & canola, and corresponding economic thresholds (ET) for heliothis