Helicoverpa activity and spring moth trapping
Now is the time to commence monitoring for Helicoverpa larvae in pulse and canola crops at the susceptible flowering and podding stages. Larvae attack legume crops in spring, causing economic damage though direct grain consumption and reduce grain quality. Large larvae close to maturity, 30-40 mm in length, are responsible for 90% of their total grain consumption.
This spring, SARDI has established a network of pheromone traps operated by field consultants at ten locations across South Australia, to monitor the dynamics of moth flight activity and provide early warning of potential larval activity in crops. At each location, we are separately trapping both the native budworm, Helicoverpa punctigera, and the corn earworm (or cotton bollworm), H. armigera, using species-specific female sex pheromone lures in separate bucket traps. Additionally, at three locations, we are cross-validating standard bucket trap catches with automated traps supplied by ADAMA Australia. The South Australian network is part of a wider network operating in other states in collaboration with cesar, QDAF and DAFWA through the GRDC-funded National Pest Information Service (NPIS).
Traps in most locations recorded nil or low moth numbers over the last week. The Kimba traps recorded 7 native budworm and 20 corn earworm moths. We will provide moth trap data and model predictions of larval activity in crops in PestFacts issues during spring.
The native budworm () breeds during winter on native flowering plants in inland Australia. As vegetation dries off in late winter and early spring, moths migrate south on northerly weather systems into cropping regions, where females lay eggs into pulse and canola crops.
The corn earworm () is most common in northern cropping regions of Australia where it attacks cotton, pulse, canola and occasionally cereal crops. Unlike native budworm, corn earworm field populations are known to develop resistance to pyrethroid and organophosphate (but also several other) insecticides commonly used to control native budworm, and in northern pulse crops, requires a different management approach largely based around early timing of treatment of small larvae with biological insecticides. Until recently, it was assumed that the occurrence of corn earworm in southern cropping regions during spring was relatively rare. However, trapping by SARDI over the last two seasons has shown that the corn earworm is commonly trapped, and can make up a significant proportion of the total Helicoverpa catch. Where Helicoverpa larvae are found grazing cereal heads, it is most likely to be the corn earworm. However, Melina Miles (QDAF) points out that the relative numbers of H. armigera and H. punctigera moths in traps does not always reflect the relative proportions of larvae in crops, as a number of factors influence egg laying and egg/larval survival.
Monitoring for Helicoverpa larvae should be conducted using a sweep net. Take a minimum of 5 sets of 10 sweeps at various locations in the paddock, and calculate the number of larvae per 10 sweeps. Newly hatched larvae are very small (~1.5 mm in length). They develop through six or seven instars until reaching maturity (~ 40 mm in length). Larvae of both species have darkish stripes along the body and bumpy skin with stiff, dark hairs, and can show substantial variation in body colour. A characteristic feature of Helicoverpa larvae is a sharp downward angle (~45 degrees) on the last (8th) abdominal body segment, which is lacking in other Noctuid pests such as cutworm and armyworm.
Distinguishing corn earworm from native budworm requires close examination; while both species have black body hairs, native budworm larvae have black hairs around the head, while corn earworm have white hairs around the head, and mid-instar larvae have a `saddle` on the top of the 4th abdominal segment. Helicoverpa eggs are small (0.5 mm diameter) and can be found singly on the growing tips and buds of plants. Moths of both species are robust, buff to red/brown in colour with pale hindwings, around 15-18mm in length with a wingspan of around 30-45 mm. Their appearance around lights at night, particularly on warm evenings, can indicate that flights are occurring.
A dynamic economic threshold calculator for native budworm, developed by DAFWA, is available in the southern region PestNote (below). Growers should substitute in their own control costs and grain prices. Research by QDAF has shown that, under current cost structures, spray decisions in smaller seeded pulse crops (e.g. desi chickpea) can be made based on these calculated yield loss thresholds without risking penalties for reduced grain quality; however, this may not apply to larger seeded grains (e.g. Faba bean, field pea) where there is a higher likelihood of unacceptable levels of partially chewed grain in the sample. If treatment is warranted, one well-timed synthetic pyrethroid application often provides effective control and may prevent re-infestation for up to six weeks.
We would like to acknowledge our colleagues operating traps this spring:
- Nigel Myers (Landmark) – Cummins
- Andy Bates (Bates Ag Consulting) – Streaky Bay
- Adam Hancock (Elders) – Frances
- Steve Richmond (Landmark) – Mannanarie
- Chris Davey (YP Ag) – Port Broughton
- Tom Moten (Landmark) – Eudunda
- Peter Ellison (Landmark) – Keith
- Sarah Meyer & Geoff Rissmann (Cleve Rural Traders) – Cleve
- Troy Maitland (EP Ag n Fert) – Kimba
- Iain Todd (Landmark) – Crystal Brook