IPM - Understanding helicoverpa ecology and biology in southern Queensland: Know the enemy to manage it better

Quick links to topics on this page:

• About helicoverpa
• Helicoverpa AWM and IPM
• Helicoverpa armigera
• Helicoverpa punctigera
• How do we know when the moths have arraived or are emerging from diapsuse?
• Lifecycles - the moth, eggs, larvae, what do larvae eat?, pupae, & diapausing pupae
• Predators, parasitoids & pathogens
• Control without insecticides
• Pooly timed sprays are costly
• A whole-farm approach
• Further information

About helicoverpa

Helicoverpa (often called heliothis) is a serious pest of southern Queensland crops, particularly grain legumes, summer grains and cotton. Understanding the lifecycle and behaviour of helicoverpa improves our chances of managing this pest sustainably.

There are two pest species of helicoverpa in Australia that this publication refers to as 'helicoverpa': the native budworm, Helicoverpa punctigera; and the cotton bollworm or corn earworm, H. armigera . Because H. armigera has developed resistance to a wide range of insecticides, it is more difficult to control.

Helicoverpa AWM and IPM

Recent strategies such as area-wide management (AWM) and integrated pest management (IPM) aim to restrict the build-up of helicoverpa populations to below damaging levels. Successful AWM and IPM strategies (which combine biological, cultural and chemical control options) require a more sophisticated understanding of helicoverpa's lifecycle and biology than relying on insecticides alone.

Large Helicoverpa armigera larva on a mungbean pod

Female helicoverpa moth

To manage H. armigera and H. punctigera well, it is important to understand the basic differences between the two species. In particular, differences in the timing of annual events in their population build-up.

Helicoverpa armigera

Most H. armigera moths that appear in spring come from populations that survive locally from year to year. So although there may be low levels of H. armigera moths in the migratory population, this is not the major source of H. armigera.

In southern Queensland and areas further south, the majority of H. armigera moths overwinter as pupae in the soil. These overwintering pupae survive in a state of suspended development (diapause). In most seasons, diapausing pupae can be found under late summer crops such as sorghum, maize and cotton. Most moths emerge from these diapausing pupae during October. After one or two generations in spring and early summer, H. armigera numbers increase to become the dominant species in mid-summer to autumn. In central Queensland, and further north, winter temperatures are warm enough for H. armigera not to enter diapause. As a result, these regions experience year-round helicoverpa activity.

While H. punctigera can overwinter in southern cropping regions, pupae numbers are usually low. There is no simple solution to helicoverpa control in a farming system that provides a wide range of food sources throughout the year.

The continuous availability of hosts potentially allows successive generations of helicoverpa to build up in a cropping region throughout the year.

Helicoverpa punctigera

Where do H. punctigera moths come from?

Although both species of helicoverpa are capable of travelling hundreds of kilometres on high altitude winds, this characteristic is more typical of H. punctigera. H. punctigera breeds during winter on flowering plants in inland Australia , when there is sufficient rainfall for broadleaf vegetation to flourish. When this inland vegetation dies off in late winter/spring, the moths move out. Migratory moths reach the cropping areas by flying on the warm winds that precede cold fronts in spring.

When do the moths arrive?

These spring migratory flights make H. punctigera an early season pest. The likely magnitude of these influxes of H. punctigera can be estimated by monitoring the breeding areas in inland Australia.

How do we know when the moths have arrived, or are emerging from diapause?

Pheromone traps and computer modelling are two tools used routinely to monitor the arrival of the first H. punctigera moths, and the emergence from diapause of local H. armigera. Pheromone traps attract male moths using the sex pheromone (a chemical attractant) that female moths emit to attract mates. Helicoverpa armigera and H. punctigera pheromones are different, so each species can be monitored separately. Pheromone traps, baited with synthetic pheromone lures, are used to monitor the arrival of H. punctigera of moths over spring.

 
A pheromone trap used to monitor helicoverpa moths

The computer model uses current local temperatures to calculate the rate of development of pupae in the soil, and predicts the timing of emergence of moths. Pheromone traps specific to H. armigera can confirm the timing of the emergence. The graph below illustrates the relative timing of moth immigration, predicted emergence and actual emergence for the two species in 1999, a typical season.

Typical pattern of H. punctigera immigration (these are open square), predicted H. armigera emergence from diapause (filled square) and observed H. armigera emergence (straight line).

The graph shows how H. armigera activity begins when emergence from diapause occurs, showing the contribution of the local diapausing population to seasonal activity.

Calculations for emergence are based on seasonal temperatures for Jimbour, 1999.

Lifecycles

Lifecycles of H. armigera and H. punctigera take 4-6 weeks from egg to adult in summer, and 8-12 weeks in spring or autumn. The helicoverpa lifecycle stages are egg, larva, pupa and adult (moth).

Lifecycle of helicoverpa

The moth

Adult moth wingspan is 30-45 mm; the forewings are brownish or reddish-brown (females) or dull greenish to yellow or light brown (males); hindwings are pale with a broad, dark outer margin. Helicoverpa armigera moths have a pale patch near the centre of this dark region.

Moths feed on nectar. They live for around 10 days during which time females lay 1000 eggs. Eggs are laid singly, or in clusters, on leaves, flower buds, flowers and developing fruits, and sometimes on stems and growing points. Moths tend to lay eggs on the top third of healthy plants and on vigorously growing terminals.

Moths of H. punctigera (left) and H. armigera (right) showing the characteristic differences in the markings on the hindwings. Helicoverpa armigera moths have a pale patch in the dark marking on the hindwings. There is no pale patch on the hindwings of H. punctigera.

Eggs

Fertile eggs hatch in about three days during warm weather (25 °C average) and 6-10 days in cooler conditions. As they develop, eggs change from white to brown to a black-head stage before producing a hatchling. Not all eggs are fertile. Physical factors can dramatically affect egg survival and larval establishment. Heavy rainfall and wind can force eggs off leaves. High temperatures can dehydrate and kill eggs and very small larvae.

The four stages of helicoverpa egg development. Freshly laid helicoverpa eggs are white, turning a light brown colour over the next 1-2 days. Close to hatching, the black head capsule of the developing larva is visible through the eggshell.

Larvae

The hatching larva (neonate) eats through the eggshell to make an exit hole and emerges. Neonate larvae are 1-1.5 mm long, with a brown-black head and white or yellowish-white, dark-spotted body. Larvae graze on tender young foliage for 1-2 days, and then move to feed on buds, flowers or young pods, bolls or fruits.

Note: The term neonate comes from the Latin 'neo' (new) and 'nato' (to give birth), that is 'newborn'.

Larvae develop through six growth stages (instars) and become fully grown in 2-3 weeks in summer or 4-6 weeks in spring or autumn. Development is more rapid at higher temperatures, up to 38 °C, after which development slows. Larval activity and feeding stops when temperatures fall below 12 °C.

Guide to helicoverpa larval instars/size categories

*Age of larvae shown is for larvae reared at a constant 25 °C only. The development rate of larvae at 25 °C is similar to the maximum growth rate of larvae in the field during summer.

Ninety per cent of all feeding (and therefore damage) by helicoverpa is done by larva from the third instar (small medium larva that are 8-13 mm long) onwards. Large helicoverpa larvae (longer than 24 mm) are the most damaging stage, since larvae consume about 50% of their overall diet in the fifth and sixth instars. This highlights the importance of controlling helicoverpa larvae while they are still very small to small (less than 7 mm).

Full-grown sixth instar larvae are 40-50 mm long with considerable variation in colours and markings.

 
Late instar helicoverpa larvae can vary greatly in colour and markings

What do larvae eat?

Helicoverpa larvae feed on leaves, flower buds and flowers, developing pods, fruits and seeds. In most crops, young larvae will graze on leaves alone, moving on to feeding on developing pods, bolls, cobs and grain once they are third instar or older (8 mm or longer). In some crops, such as mungbeans and cotton, hatchling larvae infest reproductive structures (flowers, squares) as soon as they hatch. Once established in these concealed feeding locations, larvae are much more difficult to control with insecticides.

The two helicoverpa species prefer different hosts. Helicoverpa punctigera prefers to feed on broadleaf species (e.g. cotton, chickpea, native herbs); H. armigera eats these crops and grass-related species (e.g. corn, sorghum and wheat). Crop and pasture weeds such as noogoora burr, common sowthistle, fat hen and marshmallow are also attacked. This broad diet demonstrates why helicoverpa is a pest of the broad farming system and not just a few specific crops.

 
A large helicoverpa larva feeding inside a chickpea pod

Pupae

Once larvae are fully grown, they crawl to the base of the plant, tunnel up to 10 cm into the soil and form a chamber in which they pupate. Pupae will normally develop to produce a moth in 10-16 days. The moth emerges, feeds, mates and is then ready to begin the cycle of egg laying and larval development. As with all insect development, the duration of pupation is determined by temperature, taking around two weeks in summer and up to six weeks in spring and autumn. However, diapausing pupae take much longer to emerge.

Helicoverpa armigera pupa in pupal chamber (partially opened soil clod)	Helicoverpa pupa in pupal chamber showing the entry and exit tunnels formed before the larva pupated

Diapausing pupae

Both species survive the winter as pupae in the soil when host plants, and thus food sources, are scarce. H. punctigera are capable of overwintering in southern cropping regions, but only a few are ever found. In contrast, substantial numbers of overwintering H. armigera pupae can be found under late summer crops, particularly when helicoverpa activity has been high late into March. In southern Queensland , pupae typically start to go into diapause from around mid-March and emerge in spring. Not all pupae that form in late summer go into diapause: a proportion continues to develop, perhaps emerging during winter, or early in spring.

The graph shows the predicted rate of helicoverpa diapause induction for Dalby, Queensland, and that from mid-March onwards, a significant proportion of larvae present in crops will enter diapause and not emerge as moths until the following spring (October). These predictions are based on long-term average temperatures for Dalby.

Overwintering pupae can be killed without using chemicals. Pupae in the soil are susceptible to soil disturbance and disruption of the emergence tunnel. Cultivation is enough to create this disturbance.

For more information on managing populations of overwintering pupae, see the DPI&F brochure Chemical-free heliothis pupae control.

Predators, parasitoids and pathogens

A variety of predatory and parasitic insects, spiders, birds, bats, rodents and diseases attack helicoverpa at different stages of its lifecycle. Natural enemies will rarely eradicate all eggs or larvae, but may reduce infestations to below economic threshold if predators and parasitoids are not disrupted by broad-spectrum insecticides. The amount of disruption that insecticides cause to natural enemy activity varies depending on which chemicals are used and which natural enemies are active. For more information on the impact of insecticides, see the table 'Impact of insecticides on key beneficial groups in cotton' (in the Cotton Pest Management Guide).

Predators

Many predators are opportunity feeders that feed on prey other than helicoverpa. Some predators found commonly in crops will not feed on helicoverpa at all; some may only feed on certain stages (for example, larvae of a particular size, or only eggs). Knowing what predators eat is useful when making helicoverpa management decisions.

The most common helicoverpa predators in field crops are predatory bugs, predatory beetles, spiders, lacewings and ants. Some predators are relatively permanent residents in fields (e.g. ants); others migrate from nearby fields, other vegetation or even further away.

(a)	(b)	Common predators found in broadacre crops: (a) lynx spider (b) predatory shield bug,
(c)	(d)	(c) green lacewing larva, d) red and blue beetle.

Parasitoids

Some wasps and flies attack helicoverpa eggs, larvae and pupae. Parasitoids kill their helicoverpa host to complete their development. The parasitoids most active in field crops include smaller wasp species such as Microplitis, Trichogramma and Telenomus; relatively large parasitoid wasps (Netelia, Heteropelma, Ichneumon); and flies (Carcelia and Chaetopthalmus ).

Parasitoids that attack helicoverpa larvae do not kill their hosts immediately. However, they do stop or slow down caterpillar feeding, which reduces the impact of the pest on the crop. When parasitoids attack late instar larvae or pupae, they stop moths developing that would otherwise produce further eggs and larvae.

For more information, see IPM-Parasitoids: Natural enemies of helicoverpa .

Pathogens

Pathogens are viruses, fungi or bacteria that infect insects. Many naturally occurring diseases infect and kill helicoverpa. The commonest pathogens infect larvae, including the nucleopolyhedrovirus (NPV) and fungal pathogens (Metarhizium, Nomurea and Beauvaria ). Another disease, ascovirus, stunts larval development, and is spread by wasp parasitoids.

Two helicoverpa pathogens are available commercially to control larvae: NPV and the bacterial toxin from Bacillus thuringiensis (commonly called Bt).

NPV occurs naturally and frequently causes natural outbreaks (epizootics) in helicoverpa populations. The commercial helicoverpa NPV is a highly selective biopesticide that infects only H. armigera and H. punctigera larvae. NPV is harmless to humans, wildlife and beneficial insects.

The Bt toxin is available as a selective spray that only kills moth larvae. Genes from the Bt organism have been used to genetically modify cotton plants so that the toxin is expressed in the plant's tissues. When young helicoverpa larvae feed on a Bt cotton plant, the Bt toxin kills susceptible individuals.

For more information on NPV, see IPM-Using NPV to manage helicoverpa in field crops. For more information on the ascovirus pathogen, see IPM-Microplitis demolitor and ascovirus: Important natural enemies of helicoverpa .

Some typical symptoms of infection.

NPV-infected helicoverpa larva.
Larvae infected by NPV crawl to the top of a plant, turn black and liquefy before disintegrating

Metarhizium-infected larva.
On larvae killed by Metarhizium, the green fungus is visible growing out of the larva's body.

Ascovirus-infected larvae are small and pale and can look as though they have been grazing lightly in the one place, leaving small 'windows' in the nearby leaf.

Controlling helicoverpa effectively with insecticides depends on knowing which species are present in the crop because H. punctigera and H. armigera have differing susceptibilities to many insecticides.

Helicoverpa punctigera is easily killed by all registered products, including products to which H. armigera is resistant (e.g. synthetic pyrethroids). Because H. punctigera moths migrate annually into eastern Australian cropping regions, they lose any resistance they develop as a result of exposure to insecticides in crops. In contrast, H. armigera populations tend to remain local so they maintain their resistance to insecticides from season to season.

The need to minimise insecticide resistance in local populations has driven the development of a Farming Systems Insecticide Resistance Management Strategy (FS-IRMS). The FS-IRMS integrates the needs of grain growers into the IRMS developed for the cotton industry. The FS-IRMS aims to provide a strategy that helps growers work out which insecticides they can use without contributing to insecticide resistance. Compliance with the strategy is voluntary, but does give all industries the best chance of maintaining a suite of effective insecticides for the control of H. armigera.

Control without insecticides

Insecticides are not the only options for controlling and managing helicoverpa.

Pupae busting remains an important, non-chemical option to reduce the size of overwintering populations. Pupae busting also reduces the carryover of insecticide-resistant individuals from season to season.

Weed management in and around crops can prevent the build-up of helicoverpa and other insect pests.

Other non-insecticide control methods include spring trap crops (an area-wide management tool for reducing the size of the overall helicoverpa population) and, as discussed, using helicoverpa's natural enemies (predators, parasitoids and pathogens).

Poorly timed sprays are costly

Poorly timed sprays are costly. Poor timing can result in a poor level of control, which consequently increases crop damage and the costs of re-treating the field. Poor timing also increases insecticide resistance by exposing larvae to doses of insecticide that do not kill entire populations, but select for resistant individuals. Timing and coverage are both critical to achieving good control of helicoverpa larvae, whether using a chemical insecticide or a biopesticide (like NPV or Bt).

Good timing of spray applications against helicoverpa larvae occurs when sprays target:

• very small (1-3 mm) to small (4-7 mm) larvae (that require a lower dose to kill)
• larvae that are feeding or moving in the open, and therefore more easily contacted by spray droplets
• larvae before they move into protected feeding locations (e.g. flowers, cobs, pods or bolls).

Good coverage is increasingly important with the introduction of ingestion-active products  because the larvae must actually feed on plant material covered with the insecticide or biopesticide.

For more information on pesticide application see Agricultural chemical users' manual - Guidelines and principles for responsible agricultural chemical use.

Using a beat sheet to sample for helicoverpa larvae in a chickpea crop. Check crops regularly and thoroughly to ensure that control measures are appropriately timed.

Targeting the most susceptible stages, the first and second instars, is the best way to manage helicoverpa. Regular crop scouting determines both the number of helicoverpa larvae in the crop and the age structure of the population.

A whole-farm approach

A whole-farm approach to helicoverpa management involves:

• being familiar with the helicoverpa lifecycle and knowing what to look for, and when
• checking crops regularly to identify when crops are most susceptible (from flowering onwards) and to time control most effectively
• being familiar with the economic thresholds for different crops and using them as a basis for your control decisions
• basing chemical choices on the FS-IRMS
• applying sprays to achieve the best possible coverage
• conserving populations of predatory and parasitic insects by using selective insecticides
• managing the local population  for example, with trap cropping, pupae busting, or effective chemical control
• cultivating to destroy overwintering pupae
• destroying weed hosts within the crop and surrounding areas.

  

Further information

• To obtain copies of DPI&F publications, contact the DPI&F Business Information Centre, open from 8 a.m. to 6 p.m. Monday to Friday telephone 13 25 23 for the cost of a local call wihtin Queensland, interstate callers 07 3404 6999 or email callweb@dpi.qld.gov.au.
• DPI&F Summer and Winter Crop Management Notes on CD-ROM available from the Business Information Centre.
• Infopest - a DPI&F national database on CDROM, containing up-to-date information on all registered agricultural and veterinary chemicals. Available through the Business Information Centre.
• Crop Insects: The Ute Guide - Northern Grain Belt Edition. 2000. ISSN 0727-6273. This publication can be purchased through the DPI&F Information Centre Toowoomba, telephone 07 4688 1415 or email informU@dpi.qld.gov.au .
• Chemical-free heliothis pupae control
• IPM-Parasitoids: Natural enemies of helicoverpa
• Egg parasitoids of heliothis 2000. QI00097.
• IPM-Microplitis demolitor and ascovirus - important natural enemies of helicoverpa
• IPM-Using NPV to manage helicoverpa in field crops
• Heliothis stateline newsletter ISSN 1441-4244.
• Spring trap crop management guidelines. Australian Cotton Cooperative Research Centre . QI00049.
• SPRAYpak: Cotton growers' spray application handbook . Cotton Research & Development Corporation . ISBN 1 876354 83 6.
• Cotton pest management guide (annual publication). NSW Department of Primary Industries, Australian Cotton CRC. ISSN 1442-8792.
• The cotton pest and beneficial guide, Cotton Research & Development Corporation. ISBN 0 7242 6633 X.
• ENTOpak - A compendium of information on insect pest management for the cotton industry. Available from the Australian Cotton CRC's Technical Resource Centre at the Australian Cotton Research Institute, Narrabri. Phone (02) 67991534.
• Information on up-to-date pesticide registrations is maintained on the Australian Pesticides and Veterinary Medicines Authority (APVMA) website (www.apvma.gov.au).
• The Australian Cotton CRC website cotton.crc.org.au has information related to integrated pest management including:
  • impact of insecticides and miticides on predators in cotton
  • current insecticide resistance management strategy
  • integrated pest management guidelines for Australian cotton

The DPI&F Entomology team is a leader in the science of managing insect pests and their natural enemies in broadacre farming systems.  This work has been supported by the Australian Government Cotton Research and Development Corporation, GRDC, CRC and the Department of Primary Industries and Fisheries, Queensland.

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Last reviewed 24 January 2006