Larvae of these two moth species are the major pests of cotton, capable of dramatically reducing yield. Historically control of these pests with insecticides has led to insecticide resistance and dramatic reductions in beneficial populations which allows populations of other pests with fast life cycles such as aphids and spider mites to develop to damaging levels. They are the main target of Bollgard II cotton that contains Cry proteins that are toxic to the Helicoverpa larvae, dramatically reducing the need to control them with insecticide (1).
Identification: The two species of Helicoverpa are very similar in appearance; however correct identification is important because H. armigera has developed resistance to many insecticides. The two species can be separated visually only for some stages of their life cycle e.g. medium and large larvae, pupae and adults.
Medium or fourth instar H. armigera larvae can be identified by the presence of a 'saddle' of darker pigments on the fourth segment back from the head (as shown in the picture). Large or final instar larvae can be separated on the basis of colour of the large hairs on the first segment behind the head, white in H. armigera, black in H. punctigera. Pupal species can be distinguished by an expert H. armigera has two small 'tail' spines which are apart and slightly smaller while they are close together and longer in H. punctigera. Adults can be identified using the hind wings; H. armigera has a small light or pale patch in the dark section of the wing while the dark section is uniform in H. punctigera.
May be confused with: Newly hatched Helicoverpa larvae may be confused with newly hatched tipworm larvae. Even with a hand lens it is difficult to distinguish among the two species at this early life stage.
Lifecycle: At 25°C, Helicoverpa eggs take 3-4 days to hatch. During this time eggs turn from white to brown and close to hatching the black head capsule of the larvae is visible through the eggshell. Larvae develop through five or six growth stages (instars) and become fully grown in 2-3 weeks. With the inclusion of the time from moth emergence to fertile egg production of around 5 days, a generation is estimated to be completed in about 42 days during summer. Four to five generations occur per growing season. Final instar larvae move to the soil to pupate, usually not far from the base of the plant where they completed development. These larvae dig a tunnel leading to a chamber in which they change to the pupal or ’chrysalis’ stage. In this stage they undergo metamorphosis and emerge as adults. The final larve build an emergence tunnel from the pupal chamber to the soil surface (usually <10cm. During summer pupal maturation takes about 16 days. However, late in summer, as day length shortens and temperatures fall an increasing proportion of these pupae will enter diapause and will spend the winter as pupae. Cultivation of the soil to a depth of 10cm during winter destroys the emergence tunnels, directly kills many pupae and brings others to the soil surface where they are eaten by predators such as birds.
Damage: In conventional cotton varieties, all stages of growth may be attacked, but reproductive tissue is preferred. Seedlings may be 'tipped out' (ie. terminal buds eaten), squares and small bolls damaged then shed, and maturing bolls lost due to direct larval feeding or secondary fungal attack which enters through the feeding holes.
Large larvae (longer than 24mm) are the most damaging stage, since larvae consume about 50% of their overall diet as fifth and sixth instars. This highlights the importance of controlling larvae while they are still very small to small (less than 7mm).
Monitoring: Helicoverpa eggs and larvae should be monitored from seedling emergence through to 30-40% open bolls at least twice per week in both conventional and Bollgard II crops.
Natural enemies: The most common predators of Helicoverpa in field crops are predatory bugs, predatory beetles, spiders, lacewings and ants.
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 tachinid 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 damage to the crop. When parasitoids attack late instar larvae or pupae, they stop moths developing that would otherwise produce further eggs and larvae.
Brown Helicoverpa eggs. 0.5mm Photo: C. Mares
Helicoverpa eggs take between 3-4 days to hatch. 0.5mm Photo: C. Mares
This large Helicoverpa feeds on a small boll which will be shed from the plant. 35mm Photo: C. Mares
A large Helicoverpa armigera larvae – note the presence of a 'saddle' of darker pigments on the fourth segment back from the head. 35mm Photo: CSIRO
Destroying overwintering Helicoverpa pupae through cultivation is an important component of the industries Insecticide Resistance Management Strategy (IRMS).
Adult moths of H.Punctigera (left) and H. armigera (right) – note pale patch on hind wing. Photo: P. Room
H. punctigera -Left- Black hairs on 1st segment behind head of large larvae , no dark "saddle" 4th segment 4th instar (medium larvae)
H.armigera - Right- White hairs on 1st segment behind head of large larvae , dark "saddle" 4th segment 4th instar (medium larvae)
Left- H. punctigera pupal tail spines longer and closer
Right- H.armigera pupal tail spines shorter and apart