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Cotton bollworm

Agronomist, David Strahorn (Furneys CRT), has reported finding high numbers of caterpillars damaging pods and stems of canola in a crop north of Gilgandra, in the Central West Slopes and Plains district of New South Wales. The caterpillars have been identified as the cotton bollworm (Helicoverpa armigera), also commonly known as the corn earworm. Dave says the majority of caterpillars were large and causing significant feeding damage. At the time of reporting, the crop was approximately 2 weeks away from windrowing.

The cotton bollworm is widely known for its resistance to numerous insecticides, and Dave says the majority of caterpillars survived a recent spray application of lambda-cyhalothrin. Fortunately, a second spray of methomyl and alpha-cypermethrin applied the following week resulted in a good level of control. It is estimated that losses of up to 50% may have occurred if the caterpillars were not controlled. A number of surrounding paddocks have also been found with cotton bollworm, and Dave says there appears to a higher number of caterpillars surviving chemical sprays this season than in previous years.

The cotton bollworm is similar in appearance and is closely related to the native budworm (Helicoverpa punctigera) and the lesser budworm (Helicoverpa punctifera). Distinguishing between these three species involves using a microscope to look at the hairs on the body and collar (segment behind the head). For older larvae (> 20 mm in length), cotton bollworm have white hairs on the collar and black to blackish-brown body hairs. Lesser budworm has white hairs on the collar and on the body, while native budworm has black hairs on the collar and black to blackish-brown body hairs. Large larvae of cotton bollworm also have a saddle of darker pigment on the fourth body segment.

Cotton bollworm caterpillars grow up to 40 mm long and exhibit substantial colour variation, usually ranging between shades of brown, green and orange. Moths have a wingspan of 30 mm, and can be distinguished from native budworm and lesser budworm by the presence of a plain dark band along the lower edge of the hind wing that has a small pale or lighter patch within the band. The cotton bollworm is the only species of the three to have developed insecticide resistance, and it is the most problematic to control.

Problems with cotton bollworm generally result from populations that survive locally from year to year. During winter, they survive as pupae in the soil in a state of suspended development (diapause), with moths typically emerging in October. This is in contrast to the native budworm and lesser budworm, which generally breed on flowering plants in inland Australia during winter and then undertake long distance migratory flights into cropping regions in late winter and spring.

The cotton bollworm is more common in warmer regions such as northern New South Wales and southern Queensland. In cooler regions such as Victoria and Tasmania they are generally only problematic in summer. The caterpillars have a wide host range, attacking all field crops, particularly cotton, sorghum, maize, sunflowers, chickpeas, lupins and lucerne. Click here for more information on the identification, ecology and control of the cotton bollworm.

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