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Helicoverpa armigera resistance management strategy

Helicoverpa armigera can become
resistant to many insecticides.
(Photo: Paolo Mazzei)

P.J. Cameron¹ and G.P. Walker^2
120 Westminster Road, Mt Eden, Auckland 1003, New Zealand
²Crop & Food Research Ltd., Private Bag 92 169, Auckland, New Zealand

(Revised T.J.B. Herman, January 2025)

Reasons for strategy and update

Helicoverpa armigera (tomato fruitworm or corn earworm) can become resistant to insecticides. Management strategies aimed at preventing or minimising resistance will help maintain control and conserve the effectiveness of existing products. This is an update of earlier resistance management strategies (Cameron 1996, Cameron and Walker 2004).

Background

Tomato fruitworm, also called corn ear worm, is a pest of vegetable and flower crops. Caterpillars damage flower, fruit and leaves. Insecticides are used to control H. armigera on processing vegetable crops, such as tomatoes, sweet corn, beans and lettuce, as well as greenhouse crops and ornamentals. Populations in peas, squash, lucerne, pines and a wide variety of plants in waste areas, such as clovers, Lotus, chicory and mallow, are usually not directly treated. Helicoverpa armigera has a low damage threshold because it attacks flowers and fruit. Control on processing crops, particularly tomatoes, relied mainly on the use of synthetic pyrethroids (SPs) until the mid-1990s when fears of resistance led to the development of IPM strategies.

Overseas, resistance to synthetic pyrethroids has been of particular concern and control failures in Australia have led to the development of a strategy to limit resistance in crops in which H. armigera is a key pest (Forrester 1990). Resistance to SPs also occurs in Thailand and India, and Heliothis virescens has become resistant in the USA. Helicoverpa armigera has a history of resistance to DDT and has also developed resistance to endosulfan (Forrester et al. 1993), carbamates and organophosphates (Gunning et al. 1992).

The original genus Heliothis has now been split into Heliothis and Helicoverpa. Helicoverpa armigera is the only species resident in New Zealand. This species occurs on a wide range of crop, ornamental and weed plants. It is commonly known as tomato fruitworm, but on sweet corn or maize it is known as corn earworm. Helicoverpa punctigera adults are occasional vagrants, but larvae are rarely found. No Heliothis spp. occur in New Zealand.

Products with label claims for H. armigera control in New Zealand (November 2024)

Current status of H. armigera resistance in New Zealand

Control failures resulting from resistance have not been reported in New Zealand. The LD₅₀ for fenvalerate (a standard synthetic pyrethroid at the time) increased from 1992 to 1995, and although the frequency of individuals surviving normal field concentrations was low, increases were recorded (Cameron et al. 1995). Overseas information suggests that control failures may develop with continual selection.

An IPM programme was initiated in processing tomatoes and sweet corn in Gisborne and Hawke's Bay in 1990 (Walker & Cameron 1990). This helped reduce the number of applications of SPs to these crops. 

Two larval parasitoids of H. armigera, Cotesia kazak and Microplitis croceipes, were successfully introduced into New Zealand and total parasitism in processing tomato crops on the east coast increased from <1% prior to introductions to between 60 and 80%. However, parasitism rates are low in corn, and management of H. armigera focuses mainly on early harvest or control of the damaging third generation of larvae in corn. Another larval parasitoid, Meteorus pulchricornis, first reported in New Zealand by Berry (1997), has a wide host range which includes H. armigera and has been recorded as the dominant larval parasitoid of H. armigera in Pukekohe (Walker et al. 2005)

In tomatoes, early season insecticide applications against aphids and thrips were demonstrated to be ineffective or unnecessary for control of virus vectors and insecticide applications in early tomato crops are not normally required. Use of Bacillus thuringiensis and spinosyn insecticides increased larval parasitism rates.  Processing tomato growers initiated research to develop an adjustable action threshold whereby crop monitoring and rapid assessments of actual parasitism levels in individual crops allowed the thresholds to be adjusted to account for control by parasitism (Walker et al. 2010). Insecticide applications have decreased by 95% since the implementation of the IPM programme (Walker et al. 2010).

Resistance management and prevention strategy

General strategies for managing resistance are based on minimising insecticide applications and alternating the insecticide groups applied to each generation of H. armigera. Because this species is very mobile, the alternation relies on collaboration between industry groups growing crops that may be infested with H. armigera.

Recommendations for tomatoes, beans, sweet corn, maize, vegetable brassicas, lettuce, and other vegetables and ornamentals infested with H. armigera follow. Also see Herman (1995) for IPM in processing tomatoes.

• Grow early maturing crops to avoid the main late season populations of H. armigera.
• H. armigera should be correctly identified before insecticides are applied. Other species that damage the foliage may not require control measures.
• Apply insecticides only when essential for control. In processing tomatoes, monitoring techniques have been developed and applications are needed only when an economic action threshold of one small larva per plant is reached.
• Avoid the repeated use of one insecticide MoA. In crops that are usually infested with H. armigera, SPs should NOT be applied in the first generation of the pest, i.e. not before approximately 7 January.
• If early season insecticide applications are needed for H. armigera or other pests, these should emphasise products less toxic to beneficial species.
• Use correct label rates and application procedure for each crop.
• Emphasise cultural control techniques to remove crop residues. Thorough cultivation soon after harvesting will help destroy pupae in the soil.

Implementation

Growers should monitor their crops for H. armigera to ensure insecticides are applied only when necessary.

Synthetic pyrethroids with label claims for use against H. armigera should carry the following label statement:

IMPORTANT - RESISTANCE MANAGEMENT
Resistance to this pesticide may develop from excessive use. To minimise this risk use strictly in accordance with label instructions. For crops infested with H. armigera do not apply before 7 January and avoid unnecessary spraying. Maintain good cultural practices.

References

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