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Diamondback moth resistance management strategy

Plutella xylostella can become
resistant to many insecticides.
(Photo: Clemson University,
www.insectimages.org)

G.P. Walker¹, S.I. Davis², F.H. MacDonald¹ and T.J.B. Herman^3
1The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169 Auckland 1142, New Zealand ²LeaderBrand Produce Ltd, 188 Mill Rd, Bombay, Auckland 2675, New Zealand ³BASF New Zealand, PO Box 407, Auckland 1140, New Zealand

(Revised October 2024)

Reasons for strategy and update

Diamondback moth (DBM), Plutella xylostella, is notorious world-wide for building resistance across multiple insecticide modes of action.  Reduced sensitivity to various insecticide Mode of Action (MoA) groups (see: https://irac-online.org/mode-of-action/ and NZ Insecticide and miticide mode of action table https://resistance.nzpps.org/index.php?p=insecticides/introduction) has been recorded in New Zealand (Cameron & Walker 1998, Cameron et al. 1997) which led to the first DBM Resistance Management Strategy (RMS) being prepared.  It has subsequently been updated to integrate new insecticides and new MoAs into the RMS.

Background

DBM is a worldwide pest of cruciferous plants and is considered the worst pest of Brassica crops (Wikipedia 2024) and was estimated to cost the world economy US$4-5 billion annually (Zalucki et al 2012). The relatively short generation time and high fecundity of DBM allows populations to rapidly build up in crops and cause significant defoliation of crops. 

In New Zealand it can be present year-round, although the life cycle slows right down in winter.  If not controlled, DBM can render vegetable brassica crops unmarketable – through damage to leaves or contamination of harvested parts by larvae and pupae.

Over the last 35 years, researchers and the vegetable industry have developed (Beck 1991, Beck and Cameron 1990; Berry et al. 2000), demonstrated (Beck et al. 1992) and implemented an integrated pest management (IPM) programme for vegetable brassicas in New Zealand (Berry et al. 2000; Walker et al. 2009). The IPM programme was first implemented after resistance being reported in diamondback moth (DBM), Plutella xylostella, to a synthetic pyrethroid, lambda-cyhalothrin (Cameron and Walker 1998).

An updated RMS was included in an IPM manual for vegetable brassicas (Berry et al. 2000) which split the growing season into two windows: September to late January and February onwards. The windows divided the year equally into about six DBM generations each, based on heat unit accumulations. Insecticides were split between the windows: spinosad (Group 5) in the first and indoxacarb (Group 22A) in the second.

The IPM programme was updated in 2009 (Walker et al. 2009) and a DBM insecticide resistance management working group formed in 2009.  The group liaised with growers and the Brassica Crop Advisory Group of Vegetables New Zealand, to update the insecticide rotation strategy (Walker et al 2012).  In the revised strategy the diamide insecticides (Group 28) (including transplant drenches) was restricted to a window from December to March (drenches) or April (foliar sprays).  The windows formed in an earlier RMS, were kept but the groups in the windows were swapped around with Group 22A being moved to the first window.  This was done to move spinosad to the later window to match with the lettuce strategy where thrips control late is important and the two crops are often grown on the same locations.

Current status of insecticide resistance in diamondback moth in New Zealand

Surveys of vegetable brassicas in Pukekohe, Gisborne, Horowhenua and mid Canterbury (Walker et al. 2011) indicated that resistance to synthetic pyrethroids had increased in all regions, but there was little or no resistance to spinosad and indoxacarb.  Resistance to organophosphates had reduced.

The highest level of resistance to lambda-cyhalothrin was 885 times in a population sourced from Pukekohe (compared with the susceptible reference colony), and DBM showed high levels of resistance to pyrethroids in all of the surveyed regions. The results also indicated that resistance was variable not only between regions but also within regions (Walker et al. 2011). DBM resistance to lambda-cyhalothrin was significantly higher in South Auckland than elsewhere and resistance to this product has been increasing over time (Walker et al. 2004; Walker et al. 2011).

This problem may well be due to continued use of this product for control of other insect pests in the warmer, northern regions of New Zealand, where the noctuid species; soybean looper, Thysanoplusia orichalcea, and tomato fruitworm, Helicoverpa armigera, are important caterpillar pests in summer and autumn crops (Walker et al. 2009). Lambda-cyhalothrin is also used in this region to control infestations of other insect pests in leafy vegetable crops (Young 2012).

The insecticide resistance management rotation strategy for diamondback moth

Resistance in DBM to insecticide classes other than the pyrethroids appears to have been well managed over the last decade because of the country-wide adoption of an insecticide resistance management rotation strategy (Walker et al. 2004; Walker et al. 2011). This strategy depended on growers on a regional basis following IPM practices and using action thresholds to minimise spraying insecticides. When applications of insecticides were required, the two-window strategy was used, with products positioned for use either in an early- or a late-window period, with growers changing to an insecticide from a different group (with a different mode of action) at the end of each window period. It is believed this practice is the reason resistance in DBM has not developed to spinosad and indoxacarb in New Zealand. This strategy has had to be modified with the advent of new insecticide products, particularly to accommodate the use of pre-transplant drenches. The updated strategy is presented in Table 2.

¹ If chlorantraniliprole is used as a transplant drench, it should not be followed with a foliar application on that crop.

² From any one insecticide group, use a maximum of two consecutive applications (less than 15 days apart) in the life of a single crop.

³Apply Bt to small larvae on small plants.

⁴There is resistance to synthetic pyrethroids in some populations of diamondback moth (DBM). The use of this group should be avoided in areas where resistance is known to exist. Consult your local pest control advisor before use.

⁵Organophosphates are best used sparingly either against high infestations of early season seedling pests or as an end of crop 'clean-up' application.

⁶Green peach aphid, Myzus persicae, and onion thrips, Thrips tabaci, are known to be resistant to broad spectrum insecticides in New Zealand. Unnecessary application of these groups should be avoided as much as possible.

The strategy has been designed to ensure that diamides are not used all year round, in order to preserve the effectiveness of these products for as long as possible. The window for use of Durivo^™ drench has been moved to cover the main part of the year where DBM is an issue, i.e. drenching transplants from 1 December to 31 March (approximate harvest period 1 February to 30 June). This window has been designed so that there is an adequate window for at least the season's first generation of DBM to be completed in a diamide-free period. In the absence of Durivo-drenched plants, the foliar spraying of chlorantraniliprole may be used within the same window period as for Durivo. However, chlorantraniliprole should never be applied to plants drenched with Durivo.

It is suggested that using chlorantraniliprole as a foliar spray is preferrable for preventing resistance as it can be applied, as needed, using action thresholds. In contrast, using routinely chlorantraniliprole as a drench (Durivo) exposes a population for a prolonged period which increases selection for resistance.

Spinosad in the second window allows its activity against thrips (as well as caterpillars) to be better utilised.  This also aligns better with the lettuce IPM programme which may use spinosad for thrips control through the summer months (S. Davis, pers. comm.).

The strategy has been designed assuming that the industry will retain the ability to use organophosphates as a "break group" for resistance management when required.

Low use of organophosphates over recent years has resulted in a decline in organophosphate resistance in New Zealand populations of DBM (Walker et al. 2011), illustrating the importance of having multiple groups of insecticides available for effective resistance management.

The current main uses of organophosphates are in transplant nurseries (as a replacement for groups that will be subsequently used in the field) and as late-season short withholding period clean-up applications (S. Davis, pers. comm.). Other uses are against occasional establishment pests, such as wheat bug, thrips, greasy cutworm, leaf miners and springtails (S. Davis, pers. comm.).

It is recommended that organophosphates should be used only when there is a real need, with a maximum of two consecutive applications on a single crop. This is to restrict organophosphates to a single pest generation in any location. This will allow their use for crop clean-up, rotation breaks, or their availability to address issues such as leaf miners in leafy brassicas that can occur at any time and where organophosphates are an effective insecticide group.

In locations where there is no resistance recorded to synthetic pyrethroids, they can be used as a one-off nursery or pre-harvest crop clean-up spray.  Their more general use as a MoA rotation group is not recommended due to previous resistance issues and their impact on beneficial species.

Growers should use the IPM programme to ensure insecticides are applied only when necessary. If the action thresholds for caterpillar pests are exceeded, or assessments from crop scouting indicate high risk, growers should rotate insecticide groups in accordance with the national rotation strategy outlined above.

Growers throughout the country need to conform to this rotation strategy so that DBM populations do not develop resistance to individual insecticide groups. The long-term sustainability of the updated IPM programme for vegetable brassicas relies on all brassica growers minimising insecticide use and using this updated insecticide rotation strategy.

Acknowledgements

The authors acknowledge the contributions of numerous others who contributed to earlier strategies and the research programmes that laid the foundation for the IPM programme and on-going development and refinement.  The funders, both public and private sector, of research, development, implementation and continuing surveys are gratefully acknowledged and thanked.

References

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