Sustaining the effectiveness of new insecticides against aphid pests

Abstract

The overall aim of the project was to gain a clearer understanding of the incidence of insecticide resistance in several important UK aphid pests (the peach-potato aphid, Myzus persicae, the potato aphid, Macrosiphum euphorbiae, and the currant-lettuce aphid, Nasonovia ribisnigri) leading to practical recommendations for using and maintaining as wide a range of effective aphicides as possible for their control.

The project continued to investigate the occurrence and practical implications of resistance to older insecticides in aphids collected from field and protected crops. For newer compounds it established or strengthened bioassays for detailed characterisation of aphid samples. Experience has shown that resistance problems initially appearing overseas can subsequently spread to the UK. Therefore, collaboration with scientists in Europe was exploited to obtain M. persicae samples and clones that carried significantly higher resistance to imidacloprid than in the UK. The project also investigated the extent which higher resistance imposes a fitness cost through altered aphid behaviour or cold tolerance. Baseline variation in aphid response to newer insecticides was also established for Nasonovia ribisnigri and Macrosiphum euphorbiae to inform resistance monitoring for these other important UK aphid pest species.

Some of the key conclusions and implications of the project are:

• No evidence of significant resistance (that may compromise control) to neonicotinoids,
• pymetrozine or flonicamid (which belong to different chemical classes) in M. persicae in the UK.
• No directional temporal change towards an increased frequency in the UK of M. persicae
• carrying low resistance to imidacloprid and other neonicotinoids, pymetrozine or flonicamid.
• Nic-R++ M. persicae resistance (strong resistance to imidacloprid) is consistent across all
• neonicotinoids tested, reinforcing the recommendation to treat all neonicotinoids as belonging to
• the same chemical class and to alternate them with products with a different mode of action.
• Nic-R++ M. persicae are not resistant to pymetrozine or flonicamid.
• There is no association between resistance to neonicotinoids and other resistance mechanisms: MACE (pirimicarb), kdr and super-kdr (pyrethroids) in M. persicae.
• There is no apparent fitness cost, measured by ability to respond to aphid alarm pheromone, associated with neonicotinoid resistance in M. persicae.
• Neonicotinoid spray applications can impose greater selection pressures on resistant aphids compared to seed or soil treatments.
• MACE resistance (to pirimicarb) continues to be common and widespread in M. persicae in the UK and in many mainland European countries.
• Since 2003, there has been a continued decline in the frequency of M. persicae carrying resistance to pyrethroids (conferred by a specific kdr mutation) even though pyrethroid usage 6 has not fallen. However, we discovered that this species carries another pyrethroid resistance
• mechanism (a new version of super-kdr), which may be present in the majority of M. persicae in the UK. This needs to be confirmed.
• There has been a marked decline in the frequency of M. persicae carrying high (R2) or extreme (R3) esterase resistance to organophosphates (OPs), which is most likely due to the disuse of these compounds in the UK. This is in contrast to M. persicae in mainland European populations where R3 aphids are still common and OP usage tends to be much greater.
• Baseline data were generated for newer modes of action, and for two other aphid species (M. euphorbiae and N. ribisnigri) that are becoming increasingly exposed to neonicotinoids.
• Reports in the UK of control failures due to possible resistance to imidacloprid, pirimicarb, lambda-cyhalothrin and pymetrozine in M. euphorbiae and N. ribisnigri, and to imidacloprid and thiacloprid in glasshouse-potato aphids (Aulacorthum solani) were not substantiated by laboratory-based screening bioassays. Therefore, the problems with control must have occurred for another reason/s (e.g. the aphids had not been contacted by the treatments).